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Blocking Detection Based on Synoptic Filters

Blocking Detection Based on Synoptic Filters Hindawi Publishing Corporation Advances in Meteorology Volume 2011, Article ID 717812, 11 pages doi:10.1155/2011/717812 Research Article Bernd Schalge, Richard Blender, and Klaus Fraedrich Meteorologisches Institut, KlimaCampus, University of Hamburg, Grindelberg 5, 20144 Hamburg, Germany Correspondence should be addressed to Richard Blender, richard.blender@zmaw.de Received 26 December 2010; Revised 1 March 2011; Accepted 21 March 2011 Academic Editor: Hann-Ming Henry Juang Copyright © 2011 Bernd Schalge et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Tibaldi-Molteni blocking index is supplemented by additional filter criteria to eliminate cut-off lows and subsynoptic structures. We introduce three blocking filters and analyse their sensitivities: (i) a quantile filter requiring a minimum geopotential height anomaly to reject cut-off lows, (ii) an extent filter to extract scales above a minimum zonal width, and (iii) a persistence filter to extract events with a minimum duration. Practical filter application is analysed in two case studies and the blocking climatologies for the Northern and the Southern Hemisphere. 1. Introduction Due to its simplicity, the index of Tibaldi and Molteni (henceforth denoted as TM-Index) [5], which is based on the original idea of Lejenas ¨ and Økland [9], has become Blocking is one of the most prominent flow patterns and has a standard in automated blocking detection analyses. An received attention during the last decades due to its influence important problem of the TM-Index is that it cannot on seasonal weather characteristics. The first qualitative effectively distinguish between blocking and cut-off low criteria for the blocked atmospheric flow was introduced by patterns, since both fulfill the TM-Index criteria. The Rex [1] in 1950 suggesting the atmospheric jet becomes split modified TM-Index MTM [8]has ahigherrejection rate in two seperated parts with a considerable associated mass for cut-off lows. Restrictions in the Rex criteria can be transport, the flow to be meridional at the splitting point considered as implementations of independent filters which and downstream, and the state to persist for at least ten show synergetic effects if they are used in succession. The days with a zonal width of at least 45 degrees. The original zonal TM-Index has been extended to latitudes by Scherrer ideas formulated by Rex became the basis for several blocking et al. [10]. indices. Most of the indices detect typical atmospheric The main goal of the present study is to define filters patterns (e.g., [2, 3]). For shorter timeseries subjective and to determine their impacts on the detected blocking criteria have been used (e.g., [2, 4]). While the majority of frequency using ERA-40 reanalysis data in a T106 resolution the analyses consider the 500 hPa geopotential height (e.g., (≈1.125 )[11]. The blocking analysis is based on the TM- [5, 6]), Pelly and Hoskins [7], for example, detected blocking Index of Tibaldi and Molteni [5] with an additional criterion by a negative meridional potential temperature gradient on a to avoid gaps within blocked regions. A major aim of potential vorticity surface (2 PVU level). additional filters is to exclude nonsynoptic blocking flow Despite the differences between the index definitions, anomalies and cut-off lows. they draw similar conclusions regarding the patterns of the Three filters are applied: Northern Hemisphere blocking climatology. However, the blocking frequency, which is the percentage of timesteps a (1) quantile filter, to ensure a positive anomaly in the certain longitude, is blocked, can be as low as 5% [8], or geopotential height field in the blocked regions, higher than 20% [5]. Themainreasons forthiswiderange of frequencies in automatic detection are modifications of the (2) extent filter, to remove structures below a zonal Rex criteria and the use of different parameters. width, 2 Advances in Meteorology with the geopotential height gradients in a northern and southern region Z φ − Z φ N 0 GHGN = , φ − φ N 0 (2) Z φ − Z φ 0 S GHGS = . 20 φ − φ 0 S In the present study, the TM-Index is modified by introduc- ing differentrangesfor the gradients(thiswillbedenoted as the basic setup in the following): 0E 45E 90E 135E 180 135W 90W 45W 0W φ = 78.75 + Δ , Longitude φ = 60 + Δ, Original (3) Basic φ = 41.25 + Δ , Difference ◦ ◦ Δ, Δ , Δ = [−3.75 ,... ,3.75 ]. Figure 1: Northern Hemisphere blocking frequency: original TM- Index (solid), TM-Index for varying Δ-ranges in (1) (dashed, basic Z is the geopotential height and φ denotes latitudes. Thus, setup), and relative increase (dotted, in %). the 500 hPa geopotential height field is analysed for a pattern with a positive geopotential height gradient in the southern region and a strongly negative gradient in the northern (3) persistence filter, to remove structures below a dura- region. Some studies neglect the criterion in the north for tion threshold. GHGN < −10 m/ lat [12]. Here, the gradients GHGN and GHGS are computed for all possible combinations The filters are applied in two case studies and to the of latitudes in the ranges specified by the Δ intervals. A blocking climatologies for the Northern and the Southern longitude is defined as blocked if for at least one case the Hemisphere. criteria of (1) are satisfied. Since higher resolution data The paper is outlined as follows. In Section 2, the block- is used here compared to [5], the intervals for Δ include ing index including modifications is defined, and the data a larger number of latitudes and therefore more possible and the comparison methods are described. In Section 3, combinations. the results for the filters are given, as well as the results for The definition of a range for Δ leads to a larger number the Southern Hemisphere. Section 4 contains the case study of detected blocking events. As an example, the possible of blocking and cut-off low activity in February 1990. In ◦ ◦ ◦ ◦ ◦ latitudes for φ are [82.43 ,81.31 ,80.19 ,79.06 ,77.94 , Section 4, the results are summarised and discussed. ◦ ◦ 76.82 ,75.70 ] on a Gaussian grid. According to this definiton, it is possible to find blocking structures with a 2. Blocking Index and Filters maximum between the centre and southern region. If, for example, only the southernmost latitude in the mid-latitude In this section, the definition of the blocking index according region has higher geopotential height than one of the more to Tibaldi et al. [5] and the modifications are presented. The northern latitudes of the southern region, GHGS > 0and following filters are introduced: blocking will be detected. Without this modification, such a combination is not possible and blocking is not identified, (i) quantile filter, with a minimum threshold for the because the high geopotential is too far south and the lower geopotential height, geopotential too far north, associated with a relatively small (ii) extent filter, demanding a minimum zonal width, and meridional extent of the block. The increase of the blocking (iii) persistence filter, for a minimum life time. frequency by the variable gradient ranges in the basic setup amounts to 20% in regions with high blocking frequency and The data used is the 500 hPa geopotential height of the to 50% in regions with low blocking activity compared to the ERA-40 reanalysis dataset [11] (1958–2001) with a 6-hour original TM-Index (Figure 1). time step and a spectral truncation of T106 (≈ 1.125 on a Due to the restrictions to latitudinal gradients, the index Gaussian grid). defined here does not require a zonal extent of the blocking structure. In a small number of events, two seperated but 2.1. Blocking Index. The original criteria of Tibaldi et al. [5] close blocked longitudes can be identified which are in fact for blocking at a longitude are parts of one synoptic blocking event. Therefore, blocked regions with a gap less than 10 are merged here. This occurs GHGS > 0, typically in the center of omega-shaped blocking where the (1) southern criterion of (1) is not satisfied although a synoptic GHGN < −10 , blocking is present. The merging has limited impact on lat Blocking frequency Advances in Meteorology 3 the blocking frequency (<5%) in the present form of the time below a prescribed number of timesteps. This filter acts detection. However, if a zonal extent filter is applied, the in a Lagrangian frame since the blocking patterns are tracked. merging can lead to a crucial exceedance of the threshold. The identification in the tracking requires an overlap of at This effect is evaluated in Section 3. least one longitude for 6-hourly time steps. This algorithm The blocking index defined in (1) together with the is equivalent to the Spatial Criteria I from Barriopedro et merging criterion still includes cut-off lows which should be al. [13] (the second criterion in this study is not used). If excluded. Furthermore, the motivation for the identification there is no overlap, the blocking event at the subsequent time of synoptic blocking events led to a comparative analysis of step is considered as a new blocking. Note that this tracking filter criteria applied in previous studies. condition is sufficient to detect individual blockings in high temporal resolutions like 6 hours. There is a wide range of thresholds from 5 to 10 days for 2.2. Filters. The analysis is based on the TM-Index (4) the persistence filter used in previous studies, with 10 days including the merging criterion, which is denoted as the suggested by Rex, while more recent studies use a threshold reference setup. The main filters which are applied to obtain of 5 days [2, 9]. The durations of blocking events can vary synoptic blocking are introduced in the following sections. substantially and can even exceed 30 days, depending on the Quantile Filter. To eliminate cut-off lows, a positive large scale flow as well as on synoptic scale features of the geopotential height anomaly is demanded in the mid- circulation [14]. latitude region in addition to the TM-Index conditions (1) In summary, the application of all the filters with appropriate threshold values defined below, yields synoptic Z λ, φ − Z λ, φ > 0. (4) 0 Q 0 blocking events. It is important to note that the filters have to Z is the Q-quantile of the geopotential height for the be applied in the sequence mentioned above. particular latitude. For Q = 0.5, for example, only longitudes with geopotential heights in the mid-latitudes higher than 2.3. Blocking Center Definition. For synoptic blocking events the median are allowed for blocking. Thus, the anomaly of the center and the strength can be calculated. In this study the geopotential height in the mid-latitudes has to be positive the center is defined by the longitude within the block with compared to the respective Q value for that latitude. This the maximum GHGS, that is, where the difference of the idea is motivated by a study of Dole and Gordon [6], who geopotential heights between the midlatitudes and southern searched for persistent positive anomalies in the 500 hPa region is highest. The strength is defined by the maximum geopotential height field. GHGS. With the criterion in (4) the number of cut-off lows Although this definition might lead to fluctuations in is reduced (see Section 3). The Q-quantile is the threshold the center for certain types of blocking (e.g., omega-shape parameter to tune the stringency. However, this filter is not patterns), there are two major advantages compared to other sufficient to demand synoptic scales of blocking highs. definitions: (i) the center cannot be at the edge of the Extent Filter. Hitherto, all longitudes are investigated for blocking structure (as observed for the geopotential height blocking seperately without regard to the zonal extent. The maximum) and (ii) the center is identical to the location of extent filter removes all blocking events with a zonal width the intensity maximum (and not simply the middle of the below a prescribed threshold. The orginal criteria from Rex blocking as defined by geometric approaches). These aspects [1], which is based on subjective synoptic analysis, suggests a are considered in two case studies in Section 3.4. width of 45 . Since automatic detection routines based on the TM-Index yield distinctly smaller regions, a lower threshold 3. Results: Sensitivity and Climatology is necessary to extract blocking events of similar extension. The reason is that the TM-Index extracts blocked longitudes To obtain an overview on the impact of the individual inside the jet-split region rather than the whole blocked area filters, the blocking frequencies are analysed for each filter [3]. Furthermore, smaller values for the extent filter ensure seperately (i.e., without the other filters). Blocking events that not only the mature state of the block remains, but also are determined using the 500 hPa geopotential height in parts of the onset and the decay phase. Note that the merging ERA-40 data during 1953–2000. The analysis of the filter process introduced above inhibits spurious elimination of impacts is performed with Northern Hemispheric data, for blocking events by the extent filter. the Southern Hemisphere a climatology is presented. Note Since the zonal extent of a blocking event may vary that the filters have to be applied in the proper sequence given during a life cycle, it is possible that the extent falls below in Section 2. the threshold temporarily, accompanied by a breakup into two temporal segments. If these segments are subject to a subsequent temporal filter requiring a minimum life time, 3.1. Application to the Northern Hemisphere. In a first step, this splitting leads to an underestimation of the blocking the impact of the merging criterion is assessed. Blocking is frequency. If such a temporal filter is not applied, counting analysed in geopotential height data using the criteria in (1) of blocking frequencies is weakly influenced. with varying Δ and without any further filter. The blocking Persistence Filter. In this section, a filter is introduced frequency is compared to the frequency obtained if all to identify blocking events on synoptic time scales. This gaps smaller than 10 between blocked regions are merged. persistence filter removes blocked regions with a total life It is evident from Figure 2 that the difference between 4 Advances in Meteorology are also the regions with the highest cut-off activity (see Figure 3(b) for Q-values ranging fron 0.5 to 0.8). Extent Filter. The extent filter shows regional differences, but with a much smaller magnitude than the quantile filter (Figure 4). The regions with the lowest reduction by the extent filter are at the longitude 0 in the Atlantic/European sector and within 140E and 170W in the Pacific sector. The largest reduction of blocking is observed at 120W. The extent filter smoothes the blocking frequency profile due to the elimination of small structures; a noticable example is the reduction of the secondary maximum in the vicinity of 120W. However, features like the bulge close to the Ural are 0E 45E 90E 135E 180 135W 90W 45W 0W not affected. Longitude The extent filter is the reason why small gaps within blocked regions had to be closed in the merging process Basic before; otherwise the extent filter would have discarded this Reference blocking event since both subregions are not large enough to Figure 2: Northern Hemisphere blocking frequency: basic setup pass the filter. Thus, the preceeding merging process leads to (solid, as Figure 1) and reference setup with closure of gaps smaller an increase in the blocking frequency if an extent filter is used than 11.25 (dashed). (compare Figure 5). Persistence Filter. The persistence filter shows a similar impact as the extent filter since it also smoothes the profile in these two methods is very small (approximately 3–5%) a comparable but weaker manner (Figure 6). Furthermore, and zonally homogenous. Obviously, the merging does the reduction of the blocking frequency is even more homogeneous with a single outstanding maximum between not favor blocking frequencies in particular regions. Since merging of disconnected blocking regions is necessary for the 120W and 80W. For all temporal thresholds within 3 and identification of synoptic blockings which are subjected to 10 days, the climatological pattern of blocking is detected. For 10 days the overall blocking frequency decreases by more the extent filter, this method is used as a reference setup in the analyses below. than 85%. All major features observed in previous blocking anal- yses are detected (compare e.g., [6, 7, 9]): High blocking frequencies over the eastern Atlantic and Europe, a small 3.2. Sensitivities of the Filters. The impacts of the persistence bulge for Ural-blocking at 60E, high frequencies in a wide and the extent filter on the blocking frequency can be East-Asia/Pacific region (smaller than over Europe) and two assessed by a gradual increase of the thresholds. Since the distinct minima at 90E and 110W. reduction of the blocking frequency by the persistence filter Quantile Filter. The quantile filter is designed to exclude is homogeneous, it is possible to consider the zonal mean cut-off lows. The impact of this filter depends strongly on reduction. The reduction for thresholds between 1 and 10 the longitude (Figure 3(a)). The regions with the largest days follows a linear increase up to seven days (Figure 7). reduction in blocking frequencies are eastern Asia between Note that this pertains to the blocking frequency, while the 120E and 180, with a peak at 145E, and North America and persistence filter acts on the durations of the individual the western Altantic from 50W to 90W. This shows that blocking events. a substantial number of blockings which were detected in In a further analysis, the distribution of the total life these regions in the reference setup are in fact cut-off lows. time of blocking events is determined. The distribution is This effect is also observed in the case studies in Section 3.4. estimated by the difference in blocking frequency divided by The shape of the blocking frequency is altered so that the the mean filter value. The life time analysis is restricted to Asia/West Pacific blocking sector is now less pronounced. 10 days so that events with a longer life time are disregarded The choice of a threshold Q is a trade-off between (Figure 8). The numbers of the blocking events decrease elimination of cut-off lows and the erroneous elimination of with duration, but short-lived events contribute less to the synoptic blockings. With Q = 0.5 (i.e., for positive anomalies blocking frequency. with regard to the median), there are still some cut-off A similar analysis is performed for the extent filter cases left especially in the mentioned sensitive areas. Using (Figure 7(b)). It is important to note that the thresholds values higher than Q = 0.7 can result in the exclusion are not equidistant (because of data resolution) since the ◦ ◦ of considerable amounts of synoptic blocking events. For width of the intervals vary between 5.625 and 4.5 in the example for Q = 0.8 the total width of blocked regions histogram; this explains the nonmonotonous behavior. Here, ◦ ◦ cannot be higher than 72 (out of 360 ); this might be a too it has to be remarked that there is a stronger dependence of stringent exclusion for the simultaneous occurrence of two the blocking frequency on the longitude. major blocking events. If this filter is used with Q = 0.5, The results for the reduction versus the corresponding the excluded fraction is the frequency of cut-off lows. The thresholds are very similar for the extent and the persistence regions that have the highest reduction in blocking frequency filters. However, the number of blocking events in the class Blocking frequency Advances in Meteorology 5 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude Q = 0.7 Reference Q = 0.5 Q = 0.7 Q = 0.5 Q = 0.8 Q = 0.8 Q = 0.6 Q = 0.6 (a) (b) Figure 3: Quantile filter: Northern Hemisphere blocking frequency (a) reference setup and varying filter thresholds Q and (b) relative changes. 10 40 5 20 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude ◦ ◦ ◦ Reference 31.5 11.25 31.5 ◦ ◦ 41.625 11.25 21.375 41.625 21.375 (a) (b) Figure 4: Extent filter: Northern Hemisphere blocking frequency (a) reference setup and varying filter thresholds, (b) relative changes. with the smallest extension (< 5.625 ) is below the number 3.3. Southern Hemisphere. The following reasoning explains ◦ ◦ in the subsequent class (5.625 –11.25 ). This indicates that the increase for short temporal thresholds (see also the huge number of short-lived blocking events are not Figure 13(c) in Section 3.4 below): (i) blocking structures with an extension slightly larger than the minimum extent necessarily small. which fall below the threshold for a short time and (ii) A closely related hypothesis is that large structures are blocking structures which surpass the extent condition for more persistent [2]. This is tested by an analysis of the a short time only. Both cases contribute to the numbers of persistence filter after the extent filter with a minimum short-lived events. extension of 21.375 (Figure 9). The result indicates that even for structures larger than 21.375 the reduction in blocking In summary, the combination of both filters, for example, frequency is nearly linear up to 7 days as in Figure 7(a). a minimum extent of 21.375 and 3 days in the persistence Blocking frequency Blocking frequency Blocking frequency Blocking frequency 6 Advances in Meteorology 2%. The maximum at 40W vanishes, and the maximum in the Pacific region shifts eastward and is now located at 110W. This result indicates that the thresholds applied in the NH have to be adapted for the SH to obtain statistically sig- nificant blocking frequencies. In Figure 11, the results found 10 by three different combinations of extent and persistence filters are presented: 6 ◦ (i) extent filter threshold reduced to 15.75 , persistence 4 filter 3 days, (ii) extent filter 21.375 , persistence filter deactivated, (iii) extent filter 15.75 , persistence filter deactivated. 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude For option (i), the difference compared to the NH set- Extent 20 WO Extent 40 WO tings is generally small, but the maximum at 40W is distinctly Extent 40 Extent 20 reduced. Option (ii), however, shows some major differences. The overall pattern is more similar to the reference TM-Index Figure 5: Extent filter and merging: Northern Hemisphere blocking ◦ ◦ than to the NH settings. The blocking frequency distribution frequency for the thresholds 21.375 and 41.625 compared to the is almost identical (on a lower level) except for the region case without merging (WO) as indicated. east of South America, where the maximum in the blocking frequency disappears. The final option (iii) leads to a similar blocking frequency as (ii), but here as in (i), the maximum filter extracts synoptic blocking events (with a prior quantile east of South America at 40W is detected. For the SH, the filter with Q = 0.7). Note that the threshold of three days quantile filter seems to be obsolete since the reduction in permits the inclusion of transient block-like events. These blocking frequency is negligible and zonally homogenous events show all characteristics of a subjectively identified (Figure 11). This might indicate that SH cut-off lows and synoptic blocking: blockings do not occur in the same region. In summary, we conclude that the automated detection algorithm can be (i) a distinct positive geopotential height anomaly, applied to the SH as well, but it needs to be adapted to the (ii) a minimum extent of 2000 km, overall lower occurences of blocking events. (iii) persistence of at least three days. This conditions can be considered as a practical imple- 3.4. Case Studies. To demonstrate that the filters restrict the menation of the Rex criteria adapted for automatic blocking results of the TM-Index analysis to synoptic blocking events, detection in gridded high resolution data. two cases are considered explicitly here. The first example Finally, in Figure 10, the blocking frequency for the is a synoptic blocking event over Russia which has to pass recommended Northern Hemisphere threshold values is all filters (otherwise, they would be too strict). The second shown. The general distribution remains, however, a lot of example is a cut-off low detected by the reference TM-Index details differ which have been discussed in the preceeding in the Pacific which has to be eliminated with the same sensitivity studies. settings. The Southern Hemisphere (SH) blocking is far less Both cases occurred in February 1990. During this frequent and not as persistent as its northern counterparts month, a blocking situation is observed over eastern Europe (e.g., [15–18]). The blocking frequency distribution shows and Russia from Feb. 13 18 UTC to Feb. 26 6 UTC and a cut- only one large maximum in the Pacific region and a off event in the Pacific sector during the same time period small one in the Atlantic, east of South America at 40W from Feb. 21 12 UTC to Feb. 25 18 UTC. The dates given are (Figure 11). The comparison of the blocking frequencies in the times of the first and the last detected blocked longitude both hemispheres without filtering shows peaks above 20% of the respective event. In Figure 12, the geopotential height in the Northern Hemisphere (NH) while the corresponding fields for both cases at their peak development are shown. SH values are barely above 6%. Another prominent feature This blocking event is exemplary, with a highest geopotential is a distinct minimum in the blocking frequency close to height around 65N and 60E. 70 hours later the highest values South America at 65W. Due to the different orography, are found at 57N and 70E; thus, the event is nearly stationary. the structure of blocking distribution on the SH is simpler The geopotential height values are about 5560 m for the compared to the NH. There is only one region with a former and 5640 m for the latter case. The Pacific cut-of-low considerable amount of blocking in the Pacific between 150E has a very low geopotential height of 5000 m at its center and 80W with a maximum at 150W. at 48N and 167W. This low, however, is embedded in a If all the filters are used and set to the same parameters as larger region with relatively high geopotential height. The in the NH, 21.375 for the extent filter and 3 days for the per- development of this system is very fast and an example for sistence filter, the blocking frequency is significantly reduced rapid cyclogenesis, but it also decays relatively quickly, as it is compared to the reference TM-Index with a maximum below detected for only 102 hours. Blocking frequency Advances in Meteorology 7 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude Reference 7days 3days 7days 10 days 5days 3days 10 days 5days (a) (b) Figure 6: Persistence filter: Northern Hemisphere blocking frequency (a) reference setup and filter thresholds 3, 5, 7, and 10 days and (b) relative changes. 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 02468 10 5 10152025303540455055 Minimal duration Minimal extent ( ) (a) (b) Figure 7: Reduction in zonal mean blocking frequency with increasing (a) persistence filter (without extent or quantile filters) and (b) extent filter (without time or quantile filters). In the reference setup without filtering, both events are This example demonstrates that cut-off lows are removed deteced as blocking event by the TMI. Since the cut-off low in by a combination of the filters, while this cannot be achieved the Pacific region is nested in a ridge with high geopotential reliably with a single-filter method. The most powerful relatively far north, the quantile filter is not sufficient to setup in this example is the combination of the extent remove the structure, especially during the development and and persistence filters. In this case, these two filters lead decay phases. to the same result as all three filters, making the quantile In Figure 13, a composite of Hovmol ¨ ler diagrams for filter obsolete. However, in general, cut-off-lows should be blocked longitudes during February 1990 for five different eliminated using all filters. filter settings is shown: (a) without filter, (b) quantile filter, The combination of all three filters reduces the temporal (c) extent filter, (d) persistence filter, and (e) all filters. It extent of actual blocking events to the synoptically relevant is evident that the combination of all filters removes the segments, neglecting development and decay phases. In cut-off low completely, preserving the blocking event, which contrast to the quantile and extent filters, the persistence is detected for about five days from Feb. 16 18 UTC to filter either removes blocking structures or preserves them Feb. 21 12 UTC. completely. If this case study is performed exclusively with Blocking frequency Blocking frequency Blocking frequency Blocking frequency 8 Advances in Meteorology 0.4 0.35 0.2 0.3 0.15 0.25 0.2 0.1 0.15 0.1 0.05 0.05 0 11.25 21.375 31.5 41.625 51.75 024 6 8 10 Extent ( ) Duration (a) (b) Figure 8: Histograms of the number of blocking events for increasing (a) persistence filter threshold and (b) extent filter threshold. 1 14 0.8 0.6 0.4 0.2 02468 10 0E 45E 90E 135E 180 135W 90W 45W 0W Minimal duration Longitude Figure 9: Reduction in zonal mean blocking frequency with Figure 10: Northern Hemisphere blocking frequency for quantile increasing persistence filter (with extent filter width 21.375 , ◦ (Q = 0.7, minimum width 21.375 , and 3 days duration). without quantile filters). the persistence filter with a threshold of seven days, the result In the final implementation, the filters cooperate and the is the unabbreviated blocking event including development thresholds in the individual filters are adjusted accordingly. For a successful implementation the sequence of the applica- and decay phases, while all other structures are eliminated. tion is relevant. The three filters are implemented as follows. Obviously, the persistence filter does not discard cut-off-lows (i) A quantile filter requires a minimum threshold for if they exceed the temporal threshold. the geopotential height and is the basic method to eliminate cut-off lows. For a quantile Q = 0.5 (median), which is 4. Summary and Discussion roughly consistent with a threshold for positive anomalies, a Some automatic blocking detection methods, for example complete elimination of cut-off lows is not observed. On the other hand, values higher than Q = 0.7 should be avoided the TM-Index suggested by Tibaldi et al. [5], are not suitable since this can lead to partial removal of synoptic blocking to find synoptic scale blocking, as they include a large patterns. Therefore, the value Q = 0.7 is recommended. This number of smaller and short-lived structures. To compensate quantile filter shows a clear dependency on longitude. this deficiency, three filter criteria are introduced with the (ii) The extent filter requires a minimum zonal extension aim to restrict the outcomes to synoptic scale blocking of the blocking pattern to eliminate subsynoptic anomalies. events. The TM-Index is modified allowing a wider range of The width of 45 proposed by Rex [1] turns out to be not gradient measures and close blocked longitudes are merged. The impacts of the three filters are analysed independently. appropriate for automatic patterns which restrict to the core Blocking frequency Blocking events Blocking events Blocking frequency Advances in Meteorology 9 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude Q = 0 Reference Red. extent only Q = 0.5 Red. extent + 3D NH settings Extent only Q = 0.7 (a) (b) Figure 11: Southern Hemisphere blocking frequency: (a) for the reference setup (no filtering, solid, uppermost curve), NH settings (solid, lowermost curve), persistence filter disabled (dashed), extent filter reduced to 15.75 (dash-dotted), persistence filter disabled and extent filter width 15.75 (dotted) and (b) with quantile filters settings off (solid). Q = 0.5 (dashed), and Q = 0.7(dotted). 80N 80N 75N 75N 70N 70N 65N 65N 5350 5200 60N 5400 60N 5550 5300 5300 55N 55N 5450 5250 5500 5300 5200 50N 50N 5300 5250 5600 5550 5050 45N 5400 45N 5600 5150 40N 40N 5400 5350 35N 5650 35N 5500 5400 5650 5450 5600 5500 5600 5550 30N 30N 5700 5650 5700 5650 25N 25N 5750 5750 5800 5750 20N 20N 150E 160E 170E 180 170W 160W 150W 140W 130W 120W 10E 20E 30E 40E 50E 60E 70E 80E 90E 100E Longitude Longitude (a) (b) Figure 12: 500 hPa Geopotential heights for (a) fully developed blocking Feb. 20th 00UTC 1990, (b) cut-off low Feb. 23rd 00UTC 1990. of the event. In our analysis a width of 20 appears as an impact of the persistence filter saturates at about 7 days, ◦ ◦ optimum condition to extract synoptic scale anomalies. and the extent filter at 40 –50 , a width which has been (iii) The persistence filter demands a minimum life proposed by Rex [1]. Extent and persistence filter show a homogeneous blocking reduction which allows to estimate time for the blocking events which do not have to be the number of events with certain spatial and temporal stationary in our analysis. While Rex [1] suggests 10 days as a minimum condition, smaller thresholds have been used in scales. other studies. In the present analysis, 3 days appear to be a The analysis of the extent filter yields that only about sufficient minimum, in particular if this filter is combined 20% of all events are smaller than 5.625 ,thusverysmall with the quantile and the extent filters, since both discard structures are rare. The persistence filter analysis shows that growth and decay phases of blocking events. about 40% of all detected cases have a duration shorter than The sensitivities of the extent and the persistence filters one day. This means that there is a large number of extended, show a linear behaviour in wide threshold ranges. The but short-lived events. Latitude Blocking frequency Latitude Blocking frequency 10 Advances in Meteorology No filter Quantile filter 20 20 40 40 60 60 80 80 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude (a) (b) 0 0 Extent filter Persistence filter 20 20 40 40 60 60 80 80 100 100 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude (c) (d) All filter 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude (e) Figure 13: Hovmol ¨ ler diagrams showing blocked longitudes for 6 h timescales and 112 time steps for February 1990 with blocked areas (black) and filtered areas (grey). The red marks indicate the center of the block (highest GHGS): (a) no filter, (b) quantile filter only, (c) extent filter only, (d) persistence filter only, and (e) all filters. The case study of February 1990 demonstrates that Therefore, the extension of the present study to the SH applying all filters with the recommended threshold values requires a readjustment of the filter thresholds. does remove any structures that are not considered synoptic- In summary, the combination of all filters with Q = 0.7 scale blocking. for the quantile filter, 21.375 for the extent filter and 3 days for the persistence filter applied to the TM-Index output In the Southern Hemisphere, blocking events are far extracts synoptic scale blocking events. less frequent and are observed solely in the South Pacific. Time Time Time Time Time Advances in Meteorology 11 Acknowledgments [15] R. D. F. C. Marques and V. B. Rao, “Interannual variations of blockings in the Southern Hemisphere and their energetics,” The authors like to thank the two reviewers for their Journal of Geophysical Research D, vol. 105, no. 4, pp. 4625– useful comments. B. Schalge acknowledges support from the 4636, 2000. KlimaCampus, and from the School of Integrated Climate [16] J. M. Wiedenmann, A. R. Lupo, I. I. Mokhov, and E. A. Tikhonova, “The climatology of blocking anticyclones for the System Sciences (SICSS). Northern and Southern Hemispheres: block intensity as a diagnostic,” Journal of Climate, vol. 15, no. 23, pp. 3459–3473, References [17] P. Berrisford, B. J. Hoskins, and E. Tyrlis, “Blocking and Rossby wave breaking on the dynamical tropopause in the Southern [1] D. Rex, “Blocking action in the middle troposphere and hemisphere,” Journal of the Atmospheric Sciences, vol. 64, no. itseffects on regional climate. I. An aerological study of 8, pp. 2881–2898, 2007. blockingaction,” Tellus, vol. 2, no. 3, pp. 196–211, 1950. [18] J. A. Renwick, “Persistent positive anomalies in the Southern [2] A. R. Lupo, “Climatological features of blocking anticyclones Hemisphere circulation,” Monthly Weather Review, vol. 133, in the Northern Hemisphere,” Tellus, Series A,vol. 47, no.4, no. 4, pp. 977–988, 2005. pp. 439–456, 1995. [3] H. Lejenas ¨ and R. A. Madden, “Traveling planetary-scale waves and blocking,” Monthly Weather Review, vol. 120, no. 12, pp. 2821–2830, 1992. [4] A. R. Lupo, R. J. Oglesby, and I. I. Mokhov, “Climatolog- ical features of blocking anticyclones: a study of Northern Hemisphere CCM1 model blocking events in present-day and double CO concentration atmospheres,” Climate Dynamics, vol. 13, no. 3, pp. 181–195, 1997. [5] S. Tibaldi, F. D’Andrea, E. Tosi, and E. Roeckner, “Climatology of Northern Hemisphere blocking in the ECHAM model,” Climate Dynamics, vol. 13, no. 9, pp. 649–666, 1997. [6] R.M.Dole andN.D.Gordon, “Persistentanomalies of the extratropical Northern Hemisphere wintertime circulation: geographical distribution and regional persistence characteris- tics,” Monthly Weather Review, vol. 111, no. 8, pp. 1567–1586, [7] J.L.Pelly andB.J.Hoskins,“Anew perspectiveonblocking,” Journal of the Atmospheric Sciences, vol. 60, no. 5, pp. 743–755, [8] J. S. Watson and S. J. Colucci, “Evaluation of ensemble predictions of blocking in the NCEP global spectral model,” Monthly Weather Review, vol. 130, no. 12, pp. 3008–3021, [9] H. Lejenas ¨ and H. Økland, “Characteristics of Northern Hemisphere blocking as determined from a long time series of observational data,” Tellus, Series A, vol. 35, no. 5, pp. 350– 362, 1983. [10] S. C. Scherrer,M.Croci-Maspoli,C.Schwierz, andC.Appen- zeller, “Two-dimensional indices of atmospheric blocking and their statistical relationship with winter climate patterns in the Euro-Atlantic region,” International Journal of Climatology, vol. 26, no. 2, pp. 233–249, 2006. [11] S. M. Uppala, P. W. Kal ˚ lberg, A. J. Simmons et al., “The ERA- 40 re-analysis,” Quarterly Journal of the Royal Meteorological Society, vol. 131, no. 612, pp. 2961–3012, 2005. [12] F. Kreienkamp, A. Spekat, and W. Enke, “Stationarity of atmosphericwaves and blocking over Europe—based on a reanalysisdataset and two climate scenarios,” Theoretical andApplied Climatology, vol. 102, no. 1-2, pp. 205–212, 2010. [13] D. Barriopedro, R. Garc´ıa-Herrera, A. R. Lupo, and E. Hernandez, ´ “A climatology of Northern Hemisphere block- ing,” Journal of Climate, vol. 19, no. 6, pp. 1042–1063, 2006. [14] H. Athar and R. Lupo, “Scale analysis of blocking eventsfrom 2002 to 2004: a case study of an unusually persistentblocking event leading to a heat wave in the Gulf of Alaskaduring August 2004,” Advances in Meteorology, vol. 2010, Article ID 610263, 2010. 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Blocking Detection Based on Synoptic Filters

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Copyright © 2011 Bernd Schalge et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Publishing Corporation Advances in Meteorology Volume 2011, Article ID 717812, 11 pages doi:10.1155/2011/717812 Research Article Bernd Schalge, Richard Blender, and Klaus Fraedrich Meteorologisches Institut, KlimaCampus, University of Hamburg, Grindelberg 5, 20144 Hamburg, Germany Correspondence should be addressed to Richard Blender, richard.blender@zmaw.de Received 26 December 2010; Revised 1 March 2011; Accepted 21 March 2011 Academic Editor: Hann-Ming Henry Juang Copyright © 2011 Bernd Schalge et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Tibaldi-Molteni blocking index is supplemented by additional filter criteria to eliminate cut-off lows and subsynoptic structures. We introduce three blocking filters and analyse their sensitivities: (i) a quantile filter requiring a minimum geopotential height anomaly to reject cut-off lows, (ii) an extent filter to extract scales above a minimum zonal width, and (iii) a persistence filter to extract events with a minimum duration. Practical filter application is analysed in two case studies and the blocking climatologies for the Northern and the Southern Hemisphere. 1. Introduction Due to its simplicity, the index of Tibaldi and Molteni (henceforth denoted as TM-Index) [5], which is based on the original idea of Lejenas ¨ and Økland [9], has become Blocking is one of the most prominent flow patterns and has a standard in automated blocking detection analyses. An received attention during the last decades due to its influence important problem of the TM-Index is that it cannot on seasonal weather characteristics. The first qualitative effectively distinguish between blocking and cut-off low criteria for the blocked atmospheric flow was introduced by patterns, since both fulfill the TM-Index criteria. The Rex [1] in 1950 suggesting the atmospheric jet becomes split modified TM-Index MTM [8]has ahigherrejection rate in two seperated parts with a considerable associated mass for cut-off lows. Restrictions in the Rex criteria can be transport, the flow to be meridional at the splitting point considered as implementations of independent filters which and downstream, and the state to persist for at least ten show synergetic effects if they are used in succession. The days with a zonal width of at least 45 degrees. The original zonal TM-Index has been extended to latitudes by Scherrer ideas formulated by Rex became the basis for several blocking et al. [10]. indices. Most of the indices detect typical atmospheric The main goal of the present study is to define filters patterns (e.g., [2, 3]). For shorter timeseries subjective and to determine their impacts on the detected blocking criteria have been used (e.g., [2, 4]). While the majority of frequency using ERA-40 reanalysis data in a T106 resolution the analyses consider the 500 hPa geopotential height (e.g., (≈1.125 )[11]. The blocking analysis is based on the TM- [5, 6]), Pelly and Hoskins [7], for example, detected blocking Index of Tibaldi and Molteni [5] with an additional criterion by a negative meridional potential temperature gradient on a to avoid gaps within blocked regions. A major aim of potential vorticity surface (2 PVU level). additional filters is to exclude nonsynoptic blocking flow Despite the differences between the index definitions, anomalies and cut-off lows. they draw similar conclusions regarding the patterns of the Three filters are applied: Northern Hemisphere blocking climatology. However, the blocking frequency, which is the percentage of timesteps a (1) quantile filter, to ensure a positive anomaly in the certain longitude, is blocked, can be as low as 5% [8], or geopotential height field in the blocked regions, higher than 20% [5]. Themainreasons forthiswiderange of frequencies in automatic detection are modifications of the (2) extent filter, to remove structures below a zonal Rex criteria and the use of different parameters. width, 2 Advances in Meteorology with the geopotential height gradients in a northern and southern region Z φ − Z φ N 0 GHGN = , φ − φ N 0 (2) Z φ − Z φ 0 S GHGS = . 20 φ − φ 0 S In the present study, the TM-Index is modified by introduc- ing differentrangesfor the gradients(thiswillbedenoted as the basic setup in the following): 0E 45E 90E 135E 180 135W 90W 45W 0W φ = 78.75 + Δ , Longitude φ = 60 + Δ, Original (3) Basic φ = 41.25 + Δ , Difference ◦ ◦ Δ, Δ , Δ = [−3.75 ,... ,3.75 ]. Figure 1: Northern Hemisphere blocking frequency: original TM- Index (solid), TM-Index for varying Δ-ranges in (1) (dashed, basic Z is the geopotential height and φ denotes latitudes. Thus, setup), and relative increase (dotted, in %). the 500 hPa geopotential height field is analysed for a pattern with a positive geopotential height gradient in the southern region and a strongly negative gradient in the northern (3) persistence filter, to remove structures below a dura- region. Some studies neglect the criterion in the north for tion threshold. GHGN < −10 m/ lat [12]. Here, the gradients GHGN and GHGS are computed for all possible combinations The filters are applied in two case studies and to the of latitudes in the ranges specified by the Δ intervals. A blocking climatologies for the Northern and the Southern longitude is defined as blocked if for at least one case the Hemisphere. criteria of (1) are satisfied. Since higher resolution data The paper is outlined as follows. In Section 2, the block- is used here compared to [5], the intervals for Δ include ing index including modifications is defined, and the data a larger number of latitudes and therefore more possible and the comparison methods are described. In Section 3, combinations. the results for the filters are given, as well as the results for The definition of a range for Δ leads to a larger number the Southern Hemisphere. Section 4 contains the case study of detected blocking events. As an example, the possible of blocking and cut-off low activity in February 1990. In ◦ ◦ ◦ ◦ ◦ latitudes for φ are [82.43 ,81.31 ,80.19 ,79.06 ,77.94 , Section 4, the results are summarised and discussed. ◦ ◦ 76.82 ,75.70 ] on a Gaussian grid. According to this definiton, it is possible to find blocking structures with a 2. Blocking Index and Filters maximum between the centre and southern region. If, for example, only the southernmost latitude in the mid-latitude In this section, the definition of the blocking index according region has higher geopotential height than one of the more to Tibaldi et al. [5] and the modifications are presented. The northern latitudes of the southern region, GHGS > 0and following filters are introduced: blocking will be detected. Without this modification, such a combination is not possible and blocking is not identified, (i) quantile filter, with a minimum threshold for the because the high geopotential is too far south and the lower geopotential height, geopotential too far north, associated with a relatively small (ii) extent filter, demanding a minimum zonal width, and meridional extent of the block. The increase of the blocking (iii) persistence filter, for a minimum life time. frequency by the variable gradient ranges in the basic setup amounts to 20% in regions with high blocking frequency and The data used is the 500 hPa geopotential height of the to 50% in regions with low blocking activity compared to the ERA-40 reanalysis dataset [11] (1958–2001) with a 6-hour original TM-Index (Figure 1). time step and a spectral truncation of T106 (≈ 1.125 on a Due to the restrictions to latitudinal gradients, the index Gaussian grid). defined here does not require a zonal extent of the blocking structure. In a small number of events, two seperated but 2.1. Blocking Index. The original criteria of Tibaldi et al. [5] close blocked longitudes can be identified which are in fact for blocking at a longitude are parts of one synoptic blocking event. Therefore, blocked regions with a gap less than 10 are merged here. This occurs GHGS > 0, typically in the center of omega-shaped blocking where the (1) southern criterion of (1) is not satisfied although a synoptic GHGN < −10 , blocking is present. The merging has limited impact on lat Blocking frequency Advances in Meteorology 3 the blocking frequency (<5%) in the present form of the time below a prescribed number of timesteps. This filter acts detection. However, if a zonal extent filter is applied, the in a Lagrangian frame since the blocking patterns are tracked. merging can lead to a crucial exceedance of the threshold. The identification in the tracking requires an overlap of at This effect is evaluated in Section 3. least one longitude for 6-hourly time steps. This algorithm The blocking index defined in (1) together with the is equivalent to the Spatial Criteria I from Barriopedro et merging criterion still includes cut-off lows which should be al. [13] (the second criterion in this study is not used). If excluded. Furthermore, the motivation for the identification there is no overlap, the blocking event at the subsequent time of synoptic blocking events led to a comparative analysis of step is considered as a new blocking. Note that this tracking filter criteria applied in previous studies. condition is sufficient to detect individual blockings in high temporal resolutions like 6 hours. There is a wide range of thresholds from 5 to 10 days for 2.2. Filters. The analysis is based on the TM-Index (4) the persistence filter used in previous studies, with 10 days including the merging criterion, which is denoted as the suggested by Rex, while more recent studies use a threshold reference setup. The main filters which are applied to obtain of 5 days [2, 9]. The durations of blocking events can vary synoptic blocking are introduced in the following sections. substantially and can even exceed 30 days, depending on the Quantile Filter. To eliminate cut-off lows, a positive large scale flow as well as on synoptic scale features of the geopotential height anomaly is demanded in the mid- circulation [14]. latitude region in addition to the TM-Index conditions (1) In summary, the application of all the filters with appropriate threshold values defined below, yields synoptic Z λ, φ − Z λ, φ > 0. (4) 0 Q 0 blocking events. It is important to note that the filters have to Z is the Q-quantile of the geopotential height for the be applied in the sequence mentioned above. particular latitude. For Q = 0.5, for example, only longitudes with geopotential heights in the mid-latitudes higher than 2.3. Blocking Center Definition. For synoptic blocking events the median are allowed for blocking. Thus, the anomaly of the center and the strength can be calculated. In this study the geopotential height in the mid-latitudes has to be positive the center is defined by the longitude within the block with compared to the respective Q value for that latitude. This the maximum GHGS, that is, where the difference of the idea is motivated by a study of Dole and Gordon [6], who geopotential heights between the midlatitudes and southern searched for persistent positive anomalies in the 500 hPa region is highest. The strength is defined by the maximum geopotential height field. GHGS. With the criterion in (4) the number of cut-off lows Although this definition might lead to fluctuations in is reduced (see Section 3). The Q-quantile is the threshold the center for certain types of blocking (e.g., omega-shape parameter to tune the stringency. However, this filter is not patterns), there are two major advantages compared to other sufficient to demand synoptic scales of blocking highs. definitions: (i) the center cannot be at the edge of the Extent Filter. Hitherto, all longitudes are investigated for blocking structure (as observed for the geopotential height blocking seperately without regard to the zonal extent. The maximum) and (ii) the center is identical to the location of extent filter removes all blocking events with a zonal width the intensity maximum (and not simply the middle of the below a prescribed threshold. The orginal criteria from Rex blocking as defined by geometric approaches). These aspects [1], which is based on subjective synoptic analysis, suggests a are considered in two case studies in Section 3.4. width of 45 . Since automatic detection routines based on the TM-Index yield distinctly smaller regions, a lower threshold 3. Results: Sensitivity and Climatology is necessary to extract blocking events of similar extension. The reason is that the TM-Index extracts blocked longitudes To obtain an overview on the impact of the individual inside the jet-split region rather than the whole blocked area filters, the blocking frequencies are analysed for each filter [3]. Furthermore, smaller values for the extent filter ensure seperately (i.e., without the other filters). Blocking events that not only the mature state of the block remains, but also are determined using the 500 hPa geopotential height in parts of the onset and the decay phase. Note that the merging ERA-40 data during 1953–2000. The analysis of the filter process introduced above inhibits spurious elimination of impacts is performed with Northern Hemispheric data, for blocking events by the extent filter. the Southern Hemisphere a climatology is presented. Note Since the zonal extent of a blocking event may vary that the filters have to be applied in the proper sequence given during a life cycle, it is possible that the extent falls below in Section 2. the threshold temporarily, accompanied by a breakup into two temporal segments. If these segments are subject to a subsequent temporal filter requiring a minimum life time, 3.1. Application to the Northern Hemisphere. In a first step, this splitting leads to an underestimation of the blocking the impact of the merging criterion is assessed. Blocking is frequency. If such a temporal filter is not applied, counting analysed in geopotential height data using the criteria in (1) of blocking frequencies is weakly influenced. with varying Δ and without any further filter. The blocking Persistence Filter. In this section, a filter is introduced frequency is compared to the frequency obtained if all to identify blocking events on synoptic time scales. This gaps smaller than 10 between blocked regions are merged. persistence filter removes blocked regions with a total life It is evident from Figure 2 that the difference between 4 Advances in Meteorology are also the regions with the highest cut-off activity (see Figure 3(b) for Q-values ranging fron 0.5 to 0.8). Extent Filter. The extent filter shows regional differences, but with a much smaller magnitude than the quantile filter (Figure 4). The regions with the lowest reduction by the extent filter are at the longitude 0 in the Atlantic/European sector and within 140E and 170W in the Pacific sector. The largest reduction of blocking is observed at 120W. The extent filter smoothes the blocking frequency profile due to the elimination of small structures; a noticable example is the reduction of the secondary maximum in the vicinity of 120W. However, features like the bulge close to the Ural are 0E 45E 90E 135E 180 135W 90W 45W 0W not affected. Longitude The extent filter is the reason why small gaps within blocked regions had to be closed in the merging process Basic before; otherwise the extent filter would have discarded this Reference blocking event since both subregions are not large enough to Figure 2: Northern Hemisphere blocking frequency: basic setup pass the filter. Thus, the preceeding merging process leads to (solid, as Figure 1) and reference setup with closure of gaps smaller an increase in the blocking frequency if an extent filter is used than 11.25 (dashed). (compare Figure 5). Persistence Filter. The persistence filter shows a similar impact as the extent filter since it also smoothes the profile in these two methods is very small (approximately 3–5%) a comparable but weaker manner (Figure 6). Furthermore, and zonally homogenous. Obviously, the merging does the reduction of the blocking frequency is even more homogeneous with a single outstanding maximum between not favor blocking frequencies in particular regions. Since merging of disconnected blocking regions is necessary for the 120W and 80W. For all temporal thresholds within 3 and identification of synoptic blockings which are subjected to 10 days, the climatological pattern of blocking is detected. For 10 days the overall blocking frequency decreases by more the extent filter, this method is used as a reference setup in the analyses below. than 85%. All major features observed in previous blocking anal- yses are detected (compare e.g., [6, 7, 9]): High blocking frequencies over the eastern Atlantic and Europe, a small 3.2. Sensitivities of the Filters. The impacts of the persistence bulge for Ural-blocking at 60E, high frequencies in a wide and the extent filter on the blocking frequency can be East-Asia/Pacific region (smaller than over Europe) and two assessed by a gradual increase of the thresholds. Since the distinct minima at 90E and 110W. reduction of the blocking frequency by the persistence filter Quantile Filter. The quantile filter is designed to exclude is homogeneous, it is possible to consider the zonal mean cut-off lows. The impact of this filter depends strongly on reduction. The reduction for thresholds between 1 and 10 the longitude (Figure 3(a)). The regions with the largest days follows a linear increase up to seven days (Figure 7). reduction in blocking frequencies are eastern Asia between Note that this pertains to the blocking frequency, while the 120E and 180, with a peak at 145E, and North America and persistence filter acts on the durations of the individual the western Altantic from 50W to 90W. This shows that blocking events. a substantial number of blockings which were detected in In a further analysis, the distribution of the total life these regions in the reference setup are in fact cut-off lows. time of blocking events is determined. The distribution is This effect is also observed in the case studies in Section 3.4. estimated by the difference in blocking frequency divided by The shape of the blocking frequency is altered so that the the mean filter value. The life time analysis is restricted to Asia/West Pacific blocking sector is now less pronounced. 10 days so that events with a longer life time are disregarded The choice of a threshold Q is a trade-off between (Figure 8). The numbers of the blocking events decrease elimination of cut-off lows and the erroneous elimination of with duration, but short-lived events contribute less to the synoptic blockings. With Q = 0.5 (i.e., for positive anomalies blocking frequency. with regard to the median), there are still some cut-off A similar analysis is performed for the extent filter cases left especially in the mentioned sensitive areas. Using (Figure 7(b)). It is important to note that the thresholds values higher than Q = 0.7 can result in the exclusion are not equidistant (because of data resolution) since the ◦ ◦ of considerable amounts of synoptic blocking events. For width of the intervals vary between 5.625 and 4.5 in the example for Q = 0.8 the total width of blocked regions histogram; this explains the nonmonotonous behavior. Here, ◦ ◦ cannot be higher than 72 (out of 360 ); this might be a too it has to be remarked that there is a stronger dependence of stringent exclusion for the simultaneous occurrence of two the blocking frequency on the longitude. major blocking events. If this filter is used with Q = 0.5, The results for the reduction versus the corresponding the excluded fraction is the frequency of cut-off lows. The thresholds are very similar for the extent and the persistence regions that have the highest reduction in blocking frequency filters. However, the number of blocking events in the class Blocking frequency Advances in Meteorology 5 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude Q = 0.7 Reference Q = 0.5 Q = 0.7 Q = 0.5 Q = 0.8 Q = 0.8 Q = 0.6 Q = 0.6 (a) (b) Figure 3: Quantile filter: Northern Hemisphere blocking frequency (a) reference setup and varying filter thresholds Q and (b) relative changes. 10 40 5 20 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude ◦ ◦ ◦ Reference 31.5 11.25 31.5 ◦ ◦ 41.625 11.25 21.375 41.625 21.375 (a) (b) Figure 4: Extent filter: Northern Hemisphere blocking frequency (a) reference setup and varying filter thresholds, (b) relative changes. with the smallest extension (< 5.625 ) is below the number 3.3. Southern Hemisphere. The following reasoning explains ◦ ◦ in the subsequent class (5.625 –11.25 ). This indicates that the increase for short temporal thresholds (see also the huge number of short-lived blocking events are not Figure 13(c) in Section 3.4 below): (i) blocking structures with an extension slightly larger than the minimum extent necessarily small. which fall below the threshold for a short time and (ii) A closely related hypothesis is that large structures are blocking structures which surpass the extent condition for more persistent [2]. This is tested by an analysis of the a short time only. Both cases contribute to the numbers of persistence filter after the extent filter with a minimum short-lived events. extension of 21.375 (Figure 9). The result indicates that even for structures larger than 21.375 the reduction in blocking In summary, the combination of both filters, for example, frequency is nearly linear up to 7 days as in Figure 7(a). a minimum extent of 21.375 and 3 days in the persistence Blocking frequency Blocking frequency Blocking frequency Blocking frequency 6 Advances in Meteorology 2%. The maximum at 40W vanishes, and the maximum in the Pacific region shifts eastward and is now located at 110W. This result indicates that the thresholds applied in the NH have to be adapted for the SH to obtain statistically sig- nificant blocking frequencies. In Figure 11, the results found 10 by three different combinations of extent and persistence filters are presented: 6 ◦ (i) extent filter threshold reduced to 15.75 , persistence 4 filter 3 days, (ii) extent filter 21.375 , persistence filter deactivated, (iii) extent filter 15.75 , persistence filter deactivated. 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude For option (i), the difference compared to the NH set- Extent 20 WO Extent 40 WO tings is generally small, but the maximum at 40W is distinctly Extent 40 Extent 20 reduced. Option (ii), however, shows some major differences. The overall pattern is more similar to the reference TM-Index Figure 5: Extent filter and merging: Northern Hemisphere blocking ◦ ◦ than to the NH settings. The blocking frequency distribution frequency for the thresholds 21.375 and 41.625 compared to the is almost identical (on a lower level) except for the region case without merging (WO) as indicated. east of South America, where the maximum in the blocking frequency disappears. The final option (iii) leads to a similar blocking frequency as (ii), but here as in (i), the maximum filter extracts synoptic blocking events (with a prior quantile east of South America at 40W is detected. For the SH, the filter with Q = 0.7). Note that the threshold of three days quantile filter seems to be obsolete since the reduction in permits the inclusion of transient block-like events. These blocking frequency is negligible and zonally homogenous events show all characteristics of a subjectively identified (Figure 11). This might indicate that SH cut-off lows and synoptic blocking: blockings do not occur in the same region. In summary, we conclude that the automated detection algorithm can be (i) a distinct positive geopotential height anomaly, applied to the SH as well, but it needs to be adapted to the (ii) a minimum extent of 2000 km, overall lower occurences of blocking events. (iii) persistence of at least three days. This conditions can be considered as a practical imple- 3.4. Case Studies. To demonstrate that the filters restrict the menation of the Rex criteria adapted for automatic blocking results of the TM-Index analysis to synoptic blocking events, detection in gridded high resolution data. two cases are considered explicitly here. The first example Finally, in Figure 10, the blocking frequency for the is a synoptic blocking event over Russia which has to pass recommended Northern Hemisphere threshold values is all filters (otherwise, they would be too strict). The second shown. The general distribution remains, however, a lot of example is a cut-off low detected by the reference TM-Index details differ which have been discussed in the preceeding in the Pacific which has to be eliminated with the same sensitivity studies. settings. The Southern Hemisphere (SH) blocking is far less Both cases occurred in February 1990. During this frequent and not as persistent as its northern counterparts month, a blocking situation is observed over eastern Europe (e.g., [15–18]). The blocking frequency distribution shows and Russia from Feb. 13 18 UTC to Feb. 26 6 UTC and a cut- only one large maximum in the Pacific region and a off event in the Pacific sector during the same time period small one in the Atlantic, east of South America at 40W from Feb. 21 12 UTC to Feb. 25 18 UTC. The dates given are (Figure 11). The comparison of the blocking frequencies in the times of the first and the last detected blocked longitude both hemispheres without filtering shows peaks above 20% of the respective event. In Figure 12, the geopotential height in the Northern Hemisphere (NH) while the corresponding fields for both cases at their peak development are shown. SH values are barely above 6%. Another prominent feature This blocking event is exemplary, with a highest geopotential is a distinct minimum in the blocking frequency close to height around 65N and 60E. 70 hours later the highest values South America at 65W. Due to the different orography, are found at 57N and 70E; thus, the event is nearly stationary. the structure of blocking distribution on the SH is simpler The geopotential height values are about 5560 m for the compared to the NH. There is only one region with a former and 5640 m for the latter case. The Pacific cut-of-low considerable amount of blocking in the Pacific between 150E has a very low geopotential height of 5000 m at its center and 80W with a maximum at 150W. at 48N and 167W. This low, however, is embedded in a If all the filters are used and set to the same parameters as larger region with relatively high geopotential height. The in the NH, 21.375 for the extent filter and 3 days for the per- development of this system is very fast and an example for sistence filter, the blocking frequency is significantly reduced rapid cyclogenesis, but it also decays relatively quickly, as it is compared to the reference TM-Index with a maximum below detected for only 102 hours. Blocking frequency Advances in Meteorology 7 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude Reference 7days 3days 7days 10 days 5days 3days 10 days 5days (a) (b) Figure 6: Persistence filter: Northern Hemisphere blocking frequency (a) reference setup and filter thresholds 3, 5, 7, and 10 days and (b) relative changes. 1 1 0.8 0.8 0.6 0.6 0.4 0.4 0.2 0.2 02468 10 5 10152025303540455055 Minimal duration Minimal extent ( ) (a) (b) Figure 7: Reduction in zonal mean blocking frequency with increasing (a) persistence filter (without extent or quantile filters) and (b) extent filter (without time or quantile filters). In the reference setup without filtering, both events are This example demonstrates that cut-off lows are removed deteced as blocking event by the TMI. Since the cut-off low in by a combination of the filters, while this cannot be achieved the Pacific region is nested in a ridge with high geopotential reliably with a single-filter method. The most powerful relatively far north, the quantile filter is not sufficient to setup in this example is the combination of the extent remove the structure, especially during the development and and persistence filters. In this case, these two filters lead decay phases. to the same result as all three filters, making the quantile In Figure 13, a composite of Hovmol ¨ ler diagrams for filter obsolete. However, in general, cut-off-lows should be blocked longitudes during February 1990 for five different eliminated using all filters. filter settings is shown: (a) without filter, (b) quantile filter, The combination of all three filters reduces the temporal (c) extent filter, (d) persistence filter, and (e) all filters. It extent of actual blocking events to the synoptically relevant is evident that the combination of all filters removes the segments, neglecting development and decay phases. In cut-off low completely, preserving the blocking event, which contrast to the quantile and extent filters, the persistence is detected for about five days from Feb. 16 18 UTC to filter either removes blocking structures or preserves them Feb. 21 12 UTC. completely. If this case study is performed exclusively with Blocking frequency Blocking frequency Blocking frequency Blocking frequency 8 Advances in Meteorology 0.4 0.35 0.2 0.3 0.15 0.25 0.2 0.1 0.15 0.1 0.05 0.05 0 11.25 21.375 31.5 41.625 51.75 024 6 8 10 Extent ( ) Duration (a) (b) Figure 8: Histograms of the number of blocking events for increasing (a) persistence filter threshold and (b) extent filter threshold. 1 14 0.8 0.6 0.4 0.2 02468 10 0E 45E 90E 135E 180 135W 90W 45W 0W Minimal duration Longitude Figure 9: Reduction in zonal mean blocking frequency with Figure 10: Northern Hemisphere blocking frequency for quantile increasing persistence filter (with extent filter width 21.375 , ◦ (Q = 0.7, minimum width 21.375 , and 3 days duration). without quantile filters). the persistence filter with a threshold of seven days, the result In the final implementation, the filters cooperate and the is the unabbreviated blocking event including development thresholds in the individual filters are adjusted accordingly. For a successful implementation the sequence of the applica- and decay phases, while all other structures are eliminated. tion is relevant. The three filters are implemented as follows. Obviously, the persistence filter does not discard cut-off-lows (i) A quantile filter requires a minimum threshold for if they exceed the temporal threshold. the geopotential height and is the basic method to eliminate cut-off lows. For a quantile Q = 0.5 (median), which is 4. Summary and Discussion roughly consistent with a threshold for positive anomalies, a Some automatic blocking detection methods, for example complete elimination of cut-off lows is not observed. On the other hand, values higher than Q = 0.7 should be avoided the TM-Index suggested by Tibaldi et al. [5], are not suitable since this can lead to partial removal of synoptic blocking to find synoptic scale blocking, as they include a large patterns. Therefore, the value Q = 0.7 is recommended. This number of smaller and short-lived structures. To compensate quantile filter shows a clear dependency on longitude. this deficiency, three filter criteria are introduced with the (ii) The extent filter requires a minimum zonal extension aim to restrict the outcomes to synoptic scale blocking of the blocking pattern to eliminate subsynoptic anomalies. events. The TM-Index is modified allowing a wider range of The width of 45 proposed by Rex [1] turns out to be not gradient measures and close blocked longitudes are merged. The impacts of the three filters are analysed independently. appropriate for automatic patterns which restrict to the core Blocking frequency Blocking events Blocking events Blocking frequency Advances in Meteorology 9 7 7 6 6 5 5 4 4 3 3 2 2 1 1 0 0 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude Q = 0 Reference Red. extent only Q = 0.5 Red. extent + 3D NH settings Extent only Q = 0.7 (a) (b) Figure 11: Southern Hemisphere blocking frequency: (a) for the reference setup (no filtering, solid, uppermost curve), NH settings (solid, lowermost curve), persistence filter disabled (dashed), extent filter reduced to 15.75 (dash-dotted), persistence filter disabled and extent filter width 15.75 (dotted) and (b) with quantile filters settings off (solid). Q = 0.5 (dashed), and Q = 0.7(dotted). 80N 80N 75N 75N 70N 70N 65N 65N 5350 5200 60N 5400 60N 5550 5300 5300 55N 55N 5450 5250 5500 5300 5200 50N 50N 5300 5250 5600 5550 5050 45N 5400 45N 5600 5150 40N 40N 5400 5350 35N 5650 35N 5500 5400 5650 5450 5600 5500 5600 5550 30N 30N 5700 5650 5700 5650 25N 25N 5750 5750 5800 5750 20N 20N 150E 160E 170E 180 170W 160W 150W 140W 130W 120W 10E 20E 30E 40E 50E 60E 70E 80E 90E 100E Longitude Longitude (a) (b) Figure 12: 500 hPa Geopotential heights for (a) fully developed blocking Feb. 20th 00UTC 1990, (b) cut-off low Feb. 23rd 00UTC 1990. of the event. In our analysis a width of 20 appears as an impact of the persistence filter saturates at about 7 days, ◦ ◦ optimum condition to extract synoptic scale anomalies. and the extent filter at 40 –50 , a width which has been (iii) The persistence filter demands a minimum life proposed by Rex [1]. Extent and persistence filter show a homogeneous blocking reduction which allows to estimate time for the blocking events which do not have to be the number of events with certain spatial and temporal stationary in our analysis. While Rex [1] suggests 10 days as a minimum condition, smaller thresholds have been used in scales. other studies. In the present analysis, 3 days appear to be a The analysis of the extent filter yields that only about sufficient minimum, in particular if this filter is combined 20% of all events are smaller than 5.625 ,thusverysmall with the quantile and the extent filters, since both discard structures are rare. The persistence filter analysis shows that growth and decay phases of blocking events. about 40% of all detected cases have a duration shorter than The sensitivities of the extent and the persistence filters one day. This means that there is a large number of extended, show a linear behaviour in wide threshold ranges. The but short-lived events. Latitude Blocking frequency Latitude Blocking frequency 10 Advances in Meteorology No filter Quantile filter 20 20 40 40 60 60 80 80 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude (a) (b) 0 0 Extent filter Persistence filter 20 20 40 40 60 60 80 80 100 100 0E 45E 90E 135E 180 135W 90W 45W 0W 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude Longitude (c) (d) All filter 0E 45E 90E 135E 180 135W 90W 45W 0W Longitude (e) Figure 13: Hovmol ¨ ler diagrams showing blocked longitudes for 6 h timescales and 112 time steps for February 1990 with blocked areas (black) and filtered areas (grey). The red marks indicate the center of the block (highest GHGS): (a) no filter, (b) quantile filter only, (c) extent filter only, (d) persistence filter only, and (e) all filters. The case study of February 1990 demonstrates that Therefore, the extension of the present study to the SH applying all filters with the recommended threshold values requires a readjustment of the filter thresholds. does remove any structures that are not considered synoptic- In summary, the combination of all filters with Q = 0.7 scale blocking. for the quantile filter, 21.375 for the extent filter and 3 days for the persistence filter applied to the TM-Index output In the Southern Hemisphere, blocking events are far extracts synoptic scale blocking events. less frequent and are observed solely in the South Pacific. Time Time Time Time Time Advances in Meteorology 11 Acknowledgments [15] R. D. F. C. Marques and V. B. Rao, “Interannual variations of blockings in the Southern Hemisphere and their energetics,” The authors like to thank the two reviewers for their Journal of Geophysical Research D, vol. 105, no. 4, pp. 4625– useful comments. B. 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Scherrer,M.Croci-Maspoli,C.Schwierz, andC.Appen- zeller, “Two-dimensional indices of atmospheric blocking and their statistical relationship with winter climate patterns in the Euro-Atlantic region,” International Journal of Climatology, vol. 26, no. 2, pp. 233–249, 2006. [11] S. M. Uppala, P. W. Kal ˚ lberg, A. J. Simmons et al., “The ERA- 40 re-analysis,” Quarterly Journal of the Royal Meteorological Society, vol. 131, no. 612, pp. 2961–3012, 2005. [12] F. Kreienkamp, A. Spekat, and W. Enke, “Stationarity of atmosphericwaves and blocking over Europe—based on a reanalysisdataset and two climate scenarios,” Theoretical andApplied Climatology, vol. 102, no. 1-2, pp. 205–212, 2010. [13] D. Barriopedro, R. Garc´ıa-Herrera, A. R. Lupo, and E. Hernandez, ´ “A climatology of Northern Hemisphere block- ing,” Journal of Climate, vol. 19, no. 6, pp. 1042–1063, 2006. [14] H. Athar and R. Lupo, “Scale analysis of blocking eventsfrom 2002 to 2004: a case study of an unusually persistentblocking event leading to a heat wave in the Gulf of Alaskaduring August 2004,” Advances in Meteorology, vol. 2010, Article ID 610263, 2010. 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