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Characterization of the Vertical Structure of Coastal Atmospheric Boundary Layer over Thumba ( πŸ– . πŸ“ ∘ N , πŸ• πŸ” . πŸ— ∘ E ) during Different Seasons

Characterization of the Vertical Structure of Coastal Atmospheric Boundary Layer over Thumba (... Hindawi Publishing Corporation Advances in Meteorology Volume 2011, Article ID 390826, 9 pages doi:10.1155/2011/390826 Research Article Characterization of the Vertical Structure of Coastal Atmospheric Boundary Layer over Thumba ◦ ◦ (8.5 N, 76.9 E) during Different Seasons 1 1 1 1 Sandhya K. Nair, T. J. Anurose, D. Bala Subrahamanyam, N. V. P. Kiran Kumar, 1 1 1 2 M. Santosh, S. Sijikumar, Mannil Mohan, andK.V.S.Namboodiri Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Indian Space Research Organization, Government of India, Thiruvananthapuram 695 022, India MET Facility, Vikram Sarabhai Space Centre, Department of Space, Indian Space Research Organization, Government of India, Thiruvananthapuram 695 022, India Correspondence should be addressed to D. Bala Subrahamanyam, subrahamanyam@gmail.com Received 24 December 2010; Revised 8 March 2011; Accepted 23 March 2011 Academic Editor: Branko Grisogono Copyright © 2011 Sandhya K. Nair 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. Vertical profiles of meteorological parameters obtained from balloon-borne GPS Radiosonde for a period of more than two years ◦ ◦ are analyzed for characterization of the coastal atmospheric boundary layer (CABL) over Thumba (8.5 N, 76.9 E, India). The study reports seasonal variability in the thickness of three different sublayers of the CABL, namely, mixed layer, turbulent flow, and sea breeze flow. Among the three, the vertical thickness of sea breeze flow showed considerable dominance on the other two throughout the year. Mixed layer heights derived through gradients in virtual potential temperature (θ ) showed large seasonal variability with a peak in the Summer and Post-Monsoon. On the other hand, the vertical thickness of turbulent flow remained steady all through the year. Results from the present study indicate that the magnitudes of mixed layer heights are often larger than the turbulent flow thickness. 1. Introduction used to characterize the vertical extent of mixing within the boundary layer and the level at which exchange with the free In Physics and Fluid Dynamics, a boundary layer is generally troposphere occurs [4–6]. In principle, the ABL thickness defined as a layer of the fluid in the immediate vicinity of can be retrieved from atmospheric global circulation models a bounding surface, where viscosity variations are dealt in since they contain algorithms which determine the intensity detail. The atmospheric boundary layer (ABL) is defined as of the turbulence as a function of height [7, 8]. However, the lowest part of the Earth’s atmosphere, where physical these data are not routinely available or on a vertical quantities such as flow velocity, temperature, and moisture resolution which is too crude in view of the application. display rapid fluctuations and vertical mixing is strong [1– In the field of boundary layer meteorology and air 3]. The ABL processes control exchanges of momentum, pollution dispersion, the vertical thickness of ABL remain water, and trace substances between the Earth’s surface and one of the important parameters governing the dispersion of the free troposphere. Substances, once emitted into the ABL, pollutants and measured concentrations of trace gases and are gradually dispersed horizontally and vertically through atmospheric aerosols [4–6]. Therefore, accurate knowledge turbulent processes and become completely mixed over this and broad overview of the factors that influence the ABL layer. All the ABL processes largely dependent on atmo- thickness is of prime importance in understanding the ABL spheric turbulence are represented in parameterized form processes over a given region. The practical and theoretical in atmospheric models. The thickness of ABL is commonly problems associated with the determination of the ABL 2 Advances in Meteorology thickness are reflected in the numerous definitions found 2. Instrumentation and Database in the literature [1, 3, 5, 9–11]. Seibertetal. [5]reviewed In the present study, upper-air meteorological observations and intercompared some of the most important operational obtained through indigenously developed low-cost GPS methods including direct measuring techniques and remote Radiosonde (hereafter referred to as Pisharoty Sonde, as sensing techniques for determination of ABL thickness. named by the Manufacturers) form the main database [20] Seidel et al. [12]usedseven different methods on 10-year, for determination of the CABL thickness over Thumba. 505-station global radiosonde data sets for the development Table 1 provides some of the details on accuracies and of a global climatology of the ABL thickness. response time of different sensors used in Pisharoty Son- Even though, several techniques are suggested in litera- des. Further technical details on functioning of Pisharoty ture, there is still no unique definition and a single overall Sonde are described elsewhere [20]. As part of the rou- accepted method of quantification of the ABL thickness [5, 9, tine meteorological observations at Thumba, balloon-borne 13–15]. The complexity in determination of thickness of the Pisharoty Sondes are launched on Wednesdays from MET- Coastal Atmospheric Boundary Layer (CABL) is generally Facility, Vikram Sarabhai Space Centre (VSSC), typically more complicated as it is effectively influenced by presence at about 14:30 local time. A total of 92 good soundings of mesoscale atmospheric circulations such as Sea/Land between September 2008 to November 2010 formed the Breeze Circulation (SLBC) [15–18]. In this paper, we make main database for the present study, and it was reasonably use of high-resolution upper-air meteorological observations good enough to characterize the vertical structure of the obtained through balloon-borne GPS Radiosonde for a CABL over Thumba. We have confined our analysis strictly to period of about two and half years for characterization of the ◦ ◦ the afternoon soundings, so as to stick to the well-developed vertical structure of CABL over Thumba (8.5 N, 76.9 E), convective activities. one of the Coastal Stations in the Indian subcontinent. Most of the earlier studies in the field of CABL processes conducted over Thumba and adjoining inland coastal stations over 3. Data Processing and Quality Checks the Indian subcontinent were confined to surface-layer characteristics based on meteorological towers, acoustic The raw data collected from balloon-borne Pisharoty Sondes remote sensing, and vertical ascents of opportunity confined consist of pressure, temperature, relative humidity, and to a few days only [16–19]. Rani et al. [15] attempted to geographical position of balloon in the terms of its latitude, explain the vertical structure of the SLBC over the western longitude, and altitude as a function of time, approximately coastline of the Indian subcontinent through numerical at the rate of 1 Hz. In the present study, we have adopted the atmospheric model, but their study was confined to pre- following procedure for smoothening of the raw data. monsoon season only and could not address the variability in the structure of SLBC over different seasons. In all these (1) Firstly, all the exceptionally unusual values including studies, one of the missing components was the seasonal the measurements corresponding to the descent variability in the vertical structure of the CABL and its phase of the balloon are rejected. linkage to large-scale atmospheric circulations such as winter (2) All the individual values beyond 2.5 times of the and summer monsoon. On one hand, the onset of summer standard deviation from the corresponding mean for monsoon over the Indian subcontinent is vastly studied each 100 m bins are considered as spikes and are with reference to the accumulated rainfall and moisture rejected. variability in the lower atmosphere; however, its role over a coastal station, particularly, in modulating the ABL processes (3) Subsequently, the vertical profiles of meteorological remained unexplored in a systematic fashion. The present parameters are smoothened through running average article attempts to fulfil these missing components by using technique and the data are regridded at regular inter- two-year database of upper-air meteorological observations vals of 10 m in vertical. obtained from Thumba to broaden our understanding on the direct impacts on large-scale monsoonal circulation on (4) Missing data for more than 100 m altitudinal bins are the CABL processes. The objectives of this paper can be left blank, and we have confined our analysis of all the summarized under two heads. profiles to an altitude of 3500 m. (1) With reference to the CABL processes prevailing over (5) After regrinding of all the meteorological parameters the study domain, we describe three techniques for to regular intervals of 10 m, virtual potential temper- the determination of mixed layer height, turbulent ature (θ ), bulk Richardson number (Ri ), and sea v B flow depth, and sea breeze flow thickness, respec- breeze component (SBC) are derived for delineation tively. The plausible linkage between these layers is of different sublayers within the CABL. also described in detail. (2) Prevailing meteorological conditions over the study 4. Method of Analysis domain are largely modulated by dry and wet In general, the ABL over a given region is classified into monsoon seasons; therefore, the study is extended towards investigation of probable modulations in the different sublayers depending on the degree of convection, above-mentioned layers during different seasons. clouds, and the amount of moisture present in the lower Advances in Meteorology 3 Table 1: Technical details of Pisharoty GPS Sondes (Make: VSSC, markation of the mixed layer and can be estimated using the ISRO, India). following equations [1, 3]: Name of the sensor Response 0.286 Range Accuracy (measured parameter) time θ = T · , (1) Platinum RTD (air −200 Cto <2second ±0.1 C temperature) 400 C θ = θ · (1+ 0.61 · r), Capacitive humidity where θ, T , P,and r represent the potential temperature (in sensor (relative 0 to 100% <4second ±1.5% Kelvin), ambient temperature (in Kelvin), air pressure (in humidity) hPa), and mixing ratio (in g/kg). In the present study, we MEMS sensor 5hPa to ±2% 2second define the top of the mixed layer at an altitude, where the (atmospheric pressure) 1500 hPa span −1 vertical gradients in θ exceed 3 K · km [22]. 4.2. Bulk Richardson Number (Ri )Method. The bulk atmosphere. Since the CABL dynamics is effectively influ- B Richardson number (Ri ) is a dimensionless number in enced by the presence of SLBC, it can be classified into three B meteorology relating vertical stability and vertical shear, different sublayers such as which provide a measure of the dynamic stability of the flow and is given by the following expression for an altitude z as (1) mixed layer: it is the layer, where atmospheric mois- ture remain uniformly well mixed, and its thickness g · (∂θ /∂z) depends on the degree of convection prevailing over Ri = , (2) 2 2 θ (∂u/∂z) + (∂v/∂z) the region. Under the influence of SLBC, when hor- v izontal advection of onshore moist flow determines where (∂θ /∂z), (∂u/∂z), and (∂v/∂z) are the gradients in the depth of the mixed layer, it is also referred to as v virtual potential temperature (θ ) and zonal and meridional the thermal internal boundary layer (TIBL); components (u, v) of horizontal winds, respectively. The (2) turbulent flow: it is a classical fluid dynamics concept, term θ in the denominator represents the mean of virtual where it is discriminated from the laminar flow potential temperature for the two levels, and g is the through a critical threshold values of Richardson acceleration due to gravity. Sorensen recommended a value number; of 0.25 for the critical value of Ri above which the turbulent flow becomes the laminar flow [23]. In the present study also, (3) sea breeze flow: it is the moist onshore wind flow we use 0.25 as the critical value of Ri for markation of the obtained through segregation of horizontal speeds turbulent flow depth. perpendicular to the Coastline. 4.3. Sea Breeze Flow Thickness. Borne et al. [24] described On one hand, the definition of mixed layer is directly a unique approach for identification of the sea breeze linked with the amount of moisture well mixed in the days under stable synoptic conditions for the Swedish West lower atmosphere; the turbulent flow is described in the Coast. Their approach was based on hourly meteorological terms of frictional influence of the Earth’s surface on lower records and empirical knowledge of the physical processes atmospheric layer. Similarly, the sea breeze flow is just one responsible for the occurrence of a sea breeze system. In of the ways to discriminate the influence of the SLBC with the present study, we have adopted an approach explicitly the prevailing wind flow based on the wind direction and basedon the ambientwinddirection andalignment of the alignment of the coastline. It is obvious that these three layers coastline only. The west coastline of Indian subcontinent form three unique approaches based on atmospheric ther- ◦ ◦ is roughly aligned into along 145 –325 ; thus the winds modynamics, fluid dynamics, and land-sea thermal contrast, ◦ ◦ blowing between 145 and 325 are considered as sea breeze, respectively. With a view to investigating the probable linkage ◦ ◦ while the seaward winds flowing between 325 and 145 between these three stratified layers described through three are considered as land breeze [15]. Given the alignment standard methods, we have quantified them through the of Indian western coastline (Figure 1), it is appropriate to following approach. resolve the wind components in perpendicular direction to the Coastline, so as to enable the measurement of sea 4.1. Virtual Potential Temperature (θ ) Gradient Method. breeze strength. Thus, the sea breeze component (SBC) and Radiosonde temperature and wind profiles in the lower part coastal breeze component (CBC) along Thumba Coastline of the troposphere are often used for a subjective estimation are defined as given below [25] of the mixed layer [5]. Holzworth [7, 21] and others have developed objective methods to simplify and homogenise the SBC = WS · sin(325 − WD), estimation of the mixed layer under convective conditions. (3) The exact definition of top of the mixed layer in many cases CBC=−WS · cos(325 − WD), is very intuitive and different methods are found in the −1 literature. In this regard, virtual potential temperature (θ ) where WS: mean wind speed (ms ) and WD: wind direction happens to be very useful thermodynamic parameter [14]for ( ). As defined in the above equation, positive values of 4 Advances in Meteorology 25N 10N CBC 20N SBC 9N 15N WE Thumba 8N 10N 7N 5N 70E 75E 80E 85E 90E 75.5E 76.5E 77.5E 78.5E Longitude Longitude (a) (b) Figure 1: Location of Thumba depicting the direction of sea and coastal breeze components. SBC indicatethe seabreezeflow whilethe negative values presence of sea breeze flow in this layer, whereas, in the high represent the land breeze flow. During the sea breeze altitudes (>700 m), the magnitudes of SBC become negative, conditions, magnitudes of SBC remain positive in the lower in turn, indicating the reversal of sea breeze flow aloft. altitudes representing the onshore flow of moist air from sea to land. Above a certain altitude, the magnitudes of 5. Background Meteorological Conditions SBC become negative, in turn, indicating the presence of a compensatory return flow aloft. Thus, the altitude, where this Local weather over the Indian subcontinent is mostly changeover in SBC from positive to negative values is seen, influenced by monsoonal and prevailing large-scale wind cir- can betaken as thetop of thesea breezeflow. culation, surface heating, and topographic friction. Thumba ◦ ◦ Figures 2(a)–2(c) show one of the typical plots of θ (8.5 N, 76.9 E), one of the Coastal stations located on the and its gradient, Ri and SBC, corresponding to afternoon West Coastline of Indian subcontinent, is a remote, plain, balloon ascent (13:50 LT) on 11th December 2009 depicting coastal area, not in proximity to any major industrial or the methodology of delineation of MLH, TFD, and SBFT urban activities, and is located about 500 m due east of the adopted in the present study. The level of the maximum Arabian Sea and 10 Kms north, north-west of the urban area vertical gradient in θ is generally indicative of a transition (Figure 1). This experimental site experiences well-defined from a convectively less stable region below to a more stable diurnal variations in wind speed and direction with persis- region above. In the lower altitudes, the magnitudes of θ tent sea breeze circulation during daytime and land breeze remain more or less constant and steady due to convective during nighttime. In general, the ambient meteorological heating and turbulent mixing within this layer, hence this conditions over Thumba during a year can be classified into part of the lower atmosphere is termed as the mixed layer, two broad categories: (1) Summer Monsoon (also referred as shown in Figure 2(a). The top of this mixed layer is to as South-West Monsoon) and (2) Winter Monsoon (also marked at an altitude of about 450 m, where dθ /dz gradient referred to as North-East Monsoon). The period between exceeds 3.0 K/Km. From the vertical profiles of Ri shown December to February, when the study domain experiences in Figure 2(b), it is apparent to notice negative values of typically dry season is termed as the Winter Monsoon, Ri , indicating the presence of turbulent flow in the lower while the period between June to September is classified as atmosphere. At an altitude of about 500 m, the atmospheric the Summer Monsoon and is generally enriched with good flow becomes laminar, as the magnitudes of Ri exceed amount of precipitation over the subcontinent. The months a critical threshold value of 0.25 [23]. The atmospheric of March to May is termed as Pre-Monsoon months, while circulation over Thumba is often modulated by presence the months of October and November are referred to as of the SLBC, hence it is equally important to quantify the Post-Monsoon months. After withdrawal of the Summer thickness of the lower atmosphere, where signature of the Monsoon and until onset of the next monsoon, that is, sea breeze flow is eminent. Figure 2(c) shows vertical profile roughly during November to May, winds in the coastal region of SBC representing the strength of onshore flow. Positive of India are dominated by sea breeze. During the Summer magnitudes of SBC below 700 m altitudinal range indicate Monsoon over Thumba, a sea breeze is superimposed on Latitude Latitude Advances in Meteorology 5 θ ( C) 11 December 2009: 1350 LT 11 December 2009: 1350 LT 31 32 33 34 35 36 37 1400 1400 1400 1200 1200 1200 1000 1000 1000 800 800 800 600 600 600 400 400 400 200 200 200 0 0 0 −12 −6 0 6 12 −90 −30 030 90 −10 −5 0 5 10 ◦ −1 −1 dθ /dz ( C.Km ) Ri SBC (m.s ) v B (a) (b) (c) Figure 2: Vertical Profiles of (a) virtual potential temperature (θ )and (dθ /dz), (b) bulk Richardson number (Ri ), and (c) sea breeze v v B component (SBC) depicting three different techniques adopted for determination of mixed layer height (MLH), turbulent flow depth (TFD), and sea breeze flow thickness (SBFT), respectively. These layers are marked by arrows in Figure. the prevailing wind. Since the flow is then already onshore, magnitudes are relatively higher than that in the Winter it does not constitute a “sea breeze” although it might be Monsoon (Figures 3(a) and 3(b)). It is also the period termed a “sea wind”. However, for the months of December when convective activities built up and the experimental to March, the prevailing wind is from the north-east and site undergoes frequent thundershowers [30]. During the is not so intense as the Summer Monsoon wind; sea breeze Summer Monsoon, onshore winds with high magnitudes activity then becomes striking [26]. Substantial work has ranging between 5 to 7 m/s dominate the ambient wind been carried out on characterization of the CABL over flow over Thumba, as can be seen from Figure 3(c),and Thumba [15–17, 19, 27, 28]. Observational and modelling the SLBC is superposed on the prevailing wind and at time studies conducted over Thumba coast show existence of is even masked [26]. During this period, the experimental onshore flow to a vertical thickness of about 1 km during site and southern part of the Indian subcontinent receive the Summer Monsoon and about 800 m during the Winter good amount of rain-bearing clouds associated with frequent Monsoon [15, 26]. precipitation. Later in the months of October and November With a view to presenting the mean characteristics of (Post-Monsoon season), the wind circulation undergoes a the atmospheric circulation pattern over Thumba during transition from onshore flow to offshore flow and the wind different seasons, we make use of the National Centre for magnitudes show a gradual decrease (Figure 3(d)). Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) Reanalysis to show the wind cir- 6. Results and Discussion culation at 1000 hPa, roughly corresponding to surface in Figure 3 [29]. Figures 3(a)–3(d) show seasonal averaged 6.1. Seasonal Variations in Vertical Structure of the CABL. In wind circulation pattern for the Winter Monsoon (DJF), Figure 4, we show the monthly mean of MLH, TFD, and Pre-Monsoon (MAM), Summer Monsoon (JJAS), and Post- SBFT with their standard deviations for each month as the Monsoon (ON), respectively. In general, the surface-layer error bars. Table 2 provides exact details on the total number winds over Thumba remain low (<2 m/s) during the Winter, of observations used for deriving these histograms. During in turn, indicating absence of any strong convective activities the period between May to August, exact discrimination (Figure 3(a)). During this season, the wind direction over between the sea breeze flow and prevailing wind circulation ◦ ◦ Thumba Coast is, generally, between 0 to 90 indicating was quite difficult as both were aligned in the same direction; a flow of airmass from landmass to oceanic counterpart therefore, we have not shown any histogram for SBFT for (Figure 3(a)). It is important to note that due to the these months. Similarly, we could not ascertain a clear-cut prevailing north-easterly winds in this season, the formation markation of MLH in the month of August and its histogram of SLBC is remarkable and easily traceable [15]. With the is also not shown for this particular month. beginning of Pre-monsoon season during March to May, From Figure 4, it can be seen that the magnitudes of the wind circulation over Thumba undergoes a gradual SBFT are generally larger than those of the MLH and transition from Winter (dry) Monsoon to Summer (wet) TFD, indicating a dominant role of the SLBC in modu- Monsoon. During this period, the wind direction is mostly lations of CABL dynamics over Thumba. During October ◦ ◦ in the fourth quadrant (between 270 to 360 ) and wind to April months, SBFT magnitudes are almost two times Altitude (m) MLH Altitude (m) TED Altitude (m) SBFT 6 Advances in Meteorology NCEP/NCAR wind at 1000 hpa NCEP/NCAR wind at 1000 hpa MAM (pre-monsoon) DJF (winter monsoon) 20N 20N 15N 15N 10N 10N TVM 5N 5N 65E 70E 75E 80E 85E 65E 70E 75E 80E 85E (a) (b) ON (post-monsoon) JJAS (summer monsoon) 20N 20N 15N 15N 10N 10N 5N 5N 65E 70E 75E 80E 85E 65E 70E 75E 80E 85E 2 4 6 8 10 12 14 2 4 6 8 10 12 14 (c) (d) Figure 3: Seasonally averaged wind circulation corresponding to 1000 hPa from NCEP/NCAR Reanalysis for (a) Winter Monsoon (December, January, and February), (b) Pre-Monsoon (March, April, and May), (c) Summer Monsoon (June, July, August, and September), and (d) Post-Monsoon (October and November). the magnitudes of MLH and TFD. Such a large difference were recorded in a range of 310 m to 650 m with a mean of between the magnitudes of SBFT and MLH are indicative of about 420 m, whereas TFD showed variations in a range of formation of the TIBL within the sea breeze flow, as generally 175 m to 560 m with a mean of about 330 m, significantly expected over a Coastal station. Atmospheric Modelling and lower than that of the MLH. In contrast to MLH and TFD observational studies on the SLBC over Thumba in the past magnitudes, the vertical thickness of SBFT showed large have also revealed similar results [15, 27]. With the available variability within a range of 500 m to 910 m with a mean of database of about two and half years, the MLH variations about 760 m. The magnitudes of MLH for different months Advances in Meteorology 7 1000 1200 1 2 3 4 5 6 7 8 9 10 11 12 (month) Winter Pre- Summer Post- Mixed layer height monsoon monsoon monsoon monsoon Turbulent flow depth Sea breeze flow thickness Mixed layer height Turbulent flow depth Figure 4: Monthly variations in mixed layer height (MLH), Sea breeze flow thickness turbulent flow depth (TFD), and sea breeze flow thickness (SBFT) shown as histograms. Error Bars associated with these parameters Figure 5: Seasonal variations in mixed layer height (MLH), indicate the standard deviations for each month, respectively. turbulent flow depth (TFD), and sea breeze flow thickness (SBFT) shown as histograms. Error Bars associated with these parameters indicate the standard deviations for each season, respectively. Table 2: Statistics on the availability of Pisharoty Sonde data. No. of Total no. of Season Month soundings soundings conditions in the season. It is interesting to note that Srivastava et al. (2010) have also shown a similar variability December 10 Winter in MLH over Ahmedabad, one of the tropical urban sites January 19 34 Monsoon in India [31]. The magnitudes of TFD exhibit a steady February 5 picture throughout the year with a mean of about 330 m, March 9 as against large variabilities in the MLH magnitudes. From Pre-Monsoon April 6 the estimates of TFD for different seasons, it is coincidental May 17 to see that larger wind speeds for a given season (such as during the Summer Monsoon and Post-Monsoon) often June 5 results in suppressed magnitudes of TFD, in turn indicating Summer- July 1 the shrinking of turbulent flow in presence of strong winds. Monsoon August 5 The vertical thickness in sea breeze flow, shown by SBFT, September 4 shows a large peak of about 900 m in the Pre-Monsoon and October 7 Post-Monsoon seasons, indicating the increased strength of Post-Monsoon 11 November 4 sea breezeflow in therespectiveseason. During theSummer Monsoon, the sea breeze flow coincides with the prevailing Complete year 92 wind circulation, hence it is difficult to comment on its variability. are comparable to TFD, except for the Summer Monsoon and Post-Monsoon season, when the thickness of mixed layer 7. Summary and Concluding Remarks is found to be almost double of the turbulent flow. This can In this paper, we made use of upper-air meteorological obser- be attributed to the fact that, during this two seasons, the vations for a period of more than two years obtained from lower atmosphere generally experiences high wind speeds, in balloon-borne Pisharoty Sondes over Thumba, a coastal turn reducing the thickness of turbulent flow, whereas the stationonthe WesternCoastline of the Indiansubcontinent, thermal convection helps in the rise of altitude of mixed layer. for characterization of the vertical structure of the CABL. In Figure 5, we show the seasonal variations in MLH, Three different techniques were adopted for quantification TFD, and SBFT by grouping all the datasets to respective of mixed layer heights, turbulent flow depth, and sea breeze seasons. The error bars associated to these parameters flow thickness. The important findings from the study are indicate their standard deviations for the season. The mean summarized below. MLH variations are found to be ranging from 300 m to 500 m, with a peak during the summer monsoon, which can be attributed to strong convective activities prevailing over (1) Mixed layer heights (MLH) quantified through θ this season. In contrast to this, the MLH exhibits relatively gradient method showed variations within a range of lower values during the Winter Monsoon, as an indication 310 m to 650 m with a mean of about 420 m, whereas of suppressed convection in the season. Seasonal variability the turbulent flow depth (TFD) derived through Ri in TFD is almost in tune with that of the MLH, except varied between 175 m to 560 m with a mean of about for the Summer Monsoon and Post-Monsoon season, where 330 m. The magnitudes of MLH were always larger MLH is high, and can be attributed to prevailing convective than those of the TFD. Altitude (m) Altitude (m) 8 Advances in Meteorology (2) Thickness of sea breeze flow (SBFT) varied between [6] B. Medeiros, A. Hall, and B. Stevens, “What controls the climatological depth of the PBL?” Journal of Climate, vol. 18, 500 m to 910 m with a mean of about 760 m. In gen- pp. 2877–2892, 2005. eral, the magnitudes of SBFT were larger than those [7] G.C.Holzworth, “Mixing depths,wind speeds and air of the MLH and TFD, indicating the dominance of pollution potential for selected locations in the United States,” sea breeze flow over the study domain. Large values of Journal of Applied Meteorology, vol. 6, pp. 1039–1044, 1967. SBFT occur in the Pre- and Post-Monsoon seasons. [8] D. H. P. Vogelezang and A. A. M. Holtslag, “Evaluation and model impacts of alternative boundary-layer height (3) Due to strong influence of the sea/land breeze cir- formulations,” Boundary-Layer Meteorology, vol. 81, no. 3-4, culation during Winter Monsoon, Pre-Monsoon, pp. 245–269, 1996. and Post-Monsoon seasons, formation of thermal [9] D. B. Subrahamanyam, R. Ramachandran, K. S. Gupta, and internal boundary layer was eminent over the study T. K. Mandal, “Variability of mixed-layer heights over the domain, which was not very clear in the Summer Indian ocean and Central Arabian sea during Indoex, IFP- Monsoon. 99,” Boundary-Layer Meteorology, vol. 107, no. 3, pp. 683–695, (4) Irrespective of the prevailing season, the magnitudes 2003. [10] D. P. Alappattu, D. B. Subrahamanyam, and P. K. Kun- of TFD remained steady all through the year, whereas hikrishnan, “On the marine atmospheric boundary layer the MLH showed large variability with a peculiar characteristics over Bay of Bengal and Arabian Sea during the peak in the Summer Monsoon and is attributed Integrated Campaign for Aerosols, gases and Radiation Budget to enhanced convective activities prevailing in the (ICARB),” Journal of Earth System Science, vol. 960, no. S1, pp. season. 281–291, 2008. [11] M. V. Ramana, P. Krishnan, S. M. Nair, and P. K. Kun- hikrishnan, “Thermodynamic structure of the atmospheric Acknowledgments boundary layer over the Arabian Sea and the Indian Ocean during pre-INDOEX and INDOEX-FFP campaigns,” Annales The authors sincerely acknowledge Dr. K. Krishnamoorthy, Geophysicae, vol. 22, no. 8, pp. 2679–2691, 2004. Director, SPL for his consistent support to Boundary Layer [12] D. J. Seidel,C.O.Ao, and K.Li, “Estimating climatological Physics and Atmospheric Modelling activities at SPL, VSSC. planetary boundary layer heights from radiosonde obser- The authors are very much grateful to Dr. S. Satyanarayana, vations: comparison of methods and uncertainty analysis,” Group Director, RF Advanced Technology and Facilities Journal of Geophysical Research D, vol. 115, no. 16, 2010. Division, VSSC and his colleagues for their technical support [13] A. K. Georgoulias, D. K. Papanastasiou, D. Melas, V. Amiridis, related to balloon-borne GPS ascents and data recording. and G. Alexandri, “Statistical analysis of boundary layer The authors wish to mention a special word of thanks to heights in a suburban environment,” Meteorology and Atmo- all the members of MET Facility, VSSC for their logistic spheric Physics, vol. 104, no. 1-2, pp. 103–111, 2009. [14] D. B. Subrahamanyam and R. Ramachandran, “Structural and technical support in smooth functioning of balloon- characteristics of marine atmospheric boundary layer and borne GPS ascents during the period of study. The NCEP- its associated dynamics over the Central Arabian Sea during FNL Reanalysis data for this study are from the Research INDOEX, IFP-99 campaign,” Current Science,vol. 85, no.9, Data Archive (RDA) which is maintained by the Com- pp. 1334–1340, 2003. putational and Information Systems Laboratory (CISL) at [15] S. I. Rani, R. Ramachandran, D. B. Subrahamanyam, D. P. the National Center for Atmospheric Research (NCAR). Alappattu, and P. K. Kunhikrishnan, “Characterization of NCAR is sponsored by the National Science Foundation sea/land breeze circulation along the west coast of Indian sub- (NSF). The original data are available from the RDA continent during pre-monsoon season,” Atmospheric Research, (http://dss.ucar.edu/) in dataset number ds083.2. One of vol. 95, no. 4, pp. 367–378, 2010. the authors, Ms. T. J. Anurose, is thankful to the Indian [16] P. K. Kunhikrishnan, K. S. Gupta, R. Radhika, J. W. J. Prakash, and K. N. Nai, “Study on thermal internal boundary layer Space Research Organization for sponsoring fellowship for structure over Thumba, India,” Annales Geophysicae, vol. 11, her Ph.D. research work. pp. 52–60, 1993. [17] J. W. Jeeva Prakash, R. Ramachandran, K. N. Nair, K. Sen References Gupta, and P. K. Kunhikrishnan, “On the structure of sea- breeze fronts observed near the coastline of Thumba, India,” [1] J. R. Garratt, The Atmospheric Boundary Layer,Cambridge Boundary-Layer Meteorology, vol. 59, no. 1-2, pp. 111–124, University Press, New York, NY, USA, 1992. [2] T. R. Oke, Boundary Layer Climates, Halsted Press, New York, [18] R. Ramachandran, J. W.J. Prakash, K.S. Gupta,K.N.Nair, NY, USA, 1988. and P. K. Kunhikrishnan, “Variability of surface roughness and [3] R. B. Stull, An Introduction to Boundary Layer Meteorology, turbulence intensities at a coastal site in India,” Boundary- Kluwer Academic, Dodrecht, The Netherlands, 1988. Layer Meteorology, vol. 70, no. 4, pp. 385–400, 1994. [4] C. M. Bhumralkar, “Parameterization of the planetary bound- [19] D. B. Subrahamanyam, K. S. Gupta, S. Ravindran, and P. ary layer in atmospheric general circulation models,” Reviews Krishnan, “Study of sea breeze and land breeze along the west of Geophysics and Space Physics, vol. 14, no. 2, pp. 215–226, coast of Indian sub-continent over the latitude range 15N to 8N during INDOEX IFP-99 (SK-141) cruise,” Current Science, [5] P. Seibert,F. Beyrich,S. E. Gryning,S. Joffre, A. Rasmussen, vol. 80, supplement, pp. 85–88, 2001. and P. Tercier, “Review and intercomparison of operational [20] S. Satyanarayana, “GPS radiosonde development,” Technology Development for Atmospheric Research and Applications,pp. methods for the determination of the mixing height,” Atmo- spheric Environment, vol. 34, no. 7, pp. 1001–1027, 2000. 142–188, 2008. Advances in Meteorology 9 [21] C. G. Holzworth, “Estimates of mean maximum mixing depths in the contiguous united states,” Monthly Weather Review, vol. 92, no. 1964, pp. 235–242, 1964. [22] X. Zeng, M. A. Brunke, M. Zhou, C. Fairall, N. A. Bond, and D. H. Lenschow, “Marine atmospheric boundary layer height over the Eastern Pacific: data analysis and model evaluation,” Journal of Climate, vol. 17, no. 21, pp. 4159–4170, 2004. [23] J. H. Sørensen, “Sensitivity of the derma long-range gaussian dispersion model to meteorological input and diffusion parameters,” Atmospheric Environment, vol. 32, no. 24, pp. 4195–4206, 1998. [24] K. Borne, D. Chen, and M. Nunez, “A method for finding sea breeze days under stable synoptic conditions and its application to the Swedish west coast,” International Journal of Climatology, vol. 18, no. 8, pp. 901–914, 1998. [25] D. Bala Subrahamanyam and T. J. Anurose, “Solar eclipse induced impacts on sea/land breeze circulation over Thumba: acase study,” Journal of Atmospheric and Solar-Terrestrial Physics, vol. 73, no. 5-6, pp. 703–708, 2011. [26] V. Narayanan, “An observational study of sea breeze at an equatorial coastal station,” Indian Journal of Meteorology and Geophysics, vol. 18, pp. 497–504, 1967. [27] P. K. Kunhikrishnan, R. Ramachandran, D. P. Alappattu, N. V.P. Kiran Kumar, and D. Bala Subrahamanyam, “A case- study of sea breeze circulation at thumba coast through observations and modelling,” in Remote Sensing and Modeling of the Atmosphere, Oceans, and Interactions, vol. 6404 of Proceedings of SPIE, Goa, India, November 2006. [28] J. W. J. Prakash, R. Ramachandran, K. N. Nair, K. S. Gupta, and P.K.Kunhikrishnan, “On the spectral behaviour of atmospheric boundary-layer parameters at Thumba, India,” Quarterly Journal—Royal Meteorological Society, vol. 119, no. 509, pp. 187–197, 1993. [29] E. Kalnay, M. Kanamitsu, R. Kistler et al., “The NCEP/NCAR 40-year reanalysis project,” Bulletin of the American Meteoro- logical Society, vol. 77, no. 3, pp. 437–471, 1996. [30] P. K. Das, The Monsoons, National Book Trust, New Delhi, India, 2009. [31] S. Srivastava, S. Lal, D. B. Subrahamanyam, S. Gupta, S. Venkataramani, and T. A. Rajesh, “Seasonal variability in mixed layer height and its impact on trace gas distribution over a tropical urban site: ahmedabad,” Atmospheric Research, vol. 96, no. 1, pp. 79–87, 2010. 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Characterization of the Vertical Structure of Coastal Atmospheric Boundary Layer over Thumba ( πŸ– . πŸ“ ∘ N , πŸ• πŸ” . πŸ— ∘ E ) during Different Seasons

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Copyright Β© 2011 Sandhya K. Nair 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 390826, 9 pages doi:10.1155/2011/390826 Research Article Characterization of the Vertical Structure of Coastal Atmospheric Boundary Layer over Thumba ◦ ◦ (8.5 N, 76.9 E) during Different Seasons 1 1 1 1 Sandhya K. Nair, T. J. Anurose, D. Bala Subrahamanyam, N. V. P. Kiran Kumar, 1 1 1 2 M. Santosh, S. Sijikumar, Mannil Mohan, andK.V.S.Namboodiri Space Physics Laboratory, Vikram Sarabhai Space Centre, Department of Space, Indian Space Research Organization, Government of India, Thiruvananthapuram 695 022, India MET Facility, Vikram Sarabhai Space Centre, Department of Space, Indian Space Research Organization, Government of India, Thiruvananthapuram 695 022, India Correspondence should be addressed to D. Bala Subrahamanyam, subrahamanyam@gmail.com Received 24 December 2010; Revised 8 March 2011; Accepted 23 March 2011 Academic Editor: Branko Grisogono Copyright © 2011 Sandhya K. Nair 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. Vertical profiles of meteorological parameters obtained from balloon-borne GPS Radiosonde for a period of more than two years ◦ ◦ are analyzed for characterization of the coastal atmospheric boundary layer (CABL) over Thumba (8.5 N, 76.9 E, India). The study reports seasonal variability in the thickness of three different sublayers of the CABL, namely, mixed layer, turbulent flow, and sea breeze flow. Among the three, the vertical thickness of sea breeze flow showed considerable dominance on the other two throughout the year. Mixed layer heights derived through gradients in virtual potential temperature (θ ) showed large seasonal variability with a peak in the Summer and Post-Monsoon. On the other hand, the vertical thickness of turbulent flow remained steady all through the year. Results from the present study indicate that the magnitudes of mixed layer heights are often larger than the turbulent flow thickness. 1. Introduction used to characterize the vertical extent of mixing within the boundary layer and the level at which exchange with the free In Physics and Fluid Dynamics, a boundary layer is generally troposphere occurs [4–6]. In principle, the ABL thickness defined as a layer of the fluid in the immediate vicinity of can be retrieved from atmospheric global circulation models a bounding surface, where viscosity variations are dealt in since they contain algorithms which determine the intensity detail. The atmospheric boundary layer (ABL) is defined as of the turbulence as a function of height [7, 8]. However, the lowest part of the Earth’s atmosphere, where physical these data are not routinely available or on a vertical quantities such as flow velocity, temperature, and moisture resolution which is too crude in view of the application. display rapid fluctuations and vertical mixing is strong [1– In the field of boundary layer meteorology and air 3]. The ABL processes control exchanges of momentum, pollution dispersion, the vertical thickness of ABL remain water, and trace substances between the Earth’s surface and one of the important parameters governing the dispersion of the free troposphere. Substances, once emitted into the ABL, pollutants and measured concentrations of trace gases and are gradually dispersed horizontally and vertically through atmospheric aerosols [4–6]. Therefore, accurate knowledge turbulent processes and become completely mixed over this and broad overview of the factors that influence the ABL layer. All the ABL processes largely dependent on atmo- thickness is of prime importance in understanding the ABL spheric turbulence are represented in parameterized form processes over a given region. The practical and theoretical in atmospheric models. The thickness of ABL is commonly problems associated with the determination of the ABL 2 Advances in Meteorology thickness are reflected in the numerous definitions found 2. Instrumentation and Database in the literature [1, 3, 5, 9–11]. Seibertetal. [5]reviewed In the present study, upper-air meteorological observations and intercompared some of the most important operational obtained through indigenously developed low-cost GPS methods including direct measuring techniques and remote Radiosonde (hereafter referred to as Pisharoty Sonde, as sensing techniques for determination of ABL thickness. named by the Manufacturers) form the main database [20] Seidel et al. [12]usedseven different methods on 10-year, for determination of the CABL thickness over Thumba. 505-station global radiosonde data sets for the development Table 1 provides some of the details on accuracies and of a global climatology of the ABL thickness. response time of different sensors used in Pisharoty Son- Even though, several techniques are suggested in litera- des. Further technical details on functioning of Pisharoty ture, there is still no unique definition and a single overall Sonde are described elsewhere [20]. As part of the rou- accepted method of quantification of the ABL thickness [5, 9, tine meteorological observations at Thumba, balloon-borne 13–15]. The complexity in determination of thickness of the Pisharoty Sondes are launched on Wednesdays from MET- Coastal Atmospheric Boundary Layer (CABL) is generally Facility, Vikram Sarabhai Space Centre (VSSC), typically more complicated as it is effectively influenced by presence at about 14:30 local time. A total of 92 good soundings of mesoscale atmospheric circulations such as Sea/Land between September 2008 to November 2010 formed the Breeze Circulation (SLBC) [15–18]. In this paper, we make main database for the present study, and it was reasonably use of high-resolution upper-air meteorological observations good enough to characterize the vertical structure of the obtained through balloon-borne GPS Radiosonde for a CABL over Thumba. We have confined our analysis strictly to period of about two and half years for characterization of the ◦ ◦ the afternoon soundings, so as to stick to the well-developed vertical structure of CABL over Thumba (8.5 N, 76.9 E), convective activities. one of the Coastal Stations in the Indian subcontinent. Most of the earlier studies in the field of CABL processes conducted over Thumba and adjoining inland coastal stations over 3. Data Processing and Quality Checks the Indian subcontinent were confined to surface-layer characteristics based on meteorological towers, acoustic The raw data collected from balloon-borne Pisharoty Sondes remote sensing, and vertical ascents of opportunity confined consist of pressure, temperature, relative humidity, and to a few days only [16–19]. Rani et al. [15] attempted to geographical position of balloon in the terms of its latitude, explain the vertical structure of the SLBC over the western longitude, and altitude as a function of time, approximately coastline of the Indian subcontinent through numerical at the rate of 1 Hz. In the present study, we have adopted the atmospheric model, but their study was confined to pre- following procedure for smoothening of the raw data. monsoon season only and could not address the variability in the structure of SLBC over different seasons. In all these (1) Firstly, all the exceptionally unusual values including studies, one of the missing components was the seasonal the measurements corresponding to the descent variability in the vertical structure of the CABL and its phase of the balloon are rejected. linkage to large-scale atmospheric circulations such as winter (2) All the individual values beyond 2.5 times of the and summer monsoon. On one hand, the onset of summer standard deviation from the corresponding mean for monsoon over the Indian subcontinent is vastly studied each 100 m bins are considered as spikes and are with reference to the accumulated rainfall and moisture rejected. variability in the lower atmosphere; however, its role over a coastal station, particularly, in modulating the ABL processes (3) Subsequently, the vertical profiles of meteorological remained unexplored in a systematic fashion. The present parameters are smoothened through running average article attempts to fulfil these missing components by using technique and the data are regridded at regular inter- two-year database of upper-air meteorological observations vals of 10 m in vertical. obtained from Thumba to broaden our understanding on the direct impacts on large-scale monsoonal circulation on (4) Missing data for more than 100 m altitudinal bins are the CABL processes. The objectives of this paper can be left blank, and we have confined our analysis of all the summarized under two heads. profiles to an altitude of 3500 m. (1) With reference to the CABL processes prevailing over (5) After regrinding of all the meteorological parameters the study domain, we describe three techniques for to regular intervals of 10 m, virtual potential temper- the determination of mixed layer height, turbulent ature (θ ), bulk Richardson number (Ri ), and sea v B flow depth, and sea breeze flow thickness, respec- breeze component (SBC) are derived for delineation tively. The plausible linkage between these layers is of different sublayers within the CABL. also described in detail. (2) Prevailing meteorological conditions over the study 4. Method of Analysis domain are largely modulated by dry and wet In general, the ABL over a given region is classified into monsoon seasons; therefore, the study is extended towards investigation of probable modulations in the different sublayers depending on the degree of convection, above-mentioned layers during different seasons. clouds, and the amount of moisture present in the lower Advances in Meteorology 3 Table 1: Technical details of Pisharoty GPS Sondes (Make: VSSC, markation of the mixed layer and can be estimated using the ISRO, India). following equations [1, 3]: Name of the sensor Response 0.286 Range Accuracy (measured parameter) time θ = T · , (1) Platinum RTD (air −200 Cto <2second ±0.1 C temperature) 400 C θ = θ · (1+ 0.61 · r), Capacitive humidity where θ, T , P,and r represent the potential temperature (in sensor (relative 0 to 100% <4second ±1.5% Kelvin), ambient temperature (in Kelvin), air pressure (in humidity) hPa), and mixing ratio (in g/kg). In the present study, we MEMS sensor 5hPa to ±2% 2second define the top of the mixed layer at an altitude, where the (atmospheric pressure) 1500 hPa span −1 vertical gradients in θ exceed 3 K · km [22]. 4.2. Bulk Richardson Number (Ri )Method. The bulk atmosphere. Since the CABL dynamics is effectively influ- B Richardson number (Ri ) is a dimensionless number in enced by the presence of SLBC, it can be classified into three B meteorology relating vertical stability and vertical shear, different sublayers such as which provide a measure of the dynamic stability of the flow and is given by the following expression for an altitude z as (1) mixed layer: it is the layer, where atmospheric mois- ture remain uniformly well mixed, and its thickness g · (∂θ /∂z) depends on the degree of convection prevailing over Ri = , (2) 2 2 θ (∂u/∂z) + (∂v/∂z) the region. Under the influence of SLBC, when hor- v izontal advection of onshore moist flow determines where (∂θ /∂z), (∂u/∂z), and (∂v/∂z) are the gradients in the depth of the mixed layer, it is also referred to as v virtual potential temperature (θ ) and zonal and meridional the thermal internal boundary layer (TIBL); components (u, v) of horizontal winds, respectively. The (2) turbulent flow: it is a classical fluid dynamics concept, term θ in the denominator represents the mean of virtual where it is discriminated from the laminar flow potential temperature for the two levels, and g is the through a critical threshold values of Richardson acceleration due to gravity. Sorensen recommended a value number; of 0.25 for the critical value of Ri above which the turbulent flow becomes the laminar flow [23]. In the present study also, (3) sea breeze flow: it is the moist onshore wind flow we use 0.25 as the critical value of Ri for markation of the obtained through segregation of horizontal speeds turbulent flow depth. perpendicular to the Coastline. 4.3. Sea Breeze Flow Thickness. Borne et al. [24] described On one hand, the definition of mixed layer is directly a unique approach for identification of the sea breeze linked with the amount of moisture well mixed in the days under stable synoptic conditions for the Swedish West lower atmosphere; the turbulent flow is described in the Coast. Their approach was based on hourly meteorological terms of frictional influence of the Earth’s surface on lower records and empirical knowledge of the physical processes atmospheric layer. Similarly, the sea breeze flow is just one responsible for the occurrence of a sea breeze system. In of the ways to discriminate the influence of the SLBC with the present study, we have adopted an approach explicitly the prevailing wind flow based on the wind direction and basedon the ambientwinddirection andalignment of the alignment of the coastline. It is obvious that these three layers coastline only. The west coastline of Indian subcontinent form three unique approaches based on atmospheric ther- ◦ ◦ is roughly aligned into along 145 –325 ; thus the winds modynamics, fluid dynamics, and land-sea thermal contrast, ◦ ◦ blowing between 145 and 325 are considered as sea breeze, respectively. With a view to investigating the probable linkage ◦ ◦ while the seaward winds flowing between 325 and 145 between these three stratified layers described through three are considered as land breeze [15]. Given the alignment standard methods, we have quantified them through the of Indian western coastline (Figure 1), it is appropriate to following approach. resolve the wind components in perpendicular direction to the Coastline, so as to enable the measurement of sea 4.1. Virtual Potential Temperature (θ ) Gradient Method. breeze strength. Thus, the sea breeze component (SBC) and Radiosonde temperature and wind profiles in the lower part coastal breeze component (CBC) along Thumba Coastline of the troposphere are often used for a subjective estimation are defined as given below [25] of the mixed layer [5]. Holzworth [7, 21] and others have developed objective methods to simplify and homogenise the SBC = WS · sin(325 − WD), estimation of the mixed layer under convective conditions. (3) The exact definition of top of the mixed layer in many cases CBC=−WS · cos(325 − WD), is very intuitive and different methods are found in the −1 literature. In this regard, virtual potential temperature (θ ) where WS: mean wind speed (ms ) and WD: wind direction happens to be very useful thermodynamic parameter [14]for ( ). As defined in the above equation, positive values of 4 Advances in Meteorology 25N 10N CBC 20N SBC 9N 15N WE Thumba 8N 10N 7N 5N 70E 75E 80E 85E 90E 75.5E 76.5E 77.5E 78.5E Longitude Longitude (a) (b) Figure 1: Location of Thumba depicting the direction of sea and coastal breeze components. SBC indicatethe seabreezeflow whilethe negative values presence of sea breeze flow in this layer, whereas, in the high represent the land breeze flow. During the sea breeze altitudes (>700 m), the magnitudes of SBC become negative, conditions, magnitudes of SBC remain positive in the lower in turn, indicating the reversal of sea breeze flow aloft. altitudes representing the onshore flow of moist air from sea to land. Above a certain altitude, the magnitudes of 5. Background Meteorological Conditions SBC become negative, in turn, indicating the presence of a compensatory return flow aloft. Thus, the altitude, where this Local weather over the Indian subcontinent is mostly changeover in SBC from positive to negative values is seen, influenced by monsoonal and prevailing large-scale wind cir- can betaken as thetop of thesea breezeflow. culation, surface heating, and topographic friction. Thumba ◦ ◦ Figures 2(a)–2(c) show one of the typical plots of θ (8.5 N, 76.9 E), one of the Coastal stations located on the and its gradient, Ri and SBC, corresponding to afternoon West Coastline of Indian subcontinent, is a remote, plain, balloon ascent (13:50 LT) on 11th December 2009 depicting coastal area, not in proximity to any major industrial or the methodology of delineation of MLH, TFD, and SBFT urban activities, and is located about 500 m due east of the adopted in the present study. The level of the maximum Arabian Sea and 10 Kms north, north-west of the urban area vertical gradient in θ is generally indicative of a transition (Figure 1). This experimental site experiences well-defined from a convectively less stable region below to a more stable diurnal variations in wind speed and direction with persis- region above. In the lower altitudes, the magnitudes of θ tent sea breeze circulation during daytime and land breeze remain more or less constant and steady due to convective during nighttime. In general, the ambient meteorological heating and turbulent mixing within this layer, hence this conditions over Thumba during a year can be classified into part of the lower atmosphere is termed as the mixed layer, two broad categories: (1) Summer Monsoon (also referred as shown in Figure 2(a). The top of this mixed layer is to as South-West Monsoon) and (2) Winter Monsoon (also marked at an altitude of about 450 m, where dθ /dz gradient referred to as North-East Monsoon). The period between exceeds 3.0 K/Km. From the vertical profiles of Ri shown December to February, when the study domain experiences in Figure 2(b), it is apparent to notice negative values of typically dry season is termed as the Winter Monsoon, Ri , indicating the presence of turbulent flow in the lower while the period between June to September is classified as atmosphere. At an altitude of about 500 m, the atmospheric the Summer Monsoon and is generally enriched with good flow becomes laminar, as the magnitudes of Ri exceed amount of precipitation over the subcontinent. The months a critical threshold value of 0.25 [23]. The atmospheric of March to May is termed as Pre-Monsoon months, while circulation over Thumba is often modulated by presence the months of October and November are referred to as of the SLBC, hence it is equally important to quantify the Post-Monsoon months. After withdrawal of the Summer thickness of the lower atmosphere, where signature of the Monsoon and until onset of the next monsoon, that is, sea breeze flow is eminent. Figure 2(c) shows vertical profile roughly during November to May, winds in the coastal region of SBC representing the strength of onshore flow. Positive of India are dominated by sea breeze. During the Summer magnitudes of SBC below 700 m altitudinal range indicate Monsoon over Thumba, a sea breeze is superimposed on Latitude Latitude Advances in Meteorology 5 θ ( C) 11 December 2009: 1350 LT 11 December 2009: 1350 LT 31 32 33 34 35 36 37 1400 1400 1400 1200 1200 1200 1000 1000 1000 800 800 800 600 600 600 400 400 400 200 200 200 0 0 0 −12 −6 0 6 12 −90 −30 030 90 −10 −5 0 5 10 ◦ −1 −1 dθ /dz ( C.Km ) Ri SBC (m.s ) v B (a) (b) (c) Figure 2: Vertical Profiles of (a) virtual potential temperature (θ )and (dθ /dz), (b) bulk Richardson number (Ri ), and (c) sea breeze v v B component (SBC) depicting three different techniques adopted for determination of mixed layer height (MLH), turbulent flow depth (TFD), and sea breeze flow thickness (SBFT), respectively. These layers are marked by arrows in Figure. the prevailing wind. Since the flow is then already onshore, magnitudes are relatively higher than that in the Winter it does not constitute a “sea breeze” although it might be Monsoon (Figures 3(a) and 3(b)). It is also the period termed a “sea wind”. However, for the months of December when convective activities built up and the experimental to March, the prevailing wind is from the north-east and site undergoes frequent thundershowers [30]. During the is not so intense as the Summer Monsoon wind; sea breeze Summer Monsoon, onshore winds with high magnitudes activity then becomes striking [26]. Substantial work has ranging between 5 to 7 m/s dominate the ambient wind been carried out on characterization of the CABL over flow over Thumba, as can be seen from Figure 3(c),and Thumba [15–17, 19, 27, 28]. Observational and modelling the SLBC is superposed on the prevailing wind and at time studies conducted over Thumba coast show existence of is even masked [26]. During this period, the experimental onshore flow to a vertical thickness of about 1 km during site and southern part of the Indian subcontinent receive the Summer Monsoon and about 800 m during the Winter good amount of rain-bearing clouds associated with frequent Monsoon [15, 26]. precipitation. Later in the months of October and November With a view to presenting the mean characteristics of (Post-Monsoon season), the wind circulation undergoes a the atmospheric circulation pattern over Thumba during transition from onshore flow to offshore flow and the wind different seasons, we make use of the National Centre for magnitudes show a gradual decrease (Figure 3(d)). Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) Reanalysis to show the wind cir- 6. Results and Discussion culation at 1000 hPa, roughly corresponding to surface in Figure 3 [29]. Figures 3(a)–3(d) show seasonal averaged 6.1. Seasonal Variations in Vertical Structure of the CABL. In wind circulation pattern for the Winter Monsoon (DJF), Figure 4, we show the monthly mean of MLH, TFD, and Pre-Monsoon (MAM), Summer Monsoon (JJAS), and Post- SBFT with their standard deviations for each month as the Monsoon (ON), respectively. In general, the surface-layer error bars. Table 2 provides exact details on the total number winds over Thumba remain low (<2 m/s) during the Winter, of observations used for deriving these histograms. During in turn, indicating absence of any strong convective activities the period between May to August, exact discrimination (Figure 3(a)). During this season, the wind direction over between the sea breeze flow and prevailing wind circulation ◦ ◦ Thumba Coast is, generally, between 0 to 90 indicating was quite difficult as both were aligned in the same direction; a flow of airmass from landmass to oceanic counterpart therefore, we have not shown any histogram for SBFT for (Figure 3(a)). It is important to note that due to the these months. Similarly, we could not ascertain a clear-cut prevailing north-easterly winds in this season, the formation markation of MLH in the month of August and its histogram of SLBC is remarkable and easily traceable [15]. With the is also not shown for this particular month. beginning of Pre-monsoon season during March to May, From Figure 4, it can be seen that the magnitudes of the wind circulation over Thumba undergoes a gradual SBFT are generally larger than those of the MLH and transition from Winter (dry) Monsoon to Summer (wet) TFD, indicating a dominant role of the SLBC in modu- Monsoon. During this period, the wind direction is mostly lations of CABL dynamics over Thumba. During October ◦ ◦ in the fourth quadrant (between 270 to 360 ) and wind to April months, SBFT magnitudes are almost two times Altitude (m) MLH Altitude (m) TED Altitude (m) SBFT 6 Advances in Meteorology NCEP/NCAR wind at 1000 hpa NCEP/NCAR wind at 1000 hpa MAM (pre-monsoon) DJF (winter monsoon) 20N 20N 15N 15N 10N 10N TVM 5N 5N 65E 70E 75E 80E 85E 65E 70E 75E 80E 85E (a) (b) ON (post-monsoon) JJAS (summer monsoon) 20N 20N 15N 15N 10N 10N 5N 5N 65E 70E 75E 80E 85E 65E 70E 75E 80E 85E 2 4 6 8 10 12 14 2 4 6 8 10 12 14 (c) (d) Figure 3: Seasonally averaged wind circulation corresponding to 1000 hPa from NCEP/NCAR Reanalysis for (a) Winter Monsoon (December, January, and February), (b) Pre-Monsoon (March, April, and May), (c) Summer Monsoon (June, July, August, and September), and (d) Post-Monsoon (October and November). the magnitudes of MLH and TFD. Such a large difference were recorded in a range of 310 m to 650 m with a mean of between the magnitudes of SBFT and MLH are indicative of about 420 m, whereas TFD showed variations in a range of formation of the TIBL within the sea breeze flow, as generally 175 m to 560 m with a mean of about 330 m, significantly expected over a Coastal station. Atmospheric Modelling and lower than that of the MLH. In contrast to MLH and TFD observational studies on the SLBC over Thumba in the past magnitudes, the vertical thickness of SBFT showed large have also revealed similar results [15, 27]. With the available variability within a range of 500 m to 910 m with a mean of database of about two and half years, the MLH variations about 760 m. The magnitudes of MLH for different months Advances in Meteorology 7 1000 1200 1 2 3 4 5 6 7 8 9 10 11 12 (month) Winter Pre- Summer Post- Mixed layer height monsoon monsoon monsoon monsoon Turbulent flow depth Sea breeze flow thickness Mixed layer height Turbulent flow depth Figure 4: Monthly variations in mixed layer height (MLH), Sea breeze flow thickness turbulent flow depth (TFD), and sea breeze flow thickness (SBFT) shown as histograms. Error Bars associated with these parameters Figure 5: Seasonal variations in mixed layer height (MLH), indicate the standard deviations for each month, respectively. turbulent flow depth (TFD), and sea breeze flow thickness (SBFT) shown as histograms. Error Bars associated with these parameters indicate the standard deviations for each season, respectively. Table 2: Statistics on the availability of Pisharoty Sonde data. No. of Total no. of Season Month soundings soundings conditions in the season. It is interesting to note that Srivastava et al. (2010) have also shown a similar variability December 10 Winter in MLH over Ahmedabad, one of the tropical urban sites January 19 34 Monsoon in India [31]. The magnitudes of TFD exhibit a steady February 5 picture throughout the year with a mean of about 330 m, March 9 as against large variabilities in the MLH magnitudes. From Pre-Monsoon April 6 the estimates of TFD for different seasons, it is coincidental May 17 to see that larger wind speeds for a given season (such as during the Summer Monsoon and Post-Monsoon) often June 5 results in suppressed magnitudes of TFD, in turn indicating Summer- July 1 the shrinking of turbulent flow in presence of strong winds. Monsoon August 5 The vertical thickness in sea breeze flow, shown by SBFT, September 4 shows a large peak of about 900 m in the Pre-Monsoon and October 7 Post-Monsoon seasons, indicating the increased strength of Post-Monsoon 11 November 4 sea breezeflow in therespectiveseason. During theSummer Monsoon, the sea breeze flow coincides with the prevailing Complete year 92 wind circulation, hence it is difficult to comment on its variability. are comparable to TFD, except for the Summer Monsoon and Post-Monsoon season, when the thickness of mixed layer 7. Summary and Concluding Remarks is found to be almost double of the turbulent flow. This can In this paper, we made use of upper-air meteorological obser- be attributed to the fact that, during this two seasons, the vations for a period of more than two years obtained from lower atmosphere generally experiences high wind speeds, in balloon-borne Pisharoty Sondes over Thumba, a coastal turn reducing the thickness of turbulent flow, whereas the stationonthe WesternCoastline of the Indiansubcontinent, thermal convection helps in the rise of altitude of mixed layer. for characterization of the vertical structure of the CABL. In Figure 5, we show the seasonal variations in MLH, Three different techniques were adopted for quantification TFD, and SBFT by grouping all the datasets to respective of mixed layer heights, turbulent flow depth, and sea breeze seasons. The error bars associated to these parameters flow thickness. The important findings from the study are indicate their standard deviations for the season. The mean summarized below. MLH variations are found to be ranging from 300 m to 500 m, with a peak during the summer monsoon, which can be attributed to strong convective activities prevailing over (1) Mixed layer heights (MLH) quantified through θ this season. In contrast to this, the MLH exhibits relatively gradient method showed variations within a range of lower values during the Winter Monsoon, as an indication 310 m to 650 m with a mean of about 420 m, whereas of suppressed convection in the season. Seasonal variability the turbulent flow depth (TFD) derived through Ri in TFD is almost in tune with that of the MLH, except varied between 175 m to 560 m with a mean of about for the Summer Monsoon and Post-Monsoon season, where 330 m. The magnitudes of MLH were always larger MLH is high, and can be attributed to prevailing convective than those of the TFD. Altitude (m) Altitude (m) 8 Advances in Meteorology (2) Thickness of sea breeze flow (SBFT) varied between [6] B. Medeiros, A. Hall, and B. Stevens, “What controls the climatological depth of the PBL?” Journal of Climate, vol. 18, 500 m to 910 m with a mean of about 760 m. In gen- pp. 2877–2892, 2005. eral, the magnitudes of SBFT were larger than those [7] G.C.Holzworth, “Mixing depths,wind speeds and air of the MLH and TFD, indicating the dominance of pollution potential for selected locations in the United States,” sea breeze flow over the study domain. Large values of Journal of Applied Meteorology, vol. 6, pp. 1039–1044, 1967. SBFT occur in the Pre- and Post-Monsoon seasons. [8] D. H. P. Vogelezang and A. A. M. Holtslag, “Evaluation and model impacts of alternative boundary-layer height (3) Due to strong influence of the sea/land breeze cir- formulations,” Boundary-Layer Meteorology, vol. 81, no. 3-4, culation during Winter Monsoon, Pre-Monsoon, pp. 245–269, 1996. and Post-Monsoon seasons, formation of thermal [9] D. B. Subrahamanyam, R. Ramachandran, K. S. Gupta, and internal boundary layer was eminent over the study T. K. Mandal, “Variability of mixed-layer heights over the domain, which was not very clear in the Summer Indian ocean and Central Arabian sea during Indoex, IFP- Monsoon. 99,” Boundary-Layer Meteorology, vol. 107, no. 3, pp. 683–695, (4) Irrespective of the prevailing season, the magnitudes 2003. [10] D. P. Alappattu, D. B. Subrahamanyam, and P. K. Kun- of TFD remained steady all through the year, whereas hikrishnan, “On the marine atmospheric boundary layer the MLH showed large variability with a peculiar characteristics over Bay of Bengal and Arabian Sea during the peak in the Summer Monsoon and is attributed Integrated Campaign for Aerosols, gases and Radiation Budget to enhanced convective activities prevailing in the (ICARB),” Journal of Earth System Science, vol. 960, no. S1, pp. season. 281–291, 2008. [11] M. V. Ramana, P. Krishnan, S. M. Nair, and P. K. Kun- hikrishnan, “Thermodynamic structure of the atmospheric Acknowledgments boundary layer over the Arabian Sea and the Indian Ocean during pre-INDOEX and INDOEX-FFP campaigns,” Annales The authors sincerely acknowledge Dr. K. Krishnamoorthy, Geophysicae, vol. 22, no. 8, pp. 2679–2691, 2004. Director, SPL for his consistent support to Boundary Layer [12] D. J. 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Ramachandran, “Structural and technical support in smooth functioning of balloon- characteristics of marine atmospheric boundary layer and borne GPS ascents during the period of study. The NCEP- its associated dynamics over the Central Arabian Sea during FNL Reanalysis data for this study are from the Research INDOEX, IFP-99 campaign,” Current Science,vol. 85, no.9, Data Archive (RDA) which is maintained by the Com- pp. 1334–1340, 2003. putational and Information Systems Laboratory (CISL) at [15] S. I. Rani, R. Ramachandran, D. B. Subrahamanyam, D. P. the National Center for Atmospheric Research (NCAR). Alappattu, and P. K. Kunhikrishnan, “Characterization of NCAR is sponsored by the National Science Foundation sea/land breeze circulation along the west coast of Indian sub- (NSF). The original data are available from the RDA continent during pre-monsoon season,” Atmospheric Research, (http://dss.ucar.edu/) in dataset number ds083.2. 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