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Convective Cold Pool Associated with Offshore Propagation of Convection System over the East Coast of Southern Sumatra, Indonesia

Convective Cold Pool Associated with Offshore Propagation of Convection System over the East... Hindawi Advances in Meteorology Volume 2021, Article ID 2047609, 13 pages https://doi.org/10.1155/2021/2047609 Research Article Convective Cold Pool Associated with Offshore Propagation of Convection System over the East Coast of Southern Sumatra, Indonesia 1 1 1 1 1 Erma Yulihastin , Ibnu Fathrio, Trismidianto, Fadli Nauval, Elfira Saufina, 1 1 2 Wendi Harjupa, Didi Satiadi, and Danang Eko Nuryanto Research Center of Atmospheric Science and Technology, Research Organization of Aeronautics and Space, National Research and Innovation Agency, Bandung, West Java 40173, Indonesia Research and Development Center, Indonesian Agency for Meteorology Climatology and Geophysics, Jakarta Pusat 10720, Indonesia Correspondence should be addressed to Erma Yulihastin; erma.yulihastin@lapan.go.id Received 20 April 2021; Revised 22 July 2021; Accepted 26 August 2021; Published 28 September 2021 Academic Editor: Mario M. Miglietta Copyright © 2021 Erma Yulihastin et al. $is 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. $e cold pool outflow has been previously shown to be generated by decaying Mesoscale Convective Complexes (MCCs) in the Maritime Continent. $e cold pool also has a main role in the development processes of oceanic convective systems inducing heavy rainfall. $is study investigated a cold pool event (January 1-2, 2021) related to a heavy rainfall system over the coastal region of Lampung, Southern Sumatra, within a high-resolution model simulation using a regional numerical weather prediction of the Weather Research and Forecasting (WRF) with convection permitting of 1 km spatial resolution, which was validated by satellite and radar data observations. It is important to note that the intensity, duration, timing, and structure of heavy rainfall simulated were in good agreement with satellite-observed rainfall. $e results also showed that a cold pool (CP) plays an important role in inducing Mesoscale Convective Complex (MCC) and was responsible for the development of an offshore propagation of land-based convective systems due to the late afternoon rainfall over inland. $is study also suggests that the −1 propagation speed of the CP 8.8 m·s occurring over the seaside of the coastal region, the so-called CP-coastal, is a plausible mechanism for the speed of the offshore-propagating convection, which is dependent on both the background prevailing wind and outflow. $ese conditions help to maintain the near-surface low temperatures and inhibit cold pool dissipation, which has implications for the development of consecutive convection. change the surface into cold pool air [5, 7, 8]. Both previous 1. Introduction observational and numerical studies suggest that CP plays an Convective cold pools are near-surface regions of downdraft important role to develop new convective cells as well as areas that are spread out horizontally along the convective maintain the formation of a long-lasting mesoscale con- line underneath precipitating clouds [1–4]. Two plausible vective system (MCS) under the squall-line mechanism mechanisms of the cold pool (CP) that generated new [5, 9–14]. On the other hand, for the coastal region, the front convective cells have been proposed by the previous studies sea breeze systems could produce cold pools associated with [5, 6]: (1) lifting of near-surface environmental air by dense the previous precipitation process. $e sea breeze system is and cold air might produce new convection cells along the also controlled by the spatial distribution of local sea surface convective line; (2) developing of convective available po- temperature under the coastal convergence line mechanism tential energy (CAPE) and decreasing of convective inhi- [15, 16] which may have high variation due to the coastal bition (CIN) by both sensible and latent heat fluxes might dynamics [17]. 2 Advances in Meteorology Herein, as the CP is a key feature to organize deep (<1 km) as a cold pool. (2) $e existence of cold pool is also convective clouds over midlatitude regions, the existence of characterised by divergent outflow from the cloud so that we used cloud and wind surface data as the data supported. CP over the lesser latitude, that is, the Maritime Continent, still had been questionable and not yet understood. How- $ese variables will be carried out from the model ever, limited studies have mentioned the CP as a responsible simulation. mechanism to develop propagating convective systems re- Furtherly, to confirm the convective activity during the lated to diurnal rainfall propagation speed over the Maritime study period, radar reflectivity records obtained from Me- Continent [18–24]. On the other hand, propagating con- teorology, Climatology, and Geophysics Agency (BMKG) vective systems over coastal regions is the main character- radar was used to observe the spatial distribution of pre- istic that may produce enhanced rainfall related to extreme cipitation. In this case, to capture the wider regions due to events [24–26]. global scale, we also used precipitation data obtained from In this study, we considered a heavy rainfall event during the Global Satellite Mapping of Precipitation (GSMaP) with January 1-2, 2021, over Lampung province, South Sumatra, 0.1 spatial resolution [29]. Other primary data to confirm which triggered severe floods in the following days. $e the convective clouds are Black Body Temperature (TBB) flood hits several parts of Lampung leading to loss and data retrieved from band 13 of Himawari satellite obser- damage of hundreds of houses and also causing 250 families vation [30] which has a spatial and temporal resolution of to be isolated, on January 5, 2021 [27]. It should be noted that 4 km and 10 minutes, respectively. the Lampung province is bordered by Java Sea and Sunda In addition, to determine convective cloud systems, we Strait and is relatively near Jakarta Bay. For several Jakarta plotted spatial analysis of TBB data in a time evolution floods, South Sumatra has an important role in developing during the event periods. $e duration of deep convective offshore propagation over Java Sea which may interact with cores is identified by low TBB values (<221 K) [31], whereas northerly wind-produced heavy rainfall associated with early the minimum threshold for convective cloud top temper- morning precipitation over the north coast of West Java ature is 241 K following the method to obtain Mesoscale [24], particularly Jakarta City, the capital of Indonesia. Convective Complex (MCC) from satellite data [32]. $is study used a numerical simulation to investigate a Identification of CP needs to be addressed to the cloud- CP event related to heavy rainfall that hits Lampung, South induced surface flows which could be calculated by wind Sumatra, on January 1-2, 2021. We used Weather Research vector anomalies following [20] from the cross-calibrated and Forecasting (WRF) model [28], with initial and multiple satellite (CCMP) reanalysis datasets [33, 34], boundary conditions derived from the National Center for covering global oceans with 6-hourly temporal resolution Environmental Prediction Final Analysis (NCEP-FNL) and 25 km spatial resolution. whose spatial and temporal resolution are 0.25 and 6 hours, In this study, we identified a cold pool by using simu- respectively, to conduct a high-resolution simulation with lation of Weather Research Prediction (WRF) models of convection permitting of 1 km resolution. We further WRF.4.2 [28] designed in one-way three nested domains analysed the simulation results by comparison with the with 9 km (D01), 3 km (D02), and 1 km (D03) spatial res- detailed characteristics of the convective clouds over olution and 33 vertical grids (Figure 1). Betts Miller Janjic´ Lampung as revealed by satellite imageries as well as radar was used as a cumulus scheme on the first and second observation during the heavy rainfall period. In the next domain, while a no-cumulus scheme was used for the third domain. Details of the scheme used in this study following sections, we discuss the data used in this study, model setup and configuration, and results of both the observation and Fonseca et al. [35] produced the best qualitatively agreement simulation. in simulating diurnal precipitation intensity over MC, as shown in Table 1. Initial and boundary conditions were obtained from the 2. Data and Methods National Center for Environmental Prediction/National In this case study of heavy rainfall, we are concerned with Global Data Analysis System (NCEP GDAS)/FNL 0.25 investigating the role of a cold pool in developing thun- Degree Global Tropospheric Analyses and Forecast Grids derstorms associated with the MCC. We then divided the [36]. $e FNL was chosen as a model input since the NCEP methodology into two stages. Firstly, we explore synoptic global prediction skills have increased for the two decades analysis to explain the background condition related to the recently (i.e., Kalnay, 1996) [37], although the FNL data over the MC region still remain lower quality compared to ob- heavy rainfall event. Secondly, in order to identify the cold pool and the MCC, we used both observed and simulated servational data. However, in this case, we have assumed that NCEP-FNL data are good enough to support our present data. We examined satellite and radar data observation as well to confirm the model data simulation. research purpose. When a thunderstorm develops, clouds may be fully formed and start producing precipitation. $is condition could create a cold pool in the lower level due to the 2.1. Model Setup and Experimental Design. In the present work, simulation was integrated for 72 hours, starting from downward advection of cold air. In order to identify a cold pool, we examined 2 criteria: (1) Potential temperature December 31, 2020, 12:00 UTC (19:00 LST) until January 03, 2021, 00:00 UTC (07:00 LST), with the first 12 hours con- decreases over near surface [5]. In this case, we used sidered as spin-up time. In previous work, one-way nesting equivalent potential temperature <340 K over near surface Advances in Meteorology 3 10.0°N north-westerly moisture transport and develops conver- gence system over the south of Sumatra. D01 5.0°N Although the effects of the predominant north-westerly moisture transport might have contributed to large amounts 0.0° of rainfall over the south of Sumatra, it seemed to be D02 concentrated to limited areas due to less convective activity 5.0°S in a daily average of January 1-2, 2021 (Figure 2(a)). To understand the causes of this condition, we further analysed 10.0°S D03 the diurnal variation of convective activity and rainfall on January 1–2 over the study area from satellite data of both 15.0°S GSMaP and Himawari (Figure 3). 20.0°S 90.0°E 100.0°E 110.0°E 120.0°E 130.0°E 3.2. Heavy Rainfall Observed. Heavy rainfall observed by GSMaP satellite data revealed a large quantity of rainfall 500 700 900 1100 1300 1500 1700 1900 2100 accumulation occurring on January 1 over the entire region Figure 1: Simulation domain of WRF model. $e red boxes in the south of Sumatra and the maximum intensity −1 represent domains 1, 2, and 3 for 1, 3, and 9 km resolution, re- (>110 mm·d ) concentrated over the east coast of Lampung spectively. Colour shaded shows height terrain over islands. and ocean around the coastal region (Figure 3(a)). $e timing of heavy rainfall starts from January 1 afternoon (18: 00 LST) over inland and persists to early morning the fol- for 3-domain simulation of WRF model could produce a lowing day (02:00 LST) (Figure 3(b)). In this case, land- good agreement to capture rainfall intensity and offshore- based convective systems seem to have an offshore propa- propagating convective system over Papua New Guinea and gation due to mesoscale convective systems with the max- vicinity, the eastern part of Maritime Continent [38]. For imum convection remaining occurring over inland qualitative as well as quantitative validations of rainfall (Figure 3(c)). $is discrepancy of location between maxi- intensity, timing, duration, and location, we used data ob- mum rainfall and maximum convection indicated that the servations from the BMKG station, radar, and GSMaP dynamical process that caused offshore propagation of di- satellite. On the other hand, it is also important to notice urnal rainfall might have been related to the “self-repli- that, in the present work, we were mainly concerned with cating” mechanism into an internal deep cloud system near simulating CP as a dynamical process responsible for the the coastal region, as suggested by [18]. mechanism of heavy rainfall events related to the Lampung $is mechanism needs to be confirmed by further in- flood on January 6, 2021. In this case, the model parameters vestigation of time evolution of the convective system (equivalent potential temperature, cloud water content, and (Figure 4(c)–4(e)). It is found that the initial convection wind) should be selected to simulate realistic atmospheric occurred over the west coast of southern Sumatra in a small flows representing a CP phenomenon in a high temporal ° ° area starting from January 1 at 13:00 LST (4.8 N, 104 E) (see resolution (10 minutes). Figures 4(a) and 4(d)). It is important to note that the convective system developed rapidly as an MCC at afternoon 3. Results and Discussion time (19:00 LST) influenced by a large convergence system between north-westerly from Java Sea and south-westerly 3.1. Synoptic Condition. Synoptic weather conditions from from the Indian Ocean (Figures 4(b) and 4(f)). A single December 30 to January 2, 2020, were examined using both the satellite and NCEP-FNL data. Figure 2(a) shows that developed to multiple convection cells of MCC appeared clearly in (Figures 4(e) and 4(f)), which need to be explored in convective activities were predominant over southwest Indonesia from December 30 to 31, as indicated by the a detail hourly time evolution in further analysis (Figure 5). distribution of low Black Body Temperature (TBB) con- centrated over the Java Ocean and the southwest Indian 3.3.ColdPool-InducedMesoscaleConvectiveComplex. It was Ocean off the south of Sumatra. On the other hand, com- noticed that the life cycle of the MCC occurred more than 6 binations between anticyclonic vortices over the South hours on January 1 (19:00–02:00 LST), confirmed by both China Sea and North Sumatra develop convergence zones rainfall radar and cloud satellite observation (Figure 5). $is over most of Sumatra in this period (Figure 2(b)). MCC starts with a contiguous cold cloud shield at about It should be noticed that the Borneo vortex which started 2 2 24.000 km and grows promptly to∼55.000 km at 20:00 LST developing from January 1, 2021, enhanced convective ac- (Figure 5(b)). $e MCC is identified by a contiguous cold tivity elongated over the east coast of Sumatra but seems not cloud shield (TBB ≤ 241 K) more than 50.000 km following to extend to the south of Sumatra. However, strong westerly [31]. More interestingly, the MCC developed from single (19: moisture transport intrusion occurred over southern 00 LST) to three convective cells (21:00 LST) with the new Sumatra coming from the northern monsoon from the convective cells propagating out of phase to offshore South China Sea. $e combination of Borneo vortex de- propagation direction (Figure 5(b)). $is developing process velopment and southeast cyclonic vortex existence over of the new convective cells in MCS consistent with previous north Australia causes strengthening of predominantly Indian Ocean 4 Advances in Meteorology Table 1: Model configuration of WRF for simulating precipitation over the Maritime Continent with horizontal resolutions: 9 km (D01), 3 km (D02), and 1 km (D03). Horizontal resolution 9 km 3 km 1 km Number of horizontal grids 300 × 300 400 × 400 634 × 532 Number of vertical grids 33 33 33 Cumulus Betts Miller Janjic´ Betts Miller Janjic´ — Microphysics WSM-3 WSM-3 WSM-3 Long-wave/short-wave radiation RRTM/Dhudia RRTM/Dhudia RRTM/Dhudia Boundary layer Yonsei University Yonsei University Yonsei University Surface layer Revised MM5 Monin–Obukhov Revised MM5 Monin–Obukhov Revised MM5 Monin–Obukhov Land surface NOAH NOAH NOAH Eq Eq 4S 20201230 20201230 Bandar 5S Lampung Java Sea 5S 5S 6S Jakarta 7S 10S 10S 104E 105E 106E 107E 108E Eq Eq 5S 5S 10S 10S Eq Eq 5S 5S 10S 10S Eq Eq 5S 5S 10S 10S 100E 110E 120E 100E 110E 120E -1 -1 -5 50 kg m s TBB (K) divergence (1/s) × 10 Reference Vector 221 225 229 233 237 241 -5 -4 -3 -2 -1 0 1 2 3 4 5 (a) (b) Figure 2: Time evolution from December 30, 2020, to January 2, 2021, for (a) spatial distribution of daily averaged TBB derived from IR1 Himawari satellite imageries; (b) daily averaged vertical integrated of moisture transport (vector) and divergence (shaded) plotted from the NCEP-FNL data, corresponding to the TBB map of the left panels. $e area of interest is indicated by a red-square box. studies explained a back-building mechanism [39–41] which entire region of southern Sumatra and Java Ocean off the mainly produced extreme rainfall [39]. east coast of Sumatra. It was also clearly exhibited that, Figure 5(b) shows that the MCC developed rapidly and during the early morning, new convective cells occurred largely and also propagated offshore and extended over the over the ocean (Figures 5(a) and 5(b)). In this case, during Sunda Strait Advances in Meteorology 5 4S 07 07 01 01 19 19 5S 13 13 07 07 01 01 6S 19 19 13 13 7S 07 07 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E Longitude Longitude Longitude 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 0 2 4 6 8 101214161820222426 0 102030405060708090100110 (a) (b) (c) Figure 3: (a) Daily accumulation of rainfall from GSMaP on January 1-2, 2021; (b) Hovmoller ¨ of time-longitude cross section of rainfall from GSMaP, averaged for 5-6 N; (c) same as (b), but for convective index from Himawari satellite. 4S 4S 4S 5S 5S 5S 6S 6S 6S 7S 7S 7S 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E Longitude Longitude Longitude 10 10 10 mm mm mm 0.1 0.5 1235 10 15 20 25 0.1 0.5 1 2 3 5 10 15 20 25 0.1 0.5 1 2 3 5 10 15 20 25 (a) (b) (c) 4S 4S 4S 5S 5S 5S 6S 6S 6S 7S 7S 7S 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E Longitude 10 Longitude Longitude 10 K K 200 205 210 215 220 225 230 235 240 245 250 200 205 210 215 220 225230 235 240 245 250 200 205 210 215 220 230 235 240 245 250 (d) (e) (f) Figure 4: (a–c) evolution of observed rainfall and surface wind derived from GSMaP and CCMP data, respectively, during January 1-2, 2021, for (a) 13:00 LST; (b) 19:00 LST; (c) 01:00 LST. (d–f) same as the upper panel, but for temperature black body (TBB) of cloud from Himawari satellite. Latitude Latitude Latitude Time (LST) Latitude Latitude Time (LST) Latitude Latitude 6 Advances in Meteorology 5S 5S 6S 6S 5S 221 5S 6S 6S 5S 195 5S 6S 6S 105E 107E 105E 107E 105E 107E 105E 107E 105E 107E 105E 107E (a) (b) Figure 5: Hourly evolution of January 1-2, 2021, from 18:00 to 02:00 LST for (a) rainfall observed by BMKG radar and (b) mesoscale convective complex from Himawari satellite. $e colour gradation from yellow to blue indicates interior cold cloud with TBB ≤ 221 K and brown colour indicates cloud shield with 221 K≤ TBB≤ 241 K. the dissipation process, the MCC also produced other cells of offshore, influenced by north-westerly monsoon flow as a the convection system in the early morning on the following predominant background wind. day (01:00-02:00 LST) (Figure 5(b)). $e MCC has a long- Moreover, to test quantitative agreement between sim- lasting existence from initiation to dissipation (>6 h) which ulated and observed rainfall results in timing, maximum was closely related to the development of the mesoscale intensity, and location, we further used station and satellite convective system (MCS) that lasted for more than 10 h data observation. At first, the simulated data that needs to be [40, 41]. It is also interesting to note that oceanic systems confirmed with the terrestrial-based data was revealed from BMKG station over three locations around Lampung have a longer duration (∼14 h) and hit a slightly smaller region compared to continental systems [42]. province, that is, Rajabasa, Sukabumi, and Tanjung Senang Closer inspection of the heavy rainfall evolution based (Figure 7(a)), where the detailed locations on the map are on GSMaP satellite data revealed that the initial stage of deep described in Figure 7(b). $e daily accumulation of rainfall convective cloud starts from January 1 at 18:00 LST and from December 25 to January 2, 2021, shows that modu- further develops to MCC and expands in a wide region over lation of heavy rainfall occurred on January 1, 2021, with the inland as well as coastal region (Figure 6(a)). It also noticed highest value (∼40 mm) occurring in Tanjung Senang that the core of MCC seems to be migrating offshore at 20:00 (Figure 7(a)). Secondly, the statistical analysis was applied LST during the mature stage of MCC. $e decaying process for satellite data by using composite and area-averaged of MCC at 21:00 LST was continued until 23:00 LST by methods during January 1-2, 2021, over landside and seaside developing new convections over the coastline as well as of the coastal region, respectively (Figure 7(c)), which is ocean regime. $is mechanism related to the development of related to the box areas (Figure 7(d)). new convective clouds over the Maritime Continent is For the timing of maximum rainfall, the model captured consistent with a previous study [21] that stated that MCC it in a concurring time (22:00 LST) between landside and may induce cold pool-like environments by the so-called seaside regions. $is maximum rainfall over the landside sprinkler effects. $e hourly evolution of MCC which is region captured by the model has been delayed 2 hours later represented by the onset of heavy rainfall observed seems to compared to satellite (Figure 7(c)) and radar (Figure 7(e)) be qualitatively well simulated by the model (Figure 6(b)). data (20:00 LST). $is discrepancy is more sophisticated $e timing of the initial rainfall system at 18:00 LST and the than previous studies that found 3–15 different hours of maximum rainfall at 21:00 LSTover the seaside of the coastal maximum timing in diurnal rainfall over the coastal region region could be produced well by the model. Moreover, the of Sumatra, as interpreted in Figure 5 [43]. It is important to model is also able to simulate offshore propagation of rainfall note in previous studies that although spatial model reso- system (Figure 6(b)). lution improved, the model seems still incapable to simulate It is important to note that the maximum intensity, principal processes concerning rainfall due to land-based as duration, and structure of a heavy rainfall event on January 1 well as oceanic-based convective systems. (18:00–02:00 LST) were qualitatively well simulated by It is also interesting to note that the vertical structure of model results (Figure 6(b)). $e initial convection which MCC was well-observed by the radar data with deep con- ° ° began from January 1 at 10:00 LSTover inland (5 N; 104.8 E) vective clouds reaching ∼11 km height (Figure 7(e)). $e by a single-small rainfall cell also could be produced well by existence of 3 convective cells exhibited over low levels the model (figure not shown). $e model results also depict (<4 km) with the contour was filled by grey colour. $is several rainfall cells elongated as a rainband from inland to quasistationary convection system during 19:00–20:00 LST, the coastal region at 18:00 LST. $e rainfall system repre- the so-called back-building mechanism, corresponds to sents a land-based convective system that propagates previous evidence captured by the Himawari satellite Reflectivity (dBZ) TBB (K) Advances in Meteorology 7 18:00 19:00 20:00 18:00 19:00 20:00 5S 5S 6S –1 mm h 6S 21:00 22:00 23:00 -1 mm h 5S 21:00 22:00 23:00 5S 6S 6S 00:00 01:00 02:00 5S 0.5 00:00 01:00 02:00 6S 0.1 5S 0.5 0.1 105E 107E 105E 107E 105E 107E 6S -1 20 m s 105E 107E 105E 107E 105E 107E (a) (b) Figure 6: Same as Figure 5, but for (a) observed hourly rainfall from GSMaP satellite data; (b) simulated hourly rainfall and wind vector ° ° (925 mb) from domain 3 of WRF model simulation. A-B transect delineated through coastline (X) (105.85 E, 5.15 S) used for further analysis in Figures 8 and 9. (Figure 5(b)). However, the role of cold pools in producing pool also developed over the coastline, that so-called “CP- new convective cells under the back-building mechanism coastline” from the decaying convective cloud. Additionally, needs to be further investigated by the model simulation. CP-inland seems to have dissipated in the following time For the maximum intensity issue, the rainfall observed (20:00 LST) due to strong south-eastward flow and lack of by the GSMaP satellite occurred at 23:00 and 20:00 LST for environmental support related to minimum near-surface landside and seaside, respectively. In this case, the simulated moisture. On the other hand, a decaying convective cloud results also consistently show overestimation with rough over the seaside at around 10 km from the coastline (106 E) calculation around 2-3 mm compare to rainfall maximum created a cold pool and generated a new convective cloud in observed. $ese values are also relatively small compared to the leading edge. However, we need to investigate the the previous study that simulated heavy rainfall threshold decaying process of CP-inland from 19:00 to 20:00 LST in which estimated ∼20–40 mm compared to observed rainfall more detail in the following analysis in Figure 9. (2–4 mm) over West Java, Indonesia [44]. In this case, al- From Figures 8 and 9, we could also remark that the though the model lacks the capability to capture the evolution of convective cells by the CP is clear to follow. semidiurnal signal of diurnal rainfall over the landside Deep convection appeared at 18:00 LST over inland (CP- (Land1), it is still good agreement to simulate the devel- inland) and coastline (CP-coastline). $e CP-inland still opment of rainfall system from initial to decaying stage, existed from 19:00 LST to 19:30 LSTand then dissipated with which is strongly related to reinforcement of new convective the outflow of the CP which tends to strengthen the CP- cells (Figure 7(c)). In order to understand this development coastline, which is produced from another decaying process of the convective cells, we need to further investigate it by of convective cloud over the coastline at 19:00 LST. $e CP- simulating model resemblances. coastline was triggered resembling a “back-building” mechanism in a mesoscale convective system (MCS), par- ticularly from 19:30 to 19:40 LST (Figure 9). It was clearly 3.4. Role of Cold Pool on Propagation of Convection System. shown that the new convection cell produced landward from To understand the dynamical processes of this offshore the offshore convection system which was relatively sta- rainfall propagation, we further explore the vertical-longi- tionary over C location (154 km from the coastal line). $is tudinal distributions of vertical wind, water vapor mixing new convection then merged with offshore convection and ratio, and equivalent potential temperature (Figure 8). $e propagated over almost 100 km offshore at 21:40 LST intense rainfall center extended south-eastward from inland (Figure 9). to the east coastline of Lampung, southern Sumatra, which is At 22:00 LST, the CP-coastal continued to propagate generated by several convective clouds at 18:00 LST. $e offshore and seemed to have induced deep convection decaying cloud over inland (A, 105 E) induced a cold pool leeward over the seaside of the coastal region (Figure 8). At (CP), the so-called “CP-inland,” over around 0.5 km and the same time, another deep convection windward over the created a new convective cloud over 50 km distance to the leading edge of CP was also developed resembling a “back- coastline (X) at 19:00 LST. At the same time, another cold building” mechanism. $e offshore propagation of several 8 Advances in Meteorology (a) Daily Rainfall Lampung 25 Dec 2020 - 2 Jan 2021 (b) Rajabase 5°S 35 Sukabumi Rajabasa Tanjung Senang Sukabumi Tanjung Senang 5.2°S 5.4°S 5.6°S 5.8°S (c) (d) 104.8°E 105.1°E 105.4°E 105.7°E 4S Rain tate Land1Land2 Sea1 Sea2 Maximum4.17 10.56 7.16 9.39 Average 1.2 1.53 1.8 1.53 5S 6S 7S 104E 105E 106E 107E 108E 07LST 11LST 15LST 19LST 23LST 01LST Land1 Land2 Sea1 Sea2 (e) 20210101 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 00 01 02 03 04 05 06 LST Figure 7: (a) Time series of daily accumulation of rainfall from BMKG station during December 30, 2020–January 2, 2021. (b) A map of the −1 3 station data locations. (c) Comparison of simulated diurnal rainfall (mm h ) at D03 (1 km) resolution with GSMaP satellite. Rainfall over seaside (landside) from GSMaP (WRF) is denoted as Sea1 (Sea2) and Land1 (Land2), respectively. (d) $e landside and seaside area denoted ° ° ° ° for area averaged over 105–105.8 E 5.5–5 S (black box) and 105.8–106.5 E; 5.5–5 S (red box) of the eastern coastal region of South Sumatra. $e average diurnal cycle is composited from January 1 to January 2, 2021. (e) Diurnal rainfall observed by radar reflectivity (dBZ) height- ° ° time cross section of radar reflectivity (dBZ) over X (105.85 E, 5.15 S as a center area of Lampung at January 1, 2021. –1 Rainfall (mm) Height (km) Rainfall (mm h ) 25-Dec -2020 26-Dec -2020 27-Dec -2020 28-Dec -2020 29-Dec -2020 30-Dec -2020 31-Dec -2020 01-Jan -2021 02-Jan -2021 350 Advances in Meteorology 9 (a) (e) 18:00 22:00 3 3 1 1 (b) (f) 19:00 23:00 3 3 1 1 (c) (g) 20:00 00:00 3 3 -1 1 m s 1 1 -1 (d) (h) gr kg 21:00 01:00 3 3 0.15 0.1 1 1 0.05 A XB A XB 105E 108E 105E 108E Figure 8: Vertical-longitude cross section of wind (vector; zonal component multiplied by a factor of 0.01), equivalent potential temperature (contour), and cloud mixing ratio (shaded) along the thick black line from point A to point B in Figure 6(b) with time evolution for (a-h) 18: 00–01:00 LST. $ex-axis is longitude representing the distance of A-B transect. $e black vertical dotted line (X) indicated the coastal line. For clarity, the equivalent potential temperatures (θ ) are differenced as contour lines with red for θ ≥ 350 K, blue for 350>θ ≥ 345 K, and e e e purple for 345>θ ≥ 340 K. $e purple contour line over near surface (<1 km) indicated a cold pool. deep convective cells continuing was produced by the Hence, to calculate the speed phase of CP-coastal related persistence of that CP until 02:00 LST over the middle sea. to rainfall onset propagation and to investigate the offshore $us, our model results indicate that CP propagation and environment, time-height sections of CAPE, rainfall, and 0.5 km as well as 3 km temperature perturbation were chosen advection by the north-westerly of background winds is a conceivable mechanism for the propagating convective as further analyses (Figure 10). In this case, 3 km represents a systems due to the life cycle of MCC, which is consistent depth of the maximum cool anomaly which has been re- with the study by Li et al. [45] that mentioned that the ported in previous work [38]. In a rough estimation, the direction of propagating diurnal rainfall is regulated by the speed of CP-coastal is associated with rainfall onset prop- −1 prevailing background wind. agation around 8.8 m·s (Figure 10(b)), which is in km km km km km km km km 350 10 Advances in Meteorology (a) (e) 19:10 19:50 (b) (f) 19:20 20:00 1 1 (c) (g) 19:30 20:10 3 3 -1 1 m s -1 gr kg (d) (h) 19:40 20:20 3 3 0.15 0.1 0.05 A XC B A X B 105E 108E 105E 108E Figure 9: Same as Figure 8, but from 19:10 to 21:40 LST. $e black vertical solid line (C) represents location (154 km from (X) of a stationary convection cell from 19:00 LST (see Figure 8) to 20:10 LST. agreement with previous studies [46, 47]. $is offshore with the land-based convective system under the consecu- rainfall propagation corresponds to a strong increase of tively CP-inland mechanism (Figure 10(d)). $is was also CAPE (Figure 10(a)) as well as a cooling anomaly over the shown by the existence of anomalies in a pair of warming and cooling as consecutively at low level (3 km) which is surface level at 0.5 km (Figure 10(c)). It appears that the offshore convective system was strongly maintained by the associated with developing a new convective cell from the surface cold pool (CP-coastal) which moves offshore rapidly previous decaying convection. $is CP development is also from late afternoon (18:00 LST) to early morning (02:00 triggering a deep convective cloud over the coastal region in LST). Interestingly, the initial convection around 16:00 LST the afternoon and maintaining a strong offshore propaga- over the seaside of the coastal region has strong connections tion under the long-lasting MCC mechanism. km km km km km km km km 350 Advances in Meteorology 11 04-05 -1 -1 mm h J kg 01-02 22-23 2400 19-20 19 10 2000 16-17 13-14 0.5 10-11 0.1 07-08 A XB A XB 105E 108E 105E 108E (a) (b) K K 1 1 0.8 0.8 22 22 0.6 0.6 0.4 0.4 19 19 0.2 0.2 0 0 -0.2 -0.2 -0.4 -0.4 13 13 -0.6 -0.6 -0.8 -0.8 10 10 -1 -1 07 07 A XB A XB 105E 108E 105E 108E (c) (d) Figure 10: Diurnal cycle on January 1, 2021, for (a) CAPE, (b) rainfall, and temperature perturbations from daily mean, (c) 0.5 km, and (d) 3 km. Black dashed vertical lines indicate the position of the eastern coastline of South Sumatra. $e phase speed of rainfall propagation of −1 the red line is ∼ 8.8 m·s . strengthens the CP-coastal which is generated over the 4. Conclusions seaside of the coastal region. $e CP-coastal tends to We have investigated the case study of a cold pool related to a persist and propagates further offshore influenced by a heavy rainfall system during January 1-2, 2021, over Lampung, large gradient between the near-surface equivalent tem- South Sumatra, by using the numerical weather prediction of the perature of CP and its environment. $e CP-coastal which WRF model. Heavy rainfall observed by radar as well as satellite is developed as a result of decaying a deep convective observations could be qualitatively well simulated by the model cloud also induced the MCC by triggering new several results. In this period, synoptic weather conditions due to the convective clouds rapidly as well as spread under the initial development of the Borneo vortex concurred to enhance back-building mechanism over the sea. Interestingly, the the low-level north-westerly winds as a predominant prevailing MCC developed in linear system which reported previ- wind over the wider area of Lampung, South Sumatra. ously as mainly type of MCS over Java Sea (> 65%) oc- $e development of CP related to offshore propaga- curred during January-February [47]. tion of convective systems in this case study was illus- Moreover, it can be coarsely calculated from Figures 8 trated by 2 categories: CP-inland and CP-coastal. $e CP- and 10 that the speed of the CP-coastal is around −1 inland propagates slowly and disappeared rapidly because 8.8 m·s (18:00–23:00 LST). $is simulated CP speed is of the relatively strong north-westerly wind due to the in agreement with the previous study [38] which founded −1 synoptic condition and lack of supporting environment that CP speed is 8 m·s over the eastern Maritime related to near-surface moisture over the leading edge of Continent and [46] proposed that theoretically, the speed −1 the CP-inland. 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Convective Cold Pool Associated with Offshore Propagation of Convection System over the East Coast of Southern Sumatra, Indonesia

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Hindawi Advances in Meteorology Volume 2021, Article ID 2047609, 13 pages https://doi.org/10.1155/2021/2047609 Research Article Convective Cold Pool Associated with Offshore Propagation of Convection System over the East Coast of Southern Sumatra, Indonesia 1 1 1 1 1 Erma Yulihastin , Ibnu Fathrio, Trismidianto, Fadli Nauval, Elfira Saufina, 1 1 2 Wendi Harjupa, Didi Satiadi, and Danang Eko Nuryanto Research Center of Atmospheric Science and Technology, Research Organization of Aeronautics and Space, National Research and Innovation Agency, Bandung, West Java 40173, Indonesia Research and Development Center, Indonesian Agency for Meteorology Climatology and Geophysics, Jakarta Pusat 10720, Indonesia Correspondence should be addressed to Erma Yulihastin; erma.yulihastin@lapan.go.id Received 20 April 2021; Revised 22 July 2021; Accepted 26 August 2021; Published 28 September 2021 Academic Editor: Mario M. Miglietta Copyright © 2021 Erma Yulihastin et al. $is 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. $e cold pool outflow has been previously shown to be generated by decaying Mesoscale Convective Complexes (MCCs) in the Maritime Continent. $e cold pool also has a main role in the development processes of oceanic convective systems inducing heavy rainfall. $is study investigated a cold pool event (January 1-2, 2021) related to a heavy rainfall system over the coastal region of Lampung, Southern Sumatra, within a high-resolution model simulation using a regional numerical weather prediction of the Weather Research and Forecasting (WRF) with convection permitting of 1 km spatial resolution, which was validated by satellite and radar data observations. It is important to note that the intensity, duration, timing, and structure of heavy rainfall simulated were in good agreement with satellite-observed rainfall. $e results also showed that a cold pool (CP) plays an important role in inducing Mesoscale Convective Complex (MCC) and was responsible for the development of an offshore propagation of land-based convective systems due to the late afternoon rainfall over inland. $is study also suggests that the −1 propagation speed of the CP 8.8 m·s occurring over the seaside of the coastal region, the so-called CP-coastal, is a plausible mechanism for the speed of the offshore-propagating convection, which is dependent on both the background prevailing wind and outflow. $ese conditions help to maintain the near-surface low temperatures and inhibit cold pool dissipation, which has implications for the development of consecutive convection. change the surface into cold pool air [5, 7, 8]. Both previous 1. Introduction observational and numerical studies suggest that CP plays an Convective cold pools are near-surface regions of downdraft important role to develop new convective cells as well as areas that are spread out horizontally along the convective maintain the formation of a long-lasting mesoscale con- line underneath precipitating clouds [1–4]. Two plausible vective system (MCS) under the squall-line mechanism mechanisms of the cold pool (CP) that generated new [5, 9–14]. On the other hand, for the coastal region, the front convective cells have been proposed by the previous studies sea breeze systems could produce cold pools associated with [5, 6]: (1) lifting of near-surface environmental air by dense the previous precipitation process. $e sea breeze system is and cold air might produce new convection cells along the also controlled by the spatial distribution of local sea surface convective line; (2) developing of convective available po- temperature under the coastal convergence line mechanism tential energy (CAPE) and decreasing of convective inhi- [15, 16] which may have high variation due to the coastal bition (CIN) by both sensible and latent heat fluxes might dynamics [17]. 2 Advances in Meteorology Herein, as the CP is a key feature to organize deep (<1 km) as a cold pool. (2) $e existence of cold pool is also convective clouds over midlatitude regions, the existence of characterised by divergent outflow from the cloud so that we used cloud and wind surface data as the data supported. CP over the lesser latitude, that is, the Maritime Continent, still had been questionable and not yet understood. How- $ese variables will be carried out from the model ever, limited studies have mentioned the CP as a responsible simulation. mechanism to develop propagating convective systems re- Furtherly, to confirm the convective activity during the lated to diurnal rainfall propagation speed over the Maritime study period, radar reflectivity records obtained from Me- Continent [18–24]. On the other hand, propagating con- teorology, Climatology, and Geophysics Agency (BMKG) vective systems over coastal regions is the main character- radar was used to observe the spatial distribution of pre- istic that may produce enhanced rainfall related to extreme cipitation. In this case, to capture the wider regions due to events [24–26]. global scale, we also used precipitation data obtained from In this study, we considered a heavy rainfall event during the Global Satellite Mapping of Precipitation (GSMaP) with January 1-2, 2021, over Lampung province, South Sumatra, 0.1 spatial resolution [29]. Other primary data to confirm which triggered severe floods in the following days. $e the convective clouds are Black Body Temperature (TBB) flood hits several parts of Lampung leading to loss and data retrieved from band 13 of Himawari satellite obser- damage of hundreds of houses and also causing 250 families vation [30] which has a spatial and temporal resolution of to be isolated, on January 5, 2021 [27]. It should be noted that 4 km and 10 minutes, respectively. the Lampung province is bordered by Java Sea and Sunda In addition, to determine convective cloud systems, we Strait and is relatively near Jakarta Bay. For several Jakarta plotted spatial analysis of TBB data in a time evolution floods, South Sumatra has an important role in developing during the event periods. $e duration of deep convective offshore propagation over Java Sea which may interact with cores is identified by low TBB values (<221 K) [31], whereas northerly wind-produced heavy rainfall associated with early the minimum threshold for convective cloud top temper- morning precipitation over the north coast of West Java ature is 241 K following the method to obtain Mesoscale [24], particularly Jakarta City, the capital of Indonesia. Convective Complex (MCC) from satellite data [32]. $is study used a numerical simulation to investigate a Identification of CP needs to be addressed to the cloud- CP event related to heavy rainfall that hits Lampung, South induced surface flows which could be calculated by wind Sumatra, on January 1-2, 2021. We used Weather Research vector anomalies following [20] from the cross-calibrated and Forecasting (WRF) model [28], with initial and multiple satellite (CCMP) reanalysis datasets [33, 34], boundary conditions derived from the National Center for covering global oceans with 6-hourly temporal resolution Environmental Prediction Final Analysis (NCEP-FNL) and 25 km spatial resolution. whose spatial and temporal resolution are 0.25 and 6 hours, In this study, we identified a cold pool by using simu- respectively, to conduct a high-resolution simulation with lation of Weather Research Prediction (WRF) models of convection permitting of 1 km resolution. We further WRF.4.2 [28] designed in one-way three nested domains analysed the simulation results by comparison with the with 9 km (D01), 3 km (D02), and 1 km (D03) spatial res- detailed characteristics of the convective clouds over olution and 33 vertical grids (Figure 1). Betts Miller Janjic´ Lampung as revealed by satellite imageries as well as radar was used as a cumulus scheme on the first and second observation during the heavy rainfall period. In the next domain, while a no-cumulus scheme was used for the third domain. Details of the scheme used in this study following sections, we discuss the data used in this study, model setup and configuration, and results of both the observation and Fonseca et al. [35] produced the best qualitatively agreement simulation. in simulating diurnal precipitation intensity over MC, as shown in Table 1. Initial and boundary conditions were obtained from the 2. Data and Methods National Center for Environmental Prediction/National In this case study of heavy rainfall, we are concerned with Global Data Analysis System (NCEP GDAS)/FNL 0.25 investigating the role of a cold pool in developing thun- Degree Global Tropospheric Analyses and Forecast Grids derstorms associated with the MCC. We then divided the [36]. $e FNL was chosen as a model input since the NCEP methodology into two stages. Firstly, we explore synoptic global prediction skills have increased for the two decades analysis to explain the background condition related to the recently (i.e., Kalnay, 1996) [37], although the FNL data over the MC region still remain lower quality compared to ob- heavy rainfall event. Secondly, in order to identify the cold pool and the MCC, we used both observed and simulated servational data. However, in this case, we have assumed that NCEP-FNL data are good enough to support our present data. We examined satellite and radar data observation as well to confirm the model data simulation. research purpose. When a thunderstorm develops, clouds may be fully formed and start producing precipitation. $is condition could create a cold pool in the lower level due to the 2.1. Model Setup and Experimental Design. In the present work, simulation was integrated for 72 hours, starting from downward advection of cold air. In order to identify a cold pool, we examined 2 criteria: (1) Potential temperature December 31, 2020, 12:00 UTC (19:00 LST) until January 03, 2021, 00:00 UTC (07:00 LST), with the first 12 hours con- decreases over near surface [5]. In this case, we used sidered as spin-up time. In previous work, one-way nesting equivalent potential temperature <340 K over near surface Advances in Meteorology 3 10.0°N north-westerly moisture transport and develops conver- gence system over the south of Sumatra. D01 5.0°N Although the effects of the predominant north-westerly moisture transport might have contributed to large amounts 0.0° of rainfall over the south of Sumatra, it seemed to be D02 concentrated to limited areas due to less convective activity 5.0°S in a daily average of January 1-2, 2021 (Figure 2(a)). To understand the causes of this condition, we further analysed 10.0°S D03 the diurnal variation of convective activity and rainfall on January 1–2 over the study area from satellite data of both 15.0°S GSMaP and Himawari (Figure 3). 20.0°S 90.0°E 100.0°E 110.0°E 120.0°E 130.0°E 3.2. Heavy Rainfall Observed. Heavy rainfall observed by GSMaP satellite data revealed a large quantity of rainfall 500 700 900 1100 1300 1500 1700 1900 2100 accumulation occurring on January 1 over the entire region Figure 1: Simulation domain of WRF model. $e red boxes in the south of Sumatra and the maximum intensity −1 represent domains 1, 2, and 3 for 1, 3, and 9 km resolution, re- (>110 mm·d ) concentrated over the east coast of Lampung spectively. Colour shaded shows height terrain over islands. and ocean around the coastal region (Figure 3(a)). $e timing of heavy rainfall starts from January 1 afternoon (18: 00 LST) over inland and persists to early morning the fol- for 3-domain simulation of WRF model could produce a lowing day (02:00 LST) (Figure 3(b)). In this case, land- good agreement to capture rainfall intensity and offshore- based convective systems seem to have an offshore propa- propagating convective system over Papua New Guinea and gation due to mesoscale convective systems with the max- vicinity, the eastern part of Maritime Continent [38]. For imum convection remaining occurring over inland qualitative as well as quantitative validations of rainfall (Figure 3(c)). $is discrepancy of location between maxi- intensity, timing, duration, and location, we used data ob- mum rainfall and maximum convection indicated that the servations from the BMKG station, radar, and GSMaP dynamical process that caused offshore propagation of di- satellite. On the other hand, it is also important to notice urnal rainfall might have been related to the “self-repli- that, in the present work, we were mainly concerned with cating” mechanism into an internal deep cloud system near simulating CP as a dynamical process responsible for the the coastal region, as suggested by [18]. mechanism of heavy rainfall events related to the Lampung $is mechanism needs to be confirmed by further in- flood on January 6, 2021. In this case, the model parameters vestigation of time evolution of the convective system (equivalent potential temperature, cloud water content, and (Figure 4(c)–4(e)). It is found that the initial convection wind) should be selected to simulate realistic atmospheric occurred over the west coast of southern Sumatra in a small flows representing a CP phenomenon in a high temporal ° ° area starting from January 1 at 13:00 LST (4.8 N, 104 E) (see resolution (10 minutes). Figures 4(a) and 4(d)). It is important to note that the convective system developed rapidly as an MCC at afternoon 3. Results and Discussion time (19:00 LST) influenced by a large convergence system between north-westerly from Java Sea and south-westerly 3.1. Synoptic Condition. Synoptic weather conditions from from the Indian Ocean (Figures 4(b) and 4(f)). A single December 30 to January 2, 2020, were examined using both the satellite and NCEP-FNL data. Figure 2(a) shows that developed to multiple convection cells of MCC appeared clearly in (Figures 4(e) and 4(f)), which need to be explored in convective activities were predominant over southwest Indonesia from December 30 to 31, as indicated by the a detail hourly time evolution in further analysis (Figure 5). distribution of low Black Body Temperature (TBB) con- centrated over the Java Ocean and the southwest Indian 3.3.ColdPool-InducedMesoscaleConvectiveComplex. It was Ocean off the south of Sumatra. On the other hand, com- noticed that the life cycle of the MCC occurred more than 6 binations between anticyclonic vortices over the South hours on January 1 (19:00–02:00 LST), confirmed by both China Sea and North Sumatra develop convergence zones rainfall radar and cloud satellite observation (Figure 5). $is over most of Sumatra in this period (Figure 2(b)). MCC starts with a contiguous cold cloud shield at about It should be noticed that the Borneo vortex which started 2 2 24.000 km and grows promptly to∼55.000 km at 20:00 LST developing from January 1, 2021, enhanced convective ac- (Figure 5(b)). $e MCC is identified by a contiguous cold tivity elongated over the east coast of Sumatra but seems not cloud shield (TBB ≤ 241 K) more than 50.000 km following to extend to the south of Sumatra. However, strong westerly [31]. More interestingly, the MCC developed from single (19: moisture transport intrusion occurred over southern 00 LST) to three convective cells (21:00 LST) with the new Sumatra coming from the northern monsoon from the convective cells propagating out of phase to offshore South China Sea. $e combination of Borneo vortex de- propagation direction (Figure 5(b)). $is developing process velopment and southeast cyclonic vortex existence over of the new convective cells in MCS consistent with previous north Australia causes strengthening of predominantly Indian Ocean 4 Advances in Meteorology Table 1: Model configuration of WRF for simulating precipitation over the Maritime Continent with horizontal resolutions: 9 km (D01), 3 km (D02), and 1 km (D03). Horizontal resolution 9 km 3 km 1 km Number of horizontal grids 300 × 300 400 × 400 634 × 532 Number of vertical grids 33 33 33 Cumulus Betts Miller Janjic´ Betts Miller Janjic´ — Microphysics WSM-3 WSM-3 WSM-3 Long-wave/short-wave radiation RRTM/Dhudia RRTM/Dhudia RRTM/Dhudia Boundary layer Yonsei University Yonsei University Yonsei University Surface layer Revised MM5 Monin–Obukhov Revised MM5 Monin–Obukhov Revised MM5 Monin–Obukhov Land surface NOAH NOAH NOAH Eq Eq 4S 20201230 20201230 Bandar 5S Lampung Java Sea 5S 5S 6S Jakarta 7S 10S 10S 104E 105E 106E 107E 108E Eq Eq 5S 5S 10S 10S Eq Eq 5S 5S 10S 10S Eq Eq 5S 5S 10S 10S 100E 110E 120E 100E 110E 120E -1 -1 -5 50 kg m s TBB (K) divergence (1/s) × 10 Reference Vector 221 225 229 233 237 241 -5 -4 -3 -2 -1 0 1 2 3 4 5 (a) (b) Figure 2: Time evolution from December 30, 2020, to January 2, 2021, for (a) spatial distribution of daily averaged TBB derived from IR1 Himawari satellite imageries; (b) daily averaged vertical integrated of moisture transport (vector) and divergence (shaded) plotted from the NCEP-FNL data, corresponding to the TBB map of the left panels. $e area of interest is indicated by a red-square box. studies explained a back-building mechanism [39–41] which entire region of southern Sumatra and Java Ocean off the mainly produced extreme rainfall [39]. east coast of Sumatra. It was also clearly exhibited that, Figure 5(b) shows that the MCC developed rapidly and during the early morning, new convective cells occurred largely and also propagated offshore and extended over the over the ocean (Figures 5(a) and 5(b)). In this case, during Sunda Strait Advances in Meteorology 5 4S 07 07 01 01 19 19 5S 13 13 07 07 01 01 6S 19 19 13 13 7S 07 07 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E Longitude Longitude Longitude 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 0 2 4 6 8 101214161820222426 0 102030405060708090100110 (a) (b) (c) Figure 3: (a) Daily accumulation of rainfall from GSMaP on January 1-2, 2021; (b) Hovmoller ¨ of time-longitude cross section of rainfall from GSMaP, averaged for 5-6 N; (c) same as (b), but for convective index from Himawari satellite. 4S 4S 4S 5S 5S 5S 6S 6S 6S 7S 7S 7S 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E Longitude Longitude Longitude 10 10 10 mm mm mm 0.1 0.5 1235 10 15 20 25 0.1 0.5 1 2 3 5 10 15 20 25 0.1 0.5 1 2 3 5 10 15 20 25 (a) (b) (c) 4S 4S 4S 5S 5S 5S 6S 6S 6S 7S 7S 7S 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E 104E 105E 106E 107E 108E Longitude 10 Longitude Longitude 10 K K 200 205 210 215 220 225 230 235 240 245 250 200 205 210 215 220 225230 235 240 245 250 200 205 210 215 220 230 235 240 245 250 (d) (e) (f) Figure 4: (a–c) evolution of observed rainfall and surface wind derived from GSMaP and CCMP data, respectively, during January 1-2, 2021, for (a) 13:00 LST; (b) 19:00 LST; (c) 01:00 LST. (d–f) same as the upper panel, but for temperature black body (TBB) of cloud from Himawari satellite. Latitude Latitude Latitude Time (LST) Latitude Latitude Time (LST) Latitude Latitude 6 Advances in Meteorology 5S 5S 6S 6S 5S 221 5S 6S 6S 5S 195 5S 6S 6S 105E 107E 105E 107E 105E 107E 105E 107E 105E 107E 105E 107E (a) (b) Figure 5: Hourly evolution of January 1-2, 2021, from 18:00 to 02:00 LST for (a) rainfall observed by BMKG radar and (b) mesoscale convective complex from Himawari satellite. $e colour gradation from yellow to blue indicates interior cold cloud with TBB ≤ 221 K and brown colour indicates cloud shield with 221 K≤ TBB≤ 241 K. the dissipation process, the MCC also produced other cells of offshore, influenced by north-westerly monsoon flow as a the convection system in the early morning on the following predominant background wind. day (01:00-02:00 LST) (Figure 5(b)). $e MCC has a long- Moreover, to test quantitative agreement between sim- lasting existence from initiation to dissipation (>6 h) which ulated and observed rainfall results in timing, maximum was closely related to the development of the mesoscale intensity, and location, we further used station and satellite convective system (MCS) that lasted for more than 10 h data observation. At first, the simulated data that needs to be [40, 41]. It is also interesting to note that oceanic systems confirmed with the terrestrial-based data was revealed from BMKG station over three locations around Lampung have a longer duration (∼14 h) and hit a slightly smaller region compared to continental systems [42]. province, that is, Rajabasa, Sukabumi, and Tanjung Senang Closer inspection of the heavy rainfall evolution based (Figure 7(a)), where the detailed locations on the map are on GSMaP satellite data revealed that the initial stage of deep described in Figure 7(b). $e daily accumulation of rainfall convective cloud starts from January 1 at 18:00 LST and from December 25 to January 2, 2021, shows that modu- further develops to MCC and expands in a wide region over lation of heavy rainfall occurred on January 1, 2021, with the inland as well as coastal region (Figure 6(a)). It also noticed highest value (∼40 mm) occurring in Tanjung Senang that the core of MCC seems to be migrating offshore at 20:00 (Figure 7(a)). Secondly, the statistical analysis was applied LST during the mature stage of MCC. $e decaying process for satellite data by using composite and area-averaged of MCC at 21:00 LST was continued until 23:00 LST by methods during January 1-2, 2021, over landside and seaside developing new convections over the coastline as well as of the coastal region, respectively (Figure 7(c)), which is ocean regime. $is mechanism related to the development of related to the box areas (Figure 7(d)). new convective clouds over the Maritime Continent is For the timing of maximum rainfall, the model captured consistent with a previous study [21] that stated that MCC it in a concurring time (22:00 LST) between landside and may induce cold pool-like environments by the so-called seaside regions. $is maximum rainfall over the landside sprinkler effects. $e hourly evolution of MCC which is region captured by the model has been delayed 2 hours later represented by the onset of heavy rainfall observed seems to compared to satellite (Figure 7(c)) and radar (Figure 7(e)) be qualitatively well simulated by the model (Figure 6(b)). data (20:00 LST). $is discrepancy is more sophisticated $e timing of the initial rainfall system at 18:00 LST and the than previous studies that found 3–15 different hours of maximum rainfall at 21:00 LSTover the seaside of the coastal maximum timing in diurnal rainfall over the coastal region region could be produced well by the model. Moreover, the of Sumatra, as interpreted in Figure 5 [43]. It is important to model is also able to simulate offshore propagation of rainfall note in previous studies that although spatial model reso- system (Figure 6(b)). lution improved, the model seems still incapable to simulate It is important to note that the maximum intensity, principal processes concerning rainfall due to land-based as duration, and structure of a heavy rainfall event on January 1 well as oceanic-based convective systems. (18:00–02:00 LST) were qualitatively well simulated by It is also interesting to note that the vertical structure of model results (Figure 6(b)). $e initial convection which MCC was well-observed by the radar data with deep con- ° ° began from January 1 at 10:00 LSTover inland (5 N; 104.8 E) vective clouds reaching ∼11 km height (Figure 7(e)). $e by a single-small rainfall cell also could be produced well by existence of 3 convective cells exhibited over low levels the model (figure not shown). $e model results also depict (<4 km) with the contour was filled by grey colour. $is several rainfall cells elongated as a rainband from inland to quasistationary convection system during 19:00–20:00 LST, the coastal region at 18:00 LST. $e rainfall system repre- the so-called back-building mechanism, corresponds to sents a land-based convective system that propagates previous evidence captured by the Himawari satellite Reflectivity (dBZ) TBB (K) Advances in Meteorology 7 18:00 19:00 20:00 18:00 19:00 20:00 5S 5S 6S –1 mm h 6S 21:00 22:00 23:00 -1 mm h 5S 21:00 22:00 23:00 5S 6S 6S 00:00 01:00 02:00 5S 0.5 00:00 01:00 02:00 6S 0.1 5S 0.5 0.1 105E 107E 105E 107E 105E 107E 6S -1 20 m s 105E 107E 105E 107E 105E 107E (a) (b) Figure 6: Same as Figure 5, but for (a) observed hourly rainfall from GSMaP satellite data; (b) simulated hourly rainfall and wind vector ° ° (925 mb) from domain 3 of WRF model simulation. A-B transect delineated through coastline (X) (105.85 E, 5.15 S) used for further analysis in Figures 8 and 9. (Figure 5(b)). However, the role of cold pools in producing pool also developed over the coastline, that so-called “CP- new convective cells under the back-building mechanism coastline” from the decaying convective cloud. Additionally, needs to be further investigated by the model simulation. CP-inland seems to have dissipated in the following time For the maximum intensity issue, the rainfall observed (20:00 LST) due to strong south-eastward flow and lack of by the GSMaP satellite occurred at 23:00 and 20:00 LST for environmental support related to minimum near-surface landside and seaside, respectively. In this case, the simulated moisture. On the other hand, a decaying convective cloud results also consistently show overestimation with rough over the seaside at around 10 km from the coastline (106 E) calculation around 2-3 mm compare to rainfall maximum created a cold pool and generated a new convective cloud in observed. $ese values are also relatively small compared to the leading edge. However, we need to investigate the the previous study that simulated heavy rainfall threshold decaying process of CP-inland from 19:00 to 20:00 LST in which estimated ∼20–40 mm compared to observed rainfall more detail in the following analysis in Figure 9. (2–4 mm) over West Java, Indonesia [44]. In this case, al- From Figures 8 and 9, we could also remark that the though the model lacks the capability to capture the evolution of convective cells by the CP is clear to follow. semidiurnal signal of diurnal rainfall over the landside Deep convection appeared at 18:00 LST over inland (CP- (Land1), it is still good agreement to simulate the devel- inland) and coastline (CP-coastline). $e CP-inland still opment of rainfall system from initial to decaying stage, existed from 19:00 LST to 19:30 LSTand then dissipated with which is strongly related to reinforcement of new convective the outflow of the CP which tends to strengthen the CP- cells (Figure 7(c)). In order to understand this development coastline, which is produced from another decaying process of the convective cells, we need to further investigate it by of convective cloud over the coastline at 19:00 LST. $e CP- simulating model resemblances. coastline was triggered resembling a “back-building” mechanism in a mesoscale convective system (MCS), par- ticularly from 19:30 to 19:40 LST (Figure 9). It was clearly 3.4. Role of Cold Pool on Propagation of Convection System. shown that the new convection cell produced landward from To understand the dynamical processes of this offshore the offshore convection system which was relatively sta- rainfall propagation, we further explore the vertical-longi- tionary over C location (154 km from the coastal line). $is tudinal distributions of vertical wind, water vapor mixing new convection then merged with offshore convection and ratio, and equivalent potential temperature (Figure 8). $e propagated over almost 100 km offshore at 21:40 LST intense rainfall center extended south-eastward from inland (Figure 9). to the east coastline of Lampung, southern Sumatra, which is At 22:00 LST, the CP-coastal continued to propagate generated by several convective clouds at 18:00 LST. $e offshore and seemed to have induced deep convection decaying cloud over inland (A, 105 E) induced a cold pool leeward over the seaside of the coastal region (Figure 8). At (CP), the so-called “CP-inland,” over around 0.5 km and the same time, another deep convection windward over the created a new convective cloud over 50 km distance to the leading edge of CP was also developed resembling a “back- coastline (X) at 19:00 LST. At the same time, another cold building” mechanism. $e offshore propagation of several 8 Advances in Meteorology (a) Daily Rainfall Lampung 25 Dec 2020 - 2 Jan 2021 (b) Rajabase 5°S 35 Sukabumi Rajabasa Tanjung Senang Sukabumi Tanjung Senang 5.2°S 5.4°S 5.6°S 5.8°S (c) (d) 104.8°E 105.1°E 105.4°E 105.7°E 4S Rain tate Land1Land2 Sea1 Sea2 Maximum4.17 10.56 7.16 9.39 Average 1.2 1.53 1.8 1.53 5S 6S 7S 104E 105E 106E 107E 108E 07LST 11LST 15LST 19LST 23LST 01LST Land1 Land2 Sea1 Sea2 (e) 20210101 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 00 01 02 03 04 05 06 LST Figure 7: (a) Time series of daily accumulation of rainfall from BMKG station during December 30, 2020–January 2, 2021. (b) A map of the −1 3 station data locations. (c) Comparison of simulated diurnal rainfall (mm h ) at D03 (1 km) resolution with GSMaP satellite. Rainfall over seaside (landside) from GSMaP (WRF) is denoted as Sea1 (Sea2) and Land1 (Land2), respectively. (d) $e landside and seaside area denoted ° ° ° ° for area averaged over 105–105.8 E 5.5–5 S (black box) and 105.8–106.5 E; 5.5–5 S (red box) of the eastern coastal region of South Sumatra. $e average diurnal cycle is composited from January 1 to January 2, 2021. (e) Diurnal rainfall observed by radar reflectivity (dBZ) height- ° ° time cross section of radar reflectivity (dBZ) over X (105.85 E, 5.15 S as a center area of Lampung at January 1, 2021. –1 Rainfall (mm) Height (km) Rainfall (mm h ) 25-Dec -2020 26-Dec -2020 27-Dec -2020 28-Dec -2020 29-Dec -2020 30-Dec -2020 31-Dec -2020 01-Jan -2021 02-Jan -2021 350 Advances in Meteorology 9 (a) (e) 18:00 22:00 3 3 1 1 (b) (f) 19:00 23:00 3 3 1 1 (c) (g) 20:00 00:00 3 3 -1 1 m s 1 1 -1 (d) (h) gr kg 21:00 01:00 3 3 0.15 0.1 1 1 0.05 A XB A XB 105E 108E 105E 108E Figure 8: Vertical-longitude cross section of wind (vector; zonal component multiplied by a factor of 0.01), equivalent potential temperature (contour), and cloud mixing ratio (shaded) along the thick black line from point A to point B in Figure 6(b) with time evolution for (a-h) 18: 00–01:00 LST. $ex-axis is longitude representing the distance of A-B transect. $e black vertical dotted line (X) indicated the coastal line. For clarity, the equivalent potential temperatures (θ ) are differenced as contour lines with red for θ ≥ 350 K, blue for 350>θ ≥ 345 K, and e e e purple for 345>θ ≥ 340 K. $e purple contour line over near surface (<1 km) indicated a cold pool. deep convective cells continuing was produced by the Hence, to calculate the speed phase of CP-coastal related persistence of that CP until 02:00 LST over the middle sea. to rainfall onset propagation and to investigate the offshore $us, our model results indicate that CP propagation and environment, time-height sections of CAPE, rainfall, and 0.5 km as well as 3 km temperature perturbation were chosen advection by the north-westerly of background winds is a conceivable mechanism for the propagating convective as further analyses (Figure 10). In this case, 3 km represents a systems due to the life cycle of MCC, which is consistent depth of the maximum cool anomaly which has been re- with the study by Li et al. [45] that mentioned that the ported in previous work [38]. In a rough estimation, the direction of propagating diurnal rainfall is regulated by the speed of CP-coastal is associated with rainfall onset prop- −1 prevailing background wind. agation around 8.8 m·s (Figure 10(b)), which is in km km km km km km km km 350 10 Advances in Meteorology (a) (e) 19:10 19:50 (b) (f) 19:20 20:00 1 1 (c) (g) 19:30 20:10 3 3 -1 1 m s -1 gr kg (d) (h) 19:40 20:20 3 3 0.15 0.1 0.05 A XC B A X B 105E 108E 105E 108E Figure 9: Same as Figure 8, but from 19:10 to 21:40 LST. $e black vertical solid line (C) represents location (154 km from (X) of a stationary convection cell from 19:00 LST (see Figure 8) to 20:10 LST. agreement with previous studies [46, 47]. $is offshore with the land-based convective system under the consecu- rainfall propagation corresponds to a strong increase of tively CP-inland mechanism (Figure 10(d)). $is was also CAPE (Figure 10(a)) as well as a cooling anomaly over the shown by the existence of anomalies in a pair of warming and cooling as consecutively at low level (3 km) which is surface level at 0.5 km (Figure 10(c)). It appears that the offshore convective system was strongly maintained by the associated with developing a new convective cell from the surface cold pool (CP-coastal) which moves offshore rapidly previous decaying convection. $is CP development is also from late afternoon (18:00 LST) to early morning (02:00 triggering a deep convective cloud over the coastal region in LST). Interestingly, the initial convection around 16:00 LST the afternoon and maintaining a strong offshore propaga- over the seaside of the coastal region has strong connections tion under the long-lasting MCC mechanism. km km km km km km km km 350 Advances in Meteorology 11 04-05 -1 -1 mm h J kg 01-02 22-23 2400 19-20 19 10 2000 16-17 13-14 0.5 10-11 0.1 07-08 A XB A XB 105E 108E 105E 108E (a) (b) K K 1 1 0.8 0.8 22 22 0.6 0.6 0.4 0.4 19 19 0.2 0.2 0 0 -0.2 -0.2 -0.4 -0.4 13 13 -0.6 -0.6 -0.8 -0.8 10 10 -1 -1 07 07 A XB A XB 105E 108E 105E 108E (c) (d) Figure 10: Diurnal cycle on January 1, 2021, for (a) CAPE, (b) rainfall, and temperature perturbations from daily mean, (c) 0.5 km, and (d) 3 km. Black dashed vertical lines indicate the position of the eastern coastline of South Sumatra. $e phase speed of rainfall propagation of −1 the red line is ∼ 8.8 m·s . strengthens the CP-coastal which is generated over the 4. Conclusions seaside of the coastal region. $e CP-coastal tends to We have investigated the case study of a cold pool related to a persist and propagates further offshore influenced by a heavy rainfall system during January 1-2, 2021, over Lampung, large gradient between the near-surface equivalent tem- South Sumatra, by using the numerical weather prediction of the perature of CP and its environment. $e CP-coastal which WRF model. Heavy rainfall observed by radar as well as satellite is developed as a result of decaying a deep convective observations could be qualitatively well simulated by the model cloud also induced the MCC by triggering new several results. In this period, synoptic weather conditions due to the convective clouds rapidly as well as spread under the initial development of the Borneo vortex concurred to enhance back-building mechanism over the sea. Interestingly, the the low-level north-westerly winds as a predominant prevailing MCC developed in linear system which reported previ- wind over the wider area of Lampung, South Sumatra. ously as mainly type of MCS over Java Sea (> 65%) oc- $e development of CP related to offshore propaga- curred during January-February [47]. tion of convective systems in this case study was illus- Moreover, it can be coarsely calculated from Figures 8 trated by 2 categories: CP-inland and CP-coastal. $e CP- and 10 that the speed of the CP-coastal is around −1 inland propagates slowly and disappeared rapidly because 8.8 m·s (18:00–23:00 LST). $is simulated CP speed is of the relatively strong north-westerly wind due to the in agreement with the previous study [38] which founded −1 synoptic condition and lack of supporting environment that CP speed is 8 m·s over the eastern Maritime related to near-surface moisture over the leading edge of Continent and [46] proposed that theoretically, the speed −1 the CP-inland. However, the dissipation of CP-inland of CP is in a wide range around 5–12 m·s . Additionally, LST LST LST LST 12 Advances in Meteorology it was also found that the CP may have an important role References to develop offshore-propagating convective systems [1] H. R. Byers and R. R. Braham, 3e 3understorm, p. 287, U. which are influenced by prevailing background wind. S. Government Printing Office, Washington, DC, USA, 1949. Considering that the existence of the Borneo vortex during [2] J. Charba, “Application of gravity current model to analysis of the southward monsoon flow period may have occurred [48] squall-line gust front,” Monthly Weather Review, vol. 102, and coexist with Cross Equatorial Northerly Surge which is no. 2, pp. 140–156, 1974. reinforcement by Cold Tongue [24], this heavy rainfall event [3] J. F. W. Purdom, “Some uses of high-resolution goes imagery could reoccur with varying intensity. 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