Warm‐Season Afternoon Precipitation Peak in the Central Bay of Bengal: Process‐Oriented Diagnostics

• Published in: Journal of geophysical research. Atmospheres Vol. 128; no. 13
• Main Authors: Peng, Chin‐Hsuan, Chen, Xingchao
• Format: Journal Article
• Language: English
• Published: 16.07.2023
• Subjects: Bay of Bengal; diurnal cycle; gravity wave; mesoscale convective system (MCS)
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2022JD038398

Past studies have indicated that precipitation over tropical open oceans generally peaks in the early morning. However, an intriguing departure from this pattern is observed in the central Bay of Bengal (CBoB), where rainfall exhibits a distinct afternoon peak during the South Asian summer monsoon season. By using a novel satellite‐based cloud classification and tracking data set, we found that more than 75% of the afternoon rainfall (15–17 LST) over the CBoB comes from mesoscale convective systems (MCSs). Most of the MCSs contributing to the CBoB afternoon rainfall peak originate either locally over the CBoB or near the west and east coasts of the BoB, in contrast to the northern BoB as highlighted in previous studies. Analyses show that MCSs initiated near coastlines are primarily influenced by land‐sea breezes, whereas MCSs initiated over the BoB open ocean during early morning are strongly associated with diurnal radiative forcings. In addition, there are clear diurnal propagating MCS initiation signals from the west and north coastlines of the BoB to the CBoB, which are related to diurnal gravity waves emitted from the coastlines. The thermodynamic conditions conducive to MCS initiation over different sub‐regions of the BoB are also investigated. No systematic differences found in environmental convective available potential energy between days with and without MCS initiation. However, over most sub‐regions, days with MCS initiation generally have higher total column water vapor than days without MCS initiation. This difference suggests that the lower‐free‐tropospheric moisture content plays an important role in MCS initiation over the BoB. Plain Language Summary Precipitating clouds over tropical oceans play a major role in Earth's water and energy cycles. Their precipitation generally peak in the early morning due to the diurnal changes of solar radiation. However, previous studies indicated that the summertime rainfall over the central Bay of Bengal (BoB) shows a prominent peak in the afternoon. Using satellite data, we found that this afternoon rainfall peak is mainly attributed to large, intense, and organized cloud systems. These cloud systems usually initiate near the BoB coastlines or over the BoB open oceans. Diurnal variations of land‐sea breezes, radiation, and atmospheric gravity waves contribute to the initiation of these systems. Additionally, atmospheric moisture may play as a dominant factor that determines the occurrence of these large cloud systems. These findings might be helpful to the future development of global climate models to simulate the diurnal cycle of rainfall more accurately over the BoB. Key Points More than 75% of afternoon rainfall over the central Bay of Bengal comes from mesoscale convective systems (MCSs) These MCSs are initiated over either the coastal or open ocean due to the diurnal land‐sea breezes, radiative effects, and gravity waves The lower‐free‐tropospheric moisture content plays an important role in MCS initiation over the Bay of Bengal