Mesoscale precipitation systems and their role in the rapid development of a monsoon depression over the Bay of Bengal

Abstract

The structure of mesoscale precipitation within monsoon depressions is still not as well‐known as the synoptic‐scale composite cloud and precipitation structure. Here, using observational data from multi‐satellite sensors and a cloud‐resolving regional model, we investigate the three‐dimensional structure of mesoscale precipitation systems in the different stages of the life cycle of a monsoon depression. Effects of latent heating from the precipitation systems and the Bay of Bengal (BoB) on the development of the monsoon depression are also evaluated in sensitivity experiments with the model. A typical monsoon depression developed on 17 August 2016 over the BoB. In the rapid development phase, satellite observations reveal mesoscale convective systems with deep convective precipitation cells and stratiform precipitation near the head of the BoB. Extremely deep and intense convective cells appear along a ring‐like rain band when a closed cyclonic circulation becomes obvious around the northernmost part of the BoB. The deep convection appears frequently, particularly along the western‐to‐southwestern side of the low, a convergence‐prone area between northerlies with air masses of large convective available potential energy (CAPE) that makes up the western–southwestern part of the closed cyclonic circulation, and strong monsoon westerlies with relatively stable air masses to the south of the low. Sensitivity experiments reveal that both cloud/precipitation processes and evaporation from the BoB are essential for the rapid development of the monsoon depression over the BoB. Evaporation from the BoB adds a large amount of moisture to the atmospheric boundary layer near the low. Southwesterlies and southerlies on the eastern side of the low draw in warm, humid boundary‐layer air to the closed cyclonic circulation, which maintains a large‐CAPE environment in the low and enhances deep convection. A possible positive feedback process including moist convection that leads to the rapid intensification of monsoon depressions over the BoB is discussed.

Twice‐Daily Monsoon Precipitation Maxima in the Himalayas Driven by Land Surface Effects

“…Day-to-day fluctuations (Figures 2b,2d and 2f) are actually effects of synoptic-scale atmospheric circulation at midlatitudes and in the tropics in summer. The diurnal cycles of summer precipitation are modulated by synoptic-scale low-pressure systems, such as monsoon depressions (Fujinami et al, 2020) and mid-latitude troughs (Bohlinger et al, 2017(Bohlinger et al, , 2019, and by intraseasonal oscillations that occur in many places in the midlatitudes and tropics (Fujinami & Yasunari, 2004;Fujinami et al, 2011Fujinami et al, , 2014. Understanding how diurnal precipitation cycles are modulated by these patterns on multiple spatiotemporal scales is essential for full understanding of precipitation variability, and warrants further investigation.…”

Recurvature and movement processes of tropical cyclones over the Bay of Bengal

“…The Cloud Resolving Storm Simulator (CReSS) is a three‐dimensional terrain‐following cloud‐resolving model developed at Nagoya University, Japan (Tsuboki and Sakakibara, 2002); this model can perform numerical simulations of different weather phenomena (i.e., Wang et al ., 2005; Maesaka et al ., 2006; Ohigashi and Tsuboki, 2007; Yamada, 2008; Rafiuddin et al ., 2013) with resolutions on the order of a few tens of meters to a few kilometers. The CReSS model has also successfully simulated TCs (i.e., Tsujino et al ., 2020; Tsujino and Tsuboki, 2020; Wang et al ., 2020), including TCs and monsoon depressions over the BoB (Akter and Tsuboki, 2010; 2012; Fujinami et al ., 2020). The model architecture supports a horizontal curvilinear Arakawa “C” grid and a Lorenz grid in the vertical direction.…”

Nocturnal Southerly Moist Surge Parallel to the Coastline Over the Western Bay of Bengal

“…The extent to which the strong LLJs from the Indian subcontinent and from the Palk Strait each contribute to the nocturnal moist surge and the amount of nocturnal precipitation around the Himalayas and the Meghalaya Plateau also needs to be investigated quantitatively. Moreover, the interaction between the nocturnal surge and the different wind regimes induced by intraseasonal oscillations and monsoon low pressure systems is another interesting topic that should be investigated (Chen, 2020;Fujinami et al, 2014Fujinami et al, , 2017Fujinami et al, , 2020Liu et al, 2022).…”

“…It causes strong low‐level monsoon westerlies/southwesterlies from the Arabian Sea to the western North Pacific that represent a characteristic feature of the low‐level Asian summer monsoon circulation. In South Asia, the monsoon westerlies pass over the Indian subcontinent and then cross the Bay of Bengal (BoB) where they gain latent heat from the surface waters (Fujinami et al., 2020) (Figure 1a). One branch of the monsoon westerlies turns northward over the BoB and then moves across the Gangetic Plain, transporting abundant moisture toward areas such as the Meghalaya Plateau and the Himalayas that are known for heavy rainfall (Fujinami et al., 2017, 2021).…”