Mesoscale Convection and Bimodal Cyclogenesis over the Bay of Bengal

Abstract

Mesoscale convective systems (MCSs) are an essential component of cyclogenesis, and their structure and characteristics determine the intensity and severity of associated cyclones. Case studies were performed by simulating tropical cyclones that formed during the pre- and postmonsoon periods in 2007 and 2010 over the Bay of Bengal (BoB). The pre- (post) monsoon environment was characterized by the coupling of northwesterly (southwesterly) wind to the early advance southwesterly (northeasterly) monsoonal wind in the BoB. The surges of low-level warm southwesterlies with clockwise-rotating vertical shear in the premonsoon period and moderately cool northeasterlies with anticlockwise-rotating vertical shear in the postmonsoon period transported moisture and triggered MCSs within preexisting disturbances near the monsoon trough over the BoB. Mature MCSs associated with bimodal cyclone formations were quasi linear, and they featured leading-edge deep convection and a trailing stratiform precipitation region, which was very narrow in the postmonsoon cases. In the premonsoon cases, the MCSs became severe bow echoes when intense and moist southwesterlies were imposed along the dryline convergence zone in the northern and northwestern BoB. However, the development formed a nonsevere and nonorganized linear system when the convergence zone was farther south of the dryline. In the postmonsoon cases, cyclogenesis was favored by squall-line MCSs with a north–south orientation over the BoB. All convective systems moved quickly, persisted for a long time, and contained suitable environments for developing low-level cyclonic mesovortices at their leading edges, which played an additional role in forming mesoscale convective vortices during cyclogenesis in the BoB.

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

“…Previous studies have described the synoptic‐scale environment in the BoB during the pre‐ and postmonsoon periods and specified the dynamic and thermodynamic environments suitable for seasonal cyclogenesis (Akter and Tsuboki, 2014; Akter, 2015). Although the genesis period is not included here, the lower‐ and upper‐level winds that occur during this period and their characteristics are explained in Figure 4 to express their seasonal variabilities.…”

“…The variability in the monsoon trough (MT) location and intraseasonal oscillations concurrently offer two active TC periods, that is, the premonsoon (March–May) and postmonsoon (October–December) seasons (McBride, 1995; Kikuchi and Wang, 2010; Akter and Tsuboki, 2014). Moreover, during the premonsoon period, the presence of a dryline aloft the BoB causes the basin to be vulnerable to the occurrence of severe TCs within a very short period (Akter, 2015). Hence, the variations in the environment over the BoB are essential to comprehend, especially during the recurvature of TCs, because these variations often influence the intensity and movement speed of TCs (Riehl, 1972; Burroughs and Brand, 1973; Evans and McKinley, 1998; Knaff, 2009;Yamada et al ., 2010; Bhattacharya et al ., 2015).…”

Tropical Cyclogenesis Associated with Premonsoon Climatological Dryline over the Bay of Bengal

“…Within the premonsoon TCs, four TCs out of the nine very severe TCs (MSW ≥ 65 kt) i.e., BoB_02 (2004), Akash (2007), Aila (2009), and Mora (2017) developed around the strong gradient where the dry-moist air masses congregated. Akash (2007) was simulated before by the author to examine the formation and characteristics of the MCS during its genesis period (Akter, 2015). Subsequently, the remaining three TCs have been simulated in this study to analyze the details of the MCS and identify the causes for MCVs (core vortex of TCs) in the presence of a dryline.…”

“…The outer domain (D1) with 861 × 595 grid points covers the area 0-30°N and 70-110°E (not shown here but same as Fig. 1 in Akter 2015); however, the inner domain (D2) with 1064 × 996 grid points includes 5-22°N and 80-97°E (area of Fig. 2).…”

Climatology of the premonsoon Indian dryline

“…The climate of India is dominated by strong monsoonal winds, and four seasons are classified in this region based on these winds: premonsoon (March–May), monsoon (June–September), postmonsoon (October–December) and winter (January–February). Among them, the premonsoon season is characterized by coupled dry northwesterly and moist southwesterly winds over South Asia (Akter, ), resulting in a narrow convergence zone that is almost parallel to the eastern coast of India, extending north towards the Himalayas. The dry‐moist air convergence zone that appears during the premonsoon along the eastern Indian coast is called the premonsoon Indian dryline.…”