Abstract
Tropical cyclones of the Bay of Bengal (BoB) that formed near the synoptic-scale dryline usually intensified over a distance of 600–800 km within 3 days and caused severe destruction after landfall. High-resolution simulations of very severe cyclonic storms in association with dryline indicate that the meridional shear aids in the development of a linear-shaped group of convective cells that mature as an east–west oriented quasi-linear convective system (QLCS) within the boundary between the dry-moist air masses. The leading edge deep convections are supported by low-level moist southwesterly inflow; however, the typical mid-level mesoscale convective vortex (MCV) associated with these QLCS is unremarkable due to a very narrow trailing stratiform region within the QLCS. Supercells are likely to be organized within the QLCS due to extremely unstable atmospheric conditions resulting from a strong vertical shear of 27–39 m s−1 between 0 and 6 km and large convective available potential energy of > 3000 J kg−1. The vertical shear veering with height causes several numbers of low-level mesovortices having diameters less than 10 km at the leading edge in the different convective stages of the QLCS. The dryline aloft in the BoB produces horizontal positive shear vorticity of the order 10–5 s−1 with higher values in the levels 850–600 hPa. The advection of intense cloud-scale cyclonic mesovortices (~ 10–3 s−1) assists and enhances a cyclonic vortex to the rear side of the QLCS that performs as an MCV for cyclogenesis over the BoB.
Highlights
The genesis of a tropical cyclone (TC) follows a hierarchical environment
Another important climate feature is the presence of dryline aloft over the northern Bay of Bengal (BoB) during the premonsoon season
Cyclones formed near or along the dryline quickly evolve to severe TCs within a very short distance in a small time period and frequently have severe consequences after landfall
Summary
The appropriate climatology and proper synoptic conditions primarily favor initiating tropical disturbances or cloud clusters over oceans (Gray, 1998). Within these tropical disturbances, the triggering of mesoscale convective systems (MCSs) are the fundamental precursors of cyclogenesis in any ocean basin (e.g., Zehr 1992; Harr et al 1996; Bister and Emanuel 1997; Ritchie and Holland 1997; Simpson et al 1997; Gray 1998; Houze 2010). MCSs are organized cloud clusters containing both deep convective cells and stratiform clouds and precipitation (Houze 2004). Only a few studies (Bister and Emanuel 1997; Powers and Davis 2002; Akter 2015) have examined the types of MCSs that occur during the formation of a tropical cyclone (TC) that typically develops in warm summer seasons
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