SEPTEMBER 2015 A K T E R 3495 Mesoscale Convection and Bimodal Cyclogenesis over the Bay of Bengal NASREEN AKTER Department of Physics, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh (Manuscript received 8 August 2014, in final form 11 May 2015) 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 mod- erately 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 con- tained 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. 1. Introduction and the northward-propagating deep convection phase of the intraseasonal oscillations (ISOs) trigger an earlier The South Asian premonsoon (March–May) and post- monsoon onset in the BoB than in India (Jiang and Li monsoon (October–December) seasons are the transition 2011; K. Li et al. 2013; Yu et al. 2012). periods between the summer (June–September) and The BoB is not only significant for the Asian monsoon winter (January–February) monsoons. A strong south- onset but also offers a unique setting for tropical cyclone westerly (SW) prevailing wind transports enormous (TC) activities. The TCs over the BoB are confined moist and warm air masses from the sea to the land in the within the monsoon transition periods, with a maximum boreal summer, while dry and cool northeasterly (NE) frequency in October–November and a second maxi- flows occur in the opposite direction (i.e., from the land mum in May (McBride 1995; Harr and Chan 2005; to the sea) during the boreal winter (Ramage 1971; Das Camargo et al. 2007; Kikuchi and Wang 2010; Akter and 1995). The Arabian Sea and the Bay of Bengal (BoB), Tsuboki 2014, hereafter AT14). In the boreal summer, which are two branches of the north Indian Ocean the location of the monsoon trough (MT) is well inland; (NIO), play pivotal roles in the seasonal wind reversal of the prevailing southwesterly winds and upper-level South Asian monsoons. The southern BoB experiences easterly winds create strong vertical shear that sup- the southwest monsoon in early May (Wu and Zhang presses TC formation (Jeffries and Miller 1993; McBride 1998; Mao and Wu 2007; Wu et al. 2012). The maximum 1995; Z. Li et al. 2013). Conversely, the bimodal TC annual sea surface temperature (SST) in the central BoB activity over the BoB is modulated by the seasonal mi- gration of MT locations that are usually in the northern and central BoB during the pre- and postmonsoon sea- Corresponding author address: Nasreen Akter, Department of Physics, Bangladesh University of Engineering and Technology, sons, respectively (McBride 1995; AT14). Within the Zahir Raihan Rd., Dhaka 1000, Bangladesh. transition seasons, intraseasonal ISO phases often in- E-mail: [email protected] fluence TC formation in the BoB (Kikuchi and Wang DOI: 10.1175/MWR-D-14-00260.1 Ó 2015 American Meteorological Society Unauthenticated | Downloaded 10/05/21 07:25 AM UTC 3496 MONTHLY WEATHER REVIEW VOLUME 143 2010; Kikuchi et al. 2012; Yanase et al. 2012). Both the cycles of deep, moist convective activity called vortical seasonal MT position and ISO phases are associated hot towers (VHTs), where the lower-tropospheric vor- with the formation of synoptic-scale tropical distur- tices are formed within the embryonic environment of bances or cloud clusters that favor active cyclones (Gray MCVs (Hendricks et al. 2004; Reasor et al. 2005; 1998; Roundy and Frank 2004). Montgomery et al. 2006; Braun et al. 2010). The com- Recently, AT14 revealed that the northern position of bined effect of the upscale growth of cyclonic vortices the MT in the BoB and environmental convective in- and their integration contributes to the development of hibition (CIN) are mutually responsible for the de- the TC vortex. creased cyclone frequency in May (premonsoon), even The structure and characteristics of MCSs and the as- though the BoB maintains higher SSTs that result in sociated MCVs that form in the BoB during pre- and increased convective available potential energy (CAPE; postmonsoon TCs are vital to the seasonal bimodal Glickman 2000) compared with the postmonsoon sea- cyclogenesis process. Few studies have focused on the son. In the premonsoon season, deep hot and dry air is different types of MCSs and their contributions to pre- advected from northwest India toward the BoB to pro- cipitation in South Asia during the premonsoon and vide environmental CIN that caps the boundary layer monsoon seasons (Houze et al. 2007; Romatschke et al. over the northwest BoB (Fig. 7 of AT14). Consequently, 2010; Romatschke and Houze 2011a,b). Previous studies TC genesis is reduced because of suppressed convection. have shown that the BoB experiences large systems with Therefore, seasonal environmental flow is an essential extremely large stratiform regions in both the pre- dynamical aspect that precedes TC genesis in the BoB. monsoon and monsoon seasons. During the premonsoon Ritchie and Holland (1999) and Yoshida and Ishikawa period, MCSs are primarily related to depressions and (2013) investigated five types of large-scale dynamical exhibit weak diurnal cycles. Moreover, no investigation flow patterns associated with cyclone development: on postmonsoon MCS characteristics has been conducted. monsoon shear line (SL), monsoon confluence region Specifically, synoptic-scale flow patterns and related (CR), monsoon gyre (GY), easterly wave (EW), and MCSs for TC genesis have not been identified in the BoB. Rossby wave energy dispersion (RD) in the western Therefore, the objective of the present study is to assess North Pacific; the results demonstrated that the SL, CR, how seasonal variations in the synoptic-scale flows over and GY patterns are related to the MT. Conversely, for the BoB determine the different types of MCS formations the same basin, Lee et al. (2008) discussed the EW, SL, and the relevant vorticity generation within the BoB that and CR patterns and three synoptic-scale flows: SW, contributes to the seasonal bimodal distribution of tropi- NE, and combined NE and SW; the results showed that cal cyclogenesis. To achieve this objective, simulations of all of the patterns are related to the monsoon, except for the structural characteristics of MCSs during cyclogenesis the EW. Furthermore, Lee et al. (2008) noted that me- in the BoB are the only viable option because observa- soscale convective system (MCS; Houze 2004) activities tional data are limited. are linked with such synoptic flows. Many studies have acknowledged that large-scale or 2. Model specifications and data used synoptic-scale flows are not the only major contributors to TC genesis. Individual MCSs that are associated The Advanced Hurricane Weather Research and Fore- with a preexisting tropical disturbance and cyclonic casting (WRF) Model (AHW) (version 3.3.1), which is mesoscale convective vortices (MCVs) that develop in derived from the Advanced Research version of the the stratiform precipitation region near the middle tro- WRF (ARW) Model (Davis et al. 2008; Skamarock et al. posphere are also fundamental precursors for cyclo- 2008), is used to simulate pre- and postmonsoon cy- genesis (e.g., Zehr 1992; Harr et al. 1996; Bister and clones for examining the MCSs that are associated with Emanuel 1997; Ritchie and Holland 1997; Gray 1998; bimodal cyclogenesis. A Lambert projection map is Dunkerton et al. 2009; Houze 2010). Two processes are utilized with two-way nested domains; the grid spacing is hypothesized to explain lower-tropospheric vortices 12 km for the outer domain and 4 km for the innermost produced by midtropospheric MCVs: the top-down and domain (Fig. 1b). Davis et al. (2008) showed that 12- and bottom-up paradigms for cyclogenesis. The first para- 4-km grid spacing provide the most accurate forecasts of digm emphasizes the downward advection of MCVs in a storm position and intensity. The parent domain (D1) moist environment (Emanuel 1993; Bister and Emanuel consists of 861 3 595 grid points, while the inner nest 1997), where greater penetration occurs by the merging (D2) has 1064 3 996 grid points. The 28 terrain- of individual MCVs and cyclonic low-level vorticity is following vertical levels, where the top level is 50 hPa, further enhanced (Simpson et al. 1997; Ritchie and are used. The Kain–Fritsch cumulus parameterization, Holland 1997). The second paradigm incorporates which predicts deep and shallow convection using a mass Unauthenticated | Downloaded 10/05/21 07:25 AM UTC SEPTEMBER 2015 A K T E R 3497 FIG. 1. (a) Cyclone frequency and intensity during 2001–12 based on the JTWC data.