Numerical Simulation of Cyclone Sidr Using a Cloud-Resolving Model: Characteristics and Formation Process of an Outer Rainband

Abstract

Cyclone Sidr, one of the most devastating tropical cyclones that resulted in several thousand deaths and substantial damages, developed in the north Indian Ocean and made landfall over the Bangladesh coast on 15 November 2007. Observation and simulation results show that Sidr was embedded in a nonuniform environment and contained an intense outer rainband to the east of its center and a significant frontal band to the northwest. A detailed study of the outer rainband is performed by numerical simulation.The eastern band was a long, quasi-straight shape in the meridional direction that remained stationary relative to the cyclone center. This band was composed of convective cells that developed southeast of the center within a synoptic-scale convergence zone and propagated along the band toward the northeast quadrant. The speed of the downwind-propagating cells was greater than that of the cyclone, which resulted in a convective cluster northeast of the center. Only the downwind portion of the band consisted of convection with stratiform rain, whereas the upwind and middle portions contained active convective cells without stratiform rain.The band was located between the synoptic-scale flows of a weakly sheared, gradient-balanced westerly and a strongly sheared, nongradient-balanced prevailing southerly caused by the complex terrain of the Bay of Bengal's southeast region. Low-level convergence along the band was dominated by cross-band flow from both sides of the band and was confined below 3 km. As the cyclone moved northward, the convergence zone resulted in the extension of band length up to ;800 km. The southerly at the eastern side of the center gradually accelerated and was directed toward the center by a strong pressure gradient force. The flow accumulated a substantial amount of water vapor from the sea in addition to the increased moisture in the lower troposphere, resulting in further intensification of the convective cells.

Effect of the Eyewall Cold Pool on the Inner Rainband of a Tropical Cyclone

“…This study discusses the formation of the ECP and the mechanism by which it triggers the inner rainbands, using numerical simulations of idealized TCs with the CM1 model. There is a clear cold pool just outside the eyewall at low levels, which has been found in some previous numerical studies (Akter & Tsuboki, ; Chen et al, ; Ma et al, ; Moon & Nolan, ; Sawada & Iwasaki, ; Williams, ). The ECP is related to the TC intensity.…”

“…For the formation of rainbands in the inner core, some studies proposed that inertial-gravity waves were important (Tang et al, 2018;Willoughby, 1977), while others thought that vortex Rossby waves (VRWs) were important for inner rainband (Chen & Yau, 2001;Wang, 2002aWang, , 2002b. A cold pool just outside the eyewall was also found in some simulations of idealized (Ma et al, 2015;Sawada & Iwasaki, 2010b;Williams, 2016) and real cases (Akter & Tsuboki, 2012;Chen et al, 2014;Moon & Nolan, 2015a). However, the characteristics of cold pools due to the precipitation from the eyewall just outside the eyewall and their relationship with the inner rainbands have not yet been documented in detail.…”

“…These studies indicated that the cold pools were important for the formation (Sawada & Iwasaki, 2010b) and development (Chen et al, 2014) of rainbands. Debate remains as to whether evaporation or the induced cool pools are necessary for the formation of a rainband in some real cases in the complex environment (Akter & Tsuboki, 2012). Akter and Tsuboki (2012) investigated the formation of outer rainbands and revealed that convergence of the TC gradient-balanced wind and outside sheared, nongradient-balanced wind was a major factor for the formation of Cyclone Sidr's eastern rainband.…”

The More Rain, the Better the Model Performs—The Dependency of Quantitative Precipitation Forecast Skill on Rainfall Amount for Typhoons in Taiwan

“…The CReSS model Sakakibara 2002, 2007) is a cloud-resolving model suitable for studying convection, rainfall systems, severe weathers, and TCs alike (e.g., Liu et al 2004;Maesaka et al 2006;Wang et al 2011Wang et al , 2012Akter and Tsuboki 2012). The model employs a terrain-following vertical coordinate with prognostic equations for momentum, pressure, potential temperature, and mixing ratios of water vapor and other hydrometeors.…”

Objective convective updraft identification and tracking: Part 1. Structure and thermodynamics of convection in the rainband regions of two hurricane simulations

“…Mesoscale numerical models continue to be an invaluable tool to study the dynamical and thermodynamical processes relating to the evolution of the hurricane vortex. Numerical studies on rainband formation and structure [e.g., Akter and Tsuboki , ; Li and Wang , ; Yu and Tsai , ; Tang et al ., ], secondary eyewall formation [e.g., Terwey and Montgomery , ; Judt and Chen , ; Huang et al ., ; Rozoff et al ., ; Abarca and Montgomery , ; Qiu and Tan , ; Sun et al ., ; Wang et al ., ], and land effects on intensity, structure, and track [e.g., Kimball , ; Wu et al ., , ; Huang et al ., ] are just a few recent examples. In each of the above named situations, convective processes are vital and heavily studied.…”