A numerical study of convection in rainbands of Typhoon Morakot (2009) with extreme rainfall: roles of pressure perturbations with low-level wind maxima

C.-C Wang,C.-Y Lee,R H Johnson,S.-Y Huang,Y.-H Chen,H.-C Kuo

doi:10.5194/acp-15-11097-2015

Abstract

Abstract. This paper investigates the formation and evolution of deep convection inside the east–west oriented rainbands associated with a low-level jet (LLJ) in Typhoon Morakot (2009). With the typhoon center to the northwest of Taiwan, the westerly LLJ occurred as a result from the interaction of typhoon circulation with the southwest monsoon flow, which supplied the water vapor for the extreme rainfall (of ~ 1000 mm) over southwestern Taiwan. The Cloud-Resolving Storm Simulator with 1 km grid spacing was used to simulate the event, and it successfully reproduced the slow-moving rainbands, the embedded cells, and the dynamics of merger and back-building (BB) on 8 August as observed. Our model results suggest that the intense convection interacted strongly with the westerly LLJ that provided reversed vertical wind shear below and above the jet core. Inside mature cells, significant dynamical pressure perturbations (p'd) are induced with positive (negative) p'd at the western (eastern) flank of the updraft near the surface and a reversed pattern aloft (> 2 km). This configuration produced an upward-directed pressure gradient force (PGF) to the rear side and favors new development to the west, which further leads to cell merging as the mature cells slowdown in eastward propagation. The strong updrafts also acted to elevate the jet and enhance the local vertical wind shear at the rear flank. Additional analysis reveals that the upward PGF there is resulted mainly by the shearing effect but also by the extension of upward acceleration at low levels. In the horizontal, the upstream-directed PGF induced by the rear-side positive p'd near the surface is much smaller, but can provide additional convergence for BB development upstream. Finally, the cold-pool mechanism for BB appears to be not important in the Morakot case, as the conditions for strong evaporation in downdrafts do not exist.

Highlights

The Cloud-Resolving Storm Simulator (CReSS) model-simulated column maximum mixing ratio of total precipitating hydrometeors in the 1 km run over the period of 06:00–08:00 UTC, 8 August 2009 is shown in Fig. 7, which can be compared with the radar reflectivity composites in Fig. 3 and Wang et al (2012, their Figs. 6e–g and 7)
During the period of heaviest rainfall on 8 August, when the tropical cyclones (TCs) center was over the northern Taiwan Strait, the E–W oriented, persistent, and slow-moving rainbands and the embedded deep convection that propagated eastward were responsible for the serious and wide-spread flooding over the southwestern plains of Taiwan
Developing inside the low-level convergence zone between the TC vortex and the monsoon flow over the southern strait, as observed in several other past TCs, these rainbands were collocated with a westerly level jet (LLJ) and exhibited frequent cell merging and back-building behavior that contributed to the heavy rainfall

Summary

Introduction

Rainbands develop in response to linear forcing such as fronts, dry lines, troughs, and convergence zone (e.g., Carbone, 1982; Browning, 1990; Doswell III, 2001; Johnson and Mapes, 2001) or by self-organization in a sheared environment (e.g., Bluestein and Jain, 1985; Rotunno et al, 1988; Houze Jr. et al, 1990), and are a common type of precipitation systems around the world (e.g., Houze Jr., 1977; Chen and Chou, 1993; Garstang et al, 1994; LeMone et al, 1998; Meng et al, 2013) These linear-shaped mesoscale convective systems (MCSs) are most well studied in mid-latitudes and classified by Parker and Johnson (2000, 2004) into three archetypes based on the location of stratiform region relative to the main line: trailing stratiform (TS), leading stratiform (LS), and parallel stratiform (PS), in response primarily to the different structure of environmental vertical wind shear. Outside of North America, linear MCSs with embedded cells moving along the line are often responsible for floods, such as the events in France, Australia, Hawaii, and eastern China (Sénési et al, 1996; Tryhorn et al, 2008; Murphy Jr. and Businger, 2011; Luo et al, 2014)

Results

Discussion

Conclusion