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

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 CloudResolving 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.

Influence of southwest monsoon flow and typhoon track on Taiwan rainfall during the exit phase: modelling study of typhoon Morakot (2009)

“…The Cloud-Resolving Storm Simulator (CReSS) version 2.3 is used for typhoon simulation and sensitivity tests. Developed by Nagoya University Sakakibara, 2002, 2007), this model has been applied to study typhoons and other convective systems (Ohigashi and Tsuboki, 2007;Akter and Tsuboki, 2012;Wang et al, 2015; among many others). The CReSS model is a three-dimensional (3D), non-hydrostatic, and compressible model with a terrain-following vertical coordinate.…”

“…In other words, the reversed vertical shears above and below the LLJ interact with the updraughts in mature convective cells, which may lead to dynamic pressure perturbations and reinforce the formation of new convective cells upstream and cell mergers (Wang et al . , ).…”

“…The continuous formation and merging of convective cells along the rainband are due to the back-building mechanism (Johnson and Mapes, 2001) without cold pools. In other words, the reversed vertical shears above and below the LLJ interact with the updraughts in mature convective cells, which may lead to dynamic pressure perturbations and reinforce the formation of new convective cells upstream and cell mergers (Wang et al, 2015).…”

A numerical study of back-building process in a quasistationary rainband with extreme rainfall over northern Taiwan during 11–12 June 2012

“…For these systems, the mechanism for upstream initiation of new cells at the end of the convective line, presumably also dominated by storm-scale processes as their US counterparts (Schumacher and Johnson, 2005), is not clear. Recently, the roles of pressure perturbation (p ), in particular the dynamical pressure perturbation (p d ; e.g., Rotunno and Klemp, 1982;Weisman and Klemp, 1986;Klemp, 1987, and many others), in the evolution of convective cells inside the E-W BB rainbands associated with Typhoon Morakot (2009) and extreme rainfall (e.g., Wang et al, 2012) are examined by Wang et al (2015, hereafter referred to as WKJ15). They found that in the presence of an intense westerly LLJ, the interaction between updraft and vertical wind shear (e.g., Klemp, 1987) induces positive (negative) p d at the western (eastern) flank of the updraft below the jet-core level (with westerly shear) but a reversed pattern above (with easterly shear) and thus an upward-directed perturbation pressure gradient force (PGF) at the western (rear) flank (see e.g., Fig.…”

Influence of Synoptic Pattern and Low‐Level Wind Speed on Intensity and Diurnal Variations of Orographic Convection in Summer Over Pearl River Delta, South China

“…This upstream propagation may be related to the cold air outflow propagating upstream (Miglietta & Rotunno, ). Dynamical pressure perturbations induced by low‐level wind maxima (C. C. Wang et al, ) as well as the thermally driven gravity waves (Du & Rotunno, , ) are also two possible mechanisms, and additional analyses are necessary in the future.…”