Asymmetric Structures of a Squall-Line MCS over Taiwan with Significant Hydraulic Jumps

Yu-Tai Pan,Ming-Jen Yang

doi:10.1007/s13143-021-00262-1

Yu-Tai Pan, Ming-Jen Yang

Open Access

https://doi.org/10.1007/s13143-021-00262-1

Copy DOI

Abstract

On 19 April 2019, a mature squall-line mesoscale convective system (MCS) with the characteristics of a leading convective line and trailing stratiform landed on Taiwan, resulting in strong gust wind and heavy rainfall. This squall-line MCS became asymmetric after landfall on Taiwan. Two sets of idealized numerical simulations (mountain heights and low-level vertical wind shear) using the Weather Research and Forecasting (WRF) model were conducted to examine the impacts of realistic Taiwan topography on a squall-line MCS. Results showed numerous similarities between the idealized simulations and real-case observations. The low-level Froude number which considered the terrain height (Fmt) was calculated to examine the blocking effect of the Taiwan terrain, and the cold pool (determined by − 1.5 K isotherm) was found to be completely blocked by the 500-m height contour. The northeast-southwest orientation of the Snow Mountain Range (SMR), and the north–south orientation of the Central Mountain Range (CMR) led to the upwind side asymmetry. On the other hand, the lee-side asymmetry was associated with different intensities and occurrence locations of the hydraulic jump between the SMR and southern CMR, and the cold-pool Froude number (Fcp) indicated the flow-regime transition from subcritical to supercritical.

Highlights

During the Mei-Yu season in Taiwan, the squall-line type of mesoscale convective systems (MCSs) frequently form ahead of or along the MeiYu fronts over southeastern China, which approach Taiwan from the west and lead to heavy rainfall and fierce gust wind (Fang 1985; Ninomiya et al 1988)
One previous observational study (Teng et al 2000) investigated the impacts of Taiwan terrain on a squall-line MCS during the Taiwan Area Mesoscale experiment (TAMEX; Kuo and Chen 1990), and they found the orientation of squall-line MCS became parallel to 1000-m terrain contour line after the contacts with Taiwan topography
Notice that instead of inserting a warm bubble at the model initial time, we delay the insertion time until 240 min to prevent from the interference between lee-wave convection and the squallline MCS represented by the warm bubble

Summary

Introduction

During the Mei-Yu season (from mid-May to mid-June) in Taiwan, the squall-line type of mesoscale convective systems (MCSs) frequently form ahead of or along the MeiYu fronts over southeastern China, which approach Taiwan from the west and lead to heavy rainfall and fierce gust wind (Fang 1985; Ninomiya et al 1988). Ahead of the squall line, the environment usually has substantial vertical wind shear ( at low level), and the interaction between environmental wind shear and precipitation-induced cold pool can make the squall-line MCS a self-organized system (Fovell and Tan 1998; Lin et al 1998; Lin and Joyce 2001). There are relatively few studies on the terrain effects to squall-line MCSs. Since Taiwan is a mountainous island, the relation between a squall-line MCS and realistic Taiwan terrain is worth investigating. Whether the cold pool is blocked by terrain or not affects the longevity of the squall-line MCS (Frame and Markowski 2006; Letkewicz and Parker 2011)

Objectives

Methods

Results

Conclusion