Abstract
Based on the ARPS (Advanced Regional Prediction System) at 2 km grid spacing, the convective initiation (CI) of cells that evolved into a squall line on the southern flank of a Meiyu front in East China is investigated. The initiations of four convective cells, denoted CI-A through CI-D, are reasonably captured in the simulation. For CI-A and CI-B, locally enhanced convergence bands associated with boundary layer horizontal convective rolls (HCRs) play a crucial role in determining the exact locations of CIs, whereas a cold outflow boundary from earlier frontal precipitation contributes to additional convergence forcing. For CI-C, initiation occurs directly over the gust front, with the frontal convergence providing the main forcing. CI-D occurs south of and sufficiently far from the gust front, and is mainly forced by the HCR circulations. With surface heat flux turned off in the model, CI is much delayed. Therefore, surface heating increases the convective instability of air south of the front and causes the development of HCRs; it also enhances the gust front convergence by mixing higher southwesterly momentum toward the surface. When the condensation process is turned off, HCRs and/or gust frontal forcing are still able to lift the low-level air to super-saturation where CI is expected.
Highlights
The Meiyu front, characterized by strong moisture instead of the temperature gradient, typically establishes itself as a quasi-stationary front in the Yangtze-Huai River basin between mid-June and mid-July in China [1,2]
This paper focuses on the initiation of convective cells within a Meiyu frontal system that occurred in the Intensive Observing Period (IOP) Number 8 in the field campaign of the OPACC (Observation, Prediction and Analysis of Severe Convection of China) Project [36] on 12 July 2014 over the Yangtze
As the aim of this study is to examine the mesoscale structures and associated physical processes responsible for the convective initiation (CI), further analyses based on the simulated results in control experiment (CTRL) have relatively higher reliability
Summary
The Meiyu front, characterized by strong moisture instead of the temperature gradient, typically establishes itself as a quasi-stationary front in the Yangtze-Huai River basin between mid-June and mid-July in China [1,2]. During the period known as the Meiyu season, the Yangtze River valley region is vulnerable to flooding due to persistent heavy rainfalls. A deep understanding of the formation and evolution of these MCSs can help us better predict heavy rainfall and severe flooding in the Meiyu season. There have been many studies on the Meiyu frontal system and associated MCSs and heavy precipitation, including studies on the effects of the East Asian summer monsoon [3], large-scale moisture transport by the low-level jet (LLJ) typically found south of the Meiyu front [4,5,6], the eastward propagation of MCSs or mesoscale vortices along the Meiyu front [7,8], and diurnal cycle of precipitation [9,10,11,12]. The initiation of convective cells within the Meiyu frontal system that organizes into MCSs has been little studied,
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