Understanding Simulated Causes of Damaging Surface Winds in a Derecho-Producing Mesoscale Convective System near the East China Coast based on Convection-Permitting Simulations

Abstract: A mesoscale convective system (MCS) occurred over east China coastal provinces and the East China Sea on 30 April 2021, producing damaging surface winds near coastal city Nantong with observed speeds reaching 45 m s-1. A simulation using the Weather Research and Forecasting model at 1.5-km grid spacing generally reproduces the development of convective system and the subsequent organization into an MCS, with an eastward protruding bow segment over the sea. In the simulation, an east-west-oriented high wind swath is generated behind the gust front of the MCS. Descending dry rear-to-front inflows behind the bow and trailing gust front are found to feed the downdrafts in the main precipitation regions. The inflows help establish spreading cold outflows and enhance the downdrafts through evaporative cooling. Meanwhile, front-to-rear inflows from the south are present, associated with severely rearward-tilted updrafts initially forming over the gust front. Such inflows descend behind (north of) the gust front, significantly enhancing the downdrafts and near-surface winds within the cold pool. Consistently, calculated trajectories show that these parcels contributing to the derecho primarily originate from the region ahead (south) of the east-west-oriented gust front, and dry southwesterly flows at mid-lower levels contribute to strong downdrafts within the MCS. Moreover, momentum budget analyses reveal that large westward-directed horizontal pressure gradient force within the simulated cold pool produce rapid flow acceleration towards Nantong. The analyses enrich the understanding of damaging wind characteristics over the East China coast and will be helpful to operational forecasters.

Keywords:
• damaging surface winds,
• convection permitting simulations,
• mesoscale convective system,
• gust front,
• cold pool