Mesoscale analysis of a heavy rainfall event over Hong Kong during a pre-rainy season in South China

Abstract

During the Heavy Rainfall Experiment in South China (HUAMEX) of 1998, a record heavy rainfall event occurred in the delta of the Pearl River during the 24 hours from 1200 UTC 8 June to 1200 UTC 9 June, 1998, and a 24-hour precipitation maximum of 574 mm was reported in Hong Kong. In this paper, some mesoscale characteristics of this heavy rainfall event are studied using data from satellites, Doppler radar, wind profilers, and automatic meteorological stations collected during HUAMEX. The following conclusions are drawn: (1) During this heavy rainfall event, there existed a favorable large-scale environment, that included a front with weak baroclinity in the heavy rain area and with an upward motion branch ahead of the front. (2) Unlike most extratropical or subtropical systems, the closed low in the geopotential height field does not exited. The obvious feature was that a southerly branch trough in the westerlies existed and Hong Kong was located ahead of the trough. (3) The rainfall areas were located in the warm sector ahead of the front, rather than in the frontal zone, which is one of the characteristics of heavy rainfalls during the pre-rainy season of South China. A southerly warm and moist current contributed to the heavy rainfall formation, including the transportation of rich water vapor and the creation of strong horizontal wind convergence. (4) The observations show that the heavy rainfall in Hong Kong was directly caused by a series of meso β systems rather than a mesoscale convective complex (MCC). These meso β systems moved with the steering current in the lower-mid troposphere, their life cycles were 3–6 hours, and their horizontal sizes were 10–100 km. (5) The disturbances in the lower and mid troposphere, especially that in the planetary boundary layer (PBL) were very shallow. However, they are a possible trigger mechanism for the occurrence and development of the mesoscale convective systems and related heavy rainfalls. Finally, a conceptual model of the heavy rainfall in the warm sector ahead of the front in South China is proposed.