Multi-scale characteristics of atmospheric circulation related to short-time strong rainfall events in Beijing

Auto-weather station observations and ERA-Interim reanalysis data are used to investigate large and meso-scale circulation characteristics related to short term heavy rainfall events over Beijing during June to August from 2007 to 2014. This is done by classifying large-scale synoptic circulations. The results show that:(1) according to their occurrence frequencies, weather systems generally are able to be classified into four synoptic types for short term heavy rainfall events, i.e. the interaction between subtropical high and western trough (Ⅰ), small trough moving along the westerly stream (Ⅱ), cold vortex in northeastern China (Ⅲ), and low-pressure vortex or inverted trough over Huanghuai Plain (Ⅳ). Except that water vapor comes from Bohai Sea and Yellow Sea for Type Ⅲ, for the other three patterns the water vapor are related to remote supplies from the South China Sea and the East China Sea; (2) temporal and spatial distributions of short-time strong rainfall are different with different weather systems:the rainfall distribution is along the southeast-to-northwest mountain range extending from the southeastern plain across the urban area to northwestern mountains under Type Ⅳ. However, the rainfall distribution under the other three types is generally along the southwest-to-northeast mountain range (also along the topography in Beijing area). In particular, three high frequency centers are located in front of at the southwestern and northeastern mountains and central urban area respectively. Looking at the time series, it is found that short-time strong rainfall mainly occurs in the afternoon for Type Ⅲ, from dusk till mid-night for Type Ⅰ, and at night for Type Ⅱ and Ⅳ; (3) looking at the meso-scale characteristics, convections over mountainous area in Beijing are often triggered first by cold air at the lower level in western and northern Beijing, followed by organized thunderstorm highs. The cold pool outflow from the thunderstorm high and the warm moist airflow from the southerly winds in front of the mountains converge, further intensifying convections; for Type Ⅱ, the southeasterly winds at the boundary layer are blocked by the terrain in front of the northwestern mountain over Beijing. The flow bifurcates around the two sides of the mountains. The southwestern branch generates a cyclonic circulation and triggers convective weather over western urban area; the northeastern branch generates a terrain-induced convergence line in front of the mountain, and vertical motion strengthens at the nighttime because the southern component of the southeasterly winds enhances obviously, and triggers convective weather in front of northeastern mountains. As a result, two separate high-frequency rainfall centers characterized by nighttime rainfall appear. For Type Ⅲ, cold air invades from the north or west of Beijing, and meet with easterly winds at the lower layer in front of mountain, leading to convective weather in the afternoon. For type Ⅳ, the easterly flow at the top of the low vortex over Huanghuai Plains is lifted by the terrain in front of western mountains, which triggers convections. Mesoscale cyclonic circulation gradually forms, resulting in short term heavy rainfall.