Statistical characteristics of merger-type sea-breeze fronts and associated circulation patterns in the Bohai Bay region, North China

Sea breeze front (SBF) is one of the important weather systems affecting the occurrence and development of severe convective weather in the Bohai Bay region (BBR). 226 cases of merger-type SBFs (MSBFs) merged with gust fronts (GFs) and convective systems (CSs), respectively, were identified based on Doppler weather radar data and ground-based automatic weather station data from May to September during 2009–2018 in the BBR, and their basic tempo-spatial characteristics and associated atmospheric circulation backgrounds are documented for the first time. The number of MSBFs cases merged with GFs (MSBF-GFs) and that of MSBFs merged with CSs (MSBF-CSs) were 172 and 54, respectively. The number of MSBFs varied significantly in each year, with 37 (13) in the most (least) frequent year, and with an average number of 22.6 per year. More than 93.8 % of the MSBFs occurred from June to August, especially most frequent (37.2 %) in July. The merging locations of the MSBFs were mainly distributed in the central-northern Tianjin and the southeastern Hebei province, and the horizontal scales of MSBFs were mainly distributed in the range of 130–309 km. About 29.6 % (51.9 %) of the MSBF-CSs cases resulted in significantly (slightly) enhanced convections, while 51.4 % (23.8 %) of the MSBF-GFs bring about significantly (slightly) enhanced convections. About 72.1 % of the MSBF-GFs cases are merged in near "face-to-face" form, and their 49.2 % (23.4 %) proportion lead to significantly (slightly) enhanced convections. The atmospheric circulation patterns of MSBFs identified using objective classification method showed that, the major two patterns (occupied 56.6 % cases) have similar dynamic, thermodynamic, and water vapor characteristics including westerlies or southwesterlies with intensity about 8–10 m/s at 500 hPa, showing significant warm and moist air delivered from the south and relatively weak vertical wind shear along with intense water vapor convergence at 850 hPa.