北京2009~2013年期间持续性大雾的类型、垂直结构及物理成因

北京地区持续性大雾天气近几年呈显著增加趋势,但由于缺乏高时间分辨率的雾微物理和大气廓线数据,限制了对大雾垂直结构和物理成因的深入了解。本文基于美国Radiometrics公司生产的35通道MP-3000A型微波辐射计廓线仪(Microwave Radiometer Profiler,简称MWRP)及常规气象和卫星观测资料分析研究了2009~2013年期间北京地区13次大雾天气过程的类型、垂直结构特征及其产生的物理成因。按照雾产生的基本条件,将13次大雾天气主要划为平流雾和蒸发雾两类,辐射雾存在于持续性大雾过程中,不做单独划分。平流雾的平均雾顶高度不超过1.0 km,而蒸发雾的平均雾顶高度在0.5~1.5 km之间。平流雾主要是由来自西南和东南的暖湿平流移经北京地区冷下垫面后冷却降温过程产生,强逆温的形成有利于雾的持续发展。而蒸发雾是由本地区降水蒸发冷却形成,或是冷平流移经暖湿下垫面形成。按照微波辐射计连续观测的雾宏微观垂直结构特征,将13次大雾天气过程又划分为单一雾结构和云雾共存结构,并采用卫星和地面气象观测数据对部分典型雾个例进行了比较验证研究。结果表明,在13次大雾天气中,平流雾和蒸发雾各占69%和31%。由此说明北京地区持续性大雾天气主要是由暖湿平流过程和降水蒸发冷却过程造成,与天气过程的异常密切相关。单一雾结构仅占15%,而云雾共存结构占近85%,且持续三天以上的大雾天气基本具有云雾共存结构。

The Type, Vertical Structure and Physical Formation Mechanism of Persistent Heavy Fog Events during 2009-2013 in the Beijing Region

The tendency of heavy fog events has increased rapidly in recent years in the Beijing region.A better understanding of the vertical structure and physical formation mechanism of these heavy fog events is limited due to a lack of high temporal resolution data of fog microphysics and atmospheric profiling data.In this study, we investigated the type, vertical structure and physical formation mechanism of 13 fog events in autumn and winter in Beijing based on the atmospheric profiling data of Microwave Radiometer Profiles(MWRPs) with 35 channels produced by Radiometrics Corporation and routine meteorological and satellite data.We classified the 13 fog events into advection fog and evaporation fog from their basic formation processes.Radiation fog as part of the persistent process was not listed alone. The average top height of advection fog was less than 1.0 km, and that of precipitation evaporation fog was between 0.5 km and 1.5 km.Advection fog in the Beijing region was mainly caused by the cooling process of warm and wet flow advection from the southwest and southeast moving over a cold underlying surface.The strong inversion formed was favorable to the sustained development of the fog events.Evaporation fog was caused by the cooling process of water vapor evaporated from falling raindrops, or the cooling process due to cold air advection.We also categorized the 13 fog events into those with single fog structure and those with fog-cloud structure, based on the vertical structure observed by MWRP.Some typical cases were further verified by satellite and surface meteorological observation data.The results show that advection fog and evaporation fog accounted for 69% and 31% of the 13 fog episodes, respectively, indicating that the heavy long-lasting fog events in Beijing were mainly caused by advection and precipitation processes, which was closely related to the abnormal weather process.The proportion of single-structure fog was only about 15%, while that with a co-existing structure of low-level fog and high-level cloud reached about 85%.Severe fog events, lasting for more than three days, were almost all characterized by a fog-cloud structure.