夏季青藏高原对流边界层高度东西差异对环流场的影响

Impacts of the east-west difference in convective boundary layer height on circulation over the Tibetan Plateau in summer

基于2013—2015年6—8月“第三次青藏高原大气科学试验(TIPEX-Ⅲ)”和常规气象业务探空观测资料、欧洲中期天气预报中心(ECMWF)第5代再分析(ERA-5)数据以及“国际卫星云气候计划(ISCCP)”云量资料,采用统计分析和物理量诊断分析方法,研究了夏季青藏高原(简称高原)大气对流边界层高度东西差异对高原地区天气尺度环流的影响。结果表明:高原对流边界层高度东西差异表现出明显的日变化,且这种差异呈现西高东低的分布特征,主要由西部对流边界层高度明显增大所致。当中午对流边界层高度东西向差异增大时,午后地面虚位温6 h变差呈西部高、东部低的特征,且西部变化更明显,高原西部对流边界层内温度升高,东部略降低,并伴随着高原西部对流边界层内气压降低、高层升高且低压系统较浅薄,而东部低层气压升高;高原低层东高西低的气压场特征产生了异常东西向气压梯度,引起高原中部低层出现偏南风异常,伴随着西部的低层异常辐合和高层异常辐散;同时,浅薄的低压有助于当地低云发展。

Based on"The Third Tibetan Plateau Atmospheric Scientific Experiment(TIPEX-Ⅲ)"and the routine meteorological operational sounding data,the European Centre for Medium-Range Weather Forecasts Fifth Generation Reanalysis(ERA-5)data,and the ISCCP cloud cover data during the summers(June,July,and August)of 2013—2015,influences of the east-west difference in the convective boundary layer(CBL)height over the Tibetan Plateau(TP)on synoptic-scale atmospheric circulation are analyzed by statistic and physical diagnosis methods.It is found that the east-west difference in CBL shows an obvious diurnal variation.The CBL is high in the west and low in the east from the noon to the late afternoon,which is mainly attributed to the larger rise of the western CBL height.Corresponding to the large east-west difference in the CBL,temporal change of surface virtual potential temperature displays a"high in the west-low in the east"feature and the change in the western TP(WTP)is larger than that in the eastern TP(ETP).Meanwhile,temperature within the CBL increases in the WTP but slightly decreases in the ETP.Pressure decreases in the CBL but increases in higher levels in the WTP.The anomalous low pressure in the WTP is shallow.At the same time,low-level pressure increases in the ETP.The low-level east-west pressure difference may result in east-west pressure gradient anomaly and anomalous southerly winds over the central TP,which is accompanied by low-level convergence and high-level divergence in the WTP.The shallow low pressure anomaly is favorable for the development of low clouds.