Effects of the Tibetan Plateau and its second staircase terrain on rainstorms over North China: From the perspective of water vapour transport

The effects of the Tibetan Plateau (TP) and its second staircase terrain (i.e., the extension of the TP) on summer rainstorms over North China (NC) from the perspective of large-scale water vapour transport were investigated based on the frequency of summer rainstorms from 70 observational stations in NC and National Centers for Environmental Prediction reanalysis data from 1961 to 2010. Seven rainstorm cases over NC during 1981-2016 acquired from ERA-Interim reanalysis data and observational hourly precipitation data were selected. Two water vapour transport channels provided favourable backgrounds for rainstorms over NC. One channel flowed along the northern edge of the TP via westerlies. The other channel flowed along the eastern edges of the TP and its second staircase terrain accompanied with southerly monsoon airflow, with water vapour sources from the Bay of Bengal, South China Sea, and western Pacific. The abundant water vapour that was transported along the two channels not only offered a favourable moisture background for rainstorms over NC but also resulted in the development of water vapour flux vortexes that supplied convergence conditions for rainstorms in situ. At low levels, water vapour transport mainly flowed along the eastern edge of the TP second staircase terrain. The results from the Weather and Research Forecasting model simulation of a randomly selected rainstorm case originating from the TP to NC confirmed the vital effect of the TP second staircase terrain on rainstorms over NC accompanied with water vapour transport.