Abstract: Based on the conventional meteorological data,the NCEP (National Centers for Environmental Prediction) reanalysis data,and data from a meteorological tower,a wind profiler radar,and a microwave radiometer in Tianjin,the structure characteristics and formation mechanism of a return-flow snowfall event on February 6-7,2014 at Tianjin Binhai International airport located on the west coast of Bohai Sea was analyzed.The effects of easterly flow in the boundary layer and the formation mechanisms of ascending motion during two snowfall periods in the return-flow snow event were discussed.The results show that the return-flow snowfall process can be divided into two periods,i.e.,the first period from 15:06 to 19:00 on February 6 and the second one from 00:25 to 15:46 on February 7.The first weak snowfall period on February 6 occurred at the early time of return-flow condition.The weak easterly wind at the lower levels transported abundant water vapor from the sea to the Tianjin area,and the shallow convergence and ascending motions exist below the 2 km height in combination with the mesoscale disturbance in the easterly airflow.The second snowfall period on February 7 results from a westerly trough at the middle levels and cold air in the lower levels.The dry and cold flows move southwards along the eastern path,and the humidity in the air near the sea surface (about below 600 m) obviously increases after the flows pass the Bohai Bay.Therefore,there is a significant cold pad within the boundary layer due to the wet and cold easterly wind influencing the Tianjin area.As a trough at 500 hPa moves towards Tianjin,the warm and wet airflows climb up along the cold pad,which results in the development of ascending motions.Meanwhile,the southwesterly flows play a role in transporting water vapor.

Key words: Return-flow snowfall, Boundary layer, Easterly flow, Cold air, Ascending motion