Radar echo and airmass structure of the comma head within a Jiang-Huai cyclone in winter

An extreme rainfall-snowfall event that occurred in Shandong province on 12-13 February 2016 due to the influence of the warm front associated with a Jiang-Huai cyclone. Precipitation at 42 stations in Shandong province broke their historic records. Using various observation data and thermodynamic fields simulated by the WRF model and the airflow trajectory model based on the Lagrangian method (HYSPLIT_4.9), this paper analyzes the cloud evolution, airmass structure and trajectory characteristic of comma head within the Jiang-Huai cyclone during different precipitation stages. Main results are as follows. (1) The comma head of the Jiang-Huai cyclone developed from four banded echoes. The radar echo rotated cyclonically and elongated, forming a number of strong mesoscale precipitation bands after the formation of the cyclone. (2) The comma head consisted of three vertically stacked air masses during the raining stage, i.e., from bottom to top, the boundary layer air mass from the East China Sea, the Bohai Sea or from the inland region, the warm-moist air mass from the South China Sea, and the cold-dry air mass originating from western Asia and eastern Europe. There are three rainfall zones within the comma head of the Jiang-Huai cyclone. The northern zone and southern zone, where warm-moist air was shallow and the stratification was stable, was characterized by stratiform precipitation. The middle zone, where warm-moist air was deep and conditional instability developed in the middle and upper levels, was marked by deep convective precipitation. The dry air over the southern and middle zones originated from high level airmass over western Asia, while that in the northern zone originated from mid-level airmass in the eastern Tibetan Plateau, which later transformed into moist air and contributed greatly to rainfall. (3) During the snowfall stage, from bottom to top, the comma head consisted of four vertically stacked air masses that originated from the Siberia, the East China Sea, the South China Sea and the Bay of Bengal, respectively. There were two precipitation zones within the western area of stratiform precipitation:snowfall in the northern zone and rainfall in the southern zone. Whether the cold Siberia air mass existed under the cold-moist East China Sea air mass determined the precipitation in snow or rainfall phases. In the snowfall area, the air mass originating from the East China Sea was deep and the air mass originating from the South China Sea was shallow, while the opposite was true in the rainfall area.