Activities of cold-dry air and its impact on heavy rain-snow processes in North China during March 3-5, 2007.

The activity of cold-dry air and its impact on a heavy rain-snow storm in late winter in North China is studied by using FY-2 meteorological satellite data,Doppler radar data,normal observations and 1°×1° NCEP/NCAR reanalysis data,and the water vapor source is analyzed with black body temperature(TBB),isentropic potential vorticity(IPV),water vapor flux,relativity humidity and wind fields.Results show that the east and south airflow of the Jianghuai cyclone provided abundant water vapor for the heavy rain-snow storm,meanwhile,cold-dry air played different roles at different heights and paths:1)in the lower troposphere,cold-dry air acted as a cold cushion that wedged into the bottom of warm wet air flow,resulting in frontogenesis,elevation of warm-wet air and condensation.2)in the upper troposphere,cold-dry air with high potential vorticity moved downward from high latitude along 320 K isentropic surface which offered the dynamical and thermal conditions for Jianghuai cyclone's triggering,strengthening and maintaining.The area with high positive potential vorticity(IPV≥10-6m2·s-1·K·kg-1) and the dry zone with a relative humidity≤45% on the isentropic surface correspond well to the dark zone of TBB≥-32 ℃ on the infrared cloud image,suggesting means that it is feasible to trace the high positive potential vorticity track by satellite data.3)The intrusion into the storm and overlay on the warm/wet air flow of the cold-dry air from the sub-cold transfer belt at the head of spiral cloud at the height 2—3 km was beneficial to the formation of unstable straification and the release of unstable energy,therefor,it is one the important factor for the local heavy and violent snow in the western coastal of Bohai Bay.