Characteristics and Maintaining Mechanisms of Sustained Cold-air Outbreak Snowstorm Processes in Shandong Peninsula During December 3—21, 2005

The analysis of characteristics and maintaining mechanisms of heavy snow events occurring in Shandong Peninsula during Dec 3—21, 2005 is undertaken using Doppler radar data, FY-2C satellite VIS image, observational station data and NCEP/NCAR reanalysis data. The results show when outbreaks of winter cold air occur from the Eurasian Continent, snow clouds develop over the Bohai Sea.The meso-scale coastal front along the north of Shandong Peninsula is the meso-scale system which causes the sustained cold-air outbreak heavy snow events. The spatial distribution of real precipitation during Dec 3—21, 2005 is different from the north to the south in Shandong Peninsula, and the precipitation per 6 hours on Dec 4 and 7 2005 in Weihai shows the day variation of snowstorms. Doppler radar data show that the reflectivity at 0.5° elevation of the heavy snow-band extends from Bohai sea toward Shandong Peninsula is about 35 to 40 dBz. In the velocity field, the anticlockwise movement of wind vectors with height presents cold air current and a coastal front is studied by analyzing the Doppler radar data also. In reflectivity cross section data, most radar echo tops are less than 3 km. Velocity cross section shows the vertical structure characteristics of the horizontal wind velocity of the snowstorm.The FY-2C VIS satellite image shows the cloud streaks which the alignment of SW to NE is almost perpendicular to the northwesterly wind in the lower troposphere. It shows that there exists a close relationship between the evolution of the circulation over the mid-high latitudes and the heavy snowstorm processes. Once the dry cold air originates from the mid-high latitudes, the heavy snowstorm event develops.There is a weak south-northerly wind area when heavy snow events occur in Shandong Peninsula.The relative humidity is more than 90% between 850 hPa and 750 hPa, and the line of 90% relative humidity change with a wave movement. The snowstorms are in close relationship with the relative humidity. The horizontal convergence distribution at 850 hPa and the updraft along 122°E plays an important role in the spatial distribution of real precipitation. The updrafts and the convergence in the lower layer are the dynamic maintaining mechanism of the snowstorms by analyzing the time-height cross section of vorticity and horizontal divergence from Dec 3 to Dec 21 2005.The difference between sea surface temperature and temperature at 850 hPa over Bohai Sea is more than 20℃, and the distribution of θ_se at 850 hPa shows the convective instability in Shandong Peninsula during the snowstorm processes.