Precipitation Mechanism of Mesoscale Cloud System in a Cold Vortex over Northeast China. I: Observational Analysis

Detailed analyses of the macro- and microstructure of a mesoscale cloud system in a mesoscale cold vortex over Northeast China were conducted using data including those from an airborne particle measuring system, radar, satellite, and weather charts obtained during a field campaign on 8 July 2003 in Jilin Province. A preliminary discussion of the precipitation formation mechanism is presented. The results show that the northeastern cold vortex had an apparent horizontal band echo structure with moderate intensity and low vertical height (less than 6 km), but it had a high supercooled water content, with a maximum value of 3.3 g/m³ and a value of 2.0 g/m³ even at the upper cloud levels. Above 4 km, the number concentration of ice particles from 2-DC increased rapidly with height; the rapid growth of these particles in the region of high supercooled water concentration made an important contribution to precipitation formation. Vertical observation in the convective region of the cloud system showed needle ice at upper levels of 4.6 km. There was also slight aggregation of ice and snow and high supercooled water content between 4.5 km and 3.8 km, and considerable aggregation of ice and snow over near the 0℃ layer. However, some ellipsoid droplets and non-melting ice crystals appeared just below the 0℃ layer, and precipitation particles were the main rain droplets at 200 m below the 0℃ layer, some of which were produced by melting ice particles. These results suggest that ice crystals may have been created by ice crystal multiplication in addition to ice nuclei nucleation. They grew by aggregation, collection, and accretion of supercooled water. Hence, the ice phase process is included in the precipitation formation process.