Classification and Satellite Nephogram Features of Hail Weather in North China

Based on conventional observations, automatic weather station data, geostationary satellite data and NCEP FNL data, meso-scale features of 27 hail processes occurred over Northern China during 2010-2012 are analyzed. According to synoptic circulation and cloud characteristics, these hail processes are divided into three types.The first type of hail convective storm is often embedded in the westerly trough of cold vortex system. The place where the severe convective storms initiated is frequently on the rear of the cloud band corresponding to the synoptic system. The cold front provides a strong lifting for convective initiation, while the anticyclone dry air intrusion triggers the intensive development of the hail storm. Whenever the water vapor content is plenty, heavy rainfall can also occur.The hail shooting zone of the second type convective storm is in the front of the cold vortex. The range of affected area is highly related to the southward movement of the cold vortex system. The front system often presents a forward-tilting structure, which is the main characteristic of this type of hail convective storm. The middle layer cold air mass become superimposed above 850 hPa warm ridge, which causes a wide range of potential instability, and also a continuous hail shooting weather, accompany with heavy rainfall in North China. The life span of the convective system is as long as 10 to 16 hours. The third type of hail convective storm generally occurs in a stable synoptic background, which is different from the other two. The hail storm initiates within the cold air mass, while the northerly air stream dominates the upper layer. Due to the poor moisture condition, the main disaster is hail and wind gale rather than short-during heavy rainfall. The short-wave trough at 500 hPa and the weak convective instability in the afternoon locally may be the cause for this kind of convective storm, and it is still difficult to forecast.On satellite-based (infrared and water vapor) images, over 90% of hail events produce hail when the convective storm growing rapidly. The main hail shooting zone is near the edge of a storm propagation frontal side, corresponding to a large gradient of T_BB area in infrared image. The convective storm with both the low T_BB (≤-40 ℃) and large gradient of T_BB (≥8 ℃/0.05°) features seems an important threshold for short-range forecasting a bigger hail stone.