Classification of Thunderstorm Processes with the Circumfluence Types and Environmental Field Characteristics in Ningxia

Using the thunderstorm observation data of 25 weather stations in Ningxia during 1961—2005, according to the distribution of weather stations and occurring characteristics of thunderstorms, 6 thunderstorm types are defined as thunderstorm process over whole or most of Ningxia, regional thunderstorm process Ⅰ and Ⅱ, continuous thunderstorms process, local thunderstorm process and dispersed thunderstorm process. Statistics derives that during 1961—2005, there are 110 thunderstorm processes over whole or most of Ningxia, occupying 3% of the total thunderstorm processes, and 357 regional Ⅰ thunderstorm processes, occupying 9.2%, 931 regional Ⅱ thunderstorm processes, occupying 23.9%, 1039 local thunderstorm processes, occupying 26.7%, 1461 dispersed thunderstorm processes, occupying 37.5%, and 27 continuous thunderstorm processes. 263 regional thunderstorm processes occurred from March to September during 1996—2005 are analyzed using the NCEP/NCAR global reanalyzed data, indicating that there are 4 main circulation types favorable for the occurrence of thunderstorms in Ningxia: Mongolia cold vortex, cold transversal trough at the back of northeast cold vortex, Hetao cold vortex and west cold trough. The first 3 circulation conditions often lead to continuous thunderstorm. Several types of environmental field are favorable for the occurrence of thunderstorms in Ningxia, for example, weak northwest airflow under the circulation background of high in the east and low in the west at 500 hPa, ground surface air being controlled by cyclone or warm inverted trough, cold front, shear moving eastward and southward from Mongolia and Xinjiang or west Hetao, certain moisture and convergence uplifting conditions in the mid low level, and the relative humidity reaching 50%—80% at 700 hPa. From March to September, if the 4 circulation types and the environmental characteristics mentioned above occur in the area from Europe to Asia, then attentions should be paid to the possibility of thunderstorms. The relationships between circulation background and weather phenomenon should also be investigated based on analysis of the circulation evolvement characteristics of different circulation types. That will be helpful for judging the possibility of thunderstorm and its occurring area, time and intensity.