Preliminary Study on Circulation Characteristics and Mechanism of Rainstorm in Spring and Autumn of Shandong

The climate characteristics and the influencing systems associated with the heavy rain in spring and autumn in Shandong Province are analyzed, and the mean circulation charts are also given. The circulation characteristics and the weather systems causing the two large scale rainstorm events in 2003 spring and autumn are undertaken with the thermal characteristics and water vapor transfer mechanism during the rainstorm discussed. The mechanisms of the two heavy rain processes are studied by using the k-helicity and slantwise vorticity development theory. The result shows that all the rainstorms in spring are caused by cyclones, and the majority of rainstorms in autumn are caused by cold front. The heavy rain during 17—18 April 2003 is caused by cyclone and that during 10—12 Oct 2003 is caused by wind shear line and cold front. There is a low-level southerly jet which transports the watervapor to the storm area, and the temperature and the humidity increase rapidly, also the convective instability and the moist baroclinity increase in heavy rain area. The strong cold front triggers the release of the convective instability energy which leads to the heavy rain. The low-level positive k-helicity develops impetuously during the two events. The warm and humid air in low-level flows into the rain storm area cyclonicly and goes up in spiral way, and flows out anticyclonicly in high-level. The positive k-helicity center in cyclone rainstorm appears in the east of low-level eddy and moves towards the northeast, but that in cold front rainstorm occurs near wind shear line which locates in the north of Shandong Province and moves to the southeast. The rainstorms occur in the moist baroclinic atmosphere and the development of slantwise vorticity is very important in the two cases. The wet potential vorticity augments intensively during the rainstorm period. The southward moving cold air encounters the warm air, which makes the convective instability and moist baroclinic are both prone to the forming of the rainstorm. The MPV1 < 0 and MPV2 > 0 in the warm areas ahead of cold front, namely, the convective instability and ageostrophic balance, are in favor of the vertical vorticity development and the ascending motion, triggering convective instability energy release, and strong precipitation by the moist potential vorticity (MPV) equation theory. When the cold front moves to the rainstorm areas, the equal θ_se lines in the front areas near 850 hPa are cragged and dense, the atmosphere is near neutrally or stability stratified and the positive MPV1 value augments intensively. The horizontal gradient of the θ_se in the front area and the wind vertical shear near the front in the low-level augments, the negative MPV2 value augments too. Equally by the MPV equation theory, in stable atmosphere, the augmentation of MPV1 positive value and MPV2 negative value indicates the development of the vertical vorticity, the updraft's augmentation and rainstorms occurrence. The MPV1 < 0 and MPV2 > 0 in the early phase of the heavy rain, MPV1 > 0 and MPV2 < 0 in the later phase of the heavy rain are both of advantages to slantwise vorticity development, and speed up ascending movement.