A Study of Heavy Rainfall Resulting from Transversal Trough and Shear Line-Low Vortex in the Huaihe River during the Meiyu Onset of 2007

Based on the methods of comparison, diagnosis and numerical calculation, the 6-hour-interval reanalysis data with 1.0°×1.0°resolution from NCEP/NCAR, station observational data, TBB (Blackbody Brightness Temperature) data and surface precipitation amount data are employed to analyze a special rainstorm process happened in the Huaihe River and its north during 19－20 June 2007 (the first heavy rainfall during the period of Meiyu onset). The main conclusions are as follows: (1) The obvious characteristics of rainfall both during the whole Meiyu period and during the first rainstorm process is that the rain band lies in the Huaihe River instead of the Yangtze River. Especially, the strongest rainfall zone is located in the middle reach of the river near 115°E. After entering the Meiyu period, it rains firstly in Hubei and Henan provinces, which lie in the upper reach of the Huaihe River. And the rain band jumps into southern Shandong Province abnormally and quickly, which is in the north of the Huaihe River. Some reasons have been revealed to explain those characteristics mentioned above: in the area near 35°N and 115°E, various factors (stationary Meiyu front, active cold-warm air, rich water vapor and strong ascending motion caused by the interaction between high- and low-level jets) result in heavy rainfall in the middle reach of the Huaihe River (115°E) and Meiyu rain band is located to the north of the Huaihe River, near 35°N; the blocking high pressure over the Huaihe River and its north is adjusted quickly and is replaced by the trough at 500 hPa, which could play an important role in heavy rainfall occurring during 19－20 June. In the lower troposphere, with the impact of frontogenesis band and steering flow, the vortex moves northeastward and leads the rain band to jump northwards.(2) During the first rainfall process, all of 500-hPa trough, shear line and vortex caused by the shear line are important. The trough supplies active cold air and this kind of pattern is seldom found in the previous Meiyu periods. Continuously strengthened ascending motion is very important for the appearance of several heavy rainfall. All of these cause the heavy rainfall and floods in the Huaihe River vally.(3) Some meteorological signals (including summer monsoon, subtropical high axes at 500 hPa over the western Pacific, blocking high, etc. ) change obviously before and after entering Meiyu period in 2007. In fact, it is a transitional period of middle latitude circulation when Meiyu onsets.(4) Dynamic factors could play more important role than thermal elements during the process of shear line causing vortex. After the vortex emerges, it moves northeastward. Although this is the early phase of Meiyu rainfall rather than the typical Meiyu rainfall, meso-scale rainy clusters and cloudy clusters are still very active.