| A new diagnostic quantity —moisture helicity is deduced from moisture equation, and its influence factors with time are analyzed in details by the diagnostic analysis . The mesoscale numerical model (MM5V3) is used to simulate three heavy rain processes .With successfully the processes be simulated, the model output with high spatial and temporal resolutions are used to study the relation on moisture helicity and heavy rainfall .The results show that with the moisture helicty raising, the precipitation is up .Additionally, the areas of them are correlative well, which is the moisture helicity is corresponding better with the low level jet of southeast.A heavy rain occurred in Changjiang basins on 24th June 2004 are studied in detail. Based on NCEP realayzed data with the spatial precision 1° × 1° and other observational datasets, two troughs and one ridge are the characteristics of macroscale in the study to the influential systems and mechanism of this process. The conflict of the dry, cold masses and the warm, wet masses ,mesoscale convergence on Meiyu front ,low eddy ,shear line and stably maintaining of low jet make for the rainfall.The condition of high energy and saturation benefits for the form of this rainfall.Two diagnostic variable named as helicity flux divergence and humid helical divergence in this paper are used to diagnose the heavy rain .In addition,the helicty used widely before is also contrasted with the two in this paper. There are two arithmetic of humid helical divergence being compared as well. The results show that we should use different diagnostic variable in different period of heavy rainfalls.
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