Statistical And Numerical Simulation Research Of Precipitation Of Tropical Cyclone Landing Zhejiang And Fujian Provinces

To reveal accurate temporal and spatial distribution characteristics precipitation of tropical cyclones (TCs) landing Zhejiang and Fujian provinces, an Objective Synoptic Analysis Technique (OSAT) and statistically analyses have been used on precipitation of TCs landing Zhejiang and Fujian provinces from 1956 to 2012. A further high resolution numerical simulation and sensitivity experiment of Typhoon Fitow (1323) which caused serious disasters is carried out by using non-hydrostatic mesoscale WRF model to diagnostically analyze the causes of heavy rainfall of Fitow and the role of local terrain of Zhejiang and Fujian provinces. The results show that the averaged track of TCs from 6 hours pre-landing to 24 hours after landing appears to move north-west and accumulated precipitation is characterized by asymmetric distribution, matching the water vapor convergent area. The averaged track of TCs turns to be north-south direction from 24 hours after landing to 48 hours after landing outside of Zhejiang and Fujian provinces, a significant precipitation appears near Lushan in Jiangxi and Dabieshan in Anhui while precipitation decreases obviously in Zhejiang and Fujian provinces. The distribution of accumulated rainfall during landing of different type TCs distinguishes considerably, the second type has the heaviest rainfall while the third type has the weakest one. In terms of the temporal distribution characteristics of precipitation, all types have their heaviest rainfall from 12 hours pre-landing to 6 hours after landing, especially from 6 hours before landing to 6 hours after landing. However, the rainfall distribution and evolution characteristics of different types vary notably from each other because of local topography and the location of TC circulation. High frequency of heavy rainfall appears in the coastal areas of Zhejiang and Fujian provinces from 6 hours pre-landing to 6 hours after landing, and concentrates on the common border of two provinces owing to the precipitous terrain of this region. Hourly precipitation extremes caused by landing TC of stations in these two provinces vary from 10mm to 143mm, most are between 30mm and 60mm. That stations with hourly precipitation extreme more than 50mm distribute mainly over coastal areas and gather in the common border of Zhejiang and Fujian provinces is consistent with the frequency of hourly heavy rainfall. In many factors affecting the precipitation of Fitow, the southwest jet in front of upper trough and easterly jet of Danas worked together to strengthen the vertical structure of Fitow and supplied abundant warm moist vapor.After landing, the confluence of southward cold air and warm moist flows further strengthened the rainfall of Fitow under the impacts of Zhejiang and Fujian complex local terrain. In addition, the steer flow of Fitow with a large speed which was almost perpendicular to the coastline during landing duplicating the effect of astronomical tide may be the main reason of storm surge of Fitow. That the heaviest rainfall of Fitow occurred between 6 hours before landing and 6 hours after landing according to the distribution of 6 hours accumulated rainfall was in accordance with the statistical analysis conclusions above. Sensitivity experiments indicates that different terrain has different degrees of influence to the distribution and evolution of precipitation of Fitow. The remove of terrain of common border of Zhejiang and Fujian provinces which affects the rainfall of Fitow notably leads to dramatic decreasing in southern Zhejiang and more precipitation in northern Zhejiang because of weak lifting effect and forms characteristics of asymmetric distribution which are significantly different from control experiment. The remove of Tianmu mountain has a local influence and results in rapid decreasing of precipitation because of the significant weak of favorable dynamic conditions. The rainfall of area 3 along the coast reduces remarkably while there is an enhancement of precipitation in northern Zhejiang inland when the underlying surface of eastern Zhejiang is replaced with water because of less frictions and weak lifting effect. As shown above, land surface frictions and the local terrain have obvious effects on TCs precipitation in this region.