Characteristics Analysis Of One Case Of Short-Duration Heavy Rainfall

Short-duration heavy rainfall is one of the most important reason that brings weather disasters in our country,and it is very hard to forecast.By now there is not a uniform standard to define what short-duration heavy rainfall is,but we still have a standard that comes from the national meteorological center which defines the short-duration heavy rainfall as the rain that can reach more than 20mm/h.Because of the importance of short-duration heavy rainfall,meteorological researchers have tried to understand some important characteristics of it,and get the spatial and temporal distribution of it in China,and also the environmental parameters,too.In this paper,we try to research the case of rain that happened in Nanjing on July 18,2011 by examine the characteristics and evolution of the rainfall,the characteristics of the background field,the environmental parameters and the structure of the precipitation system.By examine the characteristics and evolution of the rainfall we know that it is one case of rain that had a big amount and intensity of precipitation,and the rain had come down on some focused regions in a relative short period.When the rain came,the surface temperature fell and there formed a cold pool,which could be coldest when the rain had a strongest intensity,and the wind was weak and also would have a slight perturbation.By examine the radar reflectivity we know that the rain was caused by the precipitation echo at the northwest of Nanjing which moved to the southeast and also the one at the south of Nanjing which moved northward,generating a band region of rain which was oriented south and north.There was a large and long trough on 500hPa and a shear line on 700hPa above Nanjing.In addition,a low-level jet on 850hPa also make a contribution to rain.From the sounding data we can see that there was a deep wet layer above Nanjing,in which the CIN was weak but the CAPE was strong,the LCL and LFC was almost at surface but the EL lied on a rather high level,and also the wind shear was weak,causing strong convections and intensive rainfall.Through the reanalyzed data of numerical model,we can see that the specific humidity and relative humidity above the rainfall region is rather high,indicating an abundant amount of water vapor.The K index and the difference of the temperature between 850hPa and 500hPa is rather high,proving that the rainfall region was conditionally unstable.The divergence below 850hPa had negative regions drew a conclusion that the low level had upward movement.The maximum vertical velocity was more than 25m/s.Through the reanalyzed data of numerical model,we can also know that the temperature of the precipitation system was higher than that at the surroundings.The potential pseudo-equivalent temperature in the vertical had a field where-??se/?p<0 in the lower level,-??se/?p>0 in the higher level,and-??se/?p?0 in the middle level,that is to say,the pseudo-equivalent temperature was close to uniform in the vertical.The rain fell below the center where-??se/?p?0.The surface layer above the rainfall region had a weak cold pool.The inflow lied at the behind of the precipitation system,and the inflow went straightly up after going tilt up.The outflow lied at the middle and upper level.The region of inflow and rainfall had a relative strong water vapor flux divergence.The water vapor was convergent with inflow and strong vertical moving,the stronger the water vapor flux divergence was,the more intense the precipitation was.The horizontal vapor flux showed that the vapor was mainly from the surroundings under 800hPa,and the vertical vapor flux showed that there was plenty of water vapor moving straightly up coordinate with updraft.The heavy rain had a good corresponding relationship with the potential pseudo-equivalent temperature,the vertical velocity and the water vapor flux divergence.From the evolution of water vapor flux divergence,the wind field and the precipitation,we can divide the convective system into three periods:the initial period,the mature period and the last period.When in the initial period,the inflow was caused by the environmental wind shear and updraft started to grow,the convergence zone of water vapor flux divergence was small and the intensity of it was weak,too.Then updraft growed much stronger and was widely distributed in the mature period and downward flow appeared.When it came to the last period,the downward flow cut the inflow off,and updraft came to the middle and upper level.The convective system also had double warm centers,one was located at 500hPa and the other was located at 850hPa.In addition,there was also a cold weak pool near the surface.