A Case Study of Short -Duration Heavy Rainfall in Nanjing

On July 18, 2011 the city of Nanjing, China experienced a short -duration heavy rainfall that cause over 3 million dollars in damage. Maximum accumulations of rainfall in the downtown center of the city(Xuanwu lake) exceeded 200 mm in a 24-h period and 90 mm in a 1-h period. This study presents a meteorological overview of the event utilizing a wide variety data including Doppler-radar, AWS, reanalysis data and synoptic analyses.

Many of the meteorological features associated with the Nanjing short-duration heavy rainfall typify those of similar events in the eastern China. Prominent features in the Nanjing case included the weak CIN and the strong CAPE, the lower LCL and LFC but high EL, the weak to moderate southwesterly flow aloft with the weak wind shear; a deep, moist warm layer in the sounding; the occurrence of a quasi-stationary rainfall system, and absence of hail and other severe storm signatures.

Radar reflectivity revealed that the echo at the northwest of Nanjing which moved to the southeast and the echo at the south of Nanjing which moved northward, generating a band region of rain in the downtown which was oriented south and north.

AWS observations shown that the total rainfall formed from the 1400 through 1900BJT.Between 1400 and 1500 BJT the maximum accumulations of rainfall was 90 mm in northwestern of Nanjing. Between 1500 and 1600 BJT the maximum accumulations of rainfall was 97 mm in the downtown center of the city. Accompanied by rainfall 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.

The reanalysis data indicated 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 -δθse/δp<0 in the lower level, -δθse/δp>0 in the higher level, and -δθse/δp≈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 -δθse/δp≈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 convergence. The water vapor was convergent with inflow and strong vertical moving, the stronger the water vapor flux convergence 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.