Abstract:In China,the Southwest Vortex(a special type of mesoscale convective system) is second only to that caused by tropical cyclones when it comes to the severity of heavy precipitation.During 11-12 July 2013,heavy rainfall(referred to as the "7.18 rainfall" hereafter) occurred over Sichuan Province in China,resulting in catastrophic flooding.Based on rain gauge data-including conventional meteorological observations and those of automatic weather stations-provided by the Meteorological Information Center of the China Meteorological Administration,and NCEP FNL(Final) Operational Global Analysis data with a horizontal resolution of 1°×1° and prepared operationally every six hours,synoptic diagnostic methods and mesoscale numerical modeling were used to study the 7.18 heavy rainfall with the Weather Research and Forecasting(WRF) model.Numerical experiments were performed to (1)test the capability of WRF in simulating nighttime precipitation over complex terrain,such as that of Sichuan Province;(2)examine the characteristics of the Southwest Vortex;and (3)elucidate the cause of the 7.18 heavy rainfall with respect to the large-scale precipitation conditions.The main results can be summarized as follows:
(1)The 7.18 heavy rainfall was influenced by a typical mesoscale vortex,and the period of the largest amount of precipitation was 0100-0200 Beijing standard time(BST) 18 July 2013.Accompanying the development and disappearance of the mesoscale vortex at 500 hPa,the intensity of precipitation gradually changed from strong to weak and,finally,stable,meaning the 7.18 heavy rainfall possessed obvious characteristics of nighttime rainfall.
(2)The main weather systems of influence were a westerly trough at 500 hPa(nearby at 0000 UTC) and a mesoscale vortex at 700 hPa over Sichuan Province.Meanwhile,meridionally,the western Pacific subtropical high(WPSH) extended towards the west and formed a typical "western trough-eastern high" pattern;and under the obstruction of the stable WPSH,the westerly trough at 500 hPa developed a low pressure vortex,before weakening gradually and ultimately disappearing.Sichuan Province suffered long-duration precipitation during 11-12 July 2013 because there was a westerly trough at 500 hPa and a mesoscale vortex at 700 hPa stagnated over Sichuan Province for a long period under the blocking of the WPSH.The typical "western trough-eastern high" pattern allowed warm and wet flow to arrive from the ocean to the south,and dry and cold flow from the north,which converged over Sichuan Province.In addition,Sichuan Province was influenced by strong atmospheric upward motion,and these conditions led to the occurrence of the 7.18 heavy rainfall.
(3)The low vortex at 700 hPa had a baroclinic feature similar to that of a frontal zone,and was under the influence of unusually steep potential pseudo-equivalent temperature(θse),meaning slantwise and unstable vorticity could develop gradually.As the stronger and higher value positive vorticity center would have been beneficial to the formation of small and mesoscale vortexes,the formation and development of such vortexes could have provided favorable dynamic conditions for the heavy rainfall.
(4)The simulation results from the WRF model showed that WRF was able to simulate the location of the rain belt near Ziyang well,but the location of the rain belt near Guangyuan was shifted to the north and east.Moreover,the intensity of the simulated precipitation was greater than observed.Regarding the temporal evolution of precipitation occurrence,the simulation results of WRF were better for nighttime than daytime,indicating that WRF needs to be further improved for simulating daytime precipitation over complex terrain like that of Sichuan Province.
