Simulation Studies On An Extreme Rainstorm And The Effects Of The Local Mountain-valley Wind Around The Helan Mountain

The orographic precipitation is the result of the interaction of large and medium scale systems and complex underlying surface conditions.Numerical simulation and sensitivity experiments with mesoscale model WRFV3.8.1 on the rainstorm which occurred around the Helan Mountain of Ningxia on July 22,2018(hereafter “7.22”rainstorm)were conducted based on analyses of the precipitation characteristics and large scale pattern as well as evolution of mesoscale convective system about the rainstorm,and then the Mountain-valley circulation around the Helan Mountain was separated by the method of Ensemble Empirical Mode Decomposition(EEMD)and its effects on the “7.22” rainstorm were analyzed in this study.The main conclusions are as follows:(1)Analyses of observed data on the “7.22” rainstorm show that the rainstorm is a short-duration heavy rainfall process and which is obviously influenced by topography of the Helan Mountain;The rainstorm belt is mainly concentrated in the middle and north part of the eastern foot of the Helan mountain and has distribution from the northeast to the southwest along the mountain;The distribution of the observed lightning has a good consistency with that of the precipitation,and the heavy rainfall mainly occurred from 14:00 to 22:00(UTC)on July 22,and the maximum precipitation appeared in station of Huaxuechang(station number: Y1091)with the maximum hourly precipitation intensity of 74.1 mm/h;Evolution of Radar echo and TBB data show that the development of rainstorm process is closely related to the development of the linear mesoscale convective systems,and the convective cloud clusters almost stagnated along the Helan Mountain from 14:00 to 20:00,which caused the rainstorm and the maximum rainfall appeared at Y1091 with 277.6mm.(2)Analyses on evolution of large-scale and mesoscale circulation systems during the “7.22” rainstorm process indicate that the rainstorm has closely relationship with the development of upper-level jet at 200 h Pa and the trough at500 h Pa as well as the low-level shear line at 700 h Pa,the strengthen eastwards and retreating westwards of the South Asia high was accompanied with the westwards extending and northwards moving of the West Pacific subtropical high.(3)Comparing of simulated results according to the observed suggest that the WRF model can reproduce the large-scale circulation features and mesoscale systems(such as upper-level jet,the South Asia high,the trough,the subtropical high and the low-level shear line et.al)well of the rainstorm.The simulated location of precipitation was closed to that of observed.Evaluation of precipitation at different threshold value indicate that the performance of WRF model may be underestimated with traditional TS score method and which is relatively objective by the method TS score with KNN.(4)Analyses of simulated results indicate that stronger moisture convergence located above the center of precipitation,the mesoscale system which caused the rainstorm has characteristics such as stronger convergence at lower levels as well as stronger divergence at higher levels associated with stronger positive vorticity and vertical movement around the precipitation area.Simulated meteorological data was decomposed into three IMF fields which indicate characteristics mesoscale disturbances and a RES which represents large-scale field by the EEMD method.During the rainstorm process,the RES field showed the eastern wind has changed into west,and the southern wind strengthened.Evolution of the mesoscale convergence center which decomposed with EEMD at low level of troposphere are agreement with mesoscale convective cloud clusters well.(5)The decomposition by the EEMD method with output of WRF model for clear sky condition(July 25,2018)shows that the local Mountain-valley circulation is obvious around the Helan Mountain and it has obvious characteristic of diurnal variation.Sensitivity experiments which introduce the local Mountain-valley circulation at clear sky condition into the “7.22” rainstorm case show that increase/decrease the local mountain-valley wind have not obvious influence on the large-scale circulation situation and distribution of rainfall,which have a great influence on the local mesoscale circulation system and convection;Weakening the local circulation of Mountain-valley wind is adverse for the development of convective movement and convergence,which have unfavorable effects on the development of precipitation at night-time in mountainous areas.Strengthening the local circulation of Mountain-valley wind favors the development of convective movement and convergence,thus enhancing the occurrence and development of night-time precipitation in mountainous areas.