Mechanism For The Formation And Diurnal Variation Of Precipitation Over Sichuan Basin Regions

Sichuan Basin is one of the precipitation centers in China during warm season,influenced by multi-scale terrains and complex background circulations.The precipitation over Sichuan Basin shows an obvious diurnal variation with a nocturnal precipitation peak,called ”Bashan Yeyu”.Most previous studies use low spatial resolution models(over 10 km grid spacing)which rely on the convection parameterization and cannot resolve the steep terrains around Sichuan Basin well.They usually focus on the large-scale mountain-plain solenoid and synoptic scale precipitation system but ignore the meso-and micro-scale circulation and local-terrain effects,which makes it difficult to understand the mechanism of precipitation diurnal variation over Sichuan Basin.This condition indeed limits the development of the numerical weather forecast over Sichuan Basin.The research studies the precipitation diurnal variation and its mechanism over Sichuan Basin,using convective-resolving model,reanalysis data ERA5 from the European Centre for Medium-Range Weather Forecasts(ECWMF)and dense ground observation station data from Chinese Meteorological Administration(CMA).The main purpose is to reveal the dominated mechanism of the precipitation diurnal variation over Sichuan Basin and its precipitation center,Ya'an,focusing on the multi-scale factors' effects,which may improve the forecast accuracy in the future.We evaluate the WRF?NJU model simulation in 2013 summer(June-August)over Sichuan Basin.The precipitation over Sichuan Basin shows a distinct nocturnal peak at 0200 LST(0300 BJT)and a valley during afternoon,which is well reproduced by the 4-km WRF model,including the hourly temporal and spatial distribution.Based on this simulation,we investigate the primary mechanism of the precipitation diurnal variation over Sichuan Basin.We conduct an analysis on the 3-D dynamical and thermal-dynamical circulation over Sichuan Basin.The vertical circulation shows a semidiurnal reverse,following the switch of the thermal condition between mountain and basin.The near surface nocturnal downslope wind,which is quite thin and weak,brings a little moisture into Sichuan Basin.This proves that the thermal condition related downslope wind plays a second role in the formation of precipitation peak in the basin.The budget analysis of the moisture flux shows that the moisture flux from southeast side of the basin dominates the net moisture flux in the basin.The moisture flux from southeast boundary consists with prevailing part and perturbation part.The perturbation wind over Yunnan-Guizhou Plateau shows a diurnal clockwise rotation associated with the boundary layer inertial oscillation.During the day,the perturbation wind decreases the full wind and directs the moist air out of the basin.At night,the perturbation wind accelerates the full wind to boundary layer low-level jet and brings extra moisture flux into Sichuan Basin.Contrary to the common belief that emphasizes the solenoidal effects associated with the Tibetan and Yunnan?Guizhou Plateaus,prominent diurnal inertial oscillations of boundary layer south?southwesterly low?level jet into Sichuan Basin are shown to play more important roles in modulating the diurnal cycles of precipitation.We further focus on the precipitation center of Sichuan Basin,Ya'an,with a nickname ”Ya'an Tianlou”.The contributions of different meso-scale factors,including local terrain,low-level vortex,etc.to the precipitation diurnal variation of Ya'an and the formation of the local precipitation center is investigated,using ERA5 reanalysis data and the dense ground observation data from 2011 to 2019 summer(June to August).The result shows that precipitation rate,frequency and intensity at Ya'an region are all the local center which show a nocturnal peak.The precipitation rate peaks at midnight,2 hours earlier than that in Sichuan Basin,and the valley appears in the afternoon,consisting with the previous work.The low-level cyclonic vortex,located at the south corner of Sichuan Basin shows a northward propagation at night.The propagation of the low-level vortex is controlled by the boundary layer low-level jet associated with inertial oscillation over Yunnan-Guizhou Plateau.When the easterly wind of the north part of the vortex blows towards Ya'an,transporting warm and moist air into ”blare shape” region,the air from Sichuan basin is apt to be lifted on the slope,leading to the precipitation peak over Ya'an.The moisture flux from east of Ya'an shows a distinct nocturnal peak and a slight early morning peak.The nocturnal peak is closely related to the low-level circulation in Sichuan Basin and the slight morning peak is caused by the barrier jet which peaks at the same time.We also find the moisture flux into Ya'an region shows a very similar diurnal trend with the moisture flux into Sichuan basin from Yunnan-Guizhou Plateau,revealing that the low-level jet over Yunnan-Guizhou Plateau dominates the diurnal precipitation.The relatively stable barrier jet supports the long-lasting nocturnal precipitation over Ya'an region.Finally,we select the typical case of nocturnal heavy precipitation without the influence of large-scale precipitation system at Ya'an,using the WRF high-resolution model(two-layer nesting,outer layer 3 km grid,inner layer 1 km grid)simulation on August 5,2014 for 48 hours to obtain a more detailed precipitation process,including the precipitation temporal and spatial distribution.Based on the simulation,we generalize the primary mechanism of the extreme precipitation event over Ya'an region.The low-level cyclone and anticyclone pairs locate on the south and north side of the southeasterly boundary layer low-level jet in the basin respectively.When the wind rotates clockwise at night,the cyclonic vortex in the southern part of the basin moves northward.The low-level local circulation and the precipitation diurnal variation show a close relationship.We design a sensitivity experiment without wet process at the triggering stage of precipitation at Ya'an region.It is found that the dynamical and thermal-dynamical effect of the terrain have an important effect on the formation of the precipitation.However,the sensitivity heating plays an important role in developing deep convection.The comparison between sensitivity run and the control run shows the importance of the cold pool,including the strong convergence and uplift at the front of the cold pool,which plays a key role in precipitation northwestward propagation.