The Spatio-temporal Variation And Formation Mechanism Of Nocturnal Precipitation Over The Sichuan Basin

The Sichuan Basin(SCB)is located to the east of the Tibetan Plateau(TP).The special location and terrain condition features nocturnal precipitation,while the formation mechanism and model prediction of nocturnal precipitation over the SCB still need to be investigated.In this paper,we have explored the spatial variation of nocturnal precipitation during the warm season over the SCB using high spatiotemporal resolution merged precipitation products from2008 to 2017.The physical mechanism of nocturnal precipitation over the SCB have been studied with ERA-Interim reanalysis data and intensive radio-sounding observation data.The simulation ability of the numerical weather prediction model in Southwest China to the diurnal variation of precipitation in summer and the causes of its deviation have been also evaluated.Finally,we have analyzed the diurnal variation of wind field under different precipitation intensities at night over the SCB,and the numerical simulation of precipitation have been carried out using WRF model to study the possible interaction between latent heat released by precipitation and the diurnal variation of atmospheric circulation and wind field in lower troposphere.The main conclusions are as follows:There is a prominent northeastward timing delay of precipitation peak over the SCB,that is,the diurnal peak of precipitation occurs in the midnight over the southwestern SCB whereas in the early morning over the northeastern SCB.This time delay partly ascribed to a northeastward propagation of precipitation over the SCB,accounting for about 40% and 25%of the diurnal variation of precipitation over the central and northeastern SCB,respectively.The transition of peak time of diurnal precipitation is discontinuous obviously from the eastern TP to the SCB,suggesting the precipitation over the SCB may not be directly linked to the convection over the eastern TP.The spatial distribution of diurnal peak time of precipitation over the SCB is characterized by obvious intra-seasonal variation.In May and June,the propagation of diurnal peak time of precipitation over the SCB is more distinct,and weakens in July and August,then gradually strengthens in September.The dynamic and thermal forcing caused by complex topography is an important mechanism of nocturnal precipitation over the SCB.There is a basically cyclonic circulation over the SCB at 850 h Pa for all day.The deviation wind direction rotates clockwise throughout the day due to the inertial oscillation of atmospheric boundary layer winds.The diurnal variation of 850-h Pa wind has strong easterly wind deviations in the early evening over the southwestern SCB,which favors for an initiation of precipitation by orographic lift of air.The diurnal variation of difference in equivalent potential temperature between 500 h Pa and 850 h Pa has a maximum negative deviation in the early evening over the southwestern SCB,which enhances the instability and therefore favors an initiation of precipitation.Precipitation first occurs over the southwestern edge of the SCB at about 1900-2000 LST,and then propagates northeastward steered by the large-scale mid-tropospheric southwesterly airflow,which is consistent with the northeastward propagation of the large moisture convergence.The difference in the propagation and northeastward extent of diurnal precipitation is mainly related to the intensity of mid-latitude westerly flow and the positioning and strength of the Western Pacific Subtropical High.The Yunnan-Guizhou Plateau(YGP)may play an important role in the initiation of precipitation over the southwestern SCB in the early evening.The numerical weather prediction model in Southwest China can simulate the spatial distribution of daily mean precipitation in summer,but the simulated precipitation is significantly higher than the observation.The overestimated precipitation concentrated in the transition region between the eastern TP and western SCB.The model well reproduces the northeastward timing delay of precipitation peak over the SCB,but the simulated peak time is about 2-3 hours earlier than the observation.With the refined model resolution,the overestimated precipitation over the SCB is improved,and the simulated diurnal phase is closer to the observation.The results show that the simulated diurnal phase of precipitation is consistent at different starting time of the model,but the difference of precipitation between the simulation and the observation is the smallest at starting time of 1900 LST for different resolution models.The model simulates the cyclonic circulation over the SCB,but the simulated relative vorticity is slightly larger.The model reproduces the clockwise rotation of the 850-h Pa deviation wind field over the southeast side of the basin,while the 850-h Pa easterly deviation wind appears earlier and the wind speed is stronger.The simulated large-scale southwest flow in the midnight of the middle troposphere is weaker.From afternoon to evening,the temperature vertical deviation of the “warm below and cold above” and the strong moisture deviation of the surface layer enhance the atmospheric instability.The easterly deviation wind predicted by the model appears earlier than the observation,which make the precipitation starts earlier over the southwestern SCB,thus the diurnal peak time of precipitation appears earlier than the observation.The stronger wind speed of deviation wind and atmospheric instability are conducive to the triggering of convection,which make the excessive precipitation.The weakening of the simulated southwest flow in the middle troposphere weakens the northeastward propagation of precipitation after midnight over the SCB,further confirms the dominate role of diurnal variation of wind field on the occurrence and development of nocturnal precipitation over the SCB.The diurnal variation of 850-h Pa deviation wind is different under different precipitation intensities over the SCB.During the heavy precipitation period,the clockwise rotating wind speed is stronger than that of the weak precipitation period.WRF model reasonably reproduces the diurnal variation of the precipitation and atmospheric circulation over the SCB,especially the northeastward propagation of precipitation.The differences between the sensitivity experiment of turning off latent heat feedback over the SCB and the control experiment show that the condensation latent heating of precipitation intensifies the 850-h Pa cyclonic circulation over the basin.Without the latent heat feedback in the model,the 850-h Pa easterly diurnal deviation over the southeast side of the SCB in the evening is obviously weakened,and the deviation wind rotates faster from evening to early morning.Therefore,the diurnal variation of850-h Pa deviation wind over the SCB is not only related to the inertial oscillation of the atmospheric boundary layer,but also the feedback of local condensation latent heat over the SCB.