Study Of Diurnal Cycle Of Precipitation Over The Sichuan Basin: Characteristics And Its Causes

Up to now, climate models have difficulties in simulating the diurnal cycle of precipitation inthe East Asia, especially in the Sichuan Basin in the southwestern China, where it has moreprecipitation at night. One important reason is lacking of comprehension of nocturnal precipitation.The simulation of diurnal cycle of precipitation is related to many physical processes described inmodel, and also plays an important role in modulation and feedback for the physical processes.Nowadays, the diurnal cycle of precipitation have been paid more and more attention to and havebecome one of the most important aspects for evaluating model’s simulation ability. A good dealof insight into the observation and physical mechanism can not be ignored for improving theprevious model. The Sichuan Basin supplies a good evaluation platform for models, because of itsspecial diurnal cycle of precipitation. However, most of the previous works only focused on thediurnal cycle of summertime precipitation in the Basin, lacking the understanding in other seasons.In this paper, we expand the study on the diurnal cycle of precipitation in the Sichuan Basin to thewhole year. The characteristics of diurnal precipitation cycle and its causes have been studied.Firstly, high spatial-and temporal-resolution precipitation dataset was used to analyze tocharacteristics of diurnal precipitation cycle in four seasons. Secondly, using ERA-Interimreanalyzed data, the mechanism behind the diurnal cycle of precipitation in the Sichuan Basin wasexplored from the aspect of dynamical and thermal-dynamical effect caused by the complicatedtopography in the southwestern China. Thirdly, the abilities of CMIP5models for simulating thecharacteristics and mechanism of diurnal cycle of precipitation have been compared and evaluated.The main conclusions are as follows:1. The nocturnal precipitation has a specific quasi-stationary feature in the basin. It occurs in allseasons, especially in spring and autumn with the most remarkable nocturnal peak and the largestarea. There is a prominent eastward timing delay in the nocturnal precipitation, that is, the diurnalpeak of precipitation occurs at early-night in the western basin whereas at late-night in the centerand east of the basin. The intensity of the early-night peak precipitation is bigger than that of thelate-night peak precipitation, which may be related to different type of precipitation.2. The complicated topography in the southwestern China plays an essential role in the formationof this quasi-stationary nocturnal precipitation. The early-night peak of precipitation in the westernbasin is largely due to strong ascending over the plateau and its eastern lee side. In the central andeastern basin, three coexisting factors contribute to the late-night peak of precipitation. One is thelower-tropospheric southwesterly flow around the southeastern edge of the Tibetan Plateau, whichcreates a strong cyclonic rotation and ascendance in the basin at late-night, as well as bringsabundant water vapor. The second is the descending motion downslope along the eastern lee sideof the plateau, together with an air mass accumulation caused by the warmer air mass transport from the southeast of the Yunnan-Guizhou Plateau, creating a diabatic warming at low level of thetroposphere in the central basin. The third is a cold advection from the plateau to the basin atlate-night, which leads to a cooling in the middle troposphere over the central basin. All thesefactors are responsible for precipitation to occur at late-night in the central to eastern basin.3. Most of the CMIP5models have a good ability to present the total precipitation. There is nowide gap between the high and middle resolution models. But the simulations of different types ofprecipitation are very different among these models.4. The main differentiators of all models lie in the simulation of diurnal cycle of precipitation. Thesimulation ability of diurnal cycle of precipitation is closely related to the resolution. Highresolution shows an obviously better simulation than other models. Most of the models present agood correspondence in the simulation of diurnal variation of large-scale precipitation. Thesimulation of diurnal cycle of total precipitation depends on that of the convection. The convectiveprecipitation simulated by BCC-CSM1-1-m and BCC-CSM1-1are excessively more than othermodels, which may be related to the excessive convective precipitation and frequent large-scaleprecipitation. Preliminary analysis shows that the excessive convective precipitation may becaused by the cloud-water conversion rate of the convection parameterization.5. Because of the complicated topography, the presentation of physical mechanism behind thediurnal cycle of precipitation in the Sichuan Basin relies on the accuracy of topography describedin each model. The simulation of cyclonical vorticity at the lee side of the Tibetan Plateau isrelated to the simulation of wind field. High resolution models are able to simulate not only thethick cyclonical vorticity center and ascending motion at the lee side of the Tibetan Plateau, butalso the different diurnal cycles of water vapor between western and eastern basin, which can notbe represented by the middle and low resolution models as the poor description of the concavebasin. However, all models can exhibit the thermal characteristics of Tibetan Plateau and itscorresponding vertical circulation in the lee side of the Plateau.