Based On CWRF, The Variation Of Precipitation In My Country And Its Relationship With The Height Of The Boundary Layer Were Studied

Under global warming,the frequency of extreme events has increased significantly and the mean precipitation has also changed obviously.Located in the heart of the East Asian monsoon,China is especially vulnerable to precipitation changes because of its intensive human activities and low adaptive capacity of water resources.Consequently,reliable projection of regional climate changes is of great importance to make effective strategies to deal with these changes and associated impacts.The regional Climate-Weather Research and Forecasting model(CWRF)was used to downscale the NCAR Community Climate System Model V4.0(CCSM4)projection of China precipitation changes from the present(1974-2005)to future(2019-2050)under the high emission scenario RCP8.5.This study focused on regional precipitation changes in China,and more importantly,to understand the key physical mechanisms underlying these changes,improving confidence in the projection.Moreover,the precipitation changes were closely related to the planetary boundary layer height(PBLH)which is characterized by large uncertainties in the numerical modeling.Therefore,this study uses the departures correlation to explore the interactions between PBLH and related variables as well as their relationships with precipitation.The detailed analysis of physical mechanisms could provide theoretical support and scientific basis for improving the physical process of the model.The main conclusions are listed as follows:Phase Five of the Coupled Model Intercomparison Project(CMIP5)models were slightly sensitive to spatial resolution in model performance.CMIP5 were able to reproduce the spatial distributions of surface temperature in China while limited in representing the spatial distributions of regional precipitation.They failed to capture the center of precipitation and simulate the magnitude.Although CCSM4 presented well among these models and was representative in simulation biases and future changes,it produced fake heavy precipitation centers along the southern foothill of the Tibetan Plateau and over the western region of the Sichuan Basin.CWRF outperformed CCSM4 in reproducing observed regional characteristics:reducing overall biases and increasing pattern correlations for seasonal mean and extreme precipitation as well as rainy and dry days.In particular,CWRF captured more realistically the May to July northward advance and the August to September southward retreat of the main rainbands,crossing the Pearl,Yangtze and Yellow Rivers.For the future,CCSM4 projected the most significant mean and extreme precipitation increases over the Northeast and Southwest in both spring and summer and over South China in summer.CWRF generally reduced these changes in magnitude,projecting still significant increases mostly in summer for mean precipitation(PRA)in the Northeast,North China and Southwest and for extreme precipitation(P95)in North China,South China and the Southwest.Importantly,the CWRF minus CCSM4 differences in all seasonal precipitation characteristics were highly correlated in spatial pattern between the present and future.This indicated that CWRF downscaling corrections to the present climate simulation were systematically carried into the future climate projection.These biases,however,could not be simply removed from the changes because their correspondences were strongly nonlinear.Our results highlighted the added value of CWRF downscaling to capture regional precipitation characteristics more realistically through refined physical process representations,and thus to project more reliably their future changes by reducing model structural uncertainties.It has been well studied that global warming can cause the Hadley circulation widening and poleward expansion.Consistently,CWRF projected the Hadley circulation to widen,the subtropical high to expand northward,and the EAJ core to strengthen over Xinjiang with the largest surface warming in China.As a result,the EAJ exit was projected to shrink westward,inducing stronger ascending motions over its downstream regions and low-level cyclonic circulation perturbations transporting more moisture from Bei Hai and Sea of Japan to cause precipitation increases respectively in North China and the Northeast.Meanwhile,the widening Hadley circulation promoted stronger ascending motions between South and Central China and low-level cyclonic circulation perturbations converging more moisture from Nan Hai and Dong Hai to cause precipitation increases respectively in South China and the Southwest.The severe convective weather generally decreases PBLH.The CWRF simulated precipitation was highly negatively correlated with PBLH,which is of high uncertainty.Through the departure correlation analysis across relevant fields of planetary boundary layer,it was shown that sensible heat flux is the most correlated with PBLH,followed by outgoing longwave radiation and shortwave downwelling flux at the surface.While the role of surface wind was weak.It was suggested that the model performance could be further enhanced by improving the key physical processes concerning heat flux,radiation and PBLH.In summary,CWRF improved the present-day precipitation performance in China,leading to the enhanced reliability of future projection.CWRF projected the summer rainfall increases both to the north of the Yellow River and the south of the Yangtze River in company with the decreases between the rivers in eastern monsoon region.The understanding of the associated physical mechanism increased the credibility of future projection,which provided reference for preventing and mitigating the extreme weather and disaster.In the future,we could refine the physical processes(especially the planetary boundary layer process)and the horizontal resolution to further improve the CWRF performance.