Research On The Data Assimilation Scheme For High-resolution Regions Of The Qinghai-Tibet Plateau

The Tibetan Plateau(TP),known as the Earth's Third Pole,is the origin of many rivers and has a significant impact on the atmospheric environment and even global climate.The TP region is reported to have been undergoing a much greater rate of environmental change under global warming,which has become a global study focus.Due to that the thermodynamic and dynamic issues caused by the high-altitude complex terrain over the TP are still not entirely clear.Furthermore,the unique boundary layer and the complex cloud physics associated with precipitation,together with the sparse observations,the large uncertainty of satellite observations,and other issues,have severely restricted the systematic study on weather and climate change over the TP.Furthermore,current global reanalyses with their lower resolutions are not able to describe TP's atmospheric state accurately,especially humidity,plateau clouds,and precipitation.There are few high-resolution regional reanalysis datasets over the TP.Therefore,the study on data assimilation under the condition of scarce observations over the TP region is essential for the development of a high-resolution regional analysis dataset and the systematic study of weather,climate,and environmental change of the TP.This study focuses on the scientific issues mentioned above of the regional data assimilation over the TP,and mainly studies the data assimilation of the variational quality control(VarQC)and the ensemble Kalman Filter(EnKF)schemes.The established VarQCs are suitable for the TP's complex terrain to make full use of the scarce observations.The TP's analysis dataset was developed by the EnKF system with the flow-dependent characteristics,and the potential impact of satellite radiance data for analysis was studied.According to the non-Gaussian observation error distribution mode of “Gaussian + flat” and Huber distribution,there are two VarQCs: Flat-VarQC and Huber-VarQC.They are analyzed synchronously with the 3DVAR assimilation,which can effectively improve the utilization of observations,and improves the TP's analysis and forecast,while the EnKF system is more advantageous for the development of the TP's analysis dataset.The EnKF analysis dataset is more accurate than the ERA-Interim and ERA5 reanalysis developed by the European Center for Medium-Range Weather Forecast(ECMWF),especially for the humidity,water cycle,plateau cloud,and precipitation.The main conclusions are:(1)A new conceptual mode to show the principle of VarQC for observational data is proposed,and two VarQC schemes(Flat-VarQC and Huber-VarQC)are established based on the variational assimilation system.The actual observation weights and theoretical weight functions of the two VarQC schemes are consistent and reasonable.The Huber-VarQC scheme can accurately identify the outliers,effectively eliminate(absorb)harmful(useful)information of outliers,and has strong robustness.The simulation of geopotential height from VarQC is more accurate than that from the standard 3DVAR,especially in the middle and low troposphere,and the performance of Huber-VarQC is better than Flat-VarQC.It fully verified the correctness and effectiveness of the two VarQC schemes and provided an important scientific basis for the development of the TP's analysis dataset with the VarQC methods.(2)The TP's analysis dataset has been developed based on the established VarQC scheme,and it provides a superior technology for improving the TP's analysis dataset in variational assimilation and hybrid assimilation system.Compared with the standard 3DVAR,the VarQC's analysis dataset effectively improves the utilization rate of sparse observations over the TP and has more accurate temperature and humidity fields,and the wind field is comparable to3 DVAR.At the same time,it effectively reduces the precipitation forecast error of the 3DVAR analysis dataset that overestimates precipitation.(3)A pilot high-resolution EnKF analysis dataset that only assimilates the conventional observations in summer over the TP was constructed,and the performance of the EnKF system was evaluated.EnKF analysis dataset has a more accurate three-dimensional humidity field than ERA-Interim and ERA5.Its downscaling forecast can simulate precipitation distribution and precipitation intensity more accurately,and effectively eliminate spurious precipitation on the TP.The diurnal cycle of precipitation is closer to the observed diurnal cycle than those of the original and downscale precipitation forecasts initialed by ERA-Interim and ERA5.EnKF system shows great advantages for the development of the TP's analysis dataset,but the quality of the temperature field and wind field can be further improved.(4)The potential of EnKF assimilation of all-sky satellite radiance to further improve the quality of the TP's analysis dataset is studied.The forecasts initialized by the ensemble mean from the EnKF satellite assimilation experiment have more accurate simulations of the MCSs(Mesoscale Convective Systems)moving eastward of the TP compared with the no-dataassimilation(“No DA”)experiment without assimilating radiance,effectively reducing the simulation error of convective clouds and improving the pattern and intensity of precipitation after 24-hour forecasts,and gradually show the advantages of short-term precipitation forecasting,proving that EnKF assimilation of satellite radiance has great potential for the development and improvement of the TP's analysis dataset.