Summer Regional Climate Simulations over Tibetan Plateau: From Gray Zone to Convection Permitting Scale

The Tibetan Plateau (TP) is often referred to as ‘the Third Pole’ and plays an essential role in influencing the global climate. However, it remains challenging for most global and regional models to realistically simulate the characteristics of long-period climate over the TP. In this study, two Weather Research and Forecasting model (WRF) experiments using the regional climate simulation method of spectral nudging with gray-zone (GZ9) and convection-permitting (CP3) resolution are conducted for summer from 2009 to 2018. The surface air temperature (T2m) and precipitation from the two simulations and the global reanalysis ERA5 are evaluated against in-situ observations. The results show that ERA5 has a general cold bias over southern TP, especially in maximum T2m (Tmax), and wet bias over whole TP. Both experiments can successfully capture the spatial pattern and daily variation of T2m and precipitation, though cold bias for temperature and dry bias for precipitation exist especially over the regions south of 35°N. Compared with ERA5, the added value of the two WRF experiments is mainly reflected in the reduced cold bias especially for Tmax with more improvement found in CP3 and the reduced wet bias. However, the ability of WRF experiments in significantly improving the simulation of precipitation seems limited, which may be related to physical parameterization and lack of representativeness of station observation. Further investigation into surface radiation budget reveals that the underestimation of surface net radiation contributes a lot to the cold bias of T2m over the eastern TP in GZ9 while is improved in CP3 due to more downward solar radiation. Compared with GZ9, CP3 shows larger specific humidity at low-level (mid-high level) coexists with more precipitation (clouds) over the southern TP. This improvement is achieved by better depiction of topographic details and stronger northward water vapor transport (WVT) in CP3, providing more water vapor for precipitation at surface and much wetter condition in the mid-high level.