The Cold Bias in the Surface Air Temperature Simulations Over the Tibetan Plateau of RegCM4 Reduced by Adopting the 3D Sub‐Grid Terrain Longwave Radiative Effect Scheme

Abstract

The 3D sub‐grid terrain longwave radiative effects (3DSTLRE), which significantly regulate the surface downward longwave radiation (SDLR) in the mountainous regions, are not described in current numerical models. We incorporated a 3DSTLRE scheme into RegCM4 to assess its influence on modeling the surface air temperature (SAT) across the Tibetan Plateau (TP). Results show that the RegCM4 adopting the parallel‐plane longwave radiative scheme significantly underestimates the SAT over TP, this underestimation of SAT is clearly mitigated by considering 3DSTLRE, with the root mean square error (RMSE) decreased by 9%. The SAT simulations are improved more noticeable over western TP than entire TP and more evident at nighttime than at daytime in different seasons. Further analysis indicates that the improvement of SAT simulations over the rugged areas of TP is primarily benefited from the improved SDLR simulations. In the absence of the 3DSTLRE scheme, RegCM4 markedly underestimates SDLR by 20 W·m−2 over the entire TP, this underestimation can be greatly reduced by 15 W·m−2 through adopting the 3DSTLRE scheme, with the RMSE reduced by more than 40% over the rugged edges of TP. The increased SDLR induced by the 3DSTLRE is mainly transformed into sensible heat flux to warm the near surface air, further leading to reduced cold bias of SAT produced by the RegCM4 without 3DSTLRE. Better representing the TP thermal condition can enhance the simulation of East Asian monsoon. Therefore, incorporating the 3DSTLRE scheme in numerical models can potentially improve the ability in simulating and predicting the East Asian monsoon.