Implementation of an Orographic Drag Scheme Considering Orographic Anisotropy in All Flow Directions in the Earth System Model CAS-ESM 2.0

A reasonable representation of orographic anisotropy in earth system models is vital for improving weather and climate modeling. In this study, we implement the orographic drag scheme, including 3-D orographic anisotropy (3D-AFD), into the Chinese Academy of Sciences Earth System Model version 2 (CAS-ESM 2.0). Three groups of simulations named sensitivity run, medium-range forecast, and seasonal forecast respectively are conducted using the updated CAS-ESM model and validated against station observation and reanalysis data. These simulations are run using the three schemes—3D-AFD scheme, the 3D orographic anisotropy scheme for the eight-direction (3D-8x), and the 2D isotropic scheme (2D)—to compare their performance in CAS-ESM 2.0. Sensitivity runs show that the 3D-AFD provide more continuous transition of calculated drag as function of wind direction than the 3D-8x, while the drag calculated using the 2D scheme show no variation with change of wind direction. Enhanced drag in the medium range and seasonal forecast using the updated CAS-ESM alleviates part of the winter wind speed bias over the Tibetan Plateau (TP) and the cold bias over TP and the Siberian polar region. It is shown that the 3D-AFD scheme alleviates more bias than that of the 3D-8x scheme (by wind speed reduction of 1 ∼ 2 m/s and temperature of 1 ∼ 2 K) especially in the seasonal forecast. The results suggest that reasonable representation of the orographic anisotropy is important in climate modeling.