Parameterization and evaluation of nonhydrostatic effect in the orographic gravity wave drag in China Meteorological Administration Global Forecast System (CMA-GFS) v4.0 model

Abstract. The China Meteorological Administration Global Forecast System (CMA-GFS) v4.0 model was upgraded to a higher resolution of 0.125° in May 2023. To be compatible with its fine resolution, the parameterization scheme of orographic gravity wave drag (OGWD) in CAM-GFS is revised herein by accounting for the nonhydrostatic effect (NHE) on the wave momentum flux of subgrid-scale orographic gravity waves. The performance of the revised OGWD scheme is then evaluated for the 10 d medium-range forecast in December 2023. Results show that the revised OGWD scheme can better capture the large-scale circulation in the Northern Hemisphere (NH), particularly in the high latitudes. The easterly (westerly) wind biases in the NH polar stratosphere (troposphere) are decreased. The underestimation of East Asia subtropical jet is also alleviated. Quantitative evaluation shows that the revised OGWD scheme reduces both the mean bias and root mean square error of 500 hPa geopotential height in the NH after the 6th forecast day, reaching 11.59 % and 5.06 %, respectively, by day 10. The decrease of easterly biases in the polar stratosphere is owing to the weakening of stratospheric zonal OGWD by the NHE. For the decrease of westerly biases in the NH polar troposphere, it is due to the fact that the enhanced stratospheric winds suppress the upward propagation of Rossby waves into the stratosphere, resulting in greater convergence of Eliassen-Palm flux in the mid-upper troposphere.