Development and Test of Hydrostatic Extraction Scheme in Spectral Model

The introduction of “hydrostatic extraction” scheme, or “standard stratification approximation”, into spectral model gained some advantages compared with commonly used schemes. However, computational instability may occur for high vertical resolution versions if the stratification parameterC 0 taken as a constant. In this paper, the possible cause leading to the instability is discussed and an improved scheme presented whereC 0 is generalized to be a function of both height and latitudes. Hence the reference atmosphere gets closer to the real atmosphere and the temperature deviation field to be expanded becomes smoother everywhere. Test by real case forecasts shows good computational stability of the new scheme and better prediction performance than usual schemes of spectral model. This work has been carried out under the support of the Medium-range Numerical Weather Forecast research project.