A physical initialization method for the economical prognostic Arakawa-Schubert scheme

A physical initialization method for the economical prognostic Arakawa-Schubert scheme (EPAS) is developed to incorporate observed precipitation data into a numerical weather prediction model. The method adopted is a variational approach which minimizes the difference between the first guess and the initialized model variables, subject to strong constraints on precipitation areas and precipitation rates. The physical initialization method is divided into 2 parts: (1) Adjustment of the initial thermodynamic variables such that the model precipitation areas diagnosed from the adjusted variables become consistent with the observed precipitation areas at the initial time. (2) Adjustment of the initial cloud-base mass flux (Mb) in such a way that the model precipitation rates are equal to the observed precipitation rates. In order to examine the impact of the physical initialization method, forecast experiments for a case study of Typhoon WALT (T9407) were performed. The results of the forecast experiments indicate that the physical initialization method eliminated the spin-up error of precipitation forecast for the first one hour, and reduced the positional error of precipitation forecasts for the first few hours. The results suggest that the adjustment of the initial cloud base mass flux was essential for eliminating the spin-up error. It is also found that the adjustment of the initial cloud base mass flux contributed to reduce the positional error by strengthening the model convective precipitation around the observed heavy-rain area, and that the adjustment of the initial thermodynamic variables reduced the positional error by removing extreme moist-convective instability in the observed rain-free areas.