Improving the Observation Operator for the Phased Array Weather Radar in the SCALE-LETKF System

抄録

The observation operator for the Phased Array Weather Radar in the SCALE-LETKF data assimilation system is revisited, and the impact of its improvement on the analyses and forecasts is examined. The observation operator provides a functional relationship between equivalent radar reflectivity factor (Z_e) and hydrometeor mass density (W) of each precipitation particle category. The W–Z_e relationship is obtained by a radar simulator. This study performs a radiation code calculation with the parameters regarding particle size distribution of graupel consistent with the cloud microphysics scheme in the SCALE model. The newly obtained observation operator provides much stronger sensitivity of graupel mixing ratio to observed Z_e compared to the operator originally used in the model. To examine the impact on the SCALE-LETKF analyses and forecasts, an experiment on a 13 July 2013 heavy rain case is performed with the new observation operator and is compared with the previous study. The forecast initiated by the analysis using the new operator shows much more realistic evolution of Z_e in the middle troposphere, where a large amount of graupel is located. The overestimation of forecast Z_e is significantly alleviated by the new observational operator. The 30-minute forecast of surface precipitation rate is also improved.