Impact of Assimilating Radar Refractivity with Radial Wind and Reflectivity in the Context of Ensemble Kalman Filter

Moisture information near the surface plays a crucial role in convective initiation and heavy rain prediction, and radar-derived refractivity is able to provide the moisture information both in precipitating and non-precipitating conditions. In this study, radar- refractivity was assimilated with radial wind and reflectivity by using the high-resolution Weather Research and Forecasting local ensemble transform Kalman filter data assimilation system. Two cases in the Southwest Monsoon Experiment 2008 were selected and two sets of the experiments were conducted. The first set was applied to investigate the effects of assimilating radar refractivity along with reflectivity and radial wind in both cases. Results of the two cases in the first set experiments illustrated that assimilating reflectivity and radial velocity modified near-surface humidity on the basis of the flow-dependent error correlation estimated by the ensemble. However, the spatial distribution may not be fully accurate, causing underestimation of rainfall. With further refractivity assimilation, stronger convergence and more accurate low-level moisture, temperature, and wind field corrections were obtained, leading to superior forecasts for both light and heavy rainfall up to 6 h. The second set of the experiments was conducted for the second case to examine the benefit of increasing the frequency of refractivity assimilation and investigate the optimal strategy to assimilate refractivity. The results of the second set indicated that increasing the assimilation interval of refractivity enabled capturing the dramatic moisture variation and enhancing wind convergence, resulting in short-term forecast improvement. The strategy that solely assimilated refractivity before the appearance of the precipitating system was capable of optimizing the correction of environmental moisture, then accurately representing the humidity and strengthening the wind convergence when precipitation occurred.