Impact of Assimilating Clear-Air Radial Velocity Observations on the Forecasting of Supercell Thunderstorm: An Observing System Simulation Experiment Study

The development of phased-array radar (PAR) technology allows sampling the clear-air radial velocity more accurately at a high spatiotemporal resolution. Observing system simulation experiments (OSSEs) are conducted to explore the potential value of assimilating clear-air radial velocity observations for numerical prediction of supercells. Synthetic PAR observations of a splitting supercell are assimilated in different stages of the storm using an ensemble Kalman filter (EnKF). Results show that assimilating extra clear-air radial velocity can improve the supercell forecasting in different stages of the storm, especially the forecast after 30 min. Assimilating extra clear-air radial velocity can not only reduce the near-storm environment wind errors, but also improve the accuracy of wind fields within the storm region due to the interaction between the storm and the environment. After assimilating the extra clear-air radial velocity observations, the probabilities of updraft helicity and precipitation within the probability swaths increase up to 30–40%. Additional diagnostics suggest that the more accurate track forecast, the accelerated vertical motion and the better maintained supercell can be attributed to the better analyzed and predicted linear and nonlinear forces. Consequently, assimilating extra clear-air radial velocity improves the accuracy and duration of storm structure (rotating updrafts), produces reasonable updraft size and accurate storm track, and finally improves surface accumulated precipitation forecast. These results highlight the potential for assimilating rapid-scanning PAR clear-air radial velocity observations to improve the numerical prediction of the supercell thunderstorm.