Verification of ensemble Kalman filter analyses of a convective storm with a one- and a two-moment bulk microphysics scheme using polarimetric radar

Supercell thunderstorms are known to exhibit many distinct polarimetric signatures that are associated with the large natural variability in the size distributions, shapes, phase composition, and orientations of hydrometeors within a storm as a result of storm dynamics and microphysics. Use of a single-moment (SM) microphysics scheme substantially limits the capability of a numerical weather prediction model to simulate the observed variability in the particle size distributions (PSDs) of supercell storms, although such schemes are often used because of their lower computational costs. Double- or higher-moment microphysics schemes, on the other hand, are capable of simulating observed PSD variability to a certain extent. Additionally, polarimetric data can be very useful in the verification of convective-scale modeling studies because they contain additional information on the aforementioned hydrometeor properties.

The 29-30 May 2004 central Oklahoma tornado outbreak case was selected to investigate the ability of a SM and double-moment (DM) microphysics scheme and the ensemble Kalman filter to retrieve the polarimetric signatures reported in observational studies. Comparisons between simulated polarimetric variables from the analyses and the corresponding observations from an experimental polarimetric WSR-88D radar in Norman, Oklahoma (KOUN) revealed that analyzed Z_DR and K_DP values are too weak in the experiment using a SM scheme, despite good agreement in reflectivity. On the contrary, polarimetric signatures in the experiment using a DM scheme compare rather well with observations in terms of the general shape, location, and intensity of the signatures. The results suggest that EnKF data assimilation using a numerical model with an adequate microphysics scheme, i.e., a scheme that predicts at least two moments of the hydrometeor size distributions, is capable of producing polarimetric radar signatures similar to those seen in observations.