What Can Polarimetric Radar Signatures of Developing Downbursts Reveal about Their Intensity? Journal of the Atmospheric Sciences

Downbursts present a major operational forecasting challenge. Numerous radar-based signatures have been proposed for nowcasting downburst development, including recent research on polarimetric signatures associated with downbursts. However, the reliability of these signatures, and their relationship to downburst intensity, are not well established. In this work, we develop an idealized one-dimensional model of downburst development with bin microphysics and a coupled polarimetric radar forward operator to study the relationships, if any, between proposed downburst radar signatures (viz., descending Z and Kdp cores) and forcing mechanisms (i.e., precipitation loading and diabatic cooling). The model is able to realistically reproduce observed downburst radar signatures and evolution, with precipitation loading being the dominant forcing mechanism close to the 0°C level and diabatic cooling becoming dominant closer to the surface. Environmental sensitivity runs show that for a given initial particle size distribution, the diabatic cooling forcing/downdraft magnitude and Kdp exhibit opposite responses to variations in temperature lapse rate and RH, while Z and total precipitation loading forcing are mostly insensitive to the environment. However, ensemble simulations show that although neither Z or Kdp are well correlated with the instantaneous forcing magnitudes at most heights, Kdp below the 0°C level is well correlated with the resultant downburst intensity at the surface within a given thermodynamic environment, with higher Kdp aloft corresponding to stronger downbursts. These findings support the use and further exploration of Kdp cores near the melting level as downburst radar precursors. Grant no. NA21OAR4320204