What Can Polarimetric Radar Signatures of Developing Downbursts Reveal about Their Intensity?

Abstract

Abstract 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 K dp 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 K dp 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 K dp are well correlated with the instantaneous forcing magnitudes at most heights, K dp below the 0°C level is well correlated with the resultant downburst intensity at the surface within a given thermodynamic environment, with higher K dp aloft corresponding to stronger downbursts. These findings support the use and further exploration of K dp cores near the melting level as downburst radar precursors. Significance Statement Downbursts present a major nowcasting challenge due to their rapid evolution. While various weather radar signatures have been proposed to be indicative of the existence of developing downbursts, the purpose of this study is to better understand what these signatures may be able to tell us about their intensity. Using a detailed model of downburst generation, we found that, together with knowledge of how favorable the environment is for downbursts, the maximum magnitude of a specific differential phase core beneath the melting layer is associated with how strong a downburst will be when it eventually reaches the surface. This supports the potential use of this radar signature aloft to predict the severity of impending downbursts.