Using the ZDR Column Product to Detect Updraft Development and Track Updraft Evolution

Observations and recent high-resolution numerical model simulations indicate that liquid water and partially-frozen hydrometeors can be lofted to heights well above the freezing level in the updrafts of convective storms owing to the non-instantaneous nature of drop freezing. As a result, upward-extensions of positive differential reflectivity factor (e.g., ZDR > 1 dB) -- called ZDR columns -- may prove to be a very useful proxy for both detecting the initiation of new convective storms and examining the evolution of updrafts associated with convective storms. The use of a polarimetric emulator and high-resolution numerical simulations with spectral bin microphysics reveals a high spatial correlation between the updraft and ZDR column and highlights the effectiveness of the ZDR column relative to changes in radar reflectivity factor (Z) in assessing updraft evolution; changes in the depth of the ZDR column can precede changes in lower-tropospheric Z by 5-15 minutes. In addition, size sorting through differential sedimentation of hydrometeors at the initial onset of precipitation fall-out in nascent convective storms can enhance ZDR where Z may still be quite low. This project examines the utility of the ZDR Column Product and near-ground ZDR enhancement associated with the onset of precipitation fall-out for providing additional diagnostic and prognostic information pertinent to convective storm nowcasting. Some of the difficulties experienced when examining ZDR columns using data from current operational weather radars (e.g., suboptimal vertical resolution above the freezing level) will also be discussed.