ZDR columns as a predictive tool for hail growth and storm evolution

Within the updraft of a convective cell, liquid water in the form of relatively large raindrops can be carried aloft to an environment characterized by sub-freezing temperatures. Due to the oblate shape of these drops, they contribute positively to differential reflectivity Z_DR. If upward vertical velocities are strong enough, the number and size of the lofted drops results in a columnar region of enhanced Z_DR (sometimes as great as 5 dB at S band) which has been termed the Z_DR column. Therefore, the size and intensity of the column are closely related to the strength of the updraft itself; in turn, Z_DR columns likely provide a simple, yet powerful tool to assess and predict hail growth, which is highly dependent upon updraft strength, as well as overall storm intensity. To estimate this predictive capability, quantitative analyses relating the volume of Z_DR columns above the environmental freezing level and reflectivity Z_H below this level were performed for several cases. The results show that indeed an approximate lag of 20 minutes exists between an increase (decrease) in Z_DR column strength and an increase (decrease) in overall Z_H intensity, and presumably hail size, within the precipitation core near the surface. Furthermore, a negative correlation is observed at an approximate lag of 5-10 minutes, likely resulting from the ability of an updraft to loft hydrometeors. Lastly, as previous research has demonstrated that updraft weakening tends to precede tornadogenesis, the evolution of Z_DR column intensity within tornadic supercells is investigated to assess a possible connection between the two.