Descending reflectivity cores in supercell thunderstorms observed by mobile radars and in a high-resolution numerical simulation

This paper is motivated by the recent interest in the "descending reflectivity cores" (DRCs) that have been observed in some supercell thunderstorms prior to the development or intensification of low-level rotation. The DRCs of interest descend on the right rear flank of the storms and are small in scale, relative to the main radar echo. They are observed to descend from the echo overhang and, upon reaching low levels, have been found to contribute to the formation or evolution of hook echoes, which are perhaps the most familiar radar characteristic of supercells. Herein, observations of DRCs obtained by a mobile Doppler radar at close range are presented. The data afford higher-resolution views of DRCs and their accompanying radial velocity fields than typically are available from operational radars, although one drawback is that some of the larger-scale perspective is sacrificed (e.g., the origin of the DRC and its possible connection to the reflectivity near the updraft summit are within the cone of silence). It is found that it is difficult to generalize a relationship between the observations of DRCs and the subsequent evolution of the low-level wind field. The results of a three-dimensional numerical simulation of a supercell thunderstorm also are presented. DRCs are a common development within the simulation despite the use of a simple (warm rain) microphysics parameterization. The simulation allows for an investigation of the aspects of DRCs that cannot be ascertained using single-Doppler radar observations, for example, DRC formation mechanisms, the relationship between DRCs and the three-dimensional wind field, and the thermodynamic fields that accompany DRCs. Three different mechanisms are identified by which DRCs can develop in the model, not all of which are followed by increases in low-level rotation. This finding might account for the aforementioned difficulty in generalizing associations between DRCs and changes in the low-level wind field observed by mobile radar, as well as the fact that prior studies also have produced somewhat mixed results with respect to the potential of DRC detection to aid in the operational forecasting of tornadogenesis.