Comparison of observed and numerically-simulated rain drop size distributions in VORTEX2 supercells

Recent numerical studies with multimoment microphysics have shown that low-level rain drop size distributions (DSDs) vary substantially and systematically between different regions of the storm. Among the various microphysical processes controlling the behavior of the DSD, size sorting of hydrometeors by both the storm updraft and the storm-relative wind profile has been identified by several studies as an important contributor to the simulated spatial patterns of the DSD. Direct in situ measurements of DSDs from disdrometers in supercells are rare, but during VORTEX2 several disdrometers were deployed in multiple supercells. Polarimetric radar observables such as differential reflectivity (Z_DR) offer complementary information on the DSD with the advantage of greater spatiotemporal coverage. These observations can be compared with model output with the use of suitable polarimetric forward operators.

In this study, we perform idealized numerical simulations of VORTEX2 supercells with a triple-moment microphysics scheme, with and without size sorting allowed, and compare the resulting surface and near-surface DSDs with that observed from disdrometers. We emphasize qualitative comparisons utilizing a simple time-to-space conversion of the time series of 1-min DSDs. We then compute polarimetric radar observables from the observed and simulated DSDs and compare them with polarimetric radar observations from available mobile radar platforms.

Results indicate that good qualitative agreement with observed disdrometer time series for metrics such as the mean volume diameter (D_mr) is obtained when size sorting is allowed in the numerical model. In particular, both the simulation and observed DSDs exhibit a marked decrease in D_mr from the leading (right) flank to the trailing (left flank), as well as from the leading to trailing edge of the hook echo, for the right-moving supercells examined in this study. Differential reflectivity Z_DR exhibits similar trends. When size sorting is disallowed in the model, no such decrease is apparent in the simulated DSDs. This result is consistent with the aforementioned work implicating size sorting as an important or even dominant process controlling low-level DSD behavior in these storms.

Finally, we present preliminary results of simulations using a spectral bin scheme and compare and contrast with the results of the triple-moment bulk scheme.