Microphysical and Horizontal Grid Spacing Influences on WRF Forecasts of Stratiform Rain Regions and General Convective Morphology Evolution in Nocturnal MCSs

The use of convection-allowing models has allowed reproduction of detailed convective morphologies in model forecasts with some degree of success, however, some recent studies have found major shortcomings in forecasts of some modes, especially bow echos and squall lines with trailing stratiform regions. For a sample of 19 nocturnal convective system events where the low-level jet was present with weak synoptic forcing, this study utilizes Advanced Research WRF (WRF-ARW) simulations to examine the predictability of convective morphology. For these cases, a six-member ensemble was run at 3 km horizontal grid spacing utilizing two different microphysics schemes (Thompson and WSM6) and three different planetary boundary layer schemes (YSU, MYJ, MYNN). Additionally, subsets of these runs are being conducted at 1 km horizontal grid spacing and with additional microphysical variations (WSM6 with modified graupel fall speed and Morrison) to allow for investigation into grid spacing and microphysical sensitivities.

Recent prior work has shown an overall underprediction of linear convective modes throughout the nocturnal period with both Thompson and WSM6 microphysics in the 3 km runs, while the 1 km runs instead simulated linear modes more often. Also, runs using Thompson microphysics were found to more often predict the presence of stratiform rain regions when compared to runs using WSM6 in both 3 km and 1 km grid spacings. Detailed analysis of the forecasts of these stratiform regions will be shown based on the influences of grid spacing and microphysics. Additionally, further morphological evolution and stratiform region sensitivity tests, along with comparisons of forecast performance, will be shown using the modified WSM6 microphysics and the double-moment Morrison microphysics alongside the previous Thompson and WSM6 microphysics.