On the Acceleration of Nocturnal Mesoscale Convective Systems in Simulations with Increased Horizontal Grid Spacing

Mesoscale Convective Systems (MCSs) are primary contributors to late spring and summer precipitation across the central United States, and yet forecasts of these events have generally not been particularly skillful. Model forecasts of MCSs have improved since horizontal grid spacings have been refined enough to allow the removal of convective parameterizations that were known to introduce significant errors. However, even though the reflectivity patterns in simulated systems in convection-allowing model runs resemble the types of events often observed, substantial errors still remain, usually in location, timing, and precipitation amounts for the systems. One question that deserves further exploration is will implementing finer grid spacing in convection allowing models improve the timing, placement, and structural representation of convective elements in forecast simulations. Prior studies have offered mixed results.
The current study evaluates Great Plains nocturnal MCSs at 4 km and 1.5 km horizontal grid spacing using both a research model (WRF-ARW) and the operational NCEP NMMB model (Non-hydrostatic Multiscale Model on the B grid). Preliminary results show convection in 1.5 km simulations exhibiting a faster forward propagation compared to 4 km runs, with some improvement in spatial placement of the MCS with respect to observations. Despite better placement of MCSs, accurate representations of MCS structure are still hard to achieve with runs employing certain microphysics schemes. Specifically, preserving organized linear convection with 50+ dBZ cores along the leading edge of the MCS in Thompson runs remains a challenge, even in 1.5 km simulations.
The study will discuss causes for the differences between the 4 and 1.5 km results in both the WRF-ARW and NMMB, and will demonstrate the impacts of multiple microphysical schemes on the differences as horizontal resolution is changed. Some prior studies have indicated that increasing magnitudes of evaporative cooling that occur on the finer grids drive the increased propagation speed. The use of multiple microphysics schemes should help improve understanding on how much of a role this factor plays in nocturnal MCS evolution, and whether or not other factors are important.