10B.5 Forecast Sensitivities of Mcs Stratiform Precipitation Shields to Changes in Horizontal Grid Spacing in WRF Simulations

Wednesday, 6 June 2018: 2:30 PM
Colorado B (Grand Hyatt Denver)
Brian Joseph Squitieri, Iowa State Univ., Ames, IA; and W. A. Gallus Jr.

To date, skillful forecasts of some aspects of mesoscale convective systems (MCSs) remain elusive in NWP models, even for simulations generated by convection allowing models (CAMs). Producing a fairly accurate representation of stratiform precipitation shields associated with MCSs has been especially challenging. Previous works have shown that changes in microphysics schemes, as well as changes in prescribed hydrometeor characteristics (such as hydrometeor size, and fall speeds) have widely varying impacts on convective simulations, including stratiform precipitation shields. Still, experiments with microphysical alterations have yet to yield an ideal microphysical configuration contributing to universally improved MCS forecasts. In this study, changes in horizontal grid spacing (i.e. 3 km, 1 km, and 0.333-km) were employed to investigate changes in three dimensional structure of simulated stratiform precipitation associated with MCSs in finer vs. coarser grid spacing. Given the impact that varying microphysics schemes has on simulating convection, tests in alternating between one- and two-moment microphysics schemes were conducted on a subset of the sample cases to determine if forecast sensitivity from altering microphysics schemes would dominate forecast sensitivity from decreases in horizontal grid spacing. Simulations were compared to three dimensional observed reflectivity derived from mosaic MRMS data. While MCSs in their entirety are overall more expansive in area in finer grid simulations than in those using coarser grid spacings, preliminary results show that trailing stratiform precipitation shields are slightly smaller in areal coverage using 1-km above-ground-level reflectivity compared to observations and simulations with coarser grid spacing when area is normalized with respect to the size of the entire MCS. It is hypothesized that in finer grid spaced simulations, the better resolution of vertical moment fluxes along the leading line of convection and post-convective horizontal mixing leads to greater evaporation/sublimation of hydrometeors, resulting in potentially stronger, broader, and drier rear-inflow jets, perhaps also contributing to faster MCS propagation.
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