Effects of Microphysical Parameterizations and Drop Breakup on Supercells and Supercell Cold Pools.

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Ensemble Kalman filter data assimilation experiments are conducted using velocity data from the National Weather Service Radar 88-D and a Shared Mobile Atmospheric Research and Teaching Radar to determine how two different two-moment (Morrison and Milbrandt-Yau) microphysical parameterizations affect the supercell cold pool and the evolution of storm structure. Using in-situ data gathered by StickNet probes, the simulated cold pool can be objectively verified against the observations. This verification is done over the entire supercell and over the two main regions of the supercell (forward- and rear-flanks). The resulting cold pools from the experiments are colder in the forward flank and warmer in the rear flank than the observed StickNet values. The Morrison microphysical parameterization produces more realistic supercell structure in reflectivity shape and strength and the cold pool shape is also more similar to the observed cold pool. The Milbrandt-Yau reflectivity shape is more elongated and more narrow and does not match the observed supercell reflectivity structure. The Morrison microphysical parameterization results more accurately represent the observed supercell. High amounts of rain are present near the updraft of the Morrison parameterizations in the area where the strongest part of the cold pool resides. These high precipitation regions are caused by hail melting and rain evaporating. The evolution of rain, especially, is in part controlled by collision and coalescence. Depending on the size of the raindrops, they will collect and grow larger or breakup into smaller drops. The parameter controlling how the rain drops breakup is varied in another set of experiments. The effect of changes to the drop breakup on the supercell cold pool temperature is shown to be more than 0.5 K averaged over the entire supercell and 2 K over the rear-flank region of the supercell where drop breakup is occurring most frequently. Lowering the drop breakup threshold produces a colder cold pool over the entire supercell structure and raising this threshold warms the cold pool. Varying the hail fall speed has a minor effect on the cold pool structure and temperature. The variation of the drop breakup threshold in an ensemble framework improves the model’s representation of the supercell cold pool and the resulting storm structure. This also plays a role in altering the amount of barocliniclly generated vorticity along the rear-flank gust front. As the supercell goes through a mesocyclone replacement cycle the ensembles with the varying and more accurate drop breakup thresholds produce a stronger new updraft and mesocyclone that is closer to what was observed. The ensemble members can be subset and used outside of the mean to investigate the effects of various drop breakup thresholds on the cold pool. Lowering the drop breakup threshold cools the supercell cold pool over the entire storm, which brings the rear-flank temperature values closer to the observed StickNet values, but this produces a too-cold forward flank. Stitching together accurate rear-flank and forward-flank members produces a new way of investigating and diagnosing the supercell cold pool.

Supercell, Cold pool, Numerical modeling, Microphysics, StickNet, Ensemble, Severe storms