While a number of examples exist in the literature demonstrating storm-scale analysis and prediction (see above), almost no recent research has focused on what the limits of the predictability might be for convection. In part this is due to the lack of theoretical understanding regarding the flow dynamics, beyond Lilly's hypotheses regarding supercell longevity (and presumed longer predictability) arising from the helical flow, of the relative roles of the near-storm environment, internal dynamics, surface fluxes, and turbulence in quasi-two dimensional squall lines and three-dimensional supercell storms. Since storm-scale forecasts are now being reported in the literature, it would seem appropriate to attempt to quantify through numerical experiments the inherent limits of predictability within such storms.
This study will examine predictability in two types of convective systems: supercells and derecho squall lines. Both convective storm-type are associated with high-impact weather events; tornadoes and severe straight-line winds. A series of observing system experiments will be designed using an ensemble Kalman square-root filter as the data assimilation method. The OSE study will initially examine the predictability using a perfect model assumption. This study will study the analysis and forecast accuracy and ensemble spread of the convective storms. These will be used as a proxy for predictability in the hope of furthering our understanding about convective scale prediction. Experiments will be conducted to determine the sensitivity of the analyses/forecasts to the: a) frequency of the of data assimilation, b) length of assimilation window, and c) time when data is assimilated (evolution dependence). Many of these parameters need to be understood when we consider the new data platforms to be coming online in the next decade (dual-polarization radar and phased array radar). Initial results from imperfect model experiments where the model error is in part represented by errors in the microphysical parameterization will also be presented.
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