670 Impacts of Storm Displacement Errors on Storm-Scale Analyses and Short-Term Forecasts

Tuesday, 24 January 2017
4E (Washington State Convention Center )
Derek R. Stratman, NRC/NSSL, Norman, OK; and C. K. Potvin and L. J. Wicker

A goal of Warn-on-Forecast (WoF) is to develop forecasting systems that produce accurate analyses and forecasts of severe weather, such as supercells, for forecasters to utilize in operational warning settings. Recent WoF-related studies have indicated the need to alleviate storm displacement errors in both storm-scale analyses and short-term forecasts. One promising method to reduce these errors is the feature calibration and alignment (FCA) method. Previous studies merging FCA with variational data assimilation systems have shown substantial improvement of analyses and forecasts, especially at earlier forecast times, by minimizing displacement errors. However, the WoF project mostly employs variants of the ensemble Kalman filter (EnKF) data assimilation technique to produce analyses. The potential impact of using a displacement error corrector, such as the FCA, with an EnKF data assimilation technique on storm-scale analyses and subsequent short-term forecasts is not clear. Thus before vetting the FCA as a potential alleviator to storm displacement errors, multiple observation system simulation experiments (OSSEs) will be conducted to test the sensitivities associated with these errors. Idealized nature runs will be generated for various storm modes, such as supercells, linear mesoscale convective systems, and multi-cell storms. In each case, two sets of EnKF cycles will be started shortly after simulated convective initiation is reached using the nature run’s output for the initial background fields and to generate the pseudo-observations being assimilated. The initial background fields for one of the two sets of EnKF cycles will be altered by a subjectively determined 2-D field of large displacement vectors to represent background fields before the application of an FCA technique. Conversely, the other set of EnKF cycles will use initial background fields altered by small displacement vectors to represent background fields after the utilization of an FCA technique. After several EnKF analysis cycles, short-term forecasts will be produced to examine the potential impact an FCA technique might have on convective forecasts. Results from these sensitivity tests will be presented.
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