Ensemble forecasts of severe convective events during spring 2007: Improvements from surface data assimilation using an ensemble Kalman filter

As part of the NOAA Hazardous Weather Testbed 2007 Spring Forecasting Experiment, a 30-member mesoscale short-range ensemble forecasting system using the Weather Research and Forecasting (WRF) model was developed to explore the value of surface observations in mesoscale ensemble forecasts of severe weather events. The ensemble analyses and forecasts were used to investigate probabilistic severe weather forecasting applications. The forecast domain covers the contiguous 48 states and has a horizontal grid spacing of 30 km. Daily forecasts were made from 15 March through 30 June 2007, capturing several severe convective weather outbreaks. Routine hourly surface observations of 2-m potential temperature, 2-m dewpoint temperature, and 10-m winds were assimilated into the ensemble from 1300 to 1800 UTC and followed by a 42-h forecast. The data assimilation is conducted using the Data Assimilation Research Testbed (DART) square-root ensemble Kalman filter. In addition, each ensemble member has a different set of model physical process parameterization schemes and the ensemble initial and boundary conditions are perturbed following Torn et al. (2006). Thus, the ensemble represents both initial condition and model physics uncertainties.

Results from this ensemble are compared against an ensemble without data assimilation started at 1800 UTC to document any improvements to the probabilistic severe weather forecasts due to the assimilation of the surface observations during the first half of the daytime diurnal heating cycle. Particular attention will be paid to severe weather parameters and mesoscale features, such as drylines and cold pools, incorporated into the model through the surface data assimilation that are not present in the runs without surface data assimilation. The presentation will focus on forecasts on days with numerous severe reports, including the tornadic supercell events on 4-5 May over Kansas and 7 June over Wisconsin, and the merging bow echo event over Texas and Oklahoma on 19-20 June.