Corey Potvin, CIMMS/NSSL
Convective-scale analyses and forecasts obtained using LETKF vs. serial data assimilation
Coauthors
Dustan Wheatley, Kent Knopfmeier, Louis Wicker, and Therese LadwigAbstract:
The primary objective of the NOAA Warn-on-Forecast (WoF) program is to deploy a real-time storm-scale ensemble system that provides probabilistic forecasts of tornadoes and other thunderstorm hazards. This guidance will complement existing operational products to enable longer warning lead times and fewer false alarms. The ensemble Kalman filter (EnKF) will be an integral component of the WoF data assimilation system, possibly as part of a hybrid variational-ensemble configuration. EnKF variants currently used for convective-scale data assimilation vary in two important ways: (1) observations are processed either serially or simultaneously, and (2) covariance localization is applied to either the forecast covariances (serial filters) or the observation error variances (simultaneous filters). Each type of filter offers its own advantages for assimilating radar data on convective scales, but their relative accuracy has not been thoroughly explored using real data.
Toward this end, 3-km analyses and subsequent 1-h forecasts are generated from two WRF-ARW ensemble systems configured as identically as possible except with either an ensemble square root filter (EnSRF), which is a serial filter, or a local ensemble transform Kalman filter (LETKF), which is a simultaneous filter. Initial and boundary conditions for the 3-km grid are provided by the NSSL Experimental Warn-on-Forecast System for ensembles (NEWS-e). This initial work focuses on several supercell events that occurred within the United States. In each case, observations are assimilated onto the 3-km grid every 15 min from surface stations and three WSR-88D radars. Verification of the analyses and forecasts focuses on storm track, rainfall, and evolution of low-level rotation, and utilizes neighborhood ensemble probabilities and probability-matched ensemble means. Preliminary results suggest analyses and short-range forecasts of convective hazards have similar accuracy whether the EnSRF or LETKF is used.