Impact of Targeted Afternoon Radiosonde Observations on Convection-Permitting Forecasts of the 31 May 2013 Convective Event in Oklahoma

Recent studies have examined the interaction of deep convection with the surrounding environment using numerical weather prediction (NWP) model forecasts that assimilate surface, satellite, and radar data. However, the impact of assimilating nearby radiosonde observations in pre-convective environments on short-term (0 - 9 h) convection permitting NWP model forecasts has yet to be explored. Targeted radiosonde observations were obtained prior to the convective event on 31 May 2013 that included a large, violent tornado near El Reno and a deadly flash flood. Two of these soundings were obtained as part of the Mesoscale Predictability Experiment (MPEX) near Norman, OK (1610 UTC) and Chickasha, OK (1920 UTC). The other was obtained by a team from the State University of New York at Oswego at 1800 UTC near Stillwater, OK. The impact of these targeted radiosonde observations on forecasts of this high-impact event is the subject of this study.

First, standard radiosonde, aircraft, and surface observations from 31 May 2013 are assimilated onto a 15-km grid every hour from 0000 UTC to 1600 UTC using an Ensemble Kalman filter (EnKF) method within the Weather Research and Forecasting (WRF) Data Assimilation and Research Testbed (DART) system. The three targeted radiosonde observations are then assimilated in half-hourly periods from 1600 UTC to 2000 UTC. A convection-permitting (3-km) grid is embedded at 2000 UTC, and a 36-member ensemble of forecasts are run out to 9 h. This experiment is compared to a control run that excludes the three targeted soundings, consistent with the resources available in an operational setting. Additional experiments vary the localization radius and initial condition observation error in the EnKF assimilation, and exclude the 1800 UTC National Weather Service soundings from the control experiment in order to test the sensitivity of these results. The impact of the targeted radiosonde observations are evaluated by quantifying the accuracy of the timing and location of convective initiation, the tornadic supercell near El Reno, and the transition to a heavy-rain/high-wind producing Mesoscale Convective System (MCS) among the set of 0-9 h forecasts.