Prediction of Convective Storms at Convection-Resolving 1？km Resolution over Continental United States with Radar Data Assimilation: An Example Case of 26 May 2008 and Precipitation Forecasts from Spring 2009
For the first time ever, convection-resolving forecasts at 1？km grid spacing were produced in realtime in spring 2009 by the Center for Analysis and Prediction of Storms (CAPS) at the University of Oklahoma. The forecasts assimilated both radial velocity and reflectivity data from all operational WSR-88D radars within a domain covering most of the continental United States. In preparation for the realtime forecasts, 1？km forecast tests were carried out using a case from spring 2008 and the forecasts with and without assimilating radar data are compared with corresponding 4？km forecasts produced in realtime. Significant positive impact of radar data assimilation is found to last at least 24 hours. The 1？km grid produced a more accurate forecast of organized convection, especially in structure and intensity details. It successfully predicted an isolated severe-weather-producing storm nearly 24 hours into the forecast, which all ten members of the 4？km real time ensemble forecasts failed to predict. This case, together with all available forecasts from 2009 CAPS realtime forecasts, provides evidence of the value of both convection-resolving 1？km grid and radar data assimilation for severe weather prediction for up to 24 hours. 1. Introduction Accurate prediction of convective-scale hazardous weather continues to be a major challenge. Efforts to explicitly predict convective storms using numerical models dated back to Lilly [1] and began with the establishment in 1989 of an NSF Science and Technology Center, the Center for Analysis and Prediction of Storms at the University of Oklahoma (CAPS). Over the past two decades, steady progress has been made, aided by steady increases in available computing power. Still, the resolutions of the current-generation operational numerical weather prediction (NWP) models remain too low to explicitly resolve convection, limiting the accuracy of quantitative precipitation forecasts. For over a decade, the research community has been producing experimental real time forecasts at 3-4？km convection-allowing resolutions (e.g., [2–4]). Roberts and Lean [5] documented that convection forecasts of up to 6 hours are more skillful when run on a 1？km grid than on a 12？km grid, and more so than on a 4？km grid. On the other hand, Kain et al. [2] found no appreciable improvement with 2？km forecasts compared to 4？km forecasts beyond 12 hours. In the spring seasons of 2007 and 2008, CAPS conducted more systematic real-time experiments. Daily forecasts of 30？h or more were produced for 10-member 4？km ensembles and 2？km deterministic
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