Impact of TAMDAR on RUC forecasts: case studies

Case studies of several convective days with TAMDAR data have clearly shown that there can be significant changes in the low level winds and moisture evolving over only a few hours. These changes can occur at time and space scales below the current upper air network, and it was clear that the TAMDAR soundings provide useful information to forecasters. Whether the TAMDAR soundings also produce notable differences in numerical model solutions in the 0 to 12-h time scale is not necessarily as straightforward to determine, and in this study we address that issue.

The Forecast Systems Laboratory has been running a parallel version of the RUC 20-km model since late Spring 2005 in which TAMDAR data are ingested. This allows us to make a direct comparison between RUC model forecasts with and without TAMDAR data. Statistics comparing these RUC runs are presented elsewhere at the conference; here we concentrate on case studies for situations where it appeared possible that the TAMDAR data might have made an impact on the RUC forecast. For these cases we subjectively evaluated the model forecasts, with emphasis on comparing forecasts of temperature, humidity and wind both at the surface and aloft (particularly below 500 mb). In addition, we compared forecasts of precipitation, looking especially for differences in location and timing, and, of course, whether the forecasts were in fact better with the TAMDAR data.