CAPS Realtime Storm Scale Ensemble and High Resolution Forecasts for the NOAA Hazardous Weather Testbed 2010 Spring Experiment

In the spring of 2010, the Center for the Analysis and Prediction of Storms (CAPS) continued its participation in the NOAA Hazardous Weather Testbed (HWT) Spring Experiment by providing storm-scale ensemble and high-resolution deterministic forecasts for evaluation by HWT, and for supporting the NSF-sponsored VORTEX-2 field experiment. In addition to severe storms, aviation weather and quantitative precipitation forecasting were also given significant attention in 2010. Compared to 2009, the forecast domain was enlarged to encompass the full continental US (CONUS) and the realtime forecast period was shifted towards late June for a period of May 1 to June 18th, to cover both the severe storm season of the central Great Plains and aviation interests in early summer.

The 4-km ensemble continued to use three dynamic cores, that is, the WRF-ARW, WRF-NMM and ARPS, but the composition of the cores was adjusted to have 19 ARW, 5 NMM and 2 ARPS members for a total of 26. Six of the ARW members had differences only in the physics parameterization schemes, and were designed to facilitate the evaluation of several advanced microphysics and PBL schemes newly available in WRF-ARW. Three ARW members contained initial condition perturbations of different scales to allow for the study of multi-scale error growth and predictability. For the perturbed boundary members, forecasts from consistent NCEP SREF members were used to provide the lateral boundary conditions at a higher frequency (hourly) than before. The upgraded ensemble-transform-initialized SREF provided initial condition perturbations for most SSEF members.

As in 2008 and 2009, level-2 radial velocity and reflectivity data from over 120 operational WSR-88D radars were analyzed using the ARPS 3DVAR together with its cloud analysis package. The result serves as the control initial condition for the three models. Thirty-hour forecasts were produced once per day, initialized at 00 UTC.

Parallel to the 4-km ensemble, a 1-km high-resolution deterministic forecast was produced once a day, for the same CONUS domain. Radar and other high-resolution (e.g., Oklahoma Mesonet) data were also analyzed, on the native 1-km grid. A National TeraGrid supercomputer operated by NICS of University of Tennessee, a Cray XT-4 supercomputer with 18,000+ CPU cores, was used in a dedicate mode for about 5 hours a day for producing these forecasts.

This paper will report on the experimental design and discuss logistic issues. It will present examples of ensemble and deterministic forecasts, and preliminary verifications against radar and other observations. Particular emphasis will be given to the value of radar data assimilation and the effect of resolution on short- and longer-range forecasts. The relative performance of different physics packages will also be examined. A companion paper (Kong et al. 2010) will focus on the realtime production and the evaluation of the ensemble forecast products.