Ensemble Simulations of a Case from the 2017 NOAA Hazardous Weather Testbed using the Unified Model

Since 2013, the Met Office has been participating in the annual Spring Forecasting Experiment (SFE), conducted in the NOAA Hazardous Weather Testbed (HWT), routinely running a 2.2 km gridlength version of the UM (US2) that covers the 48 contiguous states of the US. This is a deterministic model, one-way nested within the global model and closely following the configuration of the operational UK model. This paper describes 2.2km ensemble simulations of one of the cases from the 2017 SFE. The ensemble has been generated by nesting the US2 within the Met Office global ensemble. Within the ensemble framework, several different science configurations have been trialled in the 2.2km model. The availability of reliable observations in this region combined with input from forecasters and scientists who attended the SFE makes this a useful testbed for process studies.

The ensembles provide a range of scenarios with differences in the initiation time of convection and the location of rainfall features. While the different science configurations have differences in the details of features (e.g. rainfall features in one configuration tend to be larger and more intense than those in another), the location appears to be controlled by the driving ensemble. This suggests that subtle differences in synoptic-scale features may have a substantial impact at the meso- and convective scales, which here manifests as positional and timing errors in the simulated convective storms.

All of the 2.2km simulations were found to have less CIN than the observed soundings. The CIN was found to be independent of the science configuration implying it is determined by the global model. The structure of the CIN layer is believed to play an important role in modulating the timing, location, and incidence of convective initiation. The member which produced the best simulation of the supercells which developed over Oklahoma on this day had more CIN than the other members which allowed convection to initiate later and CAPE to build up. However, comparisons with surface station data found that this member was too warm and too dry. This suggests there are compensating errors in the factors controlling initiation in these models which comparisons with observations as described here can help ellucidate.