Evaluating Model Physics in the Unified Forecast System (UFS)
Medium-Range Weather Application
Abstract
To support the development of the Global Forecast System (GFS) physics
suite and identify opportunities for improving the model physics in the
UFS, the Developmental Testbed Center (DTC) conducted an array of
analyses for evaluating the operational GFSv15 forecasts and the
experimental forecasts using the GFSv16beta physics suite distributed
with the UFS Medium-Range Weather Application v1.0 public release.
Five-day GFSv16beta forecasts for one boreal winter season were
generated using the operational GFS analyses as initial conditions. The
evaluation metrics included tools from Model Evaluation Tools (MET) and
in-house process-oriented diagnostics. The evaluations focused on the
perpetuating GFS forecast errors pertaining to the planetary boundary
layer (PBL), land-surface, cumulus, radiation, and cloud processes. The
runs using GFSv16beta outperformed the operational GFSv15 with respect
to the root-mean-square errors of large-scale environmental variables
and the anomaly correlation coefficient for 500 hPa geopotential height.
Nevertheless, larger biases associated with key physical processes were
identified in the GFSv16beta forecasts. For example, the global
precipitation forecast skill degrades and a dry bias remains in the
tropics, suggesting a persistent problem in the cumulus scheme. The
near-surface and boundary-layer cold biases are larger over most
continents and polar regions, which is partly related to the systematic
negative temperature errors in the GFS analysis. The overestimated
near-surface wind speed particularly at night in the northeastern U.S.
implies that the surface drag may be underrepresented. Excessive
short-wave radiation reaching the ground in the high-latitudes of the
summer hemisphere appears to be related to low cloud liquid and ice
water path. These and other results will be described in this
presentation.