Scientific Theme: Advanced Modeling and Observing Systems

GSD01: Numerical Weather Prediction
Theme report: Advanced Modeling and Observing Systems
GOAL:
Design and evaluate new approaches for improving regional-scale numerical weather forecasts, including forecasts
of severe weather events.
MILESTONE GSD01.1:
Begin one-hour cycling of initial version of the North American Rapid Refresh using GSI and the
chosen configuration of the WRF model.
ACCOMPLISHMENTS FOR GSD01.1:
The current Rapid Update Cycle model and assimilation system, covering the coterminous United States and
adjacent areas of Canada, Mexico and the Pacific and Atlantic Oceans as well as the Gulf of Mexico and a portion of
the Caribbean Sea (see figure below), is scheduled to be replaced by the so-called ―Rapid Refresh‖ in 2009. The
purpose of the Rapid Refresh is the same as the RUC, to provide a ―situation awareness‖ framework for forecasters
by providing very frequently updated analyses and very short-range numerical model forecasts. The figure also
shows the domain of the Rapid Refresh, which is about 2.6 times larger than the RUC domain and covers nearly all
of North America, including mainland Alaska, as well as most of the Caribbean Sea.
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In addition to the larger domain, the hydrostatic
hybrid-isentropic coordinate RUC model will be
replaced by a version of the WRF model, and the
present RUC 3dVAR analysis scheme will be
replaced by the Gridpoint Statistical Interpolation
(GSI) 3dVAR analysis scheme, which is under
continued development by NCEP and NASA
Goddard.
Work toward a cycling prototype Rapid Refresh
(RR) was an important focus of activity during the
2006-2007 period, particularly after the completion
of the RR Core Test in August. Due to a severalmonth
delay in availability of ESRL‘s new
supercomputer, and the need for ongoing testing of
important upgrades to the Rapid Update Cycle,
researchers had to make some compromises to the
scope of the efforts toward the Rapid Refresh to
accommodate the available computing resources.
This was done initially by instituting twelve-hour
cycling of the WRF-ARW using GSI on the
CONUS RR Core-Test domain, slightly smaller
than the present operational RUC CONUS domain shown in the figure. However, the version of GSI used in this
cycling has been without RUC enhancements to GSI, which are considered desirable for effective hourly cycling,
including special treatment of surface observations to spread surface information vertically when the one-hour
forecast background has an identifiable surface-based mixed layer. In order to begin to get experience running on
the whole North American RR domain with GSI and WRF, a run covering this whole domain was set up with 13-km
grid spacing on ESRL‘s older computer, and cycling was once per twelve hours. This has been running for several
months now. Although results have been acceptable, they are far inferior to those that could be achieved using onehour
cycling. Once the new computer became available in early 2007, continued problems with its file system,
which were revealed by the GSI code when it attempted to write to multiple nodes in parallel, slowed progress
further while those problems were diagnosed and fixed. Although the pieces are now mostly in place for one-hour
cycling over the full RR domain, this cycling had not yet begun as of June 2007.
A further requirement for running a WRF-based Rapid-Refresh one-hour cycle is to have a capability for digital
filter initialization in the WRF model. This is necessary to reduce fast-mode noise in the one-hour WRF forecast
used as background by the GSI 3dVAR analysis. However, the developers of WRF did not explicitly provide for