National Weather Service Headquarters Washington DC
330 PM EDT Mon Apr 16 2012
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From: Tim McClung
Chief, Science Plans Branch
Office of Science and Technology
Subject: Amended: Changing the Rapid Update Cycle (RUC) to the
Rapid Refresh (RAP) Analysis and Forecast System:
Effective date set for May 1, 2012.
Amended to set the implementation date for Tuesday May 1, 2012.
Users are cautioned that if a Critical Weather Day (CWD) designation is in effect on May 1, this implementation will be delayed until the conclusion of CWD. You can monitor the CWD status at the following webpage:
Also amended to add a clarification about shelter specific humidity replacing shelter mixing ratio in the bgrb files.
On Tuesday, May 1, 2012, beginning with the 1200 Coordinated Universal Time (UTC) run, the National Centers for Environmental Prediction (NCEP) will replace the Rapid Update Cycle (RUC) with the Rapid Refresh (RAP) model. Changes include:
-Covering a much larger domain, compared to the RUC, including Alaska and the Caribbean basin and almost all of North America
-Introducing a new modeling framework
-Installing a major upgrade to the prediction model
-Modifying the data analysis and assimilation system
-Matching most existing RUC products and adding new ones to cover the expanded domain. A select number of obsolete RUC files will be removed.
-Introducing experimental North American Rapid Refresh Ensemble using Time Lagged (NARRE-TL) forecast.
-Modifying some product output and changing the names of all directories and output filenames from *ruc* or *ruc2a* to *rap*.
Details on the various changes are provided below, along with a notice about possible changes to product generation time. NWS has tried to capture all of the changes occurring with this implementation. In the event that something was overlooked, we will amend this TIN.
Like the RUC, the RAP will be run 24 times per day, once for each hour. Each run will be integrated to 18 hours, and output will be available for each forecast hour. Due to the larger domain, lateral boundary conditions for the RAP will be provided by the Global Forecast System (GFS) instead of the North American Mesoscale (NAM) model as is done for the RUC.
The model will be fully cycled with all fields, including snow cover, carried through to the next cycle. The model will continue to trim snow cover twice per day based on the NESDIS analysis. To prevent the model from drifting away from the synoptic truth, two 6-hour partial cycles will be initiated each day at 03z and 15z by bringing in a guess for atmospheric fields only from the GFS and then performing a series of analyses and 1-hour forecasts with the final 1-hour forecast used as the first guess for the 09 and 21z cycles.
Like the RUC, the RAP has a 13 km horizontal resolution and 50 vertical levels. A sigma vertical coordinate is used in the RAP, compared to the sigma-isentropic hybrid vertical coordinate in the RUC. The pressure top of the RAP is at 10 hPa, compared to 40-70 hPa pressure top on the highest isentropic surface in the RUC. The native horizontal grid for the RAP is a rotated latitude-longitude grid.
The Gridpoint Statistical Interpolation (GSI) analysis system has been adapted for the RAP. It maintains several important components of the RUC analysis:
-Cloud hydrometeor analysis
-Assimilation of radar reflectivity data
-Diabatic digital filter initialization
Additional observations are assimilated in the RAP that are not currently assimilated in the RUC, including:
-Satellite radiances, including AMSU-A and AMSU-B, similar to that done for the North American Mesoscale (NAM) model
-Aircraft moisture observations from UPS and Southwest Airlines
-915-MHz profiler wind observations
-GOES cloud pressure/temperature from NASA Langley over full RAP domain added to NESDIS cloud data already assimilated in RUC
The RAP model component is a configuration of the Weather Research and Forecasting (WRF) model, using the Advanced Research WRF (ARW) core. It keeps, however, mostly RUC-like physics including:
-A version of the Grell convective scheme
-Thompson cloud and precipitation microphysics
-Rapid Radiation Transfer Model (RRTM) longwave radiation
-Goddard shortwave radiation
-MYJ (Mellor-Yamada-Janjic) planetary boundary layer (PBL)/turbulent mixing
-RUC/Smirnova land-surface model.
Updated versions of the cloud microphysics, Grell convection, and RUC land-surface schemes are used in the RAP over older versions used in the RUC.
Output Product Changes
The RUC currently generates output on pressure levels (pgrb) and native levels (bgrb) at horizontal resolutions of 13, 20 and 40 km for every forecast hour (0-18). Smaller files with near-surface data (sgrb) are generated at a smaller number of forecast hours at the same resolutions, and an 80 km data set generated for a few forecast hours is also available. The RUC output is made available to users on the NWS ftp server, the NCEP server, and a subset of the output is available on NOAAPORT.
Message this morning from NCEP/NCO (Chris Caruso Magee):
"The Rapid Refresh was implemented this morning, effective with the 12Z cycle. The RUC has been turned off. Thanks to all of you who participated in the evaluation of this model!"