As part of the hourly post-processing in the NCEP-operational 13-km RUC, a downscaling technique was developed to produce 5-km gridded fields from the full-resolution native (hybrid sigma-isentropic) RUC coordinate data to calculate values consistent with the higher-resolution 5-km RTMA terrain elevation field. The RUC-RTMA downscaling technique includes both horizontal and vertical components. The vertical component uses near-surface stability from the RUC native data to adjust to the RTMA 5-km terrain. In the horizontal, for example, coastline definition is enhanced as part of this RUC-RTMA downscaling using a 5-km roughness length field used to distinguish land-water boundaries on the 5-km RTMA grid. The fields downscaled to the 5-km grid include o 2-m temperature o 2-m dewpoint o surface pressure o 10-m wind components o 2-m specific humidity o gust wind speed o cloud base height (ceiling) o visibility
Ceiling and visibility (not yet required for RTMA) are defined with some accuracy due to RUC hourly assimilation of METAR cloud and visibility observations.
RTMA downscaling for temperature uses virtual potential temperature, the related prognostic/analysis variable in the RUC model/assimilation system, an advantage for interpolation in irregular terrain in mixed layer conditions. Different techniques were developed for these different variables, including special approaches for vertical extrapolation vs. interpolation dependent on whether RTMA terrain elevation is higher or lower than RUC terrain. The accuracy of the RTMA fields is dependent on this RUC-RTMA downscaling, and therefore, of considerable interest to NWS RTMA users.
Further improvements were implemented operationally into the RUC post-processing in December 2006. The latest RUC-RTMA downscaling techniques will be presented, including unique treatments for vertical extrapolation vs. interpolation and coastline sharpening.