The National Centers for Environmental Prediction (NCEP) meteorological DowNscalinG system (DNG)

The NWS National Centers for Environmental Prediction (NCEP) North American Model (NAM) downscaling (DNG) system developed by NOAA/Global System Division and NCEP/ Environmental Modeling Center (EMC) downscales meteorological model fields to a National Digital Forecast Database (NDFD) high resolution grid at either 5 or 2.5 km over the Continental U.S., Alaska, Hawaii, Puerto Rico and recently Guam. One use of the downscaled fields is to provide first-guess background grid for NCEP's Real-Time Mesoscale Analysis (RTMA) and Updated Real-time Mesoscale Analysis (URMA) where fields are further adjusted to the nearby observations using the two dimensional version of the NCEP variational Global Statisical Interpolation (GSI) assimilation system. NAM parent (12 km) hourly predictions are downscaled out through 84 hours whereas NAM nests (6-, 4- and 3-km) are downscaled through 60 hours. 2 m temperature and dewpoint temperature, 10 m winds and surface pressure are adjusted to a high resolution topographical database. Terrain effects are also introduced for winds while temperatures are also downscaled to the high resolution topography using similarity theory. Such diagnostic downscaled meteorological analyses and predictions that account for complex terrain and coastal effects are often used to drive air pollution models. Coupling with the NCEP wave forecast model is also being evaluated. The DNG system also sharpens gradients of all fields around coastal areas using the land mask dataset. Basically, temps, winds, and dew point from the downscaled NAM land-water point are replaced by downscaled winds from the nearest neighbor point that agrees with the Analysis (NDFD) land-water point. Snow depth is then adjusted depending on the interpolated temperature. Chance of wetting rain, probability of precipitation from the NCEP Short Range Ensemble System (SREF) and mixed layer depth winds, temperature and humidity are also computed from the downscaled fields.

In the past year, the system has been unified form more direct application for a user defined grid domain as well as ensuring consistent downscaling approaches for all domains. Application of the DNG have been extended to downscale the NCEP Downscaled Global Extended (DGEX) to 192 forecast hours, the Global Forecast System (GFS) and the High Resolution Window (based on WRF and NEMS/NMMB models) models.

A more representative way to include terrain effects on winds could be accomplished for the case when the NAM first guess topography lie below the output analysis topography. A spatially constant γ_ terrain scaling term could be replaced by using high resolution analysis land-use roughness (e.g.: De Rooy and KOK, 2004, W&F) or similarity theory (Troen and Mahrt, 1986). Recently, a diagnostic mass consistent wind downscaling scheme was tested to include terrain effects on winds. Winds are adjusted by computing terrain gradients and then velocity potential using the Poisson equation (Sherman, 1977; Calmet, Scire, 1986; Ratto, 1996). In this study, the impact of these approaches for improving the downscaled winds will be presented and evaluated against observations.