| New global orography data sets - :11418 | National Weather Service (NWS)
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New global orography data sets
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    New orography data sets are constructed based on a United States Geological Survey (USGS) global digital elevation model (DEM) with a horizontal grid spacing of 30 arc seconds (approximately 1 km). Orography statistics including average height, mountain variance, maximum orography, land-sea-lake masks are directly derived from a 30-arc second DEM for a given resolution. Computed global orography data sets are available for three resolutions of 8, 4, and 2 minutes which correspond to approximately 16, 8, 4 km, respectively. The average height in a 10-min resolution is compared against the NAVY 10 min data that has been used at NCEP. Overall, the new data is more realistic than the NAVY data. The biggest difference between the two data appears in the antarctic region having a dipole patterns of maximum differences as high as 1 km. Near the coastlines along the antarctic and Greenland also are shown differences of 1-km which is mostly due to land-sea mask difference between the two data sets. Over mountainous regions, NAVY data show higher peaks than the new data in some places, but lose mountain peaks in other places, which is due to inconsistent treatment of orography in the NAVY data. This new orography will provide more realistic topographic forcing for NCEP forecast models and may provide improved orography-induced precipitation. In addition, orography statistics based on the new data set are expected to provide realistic treatment of gravity wave drag, for example, in the NCEP Medium-Range Forecast (MRF) model.

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