Summary

Description

U.S. Enhanced Hourly Wind Station Data is digital data set DSI-6421, archived at the National Centers for Environmental Information (NCEI; formerly National Climatic Data Center, NCDC). During earlier work at NCDC, it was noted that anemometer elevations at U.S. weather stations (for which metadata related to anemometer height was available) varied widely with time. Between 1931 and 2000, there were up to 12 significant anemometer height changes at some of these stations, and on average there was one change per decade at any station with more than 10 years of record. For example, at Los Angeles International Airport, the anemometer height changed 4 times during the 60 years, varying from 59 ft to 20 ft, while at Edwards Air Force Base, the anemometer height was changed 10 times and varied from 13 ft to 75 ft. Therefore, the elevation homogenization of the near-surface wind time series is a necessary pre-requisite for any climatological assessments. This was done at NCDC, creating the DSI-6421 data set. Stations were included in DSI-6421 on a year-by-year basis, depending upon the availability of anemometer metadata and the number of observations made during a year. The earliest data was from 1931, with very few stations. The number of stations increased during World War II to about 200, decreased briefly after the war, and increased to about 350 during the period 1948-1972 because most first-order (primary) stations qualified for inclusion. After 1972, as the importance of metadata was more widely recognized, the number of qualified stations rose to near 1000 by 1985, and continued at about that number through year 2000. The formulae used were U10g = Ua log[(10-Hsnod)/z0]/log[(Ha - Hsnod)/z0], and U10s = Ua log[10/z0]/log[(Ha - Hsnod)/z0], where z0 is the surface roughness (a function of the presence of snow cover at the site); Hsnod is the snow depth; Ha is the anemometer height above the ground; Ua is the wind speed at the anemometer height; U10g is the speed at 10 m above the ground; and U10s is the speed at 10 m above the surface.