Monday, 13 May 2002
Homogeneous blended wind data over the contiguous United States
We have compiled and quality controlled an hourly wind data set from 1655 stations over the lower 48 states for the period of record (up to year 2000), collected and digitized the station metadata related to the history of the anemometer elevation at most of these stations, and blended the station records with historical snow on the ground information. All this has allowed us to generate homogeneous time series of wind speed at the 10-meter height above the ground/surface for the entire period of digital record (usually, since 1948 for the First Order Stations, since early 1970s for other sites with complete metadata, and since the ASOS implementation at the sites without metadata). Archive TD-6421 "Enhanced hourly wind station data for the contiguous United States" is now available from the National Climatic Data Center.
Anemometer elevations throughout the U.S. stations varied widely with time (on average there was one change per decade at any station with more than 10 years of record). Therefore, the elevation homogenization of the near-surface wind time series is a necessary pre-requisite for any climatological assessments. In this process we were using the
U10g=Ua log[(10-Hsnod)/z0]/log[(Ha -Hsnod)/z0],
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 meters above the ground, and U10s is the speed at 10 meters above the surface.
A group of derivative climatologies and time series for the same stations has been generated using this archive as a source. Among them are:
· mean daily and daytime wind speed time series;
· wind roses; monthly/seasonal wind climatology for major wind directions with preserved diurnal cycle; frequency of calm winds; frequency of strong winds above several thresholds;
· climatology and time series of wind speed during various precipitation events;
· normalized wind speed distribution for each station and state for the assessment of their changes after the Automated Surface Observation System (ASOS) introduction;
· climatology and time series of "wilting" winds frequency (i.e., the frequency of those winds that occur with surface air temperature above 10°C and relative humidity below 30%), of wind chill and of the heat index.
The comparison of observations with the 10 m wind diagnostic output from ETA regional model has been performed for the period since April 1995. It shows a graduate increase in correlations between observed and modeled wind speeds, which corresponds to increases in the model spatial resolution and improvements in physics, with a very good correlation since the 1998 for the eastern two-thirds of the contiguous U.S.