Procedures that may be used to evaluate the operational performance of a wide spectrum of geophysical models are introduced. Primarily using a complementary set of difference measures, both model accuracy and precision can be meaningfully estimated, regardless of whether the model predictions are manifested as scalars, directions, or vectors. It is additionally suggested that the reliability of the accuracy and precision measures can be determined from bootstrap estimates of confidence and significance. Recommended procedures are illustrated with a comparative evaluation of two models that estimate wind velocity over the South Atlantic Bight.

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This scientific assessment examines changes in three climate extremes—extratropical storms, winds, and waves—with an emphasis on U.S. coastal regions during the cold season. There is moderate evidence of an increase in both extratropical storm frequency and intensity during the cold season in the Northern Hemisphere since 1950, with suggestive evidence of geographic shifts resulting in slight upward trends in offshore/coastal regions. There is also suggestive evidence of an increase in extreme winds (at least annually) over parts of the ocean since the early to mid-1980s, but the evidence over the U.S. land surface is inconclusive. Finally, there is moderate evidence of an increase in extreme waves in winter along the Pacific coast since the 1950s, but along other U.S. shorelines any tendencies are of modest magnitude compared with historical variability. The data for extratropical cyclones are considered to be of relatively high quality for trend detection, whereas the data for extreme winds and waves are judged to be of intermediate quality. In terms of physical causes leading to multidecadal changes, the level of understanding for both extratropical storms and extreme winds is considered to be relatively low, while that for extreme waves is judged to be intermediate. Since the ability to measure these changes with some confidence is relatively recent, understanding is expected to improve in the future for a variety of reasons, including increased periods of record and the development of “climate reanalysis” projects.

Means and variances of monthly mean wind speed, direction and velocity (the mean resultant vector) are derived for the period 1961-1990 at 216 stations in the coterminous United States. Direction and velocity means and variances are calculated using a complex-arithmetic extension of the equations for scalar mean and variance. Variance is derived from the 30-year time series of monthly means. While analyses of monthly mean wind fields are common, accompanying analyses of speed, direction, and velocity variance do not generally accompany them. Mean monthly wind direction and velocity fields show a typical seasonal progression from westerly and northwesterly winds in winter, to southerly winds in summer. Scalar and vector wind speeds are highest in winter and spring, and lowest in the summer. Seasonal variation in the mean fields is related to seasonal changes in mean sea level pressure, particularly east of the Rocky Mountains. In the western United States, mean winds often reflect channeling by local topography. The interannual variance of mean monthly wind speed, direction and velocity are related to seasonal variability in synoptic-scale features, such as the frequency of cyclones and anticyclones. Low variance occurs at a number of stations in the west, where topography restricts the range of wind variability. High velocity variance appears when both speed and direction variability are high, but it can occur also when speed variance is high and direction variance is low (or 6ice 6ersa). Low velocity variance can result from low speed and direction variance, or from low mean wind velocities. The mean and variance characteristics of surface winds provide additional information on the surface climatology of the coterminous United States, and serve as a useful adjunct to other extant land-surface climatologies.