Enhanced frequency spectra of winds at the mesoscale based on radar profiler observations

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Abstract
Frequency spectra of horizontal winds based on the radar profilers at Poker Flat, Alaska, and Platteville, Colorado, are studied over the range of periods from about 10 min to a day. At Platteville in the troposphere the spectrum of both zonal and meridional winds is found to obey a power law dependence on frequency at periods longer than a few hours. At shorter periods the spectra show a transition to a second power law regime which has enhanced energy relative to a simple extrapolation of the curve which fits the low‐frequency regime. The magnitude of the enhancement is about a factor of 4. The slope of the spectrum in both regions appears to be near −5/3. In the transition region, between periods of about 1/2 and 2 1/2 hours, the spectrum appears to follow a slope near −1. At Poker Flat, where useful results extend to about 20 km, a similar pattern is found in the stratosphere, above 10 km, except that the magnitude of the enhancement factor is about 1.7. This two‐regime model is found to be robust, applying when the results are sorted by season, background wind speed, altitude, and total variance under the spectrum. The results are discussed in terms of a spectrum of vertically propagating gravity waves launched by flow over rough terrain, and the implications for a spectrum due to stratified turbulence are considered.