Variability of Turbulence, 4-20 km, in Colorado and Alaska from MST (Mesosphere, Stratosphere, Troposphere) Radar Observations

摘要

The variability of small scale atmospheric turbulence on time scales from a few minutes to the annual scale and at altitudes from about 4 to 20 km is studied using clear air Doppler radar data from Poker Flat, Alaska, and Platteville, colorado. The variable used for this study is the refractivity turbulence structure constant (C sub n)squared. The frequency distribution of (C sub n)squared is found to be lognormal during all seasons at all altitudes. Using data at 4 min. intervals, it is found that the autocorrelation function of (C sub n)squared can be modeled as the sum of a first order autoregressive process and a random process. The associated integral time scale decreases with altitude from 25 to 45 min in the troposphere to about 18 min in the stratosphere. The power spectrum of log (C sub n)squared follows a power law relation with frequency; at periods greater than about 2 hours the spectral slope is near -5/3 and at periods less than 2 hours the slope is near -1. Monthly mean values of log (C sub n)squared are largest in the winter and show a secondary maximum in summer. The winter peak is apparently related to increased jet stream and baroclinic storm activity, and the summer peak is believed to be due to convective activity. The correlation of 3 hour values of (C sub n)squared with wind speed over monthlong periods ranges as high as 0.8 and has a median value near 0.3. During certain periods, (C sub n)squared also depends on other variables such as boundary layer inversions and gravity wave activity.