Improved Estimates Of Turbulence From Mesosphere-Stratosphere-Troposphere (MST) Radars

St. Cloud State University, Saint Cloud MN

Investigators

Abstract

Doppler spectra measured by MST (mesosphere-stratosphere-troposphere) radars are broadened by turbulent velocity fluctuations of the scattering elements in the beam. Measuring the spectral spread is therefore one of the ways of using MST radar to estimate the intensity of turbulence. Because the beams of such radars are fairly broad (typically several degrees) there are additional contributions to the Doppler spread caused by the cross-beam wind (so-called beam broadening) and by shear of the radial wind component in the direction of the beam axis. Recent studies indicate that the methods currently used to correct for these effects are inaccurate. Such methods are usually based on the assumption that the contributing volume is uniformly filled with turbulence. In fact, the turbulence is often confined to one or more horizontal layers that are thinner than the vertical extent of the contributing volume. Corrections based on the assumption of a filled beam are then generally too large. The present award supports development of a new experimental technique that will enable improved radar estimates of turbulence. Called the dual-beamwidth technique, it has the advantage of obtaining the turbulence intensity directly, without the need for the usual corrections to the Doppler spectral width. The assumption underlying the method is that the (generally unknown) contribution of beam broadening to the Doppler spread is proportional to the beamwidth. Simultaneous measurements at the same range with two beamwidths, which is possible with advanced MST radars such as those in Japan and India, then make it possible to measure the contribution of beam-broadening to the total Doppler spread and to compensate for it in estimating the remaining spread due to turbulence. The turbulence intensities determined this way will be compared with those estimated from a single beam employing the concept of an "effective beamwidth," which is less than the geometric beamwidth to allow for the limited vertical extent of the turbulent layers. If the method employing a single effective beamwidth is proven accurate, it will then be applied to recompute turbulence intensities for radar data from Japan and White Sands, New Mexico, that have already been reported in the literature.