Measurement of Clear-Air Gradients and Turbulence Properties with Radar Wind Profilers

Abstract

An experiment comparing balloon and profiler observations was carried out to evaluate the capability of Doppler radar wind profilers to remotely measure useful meteorological quantities other than wind. The site chosen was in Southern California during a time of year when it offers a natural laboratory for investigating extreme contrasts in temperature and humidity. To evaluate the new capabilities, it was found that new and additional treatment of the radar data was necessary. For example, the adequacy of the usual radar wind observations obtained by editing the Doppler spectral moments was found to be very questionable for short-term observations, so the authors extended the editing to the raw spectra, and substantial improvement was found. The advantages of the redundancy in five-beam systems are investigated and are also found to be very necessary to obtain the accuracy needed. A technique for minimizing the variances of the differences of the four redundant pairs of radials is described. The resulting improved vertical velocity estimates substantially improve the agreement between radio acoustic sounding system (RASS) temperature retrievals and balloon-measured temperatures. The ability of the profilers to measure turbulence intensity was tested, and the accuracy of techniques using the spectral width to measure turbulent dissipation rate when complicated spectra are present is examined. Two different techniques for optimizing the calculation of spectral width are compared and the errors assessed. One technique integrates over the uncontaminated range of the chosen spectral peak and then extrapolates a Gaussian function to infinity. The other method uses the slope of the log least squares best fit of the uncontaminated points to a Gaussian function. Profiler-measured length scales of wind and scalar quantities are measured and compared. Profiles of radar-measured gradients of refractive index are compared with gradients measured by balloon. It is shown how gradients of humidity can be calculated to about the same accuracy as refractive-index gradients by combining the temperature gradients from RASS with the refractive-index gradient observations from the radar.