Eddy Dissipation Rates in Thunderstorms Estimated by Doppler Radar in Relation to Aircraft In Situ Measurements

Abstract

High-resolution aircraft turbulence measurements, well coordinated with radar Doppler spectral width measurements, have been used to verify radar-estimated energy dissipation rates within thunderstorms anvils. The radar-estimated eddy dissipation rates have been carefully interpolated in time and space to the aircraft position. From the aircraft measurements, turbulence was estimated using a three-component averaged structure function D(r). The sensitivity of this structure function to r, to wind components selected, and to the sampling rate was investigated. Power spectra for all three wind components have been derived for selected flight sections. Differences between these three spectra and deviations from the k?5/3 law indicate areas of energy input. At areas where all three spectra follow the k?5/3 law, indicating the inertial subrange of turbulence, both estimates of the energy dissipation rate by aircraft and radar agree rather well. On average an overestimation of the dissipation rate by radar in comparison to the aircraft measurements is found, especially for weaker storm cells and in the case of strong shear. The estimated energy dissipation spans a range from some 10?6 m2 s?3 to 5 ? 10?2 m2 s?3. The lower limit detectable with the C-band Doppler radar used, representing operational C-band Doppler weather radar systems, is about 10?3 m2 s?3. Closer agreement between radar and aircraft estimates has been found for the variances. Doppler spectrum width as measured by operational weather radars and indicative for both turbulence and shear will be of use for warning applications.