Simultaneous Radar and Instrumented Aircraft Observations in a Clear Air Turbulent Layer

Abstract

Some characteristics of a scattering, turbulent region near the 3-km level were measured with radar and an instrumented aircraft. It was found that the weakly scattering region was near a thin (20 m) layer of extreme stability (0.1C m−1) which seemed to cap a region of light to moderate turbulence. For the most turbulent run lasting 110 sec, an average eddy dissipation rate was found to be 16 cm2 sec−2. Smaller intervals (7.5 see) showed ε to be as large as 45 cm2 sec−2. An RHI photograph taken just before the measurements showed two wave-like layers similar to structures previously interpreted as Kelvin-Helmholtz waves. The wave-like structure disappeared within 15 min and only an extremely weak single layer remained after ∼30 min. In general, stronger turbulence was associated with larger temperature variations and thus with greater reflectivity. Radar and radiosonde data were used for the first time to calculate eddy dissipation rates. These agreed to within a factor of 4 with the more dependable estimates of dissipation derived from airborne hot wire anemometer velocity spectra. Provided the radar scattering occurs within a layer ∼35 m thick (as appears reasonable from the aircraft data) rather than from the entire radar sample volume, then the eddy dissipation rates calculated from the radar and radiosonde data become consistent with those derived from velocity spectra.