Estimation of the turbulent fraction in the free atmosphere from MST radar measurements
Résumé
Small-scale turbulence in the free atmosphere is known to be intermittent in space and time. The turbulence fraction of the atmosphere is a key parameter in order to evaluate the transport properties of small-scale motions and to interpret clear-air radar measurements as well. Mesosphere-stratosphere-troposphere (MST)/stratosphere-troposphere (ST) radars provide two independent methods for the estimation of energetic parameters of turbulence. First, the Doppler spectral width 2 is related to the dissipation rate of kinetic energy ⑀ k. Second, the radar reflectivity, or C 2 n , relates to the dissipation rate of available potential energy ⑀ p. However, these two measures yield estimates that differ with respect to an important point. The Doppler width measurements, and related ⑀ k , are reflectivityweighted averages. On the other hand, the reflectivity estimate is a volume-averaged quantity. The values of ⑀ p depend on both the turbulence intensity and the turbulent fraction within the radar sampling volume. Now, the two dissipation rates ⑀ p and ⑀ k are related quantities as shown by various measurements within stratified fluids (atmosphere, ocean, lakes, or laboratory). Therefore, by assuming a "canonical" value for the ratio of dissipation rates, an indirect method is proposed to infer the turbulent fraction from simultaneous radar measurements of reflectivity and Doppler broadening within a sampling volume. This method is checked by using very high resolution radar measurements (30 m and 51 s), obtained by the PROUST radar during a field campaign. The method is found to provide an unbiased estimation of the turbulent fraction, within a factor of 2 or less.
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%5B15200426%20-%20Journal%20of%20Atmospheric%20and%20Oceanic%20Technology%5D%20Estimation%20of%20the%20Turbulent%20Fraction%20in%20the%20Free%20Atmosphere%20from%20MST%20Radar%20Measurements.pdf (1.05 Mo)
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