Estimation of the Turbulent Fraction in the Free Atmosphere from MST Radar Measurements

Abstract

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 σ2 is related to the dissipation rate of kinetic energy ?k. Second, the radar reflectivity, or C2n, 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 reflectivity-weighted 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.