Turbulent diffusivity in the free atmosphere inferred from MST radar measurements: a review

Abstract

Abstract. The actual impact on vertical transport of smallscale turbulence in the free atmosphere is still a debated issue. Numerous estimates of an eddy diffusivity exist, clearly showing a lack of consensus. MST radars were, and continue to be, very useful for studying atmospheric turbulence, as radar measurements allow one to estimate the dissipation rates of energy (kinetic and potential) associated with turbulent events. The two commonly used methods for estimating the dissipation rates, from the backscattered power and from the Doppler width, are discussed. The inference methods of a local diffusivity (local meaning here "within" the turbulent patch) by using the dissipation rates are reviewed, with some of the uncertainty causes being stressed. Climatological results of turbulence diffusivity inferred from radar measurements are reviewed and compared.As revealed by high resolution MST radar measurements, atmospheric turbulence is intermittent in space and time. Recent theoretical works suggest that the effective diffusivity of such a patchy turbulence is related to statistical parameters describing the morphology of turbulent events: filling factor, lifetime and height of the patches. It thus appears that a statistical description of the turbulent patches' characteristics is required in order to evaluate and parameterize the actual impact of small-scale turbulence on transport of energy and materials. Clearly, MST radars could be an essential tool in that matter.

The Lagrangian chemistry and transport model ATLAS: validation of advective transport and mixing

“…There is considerable uncertainty in the values of the vertical diffusion coefficient up to the present day (e.g. Wilson, 2004;Legras et al, 2005), and our parameterization, which is based on some rather old, but easily available data, fits well into the published range of values. Note also that the climatology was derived from tracer data by neglecting advective transport, which may introduce uncertainty.…”

“…by breaking gravity waves), which mixes the structures down to the molecular scale (e.g. McKenna et al, 2002;Legras et al, 2005;Balluch and Haynes, 1997;Wilson, 2004). This turbulence is intermittent in space and time due to the stable nature of the stratosphere.…”

Can one detect small-scale turbulence from standard meteorological radiosondes?

“…The selection of artificial inversions as overturns, may result in a dramatic overestimation of their frequency of occurrence, thus leading to overestimates of both the turbulent fraction of the atmosphere and the turbulence energy. The space-time inhomogeneity of turbulence can give rise to diffusion coefficient estimates ranging over several orders of magnitude Wilson (2004). It is therefore crucial to apply a quantitative procedure allowing to discriminate overturns from noise-induced inversions.…”

MST radar observations of turbulent altocumulus layers

“…Radiosonde ascent rate is approximately 5 m s − 1 and time resolution 2 s giving height resolution approximately 10 m. The MST radar dataset was searched visually for turbulent layers shown by increased vertical-beam spectral width corrected for beam broadening (Hocking, 1985;Wilson, 2004), in conditions of small vertical shear of horizontal wind and no breaking mountain waves. These atypical turbulent layers are unexplained by the mechanisms studied by Pepler et al (1998), Worthington and Thomas (1996), Worthington (1998), Pavelin et al (2001), Reid and Vaughan (2004), Hooper et al (2005), and Choi et al (2006) using the same radar.…”