Main Results

Thus, in Section 3.2, we presented the data of long-term experimental and theoretical studies of properties of atmospheric turbulence in the anisotropic boundary layer. These data should be taken into account for the correct prediction of characteristics of optical radiation in the atmosphere. The main results can be formulated as follows:

  • 1. It was found that the Monin—Obukhov theory of similarity of turbulent flows can be extended to an arbitrary anisotropic boundary layer. With the use of the semiempirical hypotheses of the theory of turbulence, it was shown theoretically and experimentally that the arbitrary anisotropic boundary layer can be considered as locally weakly anisotropic. In this layer, the weakly anisotropic similarity theory is locally true (in some vicinity of every point in the layer).
  • 2. At the known turbulent scales of velocity and temperature, the variable Monin—Obukhov number (ratio of the height to the Monin—Obukhov scale), different at every point of the layer, has been shown to be the main parameter of turbulence in the anisotropic boundary layer.
  • 3. It was demonstrated that the semiempirical theory of turbulence for the isotropic layer is a particular case of the semiempirical theory for the anisotropic layer. In this particular case, both anisotropy functions are simultaneously equal to zero. In the isotropic boundary layer, the absolute values of the Monin—Obukhov number usually do not exceed few units.
  • 4. In the anisotropic boundary layer, there is a range of variation of the Monin— Obukhov number, in which the conditions of the isotropic layer take place (both anisotropy functions in this range are close to zero simultaneously).
  • 5. It was found that at the known characteristic scales of temperature and velocity average for the observation region, the anisotropic boundary layer can be replaced with the isotropic one. This provides the possibility of using the optical models of turbulence developed for the isotropic boundary layer.
  • 6. Theoretical equations have been derived for the outer scale of turbulence in the anisotropic boundary layer; they have been confirmed experimentally. The relations have been found between the outer scales determined by five different methods; they connect the Tatarskii outer scale of turbulent mixing and parameters of different models of the turbulence spectrum.
  • 7. In the atmospheric boundary layer, the turbulent Prandtl number has been shown to be close to unity in a rather wide range of the Monin—Obukhov number (neutral and weakly unstable stratification). The decrease of the inverse turbulent Prandtl number at the strong instability was noted.
  • 8. New results have been presented for the turbulent scales of velocity field and temperature measured at different times under different climatic conditions, and in different regions. The data confirm the conclusion on the local applicability of the Monin—Obukhov similarity theory in the anisotropic atmospheric boundary layer and extend it to extreme temperature stratifications. In our experiments, we observed the Monin—Obukhov numbers in the range from +197 (strong stability) to -102,819 (ultrastrong instability). The large (and extremely large), in the absolute value, Monin—Obukhov numbers are characteristic only of the semiempirical theory in the anisotropic boundary layer. The data of all measurement sessions are in a good agreement with each other as functions of the Monin—Obukhov number, and they can be joined regardless of the type of regional meteorological situations. Therefore, the study of functional dependences of the turbulent scales of velocity and temperature on the Monin—Obukhov number can be considered as one of the top priority problems of the semiempirical theory of turbulence.
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