Vol 55, No 4 (2019)

Data of a numerical simulation of a stably stratified turbulent Couette flow are analyzed for various values of the Richardson number. Two different methods are used: direct numerical simulation (DNS) and large-eddy simulation (LES). It is shown that the flow contains large organized structures, along with chaotic turbulence, regardless of the simulation method. These structures appear as inclined layers in the temperature field with weakly stable stratification, separated by very thin layers with large temperature gradients. The existence of such layered structures in nature is indirectly confirmed by the analysis of data from field measurements on the meteorological mast, where temperature gradient histograms are found to be far from the normal distribution and similar to temperature gradient probability distributions obtained in numerical model data. The simulations indicate an increase in the turbulent Prandtl number with an increase in the gradient Richardson number. It is likely that the identified structures serve as efficient barriers for vertical turbulent heat flux without blocking the momentum transfer. We propose a hypothesis that it is these structures which serve as a physical mechanism for maintaining turbulence under supercritically stable stratification.

The 100 000-year periodicity of climate changes during the Late Pleistocene (in the last 800 ka) may be related to the respective oscillations in both insolation and submarine volcanic activity, with the latter being affected by gravity forces in the solar system. This is concluded on the basis of wavelet analysis of long-term data series on oscillations of the Earth’s orbital eccentricity, variations in various paleoclimatic characteristics, their known spectral estimates, and data on submarine volcanic activity.

In using the nonlinear analytical model of the flow over the mountains, orographic disturbances and model adequacy are studied. Theoretically calculated trajectories of motion and disturbances of temperature and humidity are compared to stereo-photogrammetric measurements of wave clouds. It is shown that the model successfully describes the spatial structure and amplitudes of disturbances in the troposphere beyond the turbulent surface air. It is established that, on days of cloud observations, turbulent processes in the surface air do not strongly affect wavy processes at heights over 2.5 km.

Bioparticles constitute a significant fraction of atmospheric aerosol. Their size range varies from nanometers (macromolecules) to hundreds of micrometers (plant pollen and vegetation residues). Like other atmospheric aerosol particles, the degree of involvement of bioaerosols in atmospheric processes largely depends on their hygroscopic and condensation properties. This paper studies the ability of subpollen particles of pine, birch, and rape to serve as cloud condensation nuclei. Secondary particles are obtained by the aqueous extraction of biological material from pollen grains and the subsequent solidification of atomized liquid droplets. The parameters of cloud activation are determined in the size range of 20–270 nm and water-vapor supersaturations of 0.1–1.1%. Measurement data were used to determine the hygroscopicity parameter that characterizes the effect of the chemical composition of subparticles on their condensation properties. The hygroscopic parameter varies in the range from 0.12 to 0.13. In general, the results of measurements have shown that the condensation activity of subpollen particles is comparable with the condensation activity of secondary organic aerosols and depends weakly on the type of primary pollen.

The influence of nonlinear interaction of oppositely directed nonlinear waves in a shallow basin is studied theoretically and numerically within the nonlinear theory of shallow water. It is shown that this interaction leads to a change in the phase of propagation of the main wave, which is forced to propagate along the flow induced by the oncoming wave. The estimates of the undisturbed wave height at the time of interaction agree with the theoretical predictions. The phase shift during the interaction of undisturbed waves is sufficiently small, but becomes noticeable in the case of the propagation of breaking waves.