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  • Author or Editor: S. P. S. Arya x
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S. P. S. Arya

Abstract

The free convection similarity theory is examined in the light of recent observations in the atmosphere, convection chambers and wind tunnels. The theory describes the fluctuations of temperature and vertical velocity fairly well, but only in flows with finite shear. The horizontal components of velocity and mean temperature may not be scaled by the same in the range of stability ordinarily encountered in the surface layer of the atmosphere. Free convection similarity scaling of the outer layer is expected to be more successful, although sufficient atmospheric data are not available to test this assertion. Preliminary results of numerical calculations by Deardorff are very encouraging, and so are our limited observations in a wind tunnel boundary layer. Because of extremely variable conditions in the atmosphere under free convection, some aspects of this flow may be better studied in the laboratory under carefully controlled conditions. Both convection chamber (no shear) and wind tunnel (finite shear) flows have been used for this purpose; the latter is shown to give better similarity with the atmospheric boundary layer.

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S. P. S. Arya

Abstract

The effects of baroclinity and the scale-height ratio on the drag laws of the planetary boundary layer (PBL) are examined theoretically and compared to those of stability. The similarity drag relations using surface geostrophic winds are found to be more sensitive to these parameters than the drag relations based on the layer-averaged winds. Since baroclinity can be more safely ignored in the latter, these are considered more suitable for parameterizing the PBL in general circulation models. The geostrophic drag relations based on the generalized similarity theory are used to explain (simulate) the observed increasing trend of the surface cross-isobar angle in going toward the equator. It is shown that this trend is partly due to the change in the scale-height ratio and partly due to baroclinity. Clarke and Mess (1975), on the other hand, have suggested that baroclinity is wholly responsible for this trend. It is shown here that baroclinity effects are very much exaggerated in their formulation.

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S. P. S. Arya and E. J. Plaie

Abstract

The similarity which exists between the stably stratified atmospheric boundary layer and a wind tunnelboundary layer developing over a cold plate is illustrated. It is shown that mean flow and turbulence characteristics in the near wall region of the stratified boundary layer are well described by Monin and Obukhovssimilarity theory, and that this theory provides a good basis for modeling in the laboratory of similar characteristics of the atmospheric surface layer. Various forms of stability parameters are shown to be universallyrelated.

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S. P. S. Arya and J. C. Wyngaard

Abstract

By using a simple physical model of the baroclinic convective planetary boundary layer, the similarity functions of the geostrophic drag law are expressed as sums of a barotropic part, dependent only on the stability and boundary layer height parameters, and a baroclinicity dependent part. The latter are predicted to he sinusoidal functions of the angle between surface wind and geostrophic shear, their amplitudes being proportional to the normalized magnitude of geostrophic shear. These drag laws are confirmed by the results of a more sophisticated higher-order closure model, which also predict the magnitude of actual wind shears in the bulk of the mixed layer remaining much smaller than the magnitude of imposed geostrophic shear. The results are shown to be supported by some observations from the recent Wangara and ATFX experiments. The surface cross-isobar angle is predicted to increase toward the equator, a trend well confirmed by observations, but in obvious conflict with the drag laws proposed by others who have ignored the height of the lowest inversion base from their similarity considerations.

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J. E. Stout, Y-L. Lin, and S. P. S. Arya

Abstract

Trajectories of 500- and 1000-μm diameter particles are calculated as they fall through the spatially varying flow field above sinusoidal terrain for various combinations of atmospheric stability, wind speed, and terrain wavelength. In each case, a set of 20 uniformly spaced particles are released simultaneously above sinusoidal topography and their trajectories are obtained numerically by coupling a linear wave solution for flow over sinusoidal topography with equations for particle motion. The flow field and the associated patterns of deposition are shown to be strongly influenced by atmospheric stratification. For strong stratification, the presence of vertically propagating waves produces relatively concentrated “particle streams.” For less stratified conditions with evanescent waves, little focusing of particle trajectories is apparent. The ability of the atmosphere to focus or concentrate falling particles may ultimately produce regions along the surface with enhanced deposition.

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J. C. Wyngaard, S. P. S. Arya, and O. R. Coté

Abstract

It is shown that although Coriolis forces cause large production rates of stress in a convective planetary boundary layer, there is a control mechanism, involving mean wind shear which prevents stress levels from becoming large. Higher-order-closure model calculations are presented which show that the stress profiles are essentially linear, regardless of wind direction, providing the geostrophic wind shear vanishes and the wind speed jump across the capping inversion is negligible. It is shown that it will he very difficult to verify these predicted stress profiles experimentally because of averaging time problems. A simple two-layer model is developed which leads to geostrophic drag and heat transfer expressions in fairly good agreement with Wangara data.

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J. E. Stout, S. P. Arya, and E. L. Genikhovich

Abstract

The effects of nonlinear drag on the motion and settling velocity of heavy particles in a turbulent atmosphere are investigated. The authors approach the problem rather systematically by first considering the response of particles to much simpler fluid motions that are subprocesses of the more complex turbulent field. The authors first consider the motion and time response of particles falling under gravity in still fluid. Then the effects of a sudden gust or step change in relative velocity between a falling particle and its surrounding fluid are investigated. The authors demonstrate that horizontal relative motion produced by a sudden gust tends to reduce the settling velocity of a panicle. In simple oscillating fluids it is shown that the reduction of settling velocity increases with increasing amplitude of fluid oscillation. The authors also explore the effects of oscillation frequency on the settling velocity and show that if the period of fluid oscillation is less than the particle response time, then the settling velocity reduction becomes independent of oscillation frequency. Finally, the authors explore the motion of heavy particles within simulated isotropic turbulence and show that the effect of nonlinear drag is to produce a slowing of particle settling velocity.

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