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- Author or Editor: David P. Dempsey x
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Abstract
The fundamental assumptions underlying mesoscale mixed-layer models of the atmospheric boundary layer are that 1) the flow is hydrostatic, and 2) the Reynolds-averaged potential temperature and horizontal velocity are vertically well mixed. These assumptions determine completely the Reynolds-stress profile, which to a good approximation is a quadratic function of height, and has curvature proportional to the horizontal buoyancy gradient. The only significant source of the vertical component of vorticity in mesoscale mixed-layer models is the curl of the divergence of the Reynolds stress, which can generate quasi-stationary vortices downstream of three-dimensional topography in flow containing buoyancy gradients. We provide quantitative guidance about conditions sufficient for these vortices to form under the mixed-layer modeling assumptions.
We caution that observations do not appear to support strongly the assumption that velocity is vertically well mixed in baroclinic, convective boundary layers. We also caution that, while sufficient conditions exist under the mixed-layer modeling assumptions for quasi-stationary vortices to form, these conditions are not necessary. Sufficient conditions for such vortices to form also exist under other, completely independent modeling assumptions. Hence, the mechanism by which vorticity is generated in mixed-layer models has uncertain relevance to the atmosphere.
Abstract
The fundamental assumptions underlying mesoscale mixed-layer models of the atmospheric boundary layer are that 1) the flow is hydrostatic, and 2) the Reynolds-averaged potential temperature and horizontal velocity are vertically well mixed. These assumptions determine completely the Reynolds-stress profile, which to a good approximation is a quadratic function of height, and has curvature proportional to the horizontal buoyancy gradient. The only significant source of the vertical component of vorticity in mesoscale mixed-layer models is the curl of the divergence of the Reynolds stress, which can generate quasi-stationary vortices downstream of three-dimensional topography in flow containing buoyancy gradients. We provide quantitative guidance about conditions sufficient for these vortices to form under the mixed-layer modeling assumptions.
We caution that observations do not appear to support strongly the assumption that velocity is vertically well mixed in baroclinic, convective boundary layers. We also caution that, while sufficient conditions exist under the mixed-layer modeling assumptions for quasi-stationary vortices to form, these conditions are not necessary. Sufficient conditions for such vortices to form also exist under other, completely independent modeling assumptions. Hence, the mechanism by which vorticity is generated in mixed-layer models has uncertain relevance to the atmosphere.
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Abstract
This paper describes a one-level, sigma-coordinate, mesoscale model suitable for diagnosing surface winds in mountainous and coastal regions. The model requires only modest computer resources and needs little data for initialization. Energy and momentum conservation equations are integrated under steady, specified synoptic-scale height and temperature fields to a steady state to diagnose surface wind and temperature fields forced by complex terrain. If diabatic forcing is desired, the model uses the steady state results as an initial state from which the model is integrated, with varying diabatic forcing, to the verification time. The model has no mass budget, but under the hydrostatic assumption the mass field (and therefore the surface pressure field) is determined by the vertical temperature structure, which in the model is parameterized in terms of surface temperature.
Four model runs and corresponding observed wind fields are presented. They suggest that the model can diagnose many details of mesoscale flow in complex terrain for a variety of flow directions and diabatic forcings. It is suggested that adiabatic warming and cooling play a crucial role in producing topographic deflection and channeling. Recommendations of possible improvements to the model are given.
Abstract
This paper describes a one-level, sigma-coordinate, mesoscale model suitable for diagnosing surface winds in mountainous and coastal regions. The model requires only modest computer resources and needs little data for initialization. Energy and momentum conservation equations are integrated under steady, specified synoptic-scale height and temperature fields to a steady state to diagnose surface wind and temperature fields forced by complex terrain. If diabatic forcing is desired, the model uses the steady state results as an initial state from which the model is integrated, with varying diabatic forcing, to the verification time. The model has no mass budget, but under the hydrostatic assumption the mass field (and therefore the surface pressure field) is determined by the vertical temperature structure, which in the model is parameterized in terms of surface temperature.
Four model runs and corresponding observed wind fields are presented. They suggest that the model can diagnose many details of mesoscale flow in complex terrain for a variety of flow directions and diabatic forcings. It is suggested that adiabatic warming and cooling play a crucial role in producing topographic deflection and channeling. Recommendations of possible improvements to the model are given.
Abstract
A common technique for estimating the sea surface generation functions of spray and aerosols is the so-called flux–profile method, where fixed-height concentration measurements are used to infer fluxes at the surface by assuming a form of the concentration profile. At its simplest, this method assumes a balance between spray emission and deposition, and under these conditions the concentration profile follows a power-law shape. It is the purpose of this work to evaluate the influence of waves on this power-law theory, as well as investigate its applicability over a range of droplet sizes. Large-eddy simulations combined with Lagrangian droplet tracking are used to resolve the turbulent transport of spray droplets over moving, monochromatic waves at the lower surface. The wave age and the droplet diameter are varied, and it is found that droplets are highly influenced both by their inertia (i.e., their inability to travel exactly with fluid streamlines) and the wave-induced turbulence. Deviations of the vertical concentration profiles from the power-law theory are found at all wave ages and for large droplets. The dynamics of droplets within the wave boundary layer alter their net vertical fluxes, and as a result, estimates of surface emission based on the flux–profile method can yield significant errors. In practice, the resulting implication is that the flux–profile method may unsuitable for large droplets, and the combined effect of inertia and wave-induced turbulence is responsible for the continued spread in their surface source estimates.
Abstract
A common technique for estimating the sea surface generation functions of spray and aerosols is the so-called flux–profile method, where fixed-height concentration measurements are used to infer fluxes at the surface by assuming a form of the concentration profile. At its simplest, this method assumes a balance between spray emission and deposition, and under these conditions the concentration profile follows a power-law shape. It is the purpose of this work to evaluate the influence of waves on this power-law theory, as well as investigate its applicability over a range of droplet sizes. Large-eddy simulations combined with Lagrangian droplet tracking are used to resolve the turbulent transport of spray droplets over moving, monochromatic waves at the lower surface. The wave age and the droplet diameter are varied, and it is found that droplets are highly influenced both by their inertia (i.e., their inability to travel exactly with fluid streamlines) and the wave-induced turbulence. Deviations of the vertical concentration profiles from the power-law theory are found at all wave ages and for large droplets. The dynamics of droplets within the wave boundary layer alter their net vertical fluxes, and as a result, estimates of surface emission based on the flux–profile method can yield significant errors. In practice, the resulting implication is that the flux–profile method may unsuitable for large droplets, and the combined effect of inertia and wave-induced turbulence is responsible for the continued spread in their surface source estimates.
