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Zbigniew Sorbjan

Abstract

A simple, steady-state, numerical model is used to examine the Rossby-number similarity theory of the atmospheric boundary layer over a slightly inclined terrain. The model confirms the similarity predictions. The slope-influenced universal profiles of the wind velocity defects and the stress components are obtained by the model simulation.

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Zbigniew Sorbjan

Abstract

Effects caused by variation of the potential temperature lapse rate Γ in the free atmosphere are examined based on a “large eddy simulation” model of the shear-free convective atmospheric boundary layer. The obtained results show that only near the top of the boundary layer are the statistical moments involving temperature strongly sensitive to changes of the parameter Γ. Furthermore, the moments involving only the vertical velocity are practically independent of Γ. The ratio R of the heat fluxes at the top and the bottom of the mixed layer increases when Γ increases. For the values of Γ from 1 to 10 K/km, typically observed in the atmosphere, the heat flux ratio R varies in the range −0.2 to −0.3. When Γ increases by an order of magnitude to 100 K/km, R increases only slightly to about −0.4. When Γ decreases to zero, the heat flux Hi, at the top of the mixed layer also decreases to zero. In this case, the thermal structure of the atmospheric boundary layer is found to be similar to nonpenetrative “solid lid” convection in a tank.

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Zbigniew Sorbjan

Abstract

Monin-Obukhov similarity applied to the real part of the refractive index variance n2 is discussed. It is shown that, because the observed correlation coefficient rTQ between temperature and humidity is less than unity, scaling by n * = nw/u * (where u * is the friction velocity) makes the dimensionless variance n2/n*2 dependent on two dimensionless parameters, on dimensionless height z/L (where L is the Monin-Obukhov length), and on a parameter R, which is related to the Bowen ratio. As the Monin-Obukhov similarity framework is applied, a large scatter of the dimensionless refractive index variance is expected for certain values of the Bowen ratio. Such a scatter can be eliminated if a new scale N *, defined as a combination of temperature and humidity scales, is employed.

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Zbigniew Sorbjan

Abstract

A large eddy simulation model was used to generate and compare statistics of turbulence during nonpenetrative and penetrative dry convection. In penetrative convection dimensionless vertical velocities in updrafts were found to have almost the same values as in the nonpenetrative case. The countergradient transport of heat and moisture was found to be present during nonpenetrative convection at z/z i > 0.6. For penetrative convection the countergradient transport of heat occurred only in a layer 0.5 < z/z i < 0.75, while the countergradient transport of humidity was not present. During nonpenetrative convection, temperature and humidity were perfectly correlated. In penetrative convection the correlation coefficient was found to be less than unity, varying from about 0.9 near the surface to about −0.7 at the top of the mixed layer.

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Zbigniew Sorbjan

Abstract

The “flux based” local scaling in the stably stratified boundary layer is valid only in cases with strong, continuous turbulence, when the gradient Richardson number Ri is constant and subcritical. To extend the local similarity approach to cases with weak turbulence (very stable regime), the “gradient based” local scaling is introduced and discussed in the paper. Both types of local scaling, the flux based and the gradient based, are tested based on the data, collected from a 60-m tower during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). The obtained results show that the gradient-based scaling provides a useful framework for the treatment of cases with both strong and weak turbulence and overcritical Richardson numbers.

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Zbigniew Sorbjan

Abstract

The structure of the steady-state flow, homogeneous along an inclined, flat, underlying surface, is studied. On the basis or the atmospheric boundary layer equations the resistance laws of geostrophic drag and heat transfer are obtained. The general form of the resistance law universal functions is found. The slope influence is shown in the figures, which were obtained by numerical solution of the resistance laws.

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Zbigniew Sorbjan

Abstract

The paper investigates similarity of scalar fields in a horizontally homogeneous, cloud-free, shearless, convective mixed layer. The concept of the “bottom-up” and “top-down” decomposition is verified for both passive and active scalars, based on a number of large eddy simulations. The bottom-up diffusion is not confirmed to be countergradient. The top-down scaling, based on the values of entrainment fluxes, is found to be inefficient. Alternative sets of scales are proposed and validated for mean values of scalars. For the bottom-up process, the“free-convective” scaling is applied. For the top-down processes, new scales are based on the maximum values of scalar gradients in the interfacial layer. The bottom-up and top-down similarity functions for passive and active scalars are found equal.

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Zbigniew Sorbjan

Abstract

No abstract available.

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Zbigniew Sorbjan

Abstract

The similarity theory of an atmospheric boundary layer over a slightly inclined terrain, discussed in an earlier paper (Sorbjan, 1983) is extended to the case of geostrophic wind varying with height. The forms of resistance laws and universal functions are obtained in the cases when the Ekman height or the actual boundary layer height are used as the boundary layer height scales.

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Suzanne M. Zurn-Birkhimer, Ernest M. Agee, and Zbigniew Sorbjan

Abstract

The Lake-Induced Convection Experiment provided special field data during a westerly flow cold air outbreak (CAO) on 13 January 1998, which has afforded the opportunity to examine in detail an evolving convective boundary layer. Vertical cross sections prepared from these data, extending from upstream over Wisconsin out across Lake Michigan, show the modifying effects of land–water contrast on boundary layer mixing, entrainment, heating, and moisture flux. Through this analysis, an interesting case of lake-effect airmass modification was discovered. The data show atypical differing heights in vertical mixing of heat and moisture, as well as offshore downwelling and subsidence effects in the atmosphere. Analysis shows evidence of a new observational feature, the moisture internal boundary layer (MIBL) that accords well with the often recognized thermal internal boundary layer (TIBL). The “interfacial” layer over the lake is also found to be unusually thick and moist, due in part to the upstream conditions over Wisconsin as well as the effectiveness of vertical mixing of moist plumes over the lake (also seen in the aircraft datasets presented). Results show that the atmosphere can be much more effective in the vertical mixing of moisture than heat or momentum (which mixed the same), and thus represents a significant departure from the classical bottom-up and top-down mixing formulation.

Four scales of coherent structures (CSs) with differing spatial and temporal dimensions have been identified. The CSs grow in a building block fashion with buoyancy as the dominating physical mechanism for organizing the convection (even in the presence of substantial wind shear). Characteristic turbulence statistics from aircraft measurements show evidence of these multiple scales of CSs, ranging from the smallest (microscale) in the cloud-free path region near the Wisconsin shore, to the largest (mesoscale) in the snow-filled boundary layer near the Michigan shore.

A large eddy simulation (LES) model has also been employed to study the effects of buoyancy and shear on the convective structures in lake-effect boundary layers. The model simulation results have been divided into two parts: 1) the general relationship of surface heat flux versus wind shear, which shows the interplay and dominance of these two competing forcing mechanisms for establishing convection patterns and geometry (i.e., rolls versus cells), and 2) a case study simulation of convection analogous to the CSs seen in the CFP region for the 13 January 1998 CAO event. Model simulations also show, under proper conditions of surface heating and wind shear, the simultaneous occurrence of differing scales of CSs and at different heights, including both cells and rolls and their coexisting patterns (based on the interplay between the effects of buoyancy and shear).

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