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1

Structure of numerically simulated katabatic and anabatic flows along steep slopes

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Fedorovich E. , Shapiro A.

Acta Geophysica

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2009

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tom Vol. 57, no. 4

981-1010

EN

Direct numerical simulation (DNS) is applied to investigate properties of katabatic and anabatic flows along thermally perturbed (in terms of surface buoyancy flux) sloping surfaces in the absence of rotation. Numerical experiments are conducted for homogeneous surface forcings over infinite planar slopes. The simulated flows are the turbulent analogs of the Prandtl (1942) one-dimensional laminar slope flow. The simulated flows achieve quasi-steady periodic regimes at large times, with turbulent fluctuations being modified by persistent low-frequency oscillatory motions with frequency equal to the product of the ambient buoyancy frequency and the sine of the slope angle. These oscillatory wave-type motions result from interactions between turbulence and ambient stable stratification despite the temporal constancy of the surface buoyant forcing. The structure of the mean-flow fields and turbulence statistics in simulated slope flows is analyzed. An integral dynamic similarity constraint for steady slope/wall flows forced by surface buoyancy flux is derived and quantitatively verified against the DNS data.

2

Nocturnal low-level jet over a shallow slope

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Shapiro A. , Fedorovich E.

Acta Geophysica

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2009

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tom Vol. 57, no. 4

950-980

EN

A simple theory is presented for a nocturnal low-level jet (LLJ) over a planar slope. The theory extends the classical inviscid inertialoscillation model of LLJs to include up and downslope motion in the boundary layer within a stably stratified environment. The particular scenario considered is typical of LLJs over the Great Plains of the United States: southerly geostrophic wind over terrain that gently slopes down toward the east. First, an initial value problem for the coupled equations of motion and thermodynamic energy is solved for air parcels suddenly freed of a frictional constraint near sunset. The solution is an oscillation that takes, on the hodograph plane, the form of an ellipse having an eastward-oriented major axis and an eccentricity that increases with increasing stratification and slope angle. Next, the notion of a tilted residual layer (TRL) is introduced and used to relate initial (sunset) air parcel buoyancy to free-atmosphere stratification and thermal structure of the boundary layer. Application of the TRL-estimated initial buoyancy in the solution of the initial value problem leads to expressions for peak jet strength and the slope angle that maximizes the jet strength. Analytical results are in reasonable qualitative agreement with observational data.

3

Preface to the special issue: Boundary layer flows along sloping surfaces

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Shapiro A. , Fedorovich E.

Acta Geophysica

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2009

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tom Vol. 57, no. 4

801-802

EN

This Special Issue of Acta Geophysica on boundary layer flows along sloping surfaces is concerned with the theory and modeling of a range of slopeflow phenomena in the lower atmosphere, including mountain and valley winds, low-level jets, and internal gravity waves. When a sloping surface is heated or cooled, a temperature difference is set up between the air in the surface layer and the environmental air at the same altitude. The buoyancy field associated with this temperature difference projects in the along-slope direction and induces an along-slope flow. In areas where basins are largely sheltered from synoptic effects, slope flows are the building blocks of local weather. Even in cases where synoptic forcing is important, pronounced slope-flow signals may still be apparent. In regions where heavily industrialized population centers extend across variable topography, these local flows exert major controls over energy usage, visibility, fog formation, and air pollutant dispersion. In agricultural regions, these local winds significantly affect microclimates. The slope flows also need to be taken into account in aerial spraying and fire-fighting operations. On the larger scale, persistent slope winds cover vast areas of the earth (e.g., Greenland, Antarctica), and play an important role in the weather and climate of these areas. Slope flows can be described in their most elemental forms as turbulent natural convection flows along cooled/heated sloping surfaces in a stratified environment. Katabatic flows (resulting from surface cooling) are quite shallow (typical jet maximum occurs in the height range between 1 to 100 m above ground level), and are therefore poorly resolved in most air pollution models, numerical weather prediction models and global climate models. Although much progress has been made in the conceptual understanding and quantitative description of slope flows, long-standing difficulties with the numerical description of turbulent flows in general and stably-stratified turbulent flows in particular (in the case of katabatic flows), and the myriad variety of flow interactions caused by complex topography and surface inhomogeneity, for example, from irregular snow/ice/soil cover, cloud cover, topographic shading, soil moisture, and land use, make the dynamics of slope flows a rich and challenging area of study.

4

Direct evaluation of refractive-index structure functions from large-eddy simulation output for atmospheric convective boundary layers

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Wilson C. , Fedorovich E.

Acta Geophysica

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2012

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tom Vol. 60, no. 5

1474-1492

EN

Fluctuations of the refractive index associated with atmospheric turbulence affect the propagation of electromagnetic and acoustic waves in the atmosphere. In the reported study, the effects of turbulence on wave propagation in the atmospheric convective boundary layer (CBL) are considered in terms of the second-order refractive index structure function and related to its structure-function parameter C2n . Two structure-function evaluation methods are compared. The direct evaluation method involves calculating the refractive index at each point in the simulation domain with subsequent calculation of the structure function. The second method is based on a parameterized linear relationship between the refractive-index structure function and temperature/humidity structure functions. For each evaluation method, vertical profiles of C2n computed for separations along the three coordinate directions collapse together over a significant portion of the CBL. Near-surface divergence of C2n values along the horizontal directions was noted and attributed to the influence of surface wind shear on the turbulent fluctuations of temperature and humidity. The behavior of C2n near the surface was shown to agree favorably with similarity-theory predictions.

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Katabatic flow induced by a cross-slope band of surface cooling

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Burkholder B. A. , Shapiro A. , Fedorovich E.

Acta Geophysica

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2009

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tom Vol. 57, no. 4

923-949

EN

This paper investigates the behavior of katabatic flow induced by an idealized, thermally inhomogeneous surface; a strip of surface cooling that has a finite width in the along-slope direction and is infinitely long in the cross-slope direction. Numerical simulations using the Boussinesq equations of motion and the thermodynamic energy equation are performed for various slope angles and strip lengths. The underlying dynamical processes in the katabatic jet and the near environment are explored by considering the along-slope momentum balance after a steady state has been achieved. The inhomogeneous nature of the surface forcing also induces a response in the environment that extends very far away from the sloped surface. Nearly horizontal jets close to the vertical heights of both sides of the cold strip are observed in the environment. A horizontal vorticity analysis is performed on these horizontal jets to ascertain their dynamical structure.