Three-Dimensional Buoyancy- and Shear-Induced Local Structure of the Atmospheric Boundary Layer

Khanna, Samir; Brasseur, James G.

doi:10.1175/1520-0469(1998)055<0710:tdbasi>2.0.co;2

Cited by 215 publications

(251 citation statements)

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“…The peak in the wave number -weighted alongwind spectrum is at lower wave number than the peak in the crosswind spectrum. This is consistent with observations and LES model results [e.g., Khanna and Brasseur, 1998] that show energy-containing scales elongated in the direction of near-surface shear in comparison to the cross-shear direction. It is also consistent with Langmuir circulation cells with their axes approximately aligned with the wind [e.g., Weller et al, 1985;Weller and Price, 1988].…”

Section: à2supporting

confidence: 92%

Scaled temperature spectrum in the unstable oceanic surface layer

2004

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[1] A nondimensional form of the temperature spectrum in a convective near-surface layer was derived empirically as a function of stability parameter x = z/L and surface wave parameter g = u * /(gz) 1/2 , under the assumption of horizontal isotropy, where z is the depth of the measurement, L is the Monin-Obukhov length scale, u * is the surface friction velocity, and g is the acceleration due to gravity. The wave number-weighted, onedimensional spectrum had +1 slope at low wave numbers and À2/3 slope at high wave numbers (characteristic of an inertial subrange). Spectral levels in the À2/3 range varied with g, and the spectrum width was a function of x. The wavelength of the spectral peak decreased as Àx (>0) increased. The variation of spectral level in the inertial subrange suggested that dissipation due to wave breaking was enhanced by a factor of 1.7 at 2-m depth for a wind speed of 10 m s À1 . Root-mean-square temperature fluctuations at 2-m depth versus Àx were in excellent agreement with atmospheric surface layer observations and agreed moderately well (within 30%) with the results of large-eddy simulation experiments. Root-mean-square fluctuations were proportional to (Àx) À1/3 for Àx > 0.4, consistent with the predictions of similarity theory. The skewness of temperature fluctuations varied with Àx, qualitatively similar to variation in the atmospheric surface layer. The skewness of the horizontal gradient of temperature at 2-m depth varied relative to the wind direction and was well described by a cosine function with amplitude 0.6.

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Section: à2supporting

confidence: 92%

Scaled temperature spectrum in the unstable oceanic surface layer

2004

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“…Several previous studies (e.g., Moeng and Sullivan 1994;Khanna and Brasseur 1998) have shown that the flow regime of the boundary layer in general, and the formation of roll vortices in particular, may be characterized by the stability parameter −z i /L, where z i is the boundary-layer depth and L is the Obukhov length, …”

Section: Extreme Imbalancementioning

confidence: 99%

“…Moeng and Sullivan (1994) and Khanna and Brasseur (1998) estimate roll vortex formation is characterized by smaller values for −z i /L (roughly 1.5 < −z i /L < 10), these studies were limited to the clear boundary layer. Given that nearly every day is cloudy in the YOGA dataset, and that z i is defined as the height of the steepest gradient in the buoyancy profile (Schalkwijk et al 2015), which typically occurs above the cloud layer, some discrepancy may be attributed to cloudiness.…”

Section: Extreme Imbalancementioning

confidence: 99%

An Investigation of the Eddy-Covariance Flux Imbalance in a Year-Long Large-Eddy Simulation of the Weather at Cabauw

Schalkwijk

Jonker

Siebesma

2016

Boundary-Layer Meteorol

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The low-frequency contribution to the systematic and random sampling errors in single-tower eddy-covariance flux measurements is investigated using large-eddy simulation (LES). We use a continuous LES integration that covers a full year of realistic weather conditions over Cabauw, the Netherlands, and emulate eddy-covariance measurements. We focus on the daytime flux imbalance, when the turbulent flux is sufficiently resolved. Averaged over the year, daytime single-tower eddy-covariance flux measurements lead to a significant systematic underestimation of the turbulent flux. This underestimation depends on the averaging period and measurement height. For a 3600-s averaging period at 16-m height, the systematic underestimation reduces to a few percent, but for 900-s averaged tall-tower measurements at 100-m height, the fluxes are systematically underestimated by over 20 %. The year-long dataset facilitates an investigation into the environmental conditions that influence the eddy-covariance flux imbalance. The imbalance problem is found to vary widely from day to day, strongly dependent on the flow regime. In general, the imbalance problem reduces with increased mean wind speed, but days having the largest imbalance (over twice the average) are characterized by roll vortices that occur for average wind speeds, typically having a boundary-layer height (z i ) to Obukhov length (L) ratio of 10 < −z i /L < 100.

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“…To analyze the influence of wind-shear characteristics on the evolution of the CBL, long simulations and large domains are necessary to fulfill a quasy-stationarity flow pattern that matches with the prescribed surface fluxes, and to resolve the expected pattern for forced convection (Khanna and Brasseur, 1998). With DALES, resolutions up to 25 m and 6 m in the horizontal and vertical directions, respectively were considered.…”

Section: Sheared Convective Boundary Layermentioning

confidence: 99%

Formulation of the Dutch Atmospheric Large-Eddy Simulation (DALES) and overview of its applications

et al. 2010

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Abstract. The current version of the Dutch AtmosphericLarge-Eddy Simulation (DALES) is presented. DALES is a large-eddy simulation code designed for studies of the physics of the atmospheric boundary layer, including convective and stable boundary layers as well as cloudy boundary layers. In addition, DALES can be used for studies of more specific cases, such as flow over sloping or heterogeneous terrain, and dispersion of inert and chemically active species. This paper contains an extensive description of the physical and numerical formulation of the code, and gives an overview of its applications and accomplishments in recent years.

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Three-Dimensional Buoyancy- and Shear-Induced Local Structure of the Atmospheric Boundary Layer

Cited by 215 publications

References 0 publications

Scaled temperature spectrum in the unstable oceanic surface layer

Scaled temperature spectrum in the unstable oceanic surface layer

An Investigation of the Eddy-Covariance Flux Imbalance in a Year-Long Large-Eddy Simulation of the Weather at Cabauw

Formulation of the Dutch Atmospheric Large-Eddy Simulation (DALES) and overview of its applications

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