Influence of Geostrophic Wind on Atmospheric Nocturnal Cooling

Moene

Jonker

2012

The mechanism behind the collapse of turbulence in the evening as a precursor to the onset of the very stable boundary layer is investigated. To this end a cooled, pressure-driven flow is investigated by means of a local similarity model. Simulations reveal a temporary collapse of turbulence whenever the surface heat extraction, expressed in its nondimensional form h/L, exceeds a critical value. As any temporary reduction of turbulent friction is followed by flow acceleration, the long-term state is unconditionally turbulent. In contrast, the temporary cessation of turbulence, which may actually last for several hours in the nocturnal boundary layer, can be understood from the fact that the time scale for boundary layer diffusion is much smaller than the time scale for flow acceleration. This limits the available momentum that can be used for downward heat transport. In case the surface heat extraction exceeds the so-called maximum sustainable heat flux (MSHF), the near-surface inversion rapidly increases. Finally, turbulent activity is largely suppressed by the intense density stratification that supports the emergence of a different, calmer boundary layer regime.

supporting

confidence: 57%

The Cessation of Continuous Turbulence as Precursor of the Very Stable Nocturnal Boundary Layer

Moene

Jonker

2012

“…Qualitatively similar behavior has been reported in stratified wind-tunnel experiments by Ohya et al (1997), who found dramatic changes in boundary layer characteristics in response to decreasing mechanical forcing. Likewise, regime transitions have been modeled with simple 1D models (Estournel and Guedalia 1985;Revelle 1993;McNider et al 1995;Derbyshire 1999a; Van de Wiel et al 2002a;Walters et al 2007;Acevedo et al 2012) in large-eddy simulation (LES) studies (Jimé nez and Cuxart 2005;Zhou and Chow 2011) and in studies using direct numerical simulations (Nieuwstadt 2005;Flores and Riley 2011).…”

Section: Introductionmentioning

confidence: 99%

The Minimum Wind Speed for Sustainable Turbulence in the Nocturnal Boundary Layer

Moene

Jonker

et al. 2012

132

120

The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5-7 m s 21 , depending on radiative forcing and surface properties, and is in agreement with observations at Cabauw. The critical value appears not very sensitive to model details or to the exact values of the input parameters. Finally, results are interpreted in terms of external forcings, such as geostrophic wind. As it is generally larger than the speed at crossing height, a 5 m s 21 geostrophic wind may be considered as the typical limit below which sustainable, continuous turbulence under clear-sky conditions is unlikely to exist. Below this threshold emergence of the very stable nocturnal boundary layer is anticipated.

“…Estournel and Guedalia, 1985) and typically surface temperatures were prescribed. Since the atmosphere-land surface coupling is essential for SBL evolution, we will emphasize this topic in this study.…”

Section: Introductionmentioning

confidence: 99%

Modeling the Evolution of the Atmospheric Boundary Layer Coupled to the Land Surface for Three Contrasting Nights in CASES-99

Steeneveld

Holtslag

2006

113

124

The modeling and prediction of the stable boundary layer over land is a persistent problematic feature in weather, climate and air quality topics. Here, the performance of a state-of-the-art single column boundary-layer model is evaluated with observations from the CASES-99 field experiment. Very high model resolution in the atmosphere and the soil is utilized to represent three different stable boundary-layer archetypes, namely: a fully turbulent night, an intermittently turbulent night and a radiative night with hardly any turbulence (all at clear skies). Each archetype represents a different class of atmospheric stability. In the current model, the atmosphere is fully coupled to a vegetation layer and the underlying soil. In addition, stability functions (local scaling) are utilized based on local observations. Overall it is found that the vertical structure, the surface fluxes (apart from the intermittent character) and the surface temperature in the stable boundary layer can be satisfactorily modeled for a broad stability range (at a local scale) with our current understanding of the physics of the stable boundary layer. This can also be achieved by the use of a rather detailed coupling between the atmosphere and the underlying soil and vegetation, together with high resolution in both the atmosphere and the soil. This is especially true for the very stable nights, where longwave radiative cooling is dominant. Both model outcome and observations show that in the latter case the soil heat flux is a dominant term of the surface energy budget.3