B.J.H. van de Wiel

Stable Atmospheric Boundary Layers and Diurnal Cycles: Challenges for Weather and Climate Models

The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For thi...

Intermittent Turbulence in the Stable Boundary Layer over Land. Part III: A Classification for Observations during CASES-99

Abstract In this paper a classification of stable boundary layer regimes is presented based on observations of near-surface turbulence during the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99). It is found that the different nights can be divided into three subclasses: a turbulent regime, an intermittent regime, and a radiative regime, which confirms the findings of two companion papers that use a simplified theoretical model (it is noted that its simpliflied structure limits the model generality to near-surface flows). The papers predict the occurrence of stable boundary layer regimes in terms of external forcing parameters such as the (effective) pressure gradient and radiative forcing. The classification in the present work supports these predictions and shows that the predictions are robust in a qualitative sense. As such, it is, for example, shown that intermittent turbulence is most likely to occur in clear-sky conditions with a moderately weak effective pressure gradient. The quantit...

Modeling the evolution of the atmospheric boundary layer coupled to the land surface for three contrasting nights in CASES-99

Abstract 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 1999 Cooperative Atmosphere–Surface Exchange Study (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 in situ observations. Overall it is found that the vertical structure, the surface fluxes (apart from the intermittent character) and the surface tem...

Exploring self-correlation in flux–gradient relationships for stably stratified conditions

Abstract In this paper, the degree of scatter in flux–gradient relationships for stably stratified conditions is analyzed. It is generally found that scatter in the dimensionless lapse rate ϕh is larger than in the dimensionless shear ϕm when plotted versus the stability parameter z/Λ (where Λ is the local Obukhov length). Here, this phenomenon is explained to be a result of self-correlation due to the occurrence of the momentum and the heat flux on both axes, measurement uncertainties, and other possibly relevant physical processes left aside. It is shown that the ratio between relative errors in the turbulent fluxes influences the orientation of self-correlation in the flux–gradient relationships. In stable conditions, the scatter in ϕm is largely suppressed by self-correlation while for ϕh this is not the case (vice versa for unstable stratification). An alternative way of plotting is discussed for determining the slope of the linear ϕm function.

Intermittent Turbulence and Oscillations in the Stable Boundary Layer over Land. Part I: A Bulk Model

In the stable boundary layer (SBL) it is observed often that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alternations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. It is well known that intermittent turbulence in the SBL can be generated by various mechanisms. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. It is shown that the essence of this mechanism can be captured by a 1D bulk model of three coupled nonlinear differential equations. In the present paper, numerical runs with the model show that intermittent turbulence is most likely to occur over land surfaces with low vegetation during clear-sky conditions in the presence of a moderate to low synoptical pressure gradient. The existence of a vegetation layer has a strong influence on intermittency dynamics. Due to its small heat capacity, the vegetation temperature is able to quickly respond to rapidly changing conditions. This in turn affects the stability of the lower atmosphere, causing an important feedback mechanism. In addition, it was found that intermittent behavior of SBL models occurs for various first-order closure schemes with different stability functions. However, stability functions that allow turbulent transport beyond the critical Richardson number effectively suppress intermittent‐oscillatory behavior. Currently, the latter type of formulations is often used in numerical weather prediction to prevent excessive SBL cooling in very stable conditions. The advantage of using a simplified SBL model, as proposed in the present paper, is that it allows an analytical study of the system, which, in turn, allows theoretical predictions about the occurrence of intermittent SBL behavior (see the companion paper).

Displaced-beam small aperture scintillometer test. Part II: CASES-99 stable boundary layer experiment

The performance of the Scintec displaced-beam small aperture scintillometer (DBSAS) in the stable boundary layer (SBL) is investigated using data gathered during the CASES-99 experiment in Kansas, U.S.A. The DBSAS is superior to the eddy-covariance method in determining vertical fluxes of sensible heat and momentumclose to the ground and/or over short (< 1 min) averaging intervals. Both aspects are of importance in the shallow and non-stationary SBL.The friction velocity, u*, the temperature scale, θ*, and from these the sensible heat flux, H, were calculated from the indirectly determined dissipation rate, ∈, and the structure parameter of temperature, CT2, by the DBSAS, which was operated over a path length of 112 m. All these variables are compared with eddy-covariance data for 10-minute time averages. Previously reported systematic errors in the DBSAS, overestimation of u* for low u* values and underestimation of u* for high u* values, have in part been dealt with by adjusting the beam displacement distance from 2.7 mm to 2.6 mm in the calculations. The latter adjustment is presented as a working hypothesis, not a general solution.

Intermittent Turbulence and Oscillations in the Stable Boundary Layer over Land. Part II: A System Dynamics Approach

Abstract In the stable boundary layer it is often observed that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alterations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. This atmosphere–surface intermittency (ASI) is associated with the essential elements of the stable boundary layer (SBL): strong surface cooling, the supply of mechanical energy by the synoptic pressure gradient, and the limiting effect of stratification on mixing efficiency. In Part I it is shown that the essence of ASI can be captured by a system of three coupled nonlinear differential equations. This simplified system shows both intermittent and noninter...

Exploring the possible role of small-scale terrain drag on stable boundary layers over land

This paper addresses the possible role of unresolved terrain drag, relative to the turbulent drag on the development of the stable atmospheric boundary layer over land. Adding a first-order estimate for terrain drag to the turbulent drag appears to provide drag that is similar to the enhanced turbulent drag obtained with the so-called long-tail mixing functions. These functions are currently used in many operational models for weather and climate, although they lack a clear physical basis. Consequently, a simple and practical quasi-empirical parameterization of terrain drag divergence for use in large-scale models is proposed and is tested in a column mode. As an outcome, the cross-isobaric mass flow (a measure for cyclone filling) with the new scheme, using realistic turbulent drag, appears to be equal to what is found with the unphysical long-tail scheme. At the same time, the new scheme produces a much more realistic less-deep boundary layer than is obtained by using the long-tail mixing function.

Diagnostic equations for the stable boundary layer height : Evaluation and dimensional analysis

Abstract The performance of diagnostic equations for the stable boundary layer height h is evaluated with four observational datasets that represent a broad range of latitudes, land use, and surface roughness. In addition, large-eddy simulation results are used. Special care is given to data-quality selection. The diagnostic equations evaluated are so-called multilimit equations as derived by Zilitinkevich and coworkers in a number of papers. It appears that these equations show a serious negative bias, especially for h < 100 m, and it was found that the parameters involved could not be determined uniquely with calibration. As an alternative, dimensional analysis is used here to derive a formulation for h that is more robust. The formulation depends on the surface friction velocity u*, surface buoyancy flux Bs, Coriolis parameter, and the free-flow stability N. The relevance of the Coriolis parameter for the boundary layer height estimation in practice is also discussed. If the Coriolis parameter is ignor...

Approximate solutions for the Obukhov length and the surface fluxes in terms of bulk Richardson numbers.

Simple analytic approximate solutions arepresented for the set of equations that follows fromthe Monin–Obukhov flux-profile relationships using thestability functions of Dyer (unstable case) andBeljaars–Holtslag (stable case). Several publicationsare devoted to the same subject, however the currentapproach contains some new features, namely: (a) itappears to be more accurate for unstable situationsand (b) it applies also to the general case where windspeed (u) and potential temperature(θ) are given at different levels. In order toillustrate the accuracy of the approach a comparisonwith the actual solutions is presented for someselected combinations of θ and u levelstypical for various practical applications.