Showing papers by "Albert A. M. Holtslag published in 2002"

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

Abstract: 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).

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 nonintermittent flow regimes for different circumstances. In the present paper, this system is studied analytically, following a system dynamics approach. The transition between the different flow regimes is identified as a Hopf bifurcation. This property is used to derive a dimensionless parameter, which is a function of external parameters, such as radiative forcing and pressure gradient. With this dimensionless parameter the equilibrium behavior of the system (i.e., intermittent or nonintermittent) can be predicted exactly. As such this parameter is used to classify SBL regimes. It is shown that the proposed classification parameter provides different information about the state of the SBL than other typical SBL classification parameters such as z/L and Ri.

Spatial Heterogeneity of the Soil Moisture Content and Its Impact on Surface Flux Densities and Near-Surface Meteorology

Abstract: Using a subgrid distribution for the soil moisture content derived from a macroscale hydrologic model, it is investigated how lateral subgrid variations in the soil moisture content impact both the daily and the seasonal cycle of the spatially averaged surface flux densities and near-surface meteorology. In agreement with earlier studies it is found that in wet conditions the use of one uniform volumetric soil moisture content, referred to as the bulk approach, gives larger estimates of the latent heat flux density than a quasi-distributed approach where the lateral variation in the volumetric soil moisture content is taken into account. In dry conditions the bulk approach gives lower estimates of the latent heat flux density than the quasi-distributed approach. In this study, the differences between the flux density estimates obtained by both approaches appear even when the developing convective boundary layer is allowed to feed back on the surface. It is also shown that differences in the estimated surface flux densities lead to differences between the predicted atmospheric specific humidity and the predicted near-surface temperature. The differences due to the subgrid variations in the soil moisture content appear to impact the seasonal hydrologic balance. Especially for dry climates, the quasi-distributed approach predicts a more gradual decrease of the evapotranspiration during the dry season, resulting in a larger cumulative evapotranspiration over the dry season. Thus, taking account of the spatial heterogeneity of the soil moisture content is a prerequisite for a proper representation of the seasonal hydrological cycle within largescale atmospheric models.

The surface energy balance over a desert, and the relevance of soil heat flux measurements

Abstract: This study analyses observations of the separate components of the surface energy balance, with emphasis on the Soil Heat Flux (SHF), in the Negev desert in Israel. Traditionally, the SFH measurements are performed using a thermopile sensor, with a known thermal heat conduction coefficient, diameter and thickness (Philip, 1961). These SHF plates are usually buried at a certain depth, to avoid interference with biological activity at the surface. These measurements then represent the heat flux at that depth. Corrections are needed to calculate a SHF at the surface. In this study, the SHF measurements are obtained using a new approach, whereby the SHF is measured directly at the soil surface. This new method is compared with traditional SHF measurements for validation. An accurate validation is possible, thanks to the soil being very homogeneous and the lack of vegetation cover. The very low vegetation density and the dryness of the desert, made it relatively simple to test the individual components of the surface energy balance. If the separate components of this balance are measured correctly, their sum should be zero. This check is called the SEBCT (Surface Energy Balance Closure Test). Since the SHF can be as high as the sensible heat flux, it is a very important factor in the SEBCT.

Spatial heterogeneity of the soil moisture content and its impact on the surface flux densities and the atmospheric boundary layer

Abstract: The main aim of this paper is to show that introduction of prescribed lateral variations in the soil moisture content might lead to better predictions of the surface flux densities and the near-surface temperature and specific humidity in largescale atmospheric models. The focus of our study will be on the seasonal cycle. However, we will also investigate the impact of the differences in surface energy flux densities for the temperature and specific humidity in the atmospheric boundary layer and at 2 m, the height at which most SYNOPS observations are taken.

Application of surface layer similarity theory to carbon dioxide, moisture and temperature

Abstract: J. Vila-Guerau de Arellano Meteorology and Air Quality Group, Wageningen University, Wageningen, The Netherlands O. Hartogensis Meteorology and Air Quality Group, Wageningen University, Wageningen, The Netherlands A.T. Vermeulen ECN, The Netherlands F. C. Bosveld and W. Kohsiek KNMI, The Netherlands A. A. M. Holtslag Meteorology and Air Quality Group, Wageningen University, Wageningen, The Netherlands

Comparison of observed and modeled surface fluxes of heat for the Volta river basin

Abstract: Land-surface processes and their modeling play an important role in planetary boundary modeling, due to their role of providing the surface boundary conditions to the atmosphere. In particular, processes regarding clouds and precipitation are strongly influenced by land-surface processes. To get a further understanding of those interactions we focus on the validation and calibration of a Soil-Vegetation-Atmosphere-Transfer (SVAT) model (The Oregon-State-University-Land-Surface Model) for the Volta River Basin in West Africa. This is a typical example of a semi-arid region in which studies on the interaction within soil, vegetation, atmosphere and water are still needed. For the validation of the model scintillometer measurements from three different sites in Ghana are used to calculate an areal average sensible heat flux density. The three sites show major differences concerning the vegetation, soils, land use, slopes and also climate. All data used in this study are part of long-term observations of the water- and energy balance in the Volta Basin. The calculated fluxes are compared with the output of the model. For the first step an off-line, uncoupled version of the SVAT model is used, driven by measured atmospheric forcings. These were obtained at a weather station on every research site. The second step is the adaptation of the SVAT model to the conditions in the Volta River Basin. The results from the adapted model are validated with the scintillometer measurements. Besides, a comparison with the results of the original model reveal potential shortcomings of the standard SVAT model used under semi-arid conditions

Modelling the atmospheric boundary layer in a climate model of intermediate complexity

Abstract: In this study, we describe and validate a boundary layer module that can be implemented in the climate model of intermediate complexity ECBilt (Opsteegh et al. 1998). It includes the growth of the convective boundary due to thermal-forced turbulence, the additional heating of the boundary layer because of entrainment, the collapse of the convective boundary layer during the afternoon transition and the development of a stable boundary layer in stable conditions.

Impact of soil moisture on boundary-layer cloud development

Abstract: We study the daytime land-atmosphere interaction using model with an atmospheric boundary-layer (ABL) scheme coupled with a land-surface (LS) scheme using observations taken on 31 May 1978 at Cabauw, Netherlands. In a previous study (Holtslag et al 1995) it was found that in coupled (LS-ABL) model simulations using a simple LS scheme did not accurately represent surface fluxes and coupled atmospheric boundary-layer development. Using a more sophisticated LS scheme in the study here allows the land-atmosphere system the freedom to respond interactively with the ABL where a many processes and important feedback mechanisms are represented (Figure 1). Results indicate that in coupled land-atmosphere simulations, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds.

Observations and model results for water vapor and carbon dioxide fluxes above a sparsely vegetated bog area

Abstract: In this study observations of water vapor and carbon dioxide fluxes above a sparsely vegetated bog area in the north of the Netherlands during the early growing season was analyzed. The observations are used to evaluate a combined assimilation photosynthesis (so-called A-gs) model on canopy scale and an open water model. An A-gs model couples the CO2 assimilation, A, to the stomatal conductance, gs and offers a physiological technique to simulate plant transpiration. For the patches open water of the bog area, a so-called open water evaporation model has been applied. Transpiration and open water evaporation has been mixed together by applying a weighing technique using the surface characteristics. The effects of nutrient shortage, pests, diseases and weed competition are not taken into account in most A-gs models. Nevertheless, the mechanism how it affects the assimilation process in plants is known and has been implemented in the present A-gs model. Evapotranspiration simulations resulted in an excellent agreement with the measurements over the bog area. To simulate the total net CO2 flux, a simple soil respiration model has been added to the A-gs model of the present study. The total simulated fluxes of plant assimilation and soil respiration have been compared with the measured total CO2 flux. The agreement between simulations and experiments appeared to be reasonable well