Intercomparison of single-column models for GABLS3: preliminary results

The GEWEX Atmospheric Boundary-Layer Study (GABLS) focus on the representation of stable boundary layers in atmospheric models (Holtslag, 2006). One of the main goals of GABLS is to provide a mondial platform for the atmospheric boundary layer research community through the organisation of model intercomparisons. Here we focus on single column models (SCM's), which can be both research models and SCM's derived from operational weather and climate models. Two SCM intercomparison case studies have been performed so far. One highly idealised case over snow with prescribed surface temperature (Cuxart et al., 2006) and a second case based on observations taken during the CASES 99 stable boundary layer experiment also with prescribed surface temperature (Svensson and Holtslag, 2007). In these studies it was found that especially the complexity of real world boundary conditions and the lack of interaction with the surface makes it difficult to confront the models with observed evaluation parameters. A reasonable ideal case was found in the long observational dataset of the meteorological site Cabauw in the Netherlands (Baas et al., 2008). To make comparison with observations possible care was taken to prescribe realistic advective tendency terms to the SCM's (Bosveld et al., 2008). These were estimated from both local observations and hind casts of several 3D NWP models. The specific characteristics of the Cabauw site with its flat topography (van Ulden and Wieringa, 1995; Beljaars and Bosveld, 1997) makes it well suited to study decoupling around sunset, inertial oscillation and low level jet and the morning time transition to convective conditions (Angevine et al. 2002). Preliminary results will be presented of an intercomparison between SCM's and an evaluation of the models with observations from the Cabauw site. Special attention in this study is on the moment of decoupling around sunset, the inertial oscillation and the morning time transition

Exchange processes between a coniferous forest and the atmosphere

This thesis deals with the research question: which processes are relevant in controlling the exchange fluxes between the forest and the atmosphere and how can this control be quantified? Answering this question is relevant for research in the fields of air pollution, weather and climate and remote sensing. To answer this question a measurement program has been performed over and in a dense Douglas fir forest (Speulderbos), near the village of Garderen, the Netherlands. Variables were monitored related to, the state of the atmosphere, the state of soil water and the state of the trees. Forest response was determined by measuring outgoing radiation fluxes and the surface fluxes of momentum, sensible and latent heat. Well-known concepts from micro-meteorology and hydrology were used for the interpretation of the measurement. At times, these concepts had to be adapted for the specific forest situation. One concept is surface layer similarity theory, which enables to categorise observations in a convenient way. A second concept, related to surface layer similarity, is on resistances relating spatial differences in variables to their corresponding fluxes. A third concept is the Penman-Monteith equation, which enables the discrimination between atmospheric control and plant control on transpiration.Turbulent exchange within the roughness sublayer is investigated. The importance of terrain inhomogeneity is studied with footprint analysis and with an inhomogeneous surface layer model. A windspeed dependence of roughness length for momentum and displacement height is found. Surface layer similarity theory is extended to describe the roughness-sublayer influence. This involves the introduction of an additional length scale related to the geometry of the forest. It is found that well- defined flux profile relations exist for momentum and sensible heat in the roughness layer of the current forest. In the roughness layer the exchange of temperature is more efficient than the exchange of momentum. This is in contrast to results for the surface layer, but in concord with previous findings for dense forests.The scalar excess resistance, which describes the difference between momentum transport and scalar transport at the surface/atmosphere interface, is investigated by using measurements of infrared surface temperature. Surface radiation temperature and aerodynamic surface temperature, obtained by extrapolating the air-temperature profile to the surface, are not necessarily equal. By assuming equality between the two, it is shown that a consistent description of the relation between sensible heat flux and temperature difference between the surface and the atmosphere is obtained, at least for daytime cases. The excess resistance for the current forest is much smaller than values found for low vegetation. It is shown that the enhanced exchange efficiency of heat, relative to momentum, in the roughness sublayer attributes to this low value. An alternative analysis is presented to separate this roughness layer effect from the transfer resistance at the forest/atmosphere interface. The value found for this alternative excess resistance, is more in line with low vegetation values. For neutral cases the two methods give the same results for temperature differences between the surface and the surface layer. It is shown that stability effects give rise to a discrepancy between the two methods. The observations show some evidence in favour of the alternative method. The difference between forest interior air temperature and air temperature at canopy height is related to storage heat flux and sensible heat flux by applying the concepts of gust penetration and surface renewal. The analysis suggests that the renewal of interior air caused by gust penetration is slow due to the presence of a very dense crown layer.For night-time cases, the equality between aerodynamic and radiation surface temperature breaks down when wind speeds are low and longwave cooling is high. The analysis shows that forest air becomes decoupled from the air aloft. Longwave cooling at the crown layer triggers canopy convection which transports cooled air from the crown layer to the forest interior. The existence of a convective surface temperature in the crown layer is deduced from the measurements. A two-layer radiation/energy balance model is constructed. The model explains the difference between radiation- and aerodynamic surface temperature in terms of the distribution of storage heat and sensible heat over the two model layers.Transpiration for dry conditions is investigated by using the Penman-Monteith equation with a Jarvis type of formulation for the surface resistance. First the closure of the surface energy balance is checked. Overall closure is within the range of estimated measurement error. However, at times deviations occur which can be attributed to wind direction. With respect to transpiration it is found that surface resistance reacts strongly to water vapour deficit changes. This is related to the good aerodynamic coupling of the rough forest to the atmosphere. In spring, a clear increase in transpiration is observed after shoot growth. Soil water response is clearly present before mid summer, after that the forest seems less susceptible to draught. Probably the root system adapts to the dry situation. An analysis of residuals between observed and modelled transpiration shows that deviations occurred at the same wind direction where the energy balance closure broke down. The variance in the residuals appears to be two times larger than estimated from atmospheric statistics.Important contributions to this variance are correlated over periods of one day. This suggests that standard statistical techniques lead to an underestimation of the confidence intervals of estimated model parameters. Two other models are evaluated. A new formulation suggested by Monteith, where stomatal response to moisture deficit is replaced by a response to transpiration itself, is investigated. This formulation appears to be equivalent to the Jarvis formulation with an atmospheric moisture deficit response. The Priestley-Taylor formula is adapted to include soil water response. It performs reasonably well given the simple nature of the formulation.Interception measurements and xylem sapflow measurements are exploited to investigate the interaction between evaporation and transpiration in a partially wet forest. The Penman-Monteith equation is generalised to describe this interaction. Explicit expressions are obtained for evaporation and transpiration. After optimisation the model is capable of describing both evaporation and transpiration reduction fairly well. Due to parameter interdependency, error bounds on individual parameter estimates, related to evaporation, are large. Independent estimates of the parameters, although crude, are shown to be within the confidence region of the optimisation results. It is shown that evaporation rates are smaller than the frequently used formula: wet fraction times potential evaporation. Most of the transpiration reduction comes from energy consumption by the process of evaporation and the impact of the humidity conditions close to the needles. Only a small amount of stomatal blocking due to intercepted water is needed to explain the remaining reduction. This is in concord with the observation that stomata of the Douglas fir are at the lower side of the needle, which is only partially wetted during rain.The response of the forest to external forcings can be described by a number of parameters related to model descriptions of various processes. The results for the current dense Douglas-fir forest are compared with other forest studies. A good agreement is found for the relation between the geometric parameters, canopy height, displacement height and roughness length for momentum. The exchange coefficients in the roughness layer for momentum and heat agree qualitatively with typical values found for other dense forests. Quantitatively significant and as yet unexplained differences remain. The very small scalar excess resistance found for the current forest is in agreement with the only other comparable dense forest study, which appeared recently in the literature. Transpiration rates as a function of external conditions are broadly in line with results found at other forest sites in the temperate climate.New in this thesis are the results on; the estimation of displacement height; the difference between aerodynamic surface temperature and radiation surface temperature at night time; night time convection; changing transpiration response to soil water stress during the season; and the interaction between evaporation and transpiration reduction during wet conditions.<br/

GABLS3-LES Intercomparison Study

Recently, a large-eddy simulation (LES) intercomparison study was organized under the auspices of the GEWEX Atmospheric Boundary Layer Study (GABLS). Eleven LES modelling groups around the world participated in this study to model a baroclinic, mid-latitude nighttime stable boundary layer utilizing several LES subgrid-scale (SGS) models. Some of the findings from this intercomparison study are unexpected, which make it quite unique. First of all, the LES-ensemble not only captures the statistics, but also the dynamical evolution of the observed variables remarkably well. Second, the diversity among the members of the ensemble is found to be surprisingly low. In other words, the simulated results (especially the first-order statistics) are not very sensitive to the LES-SGS parameterizations. Last, a relatively coarse spatial resolution of 6.25 m is shown to be adequate for representing the basic characteristics of a moderately/strongly stratified (the ratio of the boundary layer height and the Obukhov length is on the order of 5) boundary layer. In this conference paper, we discuss a subset of these findings

GABLS 3 SCM intercomparison and evaluation. What did we learn?

A weakly stable case is selected from the Cabauw long term archive. The case is used for an evaluation and intercomparison of nineteen SCM in the context of the GEWEX Atmospheric Boundary Layer studies. Care was taken to prescribe realistic geostrophic forcing and dynamic tendencies to the SCM’s. It is shown that in this way a direct evaluation of SCM runs with observations is feasible. The results are analyzed using a method which allows the interpretation of differences among models in terms of the dominating physical processes in the stable boundary layer, i.e. coupling to the soil, turbulent mixing and long wave radiation. Significant differences among models are found in the representation of these three processes. It is shown that on a single case basis, atmospheric forcings as obtained from a state of the art 3D atmospheric model are not accurate enough to drive the case if a direct comparison with observations is desired. Ensemble and composite approaches are applied on the basis of eight comparable nights, showing that deviations in the forcings between the model and the real-world are diminished to such extent that a very accurate comparison between models and observations becomes feasible. As a by-product, it is shown that systematic dynamical tendencies remain in the composite case, probably related to the presence of the sea

The share of the mean turbulent kinetic energy in the near-neutral surface layer for high and low wind speeds

We examine the dependence on wind speed of the share of the mean turbulent kinetic energy among the three velocity components in the near-neutral surface layer. To contrast the general behaviour and the local effects, four datasets are considered, corresponding to different surfaces and environmental conditions. For high wind speeds (i.e., wind speed ≈ 10 ms^(−1)), the shares are well-defined and about the same for all sites. As wind speed decreases (becoming ≈ 1 ms^(−1)), large record-to-record variability occurs giving, on average, an almost isotropic state for the horizontal velocity components. Through spectral analysis, we relate this behaviour to the low-frequency, submeso motions and to the lack of conditions required by Reynolds averaging. The implications for modelling are also discussed, showing that the wind speed, or a related quantity, must be accounted for, besides stability, in second-order closures

Overview of the GEWEX Atmospheric Boundary Layer Study (GABLS)

In 2001 the steering group of GEWEX (formally known as the Global Energy and Water Cycle Experiment) initiated the GEWEX Atmospheric Boundary Layer Study (GABLS). The objective of GABLS is to improve the representation of the atmospheric boundary layer in regional and large-scale atmospheric models. As such, GABLS provides a platform for model inter-comparison and development to benefit studies of Climate, Weather, Air Quality, Wind Energy and other applications. The focus of GABLS has so far been on stable boundary layers (SBLs) over land and on the representation of the diurnal cycle under clear skies. Three inter-comparison studies have been organised and below a summary of some of the results and achievements is given. Here we primarily focus on the performance of single column versions of several state-of-the-art atmospheric models

An assessment of the potential for atmospheric emission verification in The Netherlands

Doel van dit project was het ontwikkelen van een systeem voor het kwantificeren van het broeikasgasbudget op landelijke en regionale schaal. Het ME2 consortium heeft een ‘protocol’ ontwikkeld om een referentieschatting te maken ten behoeve van de verificatie van nationale emissies. Daarmee is het op termijn mogelijk de nauwkeurigheid en geloofwaardigheid van aan UNFCCC en Kyoto gerapporteerde emissies, en reducties daarvan, te verifiëren. Met verschillende inversie methoden, van data tot model gedreven, zijn emissieschattingen gemaakt. De data gedreven methoden kunnen schattingen maken voor alle drie de broeikasgassen voor NL als geheel en zijn representatief voor meerdere jaren. Met de meer model gedreven inversies zijn meer ruimtelijk en temporeel gedistribueerde schattingen te maken

Studying the spatial variability of methane flux with five eddy covariance towers of varying height

In this study, the spatial representativeness of eddy covariance (EC) methane (CH4) measurements was examined by comparing parallel CH4 fluxes from three short (6 m) towers separated by a few kilometres and from two higher levels (20 m and 60 m) at one location. The measurement campaign was held on an intensively managed grassland on peat soil in the Netherlands. The land use and land cover types are to a large degree homogeneous in the area. The CH4 fluxes exhibited significant variability between the sites on 30-min scale. The spatial coefficient of variation (CVspa) between the three short towers was 56% and it was of similar magnitude as the temporal variability, unlike for the other fluxes (friction velocity, sensible heat flux) for which the temporal variability was considerably larger than the spatial variability. The CVspa decreased with temporal averaging, although less than what could be expected for a purely random process View the MathML source(1/N), and it was 14% for 26-day means of CH4 flux. This reflects the underlying heterogeneity of CH4 flux in the studied landscape at spatial scales ranging from 1 ha (flux footprint) to 10 km2 (area bounded by the short towers). This heterogeneity should be taken into account when interpreting and comparing EC measurements. On an annual scale, the flux spatial variability contributed up to 50% of the uncertainty in CH4 emissions. It was further tested whether EC flux measurements at higher levels could be used to acquire a more accurate estimate of the spatially integrated CH4 emissions. Contrarily to what was expected, flux intensity was found to both increase and decrease depending on measurement height. Using footprint modelling, 56% of the variation between 6 m and 60 m CH4 fluxes was attributed to emissions from local anthropogenic hotspots (farms). Furthermore, morning hours proved to be demanding for the tall tower EC where fluxes at 60 m were up to four-fold those at lower heights. These differences were connected with the onset of convective mixing during the morning period