Ebba Dellwik

On the use of the Webb–Pearman–Leuning theory for closed-path eddy correlation measurements

We consider an imperfection of real closed-path eddy correlation systems—the decoupling of the water vapour and CO2 concentrations—with respect to the application of the Webb–Pearman–Leuning (WPL) theory. It is described why and how the current application of the WPL theory needs to be adapted to the processes in closed-path sensors. We show the quantitative effects of applying the WPL theory in different ways using CO2 flux measurements taken above the Danish Beech forest CarboEurope site near Sorø, Zealand. Using the WPL theory in closed-path sensors without taking amplitude damping and decoupling into account, overcorrected the annual flux by 21%, or 31 g m-2 yr-1, to which the decoupling effect contributed with 7%. We suggest either converting the raw data point-by-point to mixing ratios or using the uncorrected covariances of water vapour mole fractions with the vertical wind velocity that were calculated with the same time lag as for the scalar concentration when correcting the dilution effect.We showed that the two approaches yielded equivalent flux results. Correct ways of applying spectral corrections to CO2 fluxes calculated in either way are also shown. The findings reported here do not apply to open-path sensors.

Thermal optimality of net ecosystem exchange of carbon dioxide and underlying mechanisms.

• It is well established that individual organisms can acclimate and adapt to temperature to optimize their functioning. However, thermal optimization of ecosystems, as an assemblage of organisms, has not been examined at broad spatial and temporal scales. • Here, we compiled data from 169 globally distributed sites of eddy covariance and quantified the temperature response functions of net ecosystem exchange (NEE), an ecosystem-level property, to determine whether NEE shows thermal optimality and to explore the underlying mechanisms. • We found that the temperature response of NEE followed a peak curve, with the optimum temperature (corresponding to the maximum magnitude of NEE) being positively correlated with annual mean temperature over years and across sites. Shifts of the optimum temperature of NEE were mostly a result of temperature acclimation of gross primary productivity (upward shift of optimum temperature) rather than changes in the temperature sensitivity of ecosystem respiration. • Ecosystem-level thermal optimality is a newly revealed ecosystem property, presumably reflecting associated evolutionary adaptation of organisms within ecosystems, and has the potential to significantly regulate ecosystem-climate change feedbacks. The thermal optimality of NEE has implications for understanding fundamental properties of ecosystems in changing environments and benchmarking global models.

Effective Roughness Calculated from Satellite-Derived Land Cover Maps and Hedge-Information used in a Weather Forecasting Model

In numerical weather prediction, climate and hydrologicalmodelling, the grid cell size is typically larger than the horizontal length scales of variations in aerodynamicroughness, surface temperature and surface humidity. These local land cover variations give rise to sub-gridscale surface flux differences. Especially the roughness variations can give a significantly differentvalue between the equilibrium roughness in each of the patches as compared to the aggregated roughness value,the so-called effective roughness, for the grid cell. The effective roughness is a quantity that secures thephysics to be well-described in any large-scale model. A method of aggregating the roughness step changesin arbitrary real terrain has been applied in flat terrain (Denmark) where sub-grid scale vegetation-drivenroughness variations are a dominant characteristic of the landscape. The aggregation model is a physicaltwo-dimensional atmospheric flow model in the horizontal domain based on a linearized version of theNavier Stoke equation. The equations are solved by the Fast Fourier Transformation technique, hence the codeis very fast. The new effective roughness maps have been used in the HIgh Resolution Limited Area Model(HIRLAM) weather forecasting model and the weather prediction results are compared for a number of casesto synoptic and other observations with improved agreement above the predictions based on currentstandard input. Typical seasonal springtime bias on forecasted winds over land of +0.5 m s-1 and-0.2 m s-1 in coastal areas is reduced by use of the effective roughness maps.

Validation of ERS-2 SAR offshore wind-speed maps in the North Sea

Wind maps are retrieved from ERS-2 Synthetic Aperture Radar (SAR) scenes by the CMOD-IFR2 and CMOD4 algorithms for 61 cases at the Horns Rev site in the North Sea and compared to meteorological in situ observations from a mast located 14 km offshore. The in situ data are corrected for flow distortion and sea-level changes prior to validating the SAR wind maps. The SAR wind maps are area-averaged by a simple footprint method assuming neutral stability and with three nonlinear weighting footprint methods including correction for stability. From a physical point of view, the latter is more correct. However, between in situ and SAR-derived wind-speed estimates comparison results of the nonlinear footprint values are statistically less correlated (R 2=0.73–0.77) and the standard error (SE) is larger (>1.5 m s−1) than results from the simple footprint (R 2=0.78–0.80 and SE=1.3 m s−1). The results are found with wind direction determined from wind streaks in the SAR images by Fast Fourier Transform. Using in situ wind direction as input to the CMOD-IFR2 and CMOD4 algorithms yields even better linear regression results, e.g. for the simple footprint method R 2=0.88 and SE=0.9 m s−1. SAR wind maps may be useful for mapping of future offshore wind resources.

Complex terrain experiments in the New European Wind Atlas

The New European Wind Atlas project will create a freely accessible wind atlas covering Europe and Turkey, develop the model chain to create the atlas and perform a series of experiments on flow in many different kinds of complex terrain to validate the models. This paper describes the experiments of which some are nearly completed while others are in the planning stage. All experiments focus on the flow properties that are relevant for wind turbines, so the main focus is the mean flow and the turbulence at heights between 40 and 300 m. Also extreme winds, wind shear and veer, and diurnal and seasonal variations of the wind are of interest. Common to all the experiments is the use of Doppler lidar systems to supplement and in some cases replace completely meteorological towers. Many of the lidars will be equipped with scan heads that will allow for arbitrary scan patterns by several synchronized systems. Two pilot experiments, one in Portugal and one in Germany, show the value of using multiple synchronized, scanning lidar, both in terms of the accuracy of the measurements and the atmospheric physical processes that can be studied. The experimental data will be used for validation of atmospheric flow models and will by the end of the project be freely available. This article is part of the themed issue ‘Wind energy in complex terrains’.

Turbulence spectra, shear stress and turbulent kinetic energy budgets above two beech forest sites in Denmark

The focus of this study is the combined influence of the roughness sublayer (RSL) found above tall vegetation and the internal boundary layer (IBL) on the near-neutral flow above two forest sites. Measurements of the 3-D wind field from masts about twice the forest height were analysed. For both sites, influence from upwind conditions was detected for a short-fetch sector. For one of the sites, an additional long-fetch sector without significant IBL influence is presented. Spectral analysis, dissipation length scale analysis and evaluation of the most important terms in the turbulent kinetic energy and shear stress budgets, were performed. For all selected sectors, RSL influence was detected close to the canopy top: the dissipation length scale was greater than the height above the displacement height of the forest, and the turbulent transport terms were significant. For the short-fetch sectors, the spectral analysis of measurements taken in the RSL and in the IBL (above the RSL) showed that scaling by fixed length and velocity scales resulted in a good collapse of the spectral peaks. For the long-fetch sector, the RSL influence disappeared at greater heights, and the flow is nearly adjusted with the new surface.

How Forest Inhomogeneities Affect the Edge Flow

Most of our knowledge on forest-edge flows comes from numerical and wind-tunnel experiments where canopies are horizontally homogeneous. To investigate the impact of tree-scale heterogeneities (

Development of satellite green vegetation fraction time series for use in mesoscale modeling: application to the European heat wave 2006

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