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Changes in plant phenology as for example earlier leaf unfolding and delayed autumn senescence can result in variations in the carbon and water cycle. Studies investigating the impact of phenological shifts on biophysical processes such as water availability are still limited. Due to the sensitivity of radar satellite observations to both, structural and dielectric properties of the scattering materials, microwave remote sensing offers the potential to analyse structural (i.e. canopy biomass) and physiological (i.e. water status) dynamics in vegetation.Here, we aim to derive annual water dynamics of vegetation canopies from the Sentinel-1 C-band radar backscatter signal by removing the influence of vegetation structure on the backscatter seasonality. To decouple the phenology of vegetation structure from the moisture content dynamics, a semi-empirical backscattering model (Water Cloud Model, WCM) is combined with a canopy water balance model. The WCM aims to separate contributions of soil and vegetation to the total backscatter. When introducing physical parameters for vegetation structure like leaf area index (LAI) and and moisture like leaf fresh moisture content (LFMC) to describe the vegetation backscatter, the effect of the seasonal variability of both variables on the radar signal can be assessed. The canopy water balance model estimates interception and changes in the canopy saturation and storage capacity of the vegetation using precipitation and throughfall measurements. Both models are combined to iteratively estimate measures of vegetation moisture. To calibrate the two models, we use measurements of LFMC and of canopy interception for the Tharandt ecosystem site in Germany in 2022, which is part of the ICOS and FLUXNET network. The calibrated model is then used to analyse the individual effects of both vegetation descriptors, LAI and LFMC, by fixing either one and looking at the changes in the seasonality of the S1 signal. The combined use of both models will allow to remove the structural-related changes in the Sentinel-1 radar backscatter to finally retrieve vegetation water dynamics over larger areas.

Interception of a Norway spruce stand was analysed based on canopy water balance measurements Emass and for Eddy Covariance (EC) related measurements. The study site is located in the Tharandt Forest (Germany) and the analysis covers a long period from 2008 to 2018. Emass was calculated as residual between gross rainfall above and net rainfall below the canopy. EC related observations are based on the water equivalent either directly measured (uncorrected) by Eddy Covariance (ETEC) or as residual in the Energy Balance equation (ETEB). Additionally, evaporation of intercepted water was modelled with the Penman-Monteith equation, which was adapted for a gradually wetted canopy (ERutter) by the use of the Rutter model. The latter approach was used to integrate the time series of all methods over the duration of modelled interception events leading to different estimates of wet canopy evaporation.The canopy water balance from 2008 to 2018 shows a mean annual gross precipitation of 936±173mm and a mean annual interception evaporation of 376±56mm (Emass). The majority of rainfall events (81%) is characterized by a depth less than 5mm, which leads to a high fraction of annual precipitation being captured by the canopy surface (0.41). The application of the Rutter model yielded good results with a mean modelled annual interception ERutter of 361±47mm being very close to Emass. Thus, the model served as a good standard to define interception events. The water equivalent of wet canopy evaporation as the residual of the energy balance ETEB and from gas analyser measurements ETEC are both systematically underestimating Emass, to a higher extent for the winter than summer half-year. On a mean annual basis, ETEB and ETEC underestimate Emass by 145mm and 288mm, respectively. Comparing the totals over the majority of interception events, ETEB corresponds to only 72% and ETEC to only 33% of canopy water balance based measurements.One reason for this underestimation might be a scaling problem between the interception measurement site and the flux tower footprint, which could not be resolved by the application of a simple scaling factor. A more likely explanation is the underestimation of turbulent fluxes by the EC method. The data is most affected during raining conditions with the highest gap in winter. An annual analysis of the linear relation between the sum of turbulent fluxes and available energy shows the lowest slope (0.57±0.15) for measurements during rain, while the highest slopes occur under completely dry conditions (0.76±0.03). Wet canopy conditions without rainfall seem not to be as crucial for energy imbalance as rain, but affect the closure gap for the winter half-year.Records of EC measurements are generally too low and the magnitude of supplied sensible heat and sustained latent heat flux rates during interception events remains unclear. We conclude that gap filling and correction of both turbulent fluxes should be done separately for rain (interception) and dry (transpiration) conditions in order to determine proper amounts of evapotranspiration with the eddy-covariance method.

Wind and windstorms cause severe damage to natural and human-made environments. Thus, wind-related risk assessment is vital for the preparation and mitigation of calamities. However, the cascade of events leading to damage depends on many factors that are environment-specific and the available methods to address wind-related damage often require sophisticated analysis and specialization. Fortunately, simple indices and thresholds are as effective as complex mechanistic models for many applications. Nonetheless, the multitude of indices and thresholds available requires a careful selection process according to the target environment. Here, we first provide a basic background on wind and storm formation and characteristics, followed by a comprehensive collection of both indices and thresholds that can be used to predict the occurrence and magnitude of wind and storm damage. We focused on five key environments: forests, urban, transport, agriculture, and wind-based energy production. For each environment we described indices and thresholds relating to physical properties such as topography and land cover but also to economic aspects (e.g. disruptions in transportation or energy production). In the face of increased climatic variability, the promotion of more effective analysis of wind and storm damage could reduce the impact on society and the environment.

When measuring the energy balance at the earth’s surface using the Eddy covariance technique, the obtained budgets seldom produce a closed energy balance. The measurements often miss some of the energy fluxes. A possible reason is the neglect of non-turbulent surface fluxes of latent heat and sensible heat, i.e. advective fluxes of these quantities. We present estimates of advective latent and sensible heat fluxes for three different sites across Europe based on the ADVEX dataset. The obtained horizontal and vertical advective fluxes were site-specific and characterized by large scatter. In relative terms, the data indicated that the sensible heat budget was less affected by advection than the latent heat budget during nighttime; this is because vertical turbulent latent heat fluxes were very small or close to zero during the night. The results further showed that the additional energy gain by sensible heat advection might have triggered enhanced evaporation for two sites during nighttime. Accounting for advective fluxes improved the energy balance closure for one of the three ADVEX sites. However, the energy balance closure of the other two sites did not improve overall. A comparison with energy balance residuals (energy missed by the measurements without accounting for advection) indicated a large influence of systematic errors. An inspection of the energy balance for the sloped site of the ADVEX dataset underlined the necessity of slope-parallel measurement of radiation.

Die Wirkung von Grünflächen auf das innerstädtische Klima ist vielfältig, aber auch umstritten. Bei expliziter Betrachtung des thermischen Wirkungskomplexes steht die Schattenwirkung und Verdunstung der Ventilation scheinbar entgegen. Durch Bäume wird am Tage die Strahlungsbelastung auf Freiflächen verringert, aber auch die Ventilation, d.h. der Austausch sensibler Wärme wird eingeschränkt. In der Nacht verhindern Baumkronen zusätzlich die Ausstrahlung der über den Tag gespeicherten Energie. Die thermische Wirkung der Stadtvegetation findet zum einen gut messbar unmittelbar im Nahbereich statt. Turbulente und advektive Wärmeflüsse sind dagegen nur schwer messbar und haben eine Fernwirkung. Die Einschätzung der Stärke und Bedeutung der einzelnen Prozesse ist bei planerischen Änderungen der Stadtvegetation eine notwendige Voraussetzung. Zur Simulation der komplexen Vorgänge werden turbulenzauflösenden Modelle benötigt. Die Verwendung realistischer Vegetationsmodelle in mikrometeorologischen Modellen ist dabei essential für die Simulationen im Rauhigkeitsbereich der Grenzschicht. Bisherige Vergleiche zwischen numerischen Simulationen der turbulenten Strömung, Windkanalexperimenten und umfangreiche Feldmessungen zeigten für einen Walbestand grundlegende Verbesserungen der Simulationsergebnisse gegenüber der Verwendung von unstrukturierten Vegetationsmodellen (Queck et al. 2015). Die vorgestellte Arbeit untersucht, ob die in den letzten Jahren entwickelten mobilen Systeme mit Laserscannern die effektive Gewinnung derartiger Vegetationsmodelle erlauben (Vosselman und Maas 2010; Bienert et al. 2018). Mit terrestrischen Laserscannern wurden Vegetationsmodelle für 6 Straßenzüge in Dresden mit einer Auflösung von weniger als 1 m Gitterweite erstellt. In ersten Modellanwendungen für die Stadt wird der Einfluss des Vegetationsmodells in Abhängigkeit der räumlichen Auflösung getestet. Als Referenzdaten dienen mobile Messungen von Strahlungskomponenten, Temperatur, Feuchte und Wind.

Evapotranspiration changes with landuse, soil conditions and meteorological conditions. Landscapes in middle Europe are typically of mosaic pattern at micro to local scale with different landuses adjacent to one another and we therefore have areas of different ET. This work shall address how ET changes with landuse based on 10 years of Eddy-Covariance data for different landuses. In a first step, it will be focussed on two landuses (coniferous forest and grassland). ET obtained via measurements will be compared to ET obtained via modelling by a using a one dimensional soil-vegetation transfer scheme and a machine learning approach using gradient boosting. Results will be analysed whether typical characteristic properties of the respective landscape are preserved (e.g. Bowen ratio) as well as differences between land uses (e.g. differences in yearly ET estimates).

Ziel des BMBF-Programms 'Stadtklima im Wandel' war die Entwicklung, Validierung und Anwendung eines gebäudeauflösenden Stadtklimamodells für ganze Städte. Das Verbundprojekt 3DO übernahm die dem Modul B zugeordneten Forschungsaufgaben: Aufbereitung vorhandener Daten aus der Langzeitbeobachtung (LTO), Aufbau neuer Messstationen, Gewinnung neuer dreidimensionaler atmosphärischer Daten und die Entwicklung neuer Konzepte z.B. zur Modellevaluation. Untersucht wurden der Aufbau der atmosphärischen Grenzschicht, die Charakteristik der meteorologischen Parameter und deren Einfluss auf das thermische Empfinden des Menschen. Ein einheitlicher [UC]2-Datenstandard sowie Analysewerkzeuge wurden entwickelt und in ein Datenmanagementsystem und eine Wissensplattform für den modulübergreifenden Austausch integriert. Aim of the BMBF-Programme 'Urban Climate under Change' was development, validation and application of a building-resolving urban climate model for entire cities. The joint project 3DO took over the research tasks assigned to module B: Preparation of existing data from long-term observation (LTO), deployment of new measuring stations, acquisition of new three-dimensional atmospheric data and new concepts, e.g. for model evaluation. The structure of the atmospheric boundary layer, characteristics of meteorological parameters and their influence on the thermal sensation of humans were investigated. A uniform [UC]2 data standard as well as analysis tools were developed and integrated into a data management system and a knowledge base for cross-module exchange.

Less is known about evapotranspiration (ET) along elevation gradients of low mountain ranges, especially with regard to different land uses and concerning long-term studies. We investigate ET of four sites of different land-uses along an elevation gradient of a low mountain range over eleven years (2008–2018) based on daily values. Three different ET estimates are inspected, which can give a reasonable range of ET. These estimates are ET based on the energy balance residual (ET_residual), ET corrected for the energy balance closure gap (ET_corr) and ET not corrected for the energy balance closure gap (ET_uncorr). In general, ET_residual showed largest values and ET_uncorr showed lowest values with ET_corr in between. Average annual differences between ET_residual and ET_corr ranged between 111 mm a−1 and 196 mm a−1. Average annual differences between ET_uncorr and ET_corr ranged between 70 mm a−1 and 167 mm a−1. For two site years ET_corr was lower than ET_uncorr. This could be related to gap-filling. Differences between different estimates were site-specific and related to the respective energy balance closure gap. Principal component analysis revealed similar dependency on driving variables for all three estimates and all sites. Given the influence of the energy balance closure gap on ET_uncorr and ET_residual, we recommend using ET_corr, but ET_residual can be still useful especially for sites with low vegetation, which rarely experience water stress. Comparison of two coniferous sites situated at different altitudes showed frequently larger values for the site located at a higher altitude. This might be a result of rainfall interception, which however must be investigated at sub-daily timescale.

With the aim to simulate the exchange of energy and matter between air and vegetation, we applied the LES PALM to a typical Central European forest. The presentation shows how the level of detail within vegetation model and the orography alters the simulated flow.The site of investigation is a managed mixed forest stand (mainly Picea abies, height 30 m; a long-term CarboEurope monitoring site) within the Tharandter Wald near Dresden, Germany. Terrestrial laser scans (TLS) provided the data basis for the high-resolution vegetation model of this forest stand and a nearby clearing (50x90 m) building the inner range of the model domain. To investigate orographic effects on the flow, we extended the domain for about 1.5 km to the west. This includes the S-Berg, which is about 40 m height and therefore the highest elevation on the windward side. We used information from airborne laser scanning (ALS) along with forest inventory data to build a vegetation model as well as a digital elevation model for the extended area (2 km in streamwise and 1.5 km in lateral direction) with a resolution of (2m)3. In a first step, we restricted all simulations to a neutral atmosphere to exclude effect of buoyancy.Wind data from four measurement towers (from DFG SPP 1276 MetStröm) provided data for a validation of the simulations. They were located within the inner domain along a west-east transect over the clearing.

Drainage from the canopy is one of the sub-processes described in conceptual interception models. In theory, this refers to rainfall water that is temporally stored on the canopy and starts to drip down, when the canopy is saturated. In most Rutter-type models, storage in the canopy is described as a single linear storage (in multilayer models as a storage cascade) from which the drainage occurs after a saturation value is reached. For a precise simulation of the timing and amount of canopy drainage, this approach appears to be insufficient. E.g., with large time-steps and highly filled storage, drainage rates can become larger than the precipitation intensity, leading to a negative storage.In our study, we present a review of different approaches to simulate drainage according to literature. We test those approaches using the interception model CanWat, which allows temporally and spatially resolved simulations of the relevant processes. CanWat relies on a vegetation model derived from terrestrial laser scans for a detailed description of the stand. CanWat is written in R language allowing to process multi-annual time series for a study area up to 1 km² with a spatial resolution of 1 m3 on a PC. For validation purposes, we use rain events that are available from long-term continuous measurements of interception in a spruce stand (mainly Picea abies; a continuous flux site since started within EUROFLUX in 1996) within the Tharandter Wald Southwest of Dresden.We outline benefits and limitations of each approach and evaluate which one best fits the needs for a precise simulation of the timing and amount of canopy drainage. The implementation and refinement of the most suitable drainage simulation approach into CanWat is part of the overall attempt to improve the model description of the interception process being the focus of a German Science Foundation (DFG BE-1721/23-1) interception project.

Ziel des BMBF-Programms 'Stadtklima im Wandel' war die Entwicklung, Validierung und Anwendung eines gebäudeauflösenden Stadtklimamodells für ganze Städte. Das Verbundprojekt 3DO übernahm die dem Modul B zugeordneten Forschungsaufgaben: Aufbereitung vorhandener Daten aus der Langzeitbeobachtung (LTO), Aufbau neuer Messstationen, Gewinnung neuer dreidimensionaler atmosphärischer Daten und die Entwicklung neuer Konzepte z.B. zur Modellevaluation. Untersucht wurden der Aufbau der atmosphärischen Grenzschicht, die Charakteristik der meteorologischen Parameter und deren Einfluss auf das thermische Empfinden des Menschen. Ein einheitlicher [UC]2-Datenstandard sowie Analysewerkzeuge wurden entwickelt und in ein Datenmanagementsystem und eine Wissensplattform für den modulübergreifenden Austausch integriert.Aim of the BMBF-Programme 'Urban Climate under Change' was development, validation and application of a building-resolving urban climate model for entire cities. The joint project 3DO took over the research tasks assigned to module B: Preparation of existing data from long-term observation (LTO), deployment of new measuring stations, acquisition of new three-dimensional atmospheric data and new concepts, e.g. for model evaluation. The structure of the atmospheric boundary layer, characteristics of meteorological parameters and their influence on the thermal sensation of humans were investigated. A uniform [UC]2 data standard as well as analysis tools were developed and integrated into a data management system and a knowledge base for cross-module exchange.

Forest ecosystems play an important role in the interaction between the land surface and the atmosphere. Measurements and modelling efforts have revealed significant uncertainties in state-of-the-art flux assessments due to spatial inhomogeneities in the airflow and land surface. Here, a field experiment is used to describe the turbulent flow across a typical Central European forest clearing. A three-dimensional model of the inhomogeneous forest stand was developed using an innovative approach based on terrestrial laser-scanner technology. The comparison of the wind statistics of two measurement campaigns (5 and 12 months long) showed the spatial and temporal representativeness of the ultrasonic anemometer measurements within the canopy. An improved method for the correction of the vertical velocity enables the distinction between the instrumental offsets and the vertical winds due to the inclination of the instrument. Despite a 13 % fraction of deciduous plants within the otherwise evergreen canopy, the effects of phenological seasons on the velocity profiles were small. The data classified according to the wind speed revealed the intermittent nature of recirculating air in the clearing. Furthermore, the development of sub-canopy wind-speed maxima is explained by considering the velocity moments and the momentum equation (including measurements of the local pressure gradient). Clearings deflect the flow downward and feed the sub-canopy flow, i.e., advective fluxes, according to wind speed and, likely, clearing size, whereas local pressure gradients play an important role in the development of sub-canopy flow. The presented dataset is freely available at the project homepage.

The factors influencing the interception process such as the vegetation structure and the meteorological conditions, especially rainfall characteristics, are diverse and highly variable. Therefore, derivation of canopy parameters for interception modelling requires data from field experiments over a long period. In this study, we analyse long-term changes in canopy parameters and rainfall interception for an old-growth spruce stand (composed mainly of Picea abies at a continuous flux site started within EUROFLUX in 1996) within the Tharandter Wald, Southwest of Dresden, Germany. The 10-minute data from the continuous measurements of gross precipitation (P) and throughfall (TF) from 1997 to 2018 were used as follows: Rainfall events were separated by a minimum time of three hours without rainfall and divided into rainfall classes according to their total amount and intensity. Due to vegetation changes in the measuring period caused by two thinning operations, the complete study period was divided into three periods. Canopy parameters S (storage capacity), p (free throughfall coefficient) and Ps (canopy saturation point) were then generated applying a regression-based method using the relationship between P and TF. We generated parameter sets for the complete period, for the three periods reflecting the vegetation changes and for single years. These parameter sets were further tested using a regression model. The statistical analysis of the long-term data set showed that stand parameters and interception are largely influenced by vegetation changes. S and Ps decreased with each thinning. Interestingly, interception and rainfall only changed after the first thinning (increase in p, decrease in interception), indicating counter-acting factors after the second thinning, such as changes in meteorological conditions, rainfall patterns or thinning induced changes in the microclimate of the canopy. Generally, the reliability of the derived canopy parameters depends on the length of the dataset used for the regression analysis. However, the parameter sets that considered three periods produced the best model fit compared to parameter sets for the complete period or for single years. This clearly shows that model performance of statistical interception models depend on a reliable parameterisation that can be improved when taking into account changes in stand characteristics.

It is assumed that the description of the exchange processes between heterogeneous natural surfaces and the atmosphere within turbulence closure models is mainly limited by a realistic three-dimensional (3D) representation of the vegetation architecture. Within this contribution we present a method to record the 3D vegetation structure and to use this information to derive model parameters that are suitable for numerical flow models. A mixed conifer forest stand around a clearing was scanned and represented by a dense 3D point cloud applying a terrestrial laser scanner. Thus, the plant area density (PAD) with a resolution of one cubic meter was provided for analysis and for numerical simulations. Multi-level high-frequency wind velocity measurements were recorded simultaneously by 27 ultrasonic anemometers on 4 towers for a period of one year. The relationship between wind speed, Reynolds stress and PAD was investigated and a parametrization of the drag coefficient CD$C_D$ by the PAD is suggested. The derived 3D vegetation model and a simpler model (based on classical forest assessments of the site) were applied in a boundary layer model (BLM) and in large-eddy simulations (LES). The spatial development of the turbulent flow over the clearing is further demonstrated by the results of a wind tunnel experiment. The project showed, that the simulation results were improved significantly by the usage of realistic vegetation models. 3D simulations are necessary to depict the influence of heterogeneous canopies on the turbulent flow. Whereas we found limits for the mapping of the vegetation structure within the wind tunnel, there is a considerable potential for numerical simulations. The field measurements and the LES gave new insight into the turbulent flow in the vicinity and across the clearing. The results show that the zones of intensive turbulence development can not be restricted to the locations found in previous studies with more idealized canopies.

The effect of three-dimensional plant heterogeneity on flow past a clearing is investigated by means of large-eddy simulation. A detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately $$328\,\mathrm {m}$$ length and $$172\,\mathrm {m}$$ width at the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a highly resolved, three-dimensional plant area distribution representing the actual canopy. Hence, the vegetation maintains a rich horizontal and vertical structure including the three-dimensional clearing. The scanned plant area density is embedded in a larger domain, which is filled with a heterogeneous forest generated by the virtual canopy generator of Bohrer et al. (Tellus B 59:566–576, 2007). Based on forest inventory maps and airborne laser scanning, the characteristics of the actual canopy are preserved. Furthermore, the topography is extracted from a digital terrain model with some modifications to accommodate for periodic boundary conditions. A large-eddy simulation is performed for neutral atmospheric conditions and compared to simulations of a two-dimensional plant area density and an one-year-long field experiment conducted at the corresponding field site. The results reveal a considerable influence of the plant heterogeneity on the mean velocity field as well as on the turbulent quantities. The three-dimensional environment, e.g., the oblique edges combined with horizontal and vertical variations in plant area density and the topography create a sustained vertical and cross-flow velocity. Downstream of the windward forest edge an enhanced gust zone develops, whose intensity and relative position are influenced by the local canopy density and, therefore, is not constant along the edge. These results lead us to the conclusion that the usage of a three-dimensional plant area distribution is essential for capturing the flow features inside the canopy and within the mixing layer above.

The effect of sub-tree forest heterogeneity in the flow past a clearing is investigated by means of large-eddy simulation (LES). For this purpose, a detailed representation of the canopy has been acquired by terrestrial laser scanning for a patch of approximately 190m length in the field site “Tharandter Wald”, near the city of Dresden, Germany. The scanning data are used to produce a high resolution plant area distribution (PAD) that is averaged over approximately one tree height (30m) along the transverse direction, in order to simplify the LES study. Despite the smoothing involved with this procedure, the resulting two-dimensional PAD maintains a rich vertical and horizontal structure. For the LES study, the PAD is embedded in a larger domain covered with an idealized, horizontally homogeneous canopy. Simulations are performed for neutral conditions and compared to a LES with homogeneous PAD and recent field measurements. The results reveal a considerable influence of small-scale plant distribution on the mean velocity field as well as on turbulence data. Particularly near the edges of the clearing, where canopy structure is highly variable, usage of a realistic PAD appears to be crucial for capturing the local flow structure. Inside the forest, local variations in plant density induce a complex pattern of upward and downward motions, which remain visible in the mean flow and make it difficult to identify the “adjustment zone” behind the windward edge of the clearing.

Applications of flow models to tall plant canopies are limited, amongst other factors, by the lack of detailed information on vegetation structure. A method is presented to record 3D vegetation structure and make this information applicable to the derivation of turbulence parameters suitable for flow models. The relationship between wind speed, drag coefficient (CD) and plant area density (PAD) was experimentally investigated in a mixed conifer forest in the lower part of the Eastern Ore Mountains. Essential information was gathered by collecting multi-level high-frequency wind velocity measurements and a dense 3D representation of the forest was obtained from terrestrial laser scanner data. Wind speed dependence or streamlining was observed for most of the wind directions. Edge effects, i.e. the influence of the here not regarded pressure gradient and the advective terms of the momentum equation, are assumed to cause this heterogeneity. Contrary to the hypothetic shelter effect, which would reduce the drag on sheltered plant parts, the calculated profiles of drag coefficients revealed an increasing CD with PAD (i.e. a dependence on canopy and plant structure).

The aim of the advection initiative ADVEX within CarboEurope-IP (CEIP) was to evaluate the importance of advection on three coniferous sites. A prerequisite and crucial point for calculating the advective terms of CO 2 balance are the assumptions for the construction of wind fields. Wind measurements must be interpolated over complex control volumes. Here the plant area density is identified as a key variable. During ADVEX campaigns, the rectangular control volumes were marked by four towers at the corners, each carrying four sonic anemometers over the volume height (up to 30 m). An additional experiment was designed at the CEIP forest site Tharandter Wald to investigate the error made by interpolation between only four measurement levels. Thirteen sonic anemometers were installed on one tower up to a height of 42 m (canopy height is 30 m). In addition, plant area density profiles were measured to derive plant-specific parameterisation of the wind profiles. Profiles of velocity moments and nondimensional wind speed gradients are discussed as basic characteristics of the wind field. A new parameterisation scheme based on the mixing length approach was developed. The model was validated with the high-resolution data and applied to calculate horizontal advection for the ADVEX sites. In addition to improving the interpolation routine, this model provides the opportunity to reduce the number of large towers needed to calculate in-canopy wind fields.

Extensive field measurements have been performed at three CarboEurope-Integrated Project forest sites with different topography (Renon/Ritten, Italian Alps, Italy; Wetzstein, Thuringia, Germany; Norunda, Uppland, Sweden) to evaluate the relevant terms of the carbon balance by measuring CO2 concentrations [CO2] and the wind field in a 3D multi-tower cube setup. The same experimental setup (geometry and instrumentation) and the same methodology were applied to all the three experiments. It is shown that all sites are affected by advection in different ways and strengths. Everywhere, vertical advection (F-VA) occurred only at night. During the day, F-VA disappeared because of turbulent mixing, leading to a uniform vertical profile of [CO2]. Mean F-VA was nearly zero at the hilly site (wetzstein) and at the flat site (Norunda). However, large, momentary positive or negative contributions occurred at the flat site, whereas vertical non-turbulent fluxes were generally very small at the hilly site. At the slope site (Renon), F-VA was always positive at night because of the permanently negative mean vertical wind component resulting from downslope winds. Horizontal advection also occurred mainly at night. It was positive at the slope site and negative at the flat site in the mean diurnal course. The size of the averaged non-turbulent advective fluxes was of the same order of magnitude as the turbulent flux measured by eddy-covariance technique, but the scatter was very high. This implies that it is not advisable to use directly measured quantities of the non-turbulent advective fluxes for the estimation of net ecosystem exchange (NEE) on e.g. an hourly basis. However, situations with and without advection were closely related to local or synoptic meteorological conditions. Thus, it is possible to separate advection affected NEE estimates from fluxes which are representative of the source term. However, the development of a robust correction scheme for advection requires a more detailed site-specific analysis of single events for the identification of the relevant processes. This paper presents mean characteristics of the advective CO2 fluxes in a first site-to-site comparison and evaluates the main problems for future research.

The vertical transport of reactive nitrogen gases such as NH3 and HNO3, and of NH4 and NO3 particle fluxes, and the dry deposition of SO2 and sulphate particles was studied within the BMBF-project VERTIKO (Vertical Transport under Complex Natural Conditions). An inferential model, based on measurements of meteorological and air chemistry parameters and the throughfall method, using wet precipitation and throughfall measurements as input data for the canopy budget model, were used for dry deposition estimates. The deposition calculations were performed with the newly compiled SVAT model SPRUCEDEP. This model uses 0.5 h meteorological data, and ambient air concentration figures with a time resolution of 0.5 h (SO2, NO, NO2) and 2 weeks (HNO2, HNO3, NH3, particulate NO 3 - , SO 4 2 - , and NH 4 + ). Compensation points in the inferential model were assumed to be 0 ppbv for NH3 and 1.7 ppbv for NOx. Annual dry deposition rates were estimated for oxidised N-species (NO, NO2, HNO2, HNO3, pNO3) to amount to about 6.1 kg ha−1, for reduced N (NH3, pNH4) to 6.4 kg ha−1, and for S (SO2, pSO4) to 10.1 kg ha−1 for the years 2002–2004. The relative contribution of dry to total (dry+wet) deposition was 63% for sulphur, 61% for oxidised nitrogen, and 57% for reduced nitrogen. Total S-deposition and oxidised N-species showed good agreement with data from throughfall measurements and a canopy exchange model. For reduced N-species, the modelling approach yielded a considerably higher deposition in the vegetation period which could be explained by the inability of canopy throughfall measurements to account for stomatal uptake of NH3 and probably microbial utilisation. The good agreement of the two methods suggests that a combination of the two modelling approaches may be an alternative to cost-intensive micrometeorological measurements for the determination of dry deposition fluxes on weekly or biweekly basis and long-term monitoring of total deposition.

Latent heat, which plays a major role in the energy balance of ecosystems, is usually calculated in different types of models, e.g., water balance approaches, aggregated “big-leaf” models, multi-layer ecosystem process models, that can be generally catalogued as Soil–Vegetation–Atmosphere–Transport models (SVATs) in a broader sense, etc. In this study, four (multi-) layered models (PLant-ATmosphere INteraction model (PLATIN), MixFor-SVAT, SVAT-CN, PnET-N-DNDC) and an atmospheric boundary layer model including vegetation (HIgh Resolution Vegetation Atmosphere Coupler (HIRVAC)) were used to predict latent heat (LE) and sensible heat (H) exchange in the target area of VERTIKO, a research project on vertical transports of energy and trace gases and their spatial/temporal extrapolation under complex natural conditions. Investigated vegetation types were a Norway spruce forest, and two grassland sites in Brandenburg and Saxony (Germany) during field campaigns in 2001 and 2002. Four models had half-hourly to hourly time steps, but PnET-N-DNDC had a daily time step. All models used meteorological conditions above the canopy as input, and computed LE and H independently, or H as the residual of the energy balance. Intercomparisons Abbreviations: b, offset of linear regression; [CO 2], CO2 concentration; DoY, day of year; EC, eddy covariance; ET, evapotranspiration; E0, potential evaporation; FB, fractional bias; IA, index of agreement; LW, longwave radiation; m, slope of linear regression; MAE, mean absolute error; NIR, near infrared radiation; P, air pressure; PAR, photosynthetic active radiation; PPT, precipitation; R.H., relative humidity; Rg, global radiation, incoming shortwave radiation; RMSE, root mean square error; Rn, net radiation; S.E. of y, standard error for the y estimate; S.E., standard error of parameter; Ta, air temperature; u, wind speed

In this chapter, the basic theory and the procedures used to obtain turbulent fluxes of energy, mass, and momentum with the eddy covariance technique will be detailed. This includes a description of data acquisition, pretreatment of high-frequency data and flux calculation.