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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.

Several studies showed the impact of global climate change in Germany and Saxony including the risk of increasing precipitation extremes. Here, heavy precipitation was analyzed on the basis of daily precipitation sums using the 95th percentile (index R95p). The long term development was studied for selected stations (1917–2013). Transects with high spatial resolution (1×1 km) (1961–2015) complemented the study to gain information about spatial temporal development of the occurrence of precipitation extremes. The non parametric kernel occurrence rate estimation has been applied to reveal changes in the temporal development of daily totals. The most distinct changes have been found for the seasons and the growing seasons and only slight changes for the calendar year and the meteorological half-years. The findings of this study showed a shifting seasonality with decreasing number of heavy precipitation events in the growing season I (April, May, June) and increasing number of events in growing season II (July, August, September). Furthermore, a distinct periodicity has been revealed in all findings for all seasons, particularly striking in the growing seasons, indicating the influence of large scale drivers as potentially the North Atlantic Oscillation on local precipitation extremes. Our data showed, that kernel occurrence rate estimation is a suitable approach to analyze the temporal development of heavy precipitation with a high temporal and spatial resolution.

The algorithm for and results of a newly developed multivariate non-parametric model, the Euclidean distance model (EDM), for the hourly disaggregation of daily climate data are presented here. The EDM is a resampling method based on the assumption that the day to be disaggregated has already occurred once in the past. The Euclidean distance (ED) serves as a measure of similarity to select the most similar day from historical records. EDM is designed to disaggregate daily means/sums of several climate elements at once, here temperature (T), precipitation (P), sunshine duration (SD), relative humidity (rH), and wind speed (WS), while conserving physical consistency over all disaggregated elements. Since weather conditions and hence the diurnal cycles of climate elements depend on the weather pattern, a selection approach including objective weather patterns (OWP) was developed. The OWP serve as an additional criterion to filter the most similar day. For a case study, EDM was applied to the daily climate data of the stations Dresden and Fichtelberg (Saxony, Germany). The EDM results agree well with the observed data, maintaining their statistics. Hourly results fit better for climate elements with homogenous diurnal cycles, e.g., T with very high correlations of up to 0.99. In contrast, the hourly results of the SD and the WS provide correlations up to 0.79. EDM tends to overestimate heavy precipitation rates, e.g., by up to 15% for Dresden and 26% for Fichtelberg, potentially due to, e.g., the smaller data pool for such events, and the equal-weighted impact of P in the ED calculation. The OWPs lead to somewhat improved results for all climate elements in terms of similar climate conditions of the basic stations. Finally, the performance of EDM is compared with the disaggregation tool MELODIST (Förster et al. 2015). Both tools deliver comparable and well corresponding results. All analyses of the generated hourly data show that EDM is a very robust and flexible model that can be applied to any climate station. Since EDM can disaggregate daily data of climate projections, future research should address whether the model is capable to respect and (re)produce future climate trends. Further, possible improvements by including the flow direction and future OWPs should be investigated, also with regard to reduce the overestimation of heavy rainfall rates.

The Namib Turbulence Experiment (NamTEX) was a multinational micrometeorological campaign conducted in the central Namib Desert to investigate three-dimensional surface layer turbulence and the spatiotemporal patterns of heat transfer between the subsurface, surface, and atmosphere. The Namib provides an ideal location for fundamental research that revisits some key assumptions in micrometeorology that are implicitly included in the parameterizations describing energy exchange in weather forecasting and climate models: homogenous flat surfaces, no vegetation, little moisture, and cloud-free skies create a strong and consistent diurnal forcing, resulting in a wide range of atmospheric stabilities. A novel combination of instruments was used to simultaneously measure variables and processes relevant to heat transfer: a 3-km fiber-optic distributed temperature sensor (DTS) was suspended in a pseudo-three-dimensional array within a 300 m × 300 m domain to provide vertical cross sections of air temperature fluctuations. Aerial and ground-based thermal imagers recorded high-resolution surface temperature fluctuations within the domain and revealed the spatial thermal imprint of atmospheric structures responsible for heat exchange. High-resolution soil temperature and moisture profiles together with heat flux plates provided information on near-surface soil dynamics. Turbulent heat exchange was measured with a vertical array of five eddy-covariance point measurements on a 21-m mast, as well as by collocated small- and large-aperture scintillometers. This contribution first details the scientific goals and experimental setup of the NamTEX campaign. Then, using a typical day, we demonstrate (i) the coupling of surface layer, surface, and soil temperatures using high-frequency temperature measurements, (ii) differences in spatial and temporal standard deviations of the horizontal temperature field using spatially distributed measurements, and (iii) horizontal anisotropy of the turbulent temperature field.

Compared to the well-known drivers of spatial variability in gross primary productivity (GPP), the relative importance of climatic variables, soil properties and plant traits to the spatial variability in net ecosystem exchange of CO2 between terrestrial ecosystem and atmosphere (NEE) is poorly understood. We used principal component regression to analyze data from 147 eddy flux sites to disentangle effects of climatic variables, soil properties and plant traits on the spatial variation in annual NEE and its components (GPP and ecosystem respiration (RE)) across global forests and grasslands. Our results showed that the largest unique contribution (proportion of variance only explained by one class of variables) to NEE variance came from climatic variables for forests (24%-30%) and soil properties for grasslands (41%-54%). Specifically, mean annual precipitation and potential evapotranspiration were the most important climatic variables driving forest NEE, whereas available soil water capacity, clay content and cation exchange capacity mainly influenced grassland NEE. Plant traits showed a small unique contribution to NEE in both forests and grasslands. However, leaf phosphorus content strongly interacted with soil total nitrogen density and clay content, and these combined factors represented a major contribution for grassland NEE. For GPP and RE, the majority of spatial variance was attributed to the common contribution of climate, soil and plant traits (50% - 62%, proportion of variance explained by more than one class of variables), rather than their unique contributions. Interestingly, those factors with only minor influences on GPP and RE variability (e.g., soil properties) have significant contributions to the spatial variability in NEE. Such emerging factors and the interactions between climatic variables, soil properties and plant traits are not well represented in current terrestrial biosphere models, which should be considered in future model improvement to accurately predict the spatial pattern of carbon cycling across forests and grasslands globally.

The eddy-covariance (EC) technique is used to determine mass and energy fluxes between the Earth’s surface and the lower atmosphere at high temporal resolution. Despite the frequent and successful use of the EC technique at terrestrial sites, its application over water surfaces is rare. We present one season of EC measurements conducted on the Rappbode Reservoir, Germany’s largest drinking water reservoir. A floating observation platform in the centre of the reservoir is used for observations of fluxes that were unaffected by surrounding land surfaces and therefore representative of the actual water–atmosphere exchange. The temporal patterns of sensible heat flux are inverted compared to land sites, since the maxima and the minima occur at night and day respectively. The latent heat flux and the evaporation are unexpectedly low for a site where evaporation is not limited by the availability of water. The daily totals in summer and autumn are only 50% and 75% of the potential evaporation assessed by the FAO grass-reference evaporation, respectively. Measurement uncertainties and the effects of the energy balance closure are ruled out as potential factors, so that low values appear to be a general feature of large water surfaces. The observed carbon dioxide fluxes are characterized by distinctive diurnal variations in a typical range for lakes and reservoirs. However, the methane fluxes are low compared to other inland waters.

In this study, statistical downscaling models were used to project possible future patterns of precipitation and temperature in the Jhelum River basin shared by Pakistan and India. In-situ meteorological data were used to downscale precipitation and temperature using different General Circulation Models (i.e., CanESM2, BCC-CSM1–1, and MICROC5) relative to baseline (1961–1990) under the Representative Concentration Pathway (RCP) scenarios RCP4.5 and RCP8.5. The downscaling models used were the Statistical Downscaling Model (SDSM), which uses multiple linear regression and weather generator methods, and the Long Ashton Research Station Weather Generator (LARS-WG), which uses weather generators. The results showed that the SDSM performance was slightly better than that of LARS-WG during validation and that the representation of the simulated mean monthly precipitation was more correct than that of monthly precipitation. The results also revealed that BCC-CSM1–1 performed better than CanESM2 and MICROC5 in the study region. The future annual mean temperature and precipitation are expected to rise under both RCP scenarios. The changes in the annual mean temperature and precipitation with LARS-WG were relatively higher than those with SDSM. Out of four seasons, winter and autumn are expected to be more diverse with regard to precipitation changes. However, although both models yielded non-identical results, it is certain that the basin will face a hotter climate in the future.

Pakistan is among the countries that are highly vulnerable to climate change. The country has experienced severe floods and droughts during recent decades. The agricultural sector in Pakistan is adversely affected by climate change. This systematic review paper set out to analyze the existing literature on adaptation measures at the farm level toward climate change in Pakistan. Adopting a Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) method, a total of 62 articles were identified from the Web of Science and Scopus databases. The review paper indicates that the main adaptation strategies adopted by farmers are as follows: changing cropping practices, changing farm management techniques, advanced land use management practices, and nonagriculture livelihood options. Further, this review shows the factors influencing the farmer’s adaptation measures to climate change. Influencing factors were examined and classified into three groups: demographic, socioeconomic, and resources and institutional. Barriers hindering farmers’ adaptive capacity were identified as lack of access to information and knowledge, lack of access to extension services, lack of access to credit facility, and lack of farm resources.

East Africa is one of the most vulnerable regions of Africa to extreme weather and climate events. Regional and local information on climate extremes is critical for monitoring and managing the impacts and developing sustainable adaptation measures. However, this type of information is not readily available at the necessary spatial resolution. Therefore, here we test trends and variability of temperature (1979–2010) and precipitation (1981–2016) extremes in East Africa, particularly Ethiopia, Kenya, and Tanzania, at a spatial resolution of 0.1 and 0.05°, respectively, using the indices defined by the Expert Team on Climate Change Detection and Indices (ETCCDI). We use gridded data sets with high accuracy and resolution from the Terrestrial Hydrology Research Group, University of Princeton and Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS). Trends of 19 indices are computed by fitting a linear model and using the nonparametric Mann–Kendall test and the magnitude of change is computed using the Sen's slope method. The results show an increasing trend in monthly maximum and minimum values of daily maximum and minimum temperature in large parts of the region. This is accompanied by significant increasing trends in warm nights (TN90p), warm days (TX90p), warm spell duration index (WSDI), and summer days index (SU). In addition, cold days (TX10p) and cold nights (TN10p) showed a significant decreasing trend. In general, the results show an increasing tendency in temperatures extremes, which is in line with rising global mean temperature. In addition, most of the temperature extremes observed after 2000 are warmer than the long‐term mean (1979–2010). Precipitation indices, on the other hand, showed increasing and decreasing trends in Ethiopia, Kenya, and Tanzania, but no general pattern. The outcomes enable identifying hot spot areas and planning of adaptation and mitigation measures at much finer spatial scale than previously possible.

The seasonal and interannual variability of the terrestrial carbon cycle is regulated by the interactions of climate and ecosystem function. However, the key factors and processes determining the interannual variability of net ecosystem productivity (NEP) in different biomes are far from clear. Here, we quantified yearly anomalies of seasonal and annual NEP, net carbon uptake period (CUP), and the maximum daily NEP (NEPmax) in response to climatic variables in 24 deciduous broadleaf forest (DBF), evergreen forest (EF), and grassland (GRA) ecosystems that include at least eight years of eddy covariance observations. Over the 228 site-years studied, interannual variations in NEP were mostly explained by anomalies of CUP and NEPmax. CUP was determined by spring and autumn net carbon uptake phenology, which were sensitive to annual meteorological variability. Warmer spring temperatures led to an earlier start of net carbon uptake activity and higher spring and annual NEP values in DBF and EF, while warmer autumn temperatures in DBF, higher autumn radiation in EF, and more summer and autumn precipitation in GRA resulted in a later ending date of net carbon uptake and associated higher autumn and annual NEP. Anomalies in NEPmax s were determined by summer precipitation in DBF and GRA, and explained more than 50% of variation in summer NEP anomalies for all the three biomes. Results demonstrate the role of meteorological variability in controlling CUP and NEPmax, which in turn help describe the seasonal and interannual variability of NEP.

Terrestrial evapotranspiration (ET) for each plant functional type (PFT) is a key variable for linking the energy, water and carbon cycles of the atmosphere, hydrosphere and biosphere. Process-based algorithms have been widely used to estimate global terrestrial ET, yet each ET individual algorithm has exhibited large uncertainties. In this study, the support vector machine (SVM) method was introduced to improve global terrestrial ET estimation by integrating three process-based ET algorithms: MOD16, PT-JPL and SEMI-PM. At 200 FLUXNET flux tower sites, we evaluated the performance of the SVM method and others, including the Bayesian model averaging (BMA) method and the general regression neural networks (GRNNs) method together with three process-based ET algorithms. We found that the SVM method was superior to all other methods we evaluated. The validation results showed that compared with the individual algorithms, the SVM method driven by tower-specific (Modern Era Retrospective Analysis for Research and Applications, MERRA) meteorological data reduced the root mean square error (RMSE) by approximately 0.20 (0.15) mm/day for most forest sites and 0.30 (0.20) mm/day for most crop and grass sites and improved the squared correlation coefficient (R2) by approximately 0.10 (0.08) (95% confidence) for most flux tower sites. The water balance of basins and the global terrestrial ET calculation analysis also demonstrated that the regional and global estimates of the SVM-merged ET were reliable. The SVM method provides a powerful tool for improving global ET estimation to characterize the long-term spatiotemporal variations of the global terrestrial water budget. (Resume d'auteur)

Accurate predictions of net ecosystem productivity (NEPc) of forest ecosystems are essential for climate change decisions and requirements in the context of national forest growth and greenhouse gas inventories. However, drivers and underlying mechanisms determining NEPc (e.g., climate and nutrients) are not entirely understood yet, particularly when considering the influence of past periods. Here we explored the explanatory power of the compensation day (cDOY)-defined as the day of year when winter net carbon losses are compensated by spring assimilation-for NEPc in 26 forests in Europe, North America, and Australia, using different NEPc integration methods. We found cDOY to be a particularly powerful predictor for NEPc of temperate evergreen needleleaf forests (R2=0.58) and deciduous broadleaf forests (R2=0.68). In general, the latest cDOY correlated with the lowest NEPc. The explanatory power of cDOY depended on the integration method for NEPc, forest type, and whether the site had a distinct winter net respiratory carbon loss or not. The integration methods starting in autumn led to better predictions of NEPc from cDOY then the classical calendar method starting 1 January. Limited explanatory power of cDOY for NEPc was found for warmer sites with no distinct winter respiratory loss period. Our findings highlight the importance of the influence of winter processes and the delayed responses of previous seasons' climatic conditions on current year's NEPc. Such carry-over effects may contain information from climatic conditions, carbon storage levels, and hydraulic traits of several years back in time. (Less)

Taylor’s frozen turbulence hypothesis states that “standard” eddy-covariance measurements of fluxes at a fixed location can replace a spatial ensemble of instantaneous values at multiple locations. For testing this hypothesis, a unique turbulence measurement set-up was used for two measurement campaigns over desert (Namibia) and grassland (Germany) in 2012. This “Eddy Matrix” combined nine ultrasonic anemometer–thermometers and 17 thermocouples in a 10 m $$\times $$ 10 m regular grid with 2.5-m grid distance. The instantaneous buoyancy flux derived from the spatial eddy covariance of the Eddy Matrix was highly variable in time (from $$-0.3$$ to 1 m K s $$^{-1}$$ ). However, the 10-min average reflected 83 % of the reference eddy-covariance flux with a good correlation. By introducing a combined eddy-covariance method (the spatial eddy covariance plus the additional flux of the temporal eddy covariance of the spatial mean values), the mean flux increases by 9 % relative to the eddy-covariance reference. Considering the typical underestimation of fluxes by the standard eddy-covariance method, this is seen as an improvement. Within the limits of the Eddy Matrix, Taylor’s hypothesis is supported by the results.

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.

As a contribution to an Integrated Water Resources Management (IWRM) project in Distrito Federal, Brazil, we address several aspects for a credible downscaling of near-surface air temperature and precipitation using the Statistical DownScaling Model (SDSM4.2). For instance, we apply a detailed screening of predictors, consider the end user needs in the validation procedure, assess the added value of the downscaling model and include several sources of uncertainties until the downscaling step. Results suggest that the interpolation of large-scale predictors to the target site is a reasonable alternative to predictors derived from grid-boxes. The validation metrics, measures (i.e. bias, root-mean-square error, and Pearson's correlation coefficient) and quantile–quantile plots reveal that model tends to underestimate near-surface temperature and precipitation; whereas extreme values are subject of considerable uncertainties. Single-site projections at daily scale are derived from 27 climate models from the Coupled Model Intercomparison Project phase 5 (CMIP5) forced by Representative Concentration Pathways (i.e. RCP2.6, RCP4.5, RCP6.0 and RCP8.5) scenarios. The downscaling model adds substantial value in terms of amplitude of variability when compared to the host coarse-resolution projections. Its performance is higher than a quantile-mapping bias correction technique, particularly in reproducing observed trends. In spite of the elevated level of uncertainties in the magnitude of change, most of the downscaled projections agree on positive changes in near-surface temperature and precipitation for the period of 2036–2055 when compared to the reference period (i.e. 1986–2005). The massive amount of downscaled projections is of limited application in hydrological studies and, therefore, we suggest a summarized group of projections which are representative to the central tendency and spread of the ensemble.

The second Meteor Crater Experiment (METCRAX II) was conducted in October 2013 at Arizona’s Meteor Crater. The experiment was designed to investigate nighttime downslope windstorm−type flows that form regularly above the inner southwest sidewall of the 1.2-km diameter crater as a southwesterly mesoscale katabatic flow cascades over the crater rim. The objective of METCRAX II is to determine the causes of these strong, intermittent, and turbulent inflows that bring warm-air intrusions into the southwest part of the crater. This article provides an overview of the scientific goals of the experiment; summarizes the measurements, the crater topography, and the synoptic meteorology of the study period; and presents initial analysis results.

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.

The eddy covariance technique was used to measure the CO2 flux over four differently grazed Leymus chinensis steppe ecosystems (ungrazed since 1979 (UG79), winter grazed (WG), continuously grazed (CG), and heavily grazed (HG) sites) during four growing seasons (May to September) from 2005 to 2008, to investigate the response of the net ecosystem exchange (NEE) over grassland ecosystems to meteorological factors and grazing intensity. At UG79, the optimal air temperature for the half-hourly NEE occurred between 17 and 20 °C, which was relatively low for semi-arid grasslands. The saturated NEE (NEEsat) and temperature sensitivity coefficient (Q 10) of ecosystem respiration (RE) exhibited clear seasonal and interannual variations, which increased with canopy development and the soil water content (SWC, at 5 cm). The total NEE values for the growing seasons from 2005 to 2008 were −32.0, −41.5, −66.1, and −89.8 g C m−2, respectively. Both the amounts and distribution of precipitation during the growing season affected the NEE. The effects of grazing on the CO2 flux increased with the grazing intensity. During the peak growth stage, heavy grazing and winter grazing decreased NEEsat and gross primary production (45 % for HG and 34 % for WG) due to leaf area removal. Both RE and Q 10 were clearly reduced by heavy grazing. Heavy grazing changed the ecosystem from a CO2 sink into a CO2 source, and winter grazing reduced the total CO2 uptake by 79 %. In the early growing season, there was no difference in the NEE between CG and UG79. In addition to the grazing intensity, the effects of grazing on the CO2 flux also varied with the vegetation growth stages and SWC.

Grazing intensity effects on the evapotranspiration (ET) partitioning into evaporation (E) and transpiration (T) were investigated using eddy covariance measurement data during the growing season (May to September) from 2005 to 2008 over typical steppes in Inner Mongolia, China. Four grazing intensities were examined (UG79, ungrazed since 1979; WG, winter grazed; CG, continuously grazed; HG, heavily grazed). Soil water content (SWC) significantly influenced daily T/ET by affecting T and E in different ways. On a monthly scale, T/ET at UG79 (0.46–0.77) increased with vegetation growth regardless of the uneven distribution of precipitation. The mean T/ET at UG79 for the growing season ranged from 0.50 to 0.74 for 2005 to 2008. In the normal precipitation year (2006), WG and HG reduced T/ET because they significantly reduced T through the removal of leaf area. In the dry year (2005), this negative effect on T was depressed by summer droughts, while WG and HG decreased E primarily due to the decreased interception of rainfall by the canopy. The grazing effect on T/ET was opposite to that in 2006. These results indicate that grazing effects on ET partitioning could vary with soil water conditions in the semiarid typical steppe ecosystems.

Available climatological information of Distrito Federal does not satisfy the requirements for detailed climate diagnosis, as they do not provide the necessary spatial resolution for water resources management purposes. Annual and seasonal climatology (1971–2000) of precipitation from 6 meteorological stations and 54 rain gauges from Central Brazil were used to test eight different spatial interpolation methods. Geographical factors (i.e., altitude, longitude and latitude) explain a large portion of precipitation in the region, and therefore, multivariate models were included. The performance of estimations was assessed through independent validation using mean square error, correlation coefficient and Nash–Sutcliffe efficiency criterion. Inverse distance weighting (IDW), ordinary kriging (OK) and the multivariate regression with interpolation of residuals by IDW (MRegIDW) and OK (MRegOK) have performed the lowest errors and the highest correlation and Nash–Sutcliffe efficiency criterion. In general, interpolation methods provide similar spatial distributions of rainfall wherever observation network is dense. However, the inclusion of geographical variables to the interpolation method should improve estimates in areas where the observation network density is low. Nevertheless, the assessment of uncertainties using a geostatistical method provides supplementary and qualitative information which should be considered when interpreting the spatial distribution of rainfall.

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.

Stability indices provide simple metrics to characterize tropospheric states favouring convection. Not only for regions where precipitation is mainly related to convective processes, important aspects of the climate comprising information from different levels and variables can therefore be summarized in scalar metrics. Although the linkage between tropospheric stability and convective precipitation is blurred by additional prerequisites often not resolved in atmospheric models, analysis of stability indices from global climate models (GCMs) provides a more process-orientated assessment than the separate analysis of the individual climate variables. This paper presents an assessment of GCM capabilities to simulate tropospheric stability on the Arabian Peninsula. Therefore, six stability indices (K-Index, Total Totals Index, Vertical Totals Index, Showalter Index, SWEAT and Cross Totals Index) were calculated for several GCMs from the Climate Model Intercomparison Project 3 archive for three locations. GCM indices were compared with reanalysis data from the National Centers for Environmental Prediction in collaboration with the National Center for Atmospheric Research as well as radiosoundings from the Integrated Global Radiosounding Archive. Comparison was done by means of a classification approach based on quantile values of the index distribution. Input parameters of the indices were also analysed and biases for indices as well as input parameters were identified. There are biases found for the different indices which can be attributed to some extent to biases of the input parameters. There is no input parameter which is biased in the same direction for all indices and locations. Due to the variability of the results, rankings of the models are characterized by large rank differences so that there is no overall best performing model. Nevertheless, this study provides some insight in the performance of the GCMs to simulate tropospheric stability at individual locations and so presents an alternative method of model performance assessment.

This study investigates the effects of different urban planning scenarios on the thermal comfort of pedestrians in two districts of the city of Dresden. A total of three alternatives to reconstruct these districts were created by considering realistic approaches for urban planning projections in the next ten years. These alternatives include the development and densification of existing buildings as well as the greening of undeveloped areas or the reduction of existing green areas. Based on these planning alternatives, micro-meteorological conditions as well as bioclimatic indices within the chosen city districts of Dresden for a hot summer day were simulated with a combination of the 3D microclimatic model ENVI-met and the bioclimatic model Rayman. Using both, potential air temperature (T) and the bioclimatic index UTCI (Universal Thermal Comfort Index), spatial distribution at different times of the day, the cumulative thermal stress conditions at both exposed "receptor" points and on a virtual course of pedestrians through the inner city of Dresden, were analysed. Simulation results clearly show the local reduction of T and UTCI due to the shading effect of denser vegetation or building structures during the daytime. For example, a greening of an industrial wasteland locally reduces T and UTCI in the early afternoon of a hot and sunny summer day by up to 2 and 14 K, respectively. The same increase in T and UTCI can be found in non-shaded partitions of this area if it is sparsely rebuilt. At night, areas with widespread building placement show a higher T and UTCI (up to 1 and 4 K, respectively), and areas with dense vegetation are cooler (0.1 to 0.4 K) than open areas. Due to the small extension of modified urban structures, the mean effect over the complete area amounts to only a fraction of a degree at most. The simulation results demonstrate that a greening of urban wastelands has primarily positive effects (that means cooling) on thermal extremes during hot summer periods. A dense rebuilding causes similar positive effects during the daytime due to the generation of new shading zones as well as negative effects (warming) by night. For citizens occupying these city areas, the local effect of a modified urban structure could be more relevant than the mean effect over the whole city district. Based on these findings, modern approaches of city development should optimally combine green areas, shadow zones of large buildings and the maintenance of "street canyon ventilation".

Mixture models of precipitation frequency distributions describe the precipitation spectrum more adequately than simple probability distributions. The estimation of mixture distribution parameters by the maximum likelihood approach is commonly known. The reliability of these models and their parameters derived by the maximum likelihood method and by temporal classifications over Saxony was investigated using a radar-based precipitation product on a daily basis. Therefore, two different ‘unsupervised’ classification approaches were applied, a cluster analysis and a neural network, to derive subpopulation estimates. The temporal classification summarized high-resolution spatial grids of daily precipitation fields into 10 precipitation classes. The mixture model evaluation consists of two parts. First, an information criterion was calculated to describe the model complexity and the significance of the number of parameters. Second, the qualities of the fitted models were compared with the database through ordinary Kolmogorov–Smirnov tests and Kolmogorov–Smirnov tests in the context of field significance. Under the assumption of log-normal distributed precipitation, the neural network delivered the best mixture models for a spatial resolution of 2 and 3 km while the cluster analysis delivered the better initial parameters for an expectation-maximization algorithm on mixture distributions over the considered domain.

In the framework of the IWAS/Agua-DF project, this study focuses on changes in mean surface air temperature and accumulated precipitation in Central Brazil over the past 40 years. It has two main objectives: (1) comparison between two climatological periods (2001–2010 and 1971–2000) and (2) trend analysis of climate variables. Time series of meteorological and rain gauge stations from Central Brazil have been organized in a databank, which contains tools for homogeneity tests. From that, 4 temperature and 55 precipitation time series were sufficient homogeneous, while 1 temperature and 5 precipitation time series were identified as inhomogeneous. Reliable spatial distribution was produced using proper interpolation method. Trends and significance levels were calculated by Rapp’s estimator of slope and Mann–Kendall test, respectively. The most important results of the comparisons and trend analysis in the last four decades are: (1) marked increase in annual and seasonal mean surface air temperature, (2) evident decreases of accumulated rainfall in winter and autumn, and (3) apparent increase of precipitation amounts in the rainy season.

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.

At the study site Tharandt Anchor Station in Saxony/Germany sap flow measurements are conducted in an old Norway spruce (Picea abies [L.] KARST.) stand. During the study period from 2001 to 2007 several events like thinning, long and short drought periods and a winter storm significantly affected the amount of canopy water use. We show that intra-annual variation of Ec is strongly related to VPD and PPFD. While there is a non-linear relationship between daily Ec and VPD, daily Ec is limited by daily integrated PPFD indicating stomatal control of Ec through photosynthesis. On a monthly or seasonal basis, reduction of Ec is not only related to high VPD and non-saturating PPFD, but also to higher frequencies of precipitation. In comparison to this, nearly 55% of canopy precipitation and 20% of available energy were used for transpiration during the growing season. Intensive seasonal soil water measurements at the site revealed that on average about 74% of soil water removal within the rooting zone can be related to tree water uptake. A good correlation was found between annual Ec and Ecmax, usually occurring in June or July. Further, the monthly sums of June plus July were good predictors of annual Ec. Within the study period, the extreme drought in 2003 revealed a clear threshold of soil water content by 9.5 vol% and had the most pronounced effect on annual Ec followed by a stand thinning. The winter storm “Kyrill” in January 2007 had caused loss of green needles and twigs. It is assumed that the observed reduction in Ec during spring was related to the reduced leaf biomass and potentially to root damage of bended trees. Excluding the effect of extreme drought and forest management, a mean inter-annual variation in Ec of ±15% and in Ec/VPD of ±8% remained. It is concluded that lag-effects of drought and the winter storm add lacking explanation to the inter-annual variability of canopy transpiration besides the typical variation of atmospheric conditions.

The advection initiative ADVEX within CarboEurope-IP conducted advection experiments at three European coniferous sites in 2005 and 2006. All experiments shared the same geometry and instrumentation. Data of the ADVEX experiments were used to calculate advective fluxes of carbon dioxide and sensible heat using exactly the same method. However, the advective flux of sensible heat can be assessed more easily than the carbon dioxide flux with its associated complex measurements of gas concentrations. We explored the possibility to use advective fluxes of sensible heat as a proxy for the corresponding flux of carbon dioxide despite somewhat differing sinks and sources. On average, advective fluxes of sensible heat were of opposite sign in relation to the advective fluxes of carbon dioxide for the three investigated sites, especially during nighttime. Therefore, the respective gradients were of opposite sign, on average, for vertical and (to a lesser extent) horizontal direction. This is not as obvious for horizontal direction as for the vertical direction. A scheme is presented to explain the correlation of the respective gradients for different conditions. Based on the gained insights and regression statistics, two simple empirical models were tested to derive advective fluxes of carbon dioxide from advective fluxes of sensible heat. Our results suggest that the advective flux of sensible heat could be taken as an indicator concerning the presence and sign of carbon dioxide advection. However, the suitability of advective fluxes of sensible heat as a quantitative proxy for advective fluxes of carbon dioxide is more problematic because the representativeness including the magnitude of advection derived from advection measurements is not yet clarified. An inspection of the budget of sensible heat and carbon dioxide revealed considerable changes by advection. The results indicate that the budget of carbon dioxide might be generally more affected by the investigated non-turbulent advective fluxes than the budget of sensible heat.

The drag coefficient (C d) and heat transfer coefficient (C h) with the bulk transfer scheme are usually used to calculate the momentum and heat fluxes in meteorological models.The aerodynamic roughness length (z 0m) and thermal roughness length (z 0h) are two crucial parameters for bulk transfer equations.To improve the meteorological models,the seasonal and interannual variations of z 0m,z 0h,coefficient kB 1,C d,and C h were investigated based on eddy covariance data over different grazed semiarid grasslands of Inner Mongolia during the growing seasons (May to September) from 2005 to 2008.For an ungrazed Leymus chinensis grassland (ungrazed since 1979),z 0m and z 0h had significant seasonal and interannual variations.z 0m was affected by the amount and distribution of rainfall.kB 1 exhibited a relatively negative variation compared with z 0h,which indicates that the seasonal variation of z 0h cannot be described by kB 1.To parameterize z 0m and z 0h,the linear regressions between ln(z 0m),ln(z 0h),and the leaf area index (LAI) were performed with R 2 =0.71 and 0.83.The monthly average kB 1 was found to decrease linearly with LAI.The four-year averaged values of C d and C h were 4.5×10 3 and 3.9×10 3,respectively.The monthly average C d only varied by 8% while the variation of C h was 18%,which reflects the different impacts of dead vegetation on momentum and heat transfer at this natural grassland.Moreover,with the removal of vegetation cover,grazing intensities reduced z 0m,z 0h,C d,and C h.

The concentration of carbon dioxide in the atmosphere is influenced by land use and management. Here, the carbon (CO 2 –C) budgets of a managed forest (spruce), a grass site and a crop site (crop rotation) have been compared to examine the effects of different management practices on net ecosystem exchange (NEE) and net biome productivity (NBP). This approach enables a more comprehensive carbon budgeting as it takes into account carbon exports and imports for particular land uses. Based on NEE measured with the eddy covariance (EC) method, we quantified NBP by also considering the carbon exports due to thinning or harvest and the carbon imports due to organic fertilisation. The sites are part of the Tharandt cluster in eastern Germany, characterised as having only slight differences in weather and climate. As a result, the management effects on the C budgets can be distinguished from other drivers. For the forest site, continuous CO 2 flux measurements are available from 1997 to today, and the common study period of the grassland and cropland sites is 2005–2008 (one crop rotation). Only when considering the forest's annual NEE was there a variability in the net sink from −698 g C m −2 (1999) to −444 g C m −2 (2003), whereas the grassland and cropland sites exhibited small sinks between −177 g C m −2 (2004) and −62 g C m −2 (2005) and between −115 g C m −2 (2005) and −32 g C m −2 (2007 and 2008), respectively. The forest site is a carbon source with an NBP +221 g C m −2 in 2002 because ∼43 m 3 ha −1 solid wood was removed. The grassland alternated between carbon sources and sinks, with NBP ranging from +25 g C m −2 (2008) to −28 g C m −2 (2006) due to carbon export through several cuts per year. The cropland site was mainly influenced by the cultivated crop species and the application of organic fertiliser (manure), resulting in NBP values between +484 g C m −2 (2007) and −89 g C m −2 (2006). The different timing and length of the growing season for winter and spring crops result in different intra-annual patterns of NEE as well as lower annual net CO 2 sinks for the spring crops. Alternative accounting periods have been defined for all sites starting immediately after the harvest date at the crop site. Based on the full crop rotation period (around 4 years), the NBP showed a C sink for the spruce site and a C source for the crop rotation. The grass site was C neutral according to the calculated NBP.

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.

A weather-pattern-based multiple regression model to derive future possible changes in the level of the higher temporal resolution spectrum of heavy precipitation has been developed. The temporal spectrum was described using statistical precipitation amounts as a function of the event's duration (1–24 h) and return period (once in 5 yr to once in 100 yr). The principle of the method consists in projecting a statistical relationship between the parameters of a transformed Gumbel distribution (theoretical extreme value distribution) and the distribution of classes of objective weather patterns to time slices in the near future of climate. Changes in distribution parameters were constructed in the model from changes in the distribution of weather patterns. Possible change signals were calculated for the catchment of the Weiseritz River (Ore Mountains, Germany) for the time slices centred around 2025 (2011–2040) and 2050 (2036–2065) as changes versus the reanalyses of the reference period 1961–2000 (May–September). For the climate conditions to be expected in the near future (IPCC A1B scenario), increases in the amounts of heavy precipitation, i.e. decreases in the return periods of equal amounts of heavy precipitation from the reference period, were obtained for the entire temporal spectrum covered by this paper. Overall, the change signals derived on the basis of a concept of weather patterns seem plausible because they represent a possible continuation of the already observed increase in frequency and intensification of events of heavy precipitation in the extended study area. Copyright © 2009 Royal Meteorological Society

To regionalize precipitation on a small-scale, complex, low mountain range terrain, trigonometric relationships were constructed in a geographical information system (GIS) environment using a digital elevation model (DEM). The large-scale orographic effect and small-scale windward and leeward effects on precipitation were recorded and quantified using a grid-based combination of a terrain vector (slope angle, aspect) with a wind vector (wind direction, speed). This was based on the parameterization of a longer-term wind-precipitation distribution. Both effects were mapped for the meso-scale Weiseritz River catchment in the Ore Mountains of Germany, using an estimate of spatially distributed slope precipitation. It was shown that the small-scale windward and leeward effects modify the larger-scale orographic effect spatially. It was determined that both the amount and the phase of the small-scale windward and leeward effect correlates with the height above sea level. During the 1971–2003 growing seasons the amount of precipitation increased by 4% per 100 m, and for the 1971–2000 statistically heavy precipitation (SHP) (duration: 24 h, return period: 100 years) the increase was 1% per 100 m. The phase shift occurred because the main wind veered south-west to west to north-west with elevation increase, and it was more pronounced for mean than for heavy precipitation. Spatial slope precipitation was estimated to be within the 390–715 mm range for mean adjusted precipitation and 100–140 mm for SHP. The maximum of the windward and leeward effect on precipitation amounts to ± 25% of fallen precipitation. Copyright © 2008 Royal Meteorological Society

The eddy-covariance method is the state-of-the-art tool to measure mass and energy fluxes, though many measuring systems (particularly closed-path systems) show strong spectral attenuation that causes significant underestimates of actual mass fluxes. The standard way to correct these underestimates is to use theoretically derived transfer functions. Practical implementation has shown that the attenuation, especially of closed-path systems, is underestimated by this method, even when the additional tube damping is considered. This paper introduces and tests three alternative site-specific and hardware specific correction methods based on spectral analysis, which typically enhance—additionally to the classical correction—the calculated mass fluxes based on closed-path analysers, as their inevitable tubing and the filters used cause additional flow attenuation. Two of the three methods are based on a site-specific and hardware specific transfer functions, the third uses direct comparison of cospectra. Primarily the methods based on transfer functions proved to be easy-to-handle once established for the specific set-up and measurement site. They represent practical and robust methods to correct for spectral attenuation.

Four different approaches for the calculation of the well established decoupling coefficient Ω are compared using measurements at three experimental sites (Tharandt – spruce forest, Grillenburg and Melpitz – grass) and simulations from the soil-vegetation boundary layer model HIRVAC. These investigations aimed to quantify differences between the calculation routines regarding their ability to describe the vegetation-atmosphere coupling of grass and forest with and without water stress. The model HIRVAC used is a vertically highly resolved atmospheric boundary layer model, which includes vegetation. It is coupled with a single-leaf gas exchange model to simulate physiologically based reactions of different vegetation types to changing atmospheric conditions. A multilayer soil water module and a functional parameterisation are the base in order to link the stomata reaction of the gas exchange model to the change of soil water. The omega factor was calculated for the basic formulation according to McNaughton and Jarvis (1983) and three modifications. To compare measurements and simulations for the above mentioned spruce and grass sites, the summer period 2007 as well as a dry period in June 2000 were used. Additionally a developing water stress situation for three forest canopies (spruce, pine and beech) and for a grass site was simulated. The results showed large differences between the different omega approaches which depend on the vegetation type and the soil moisture. Between the omega values, which were calculated by the used approach, the ranking was always the same not only for the measurements but also for the adapted simulations. The lowest values came from the first modification including doubling factors and summands in all parts of omega equation in relation to the original approach. And the highest values were calculated with the second modification missing one doubling factor in the denominator of the omega equation. For example, the averages of omega ranged in the summer period 2007 from 0.11 to 0.19 for the spruce site and moderate soil wetness and from 0.42 to 0.58 for the grass site and higher soil wetness. In the case of the simulated drying out of four different canopies the forest stands showed a similar change of omega from about 0.65 (moderate soil wetness) to 0.1 (low soil wetness). The absolute change of omega for the grass canopy was smaller than for the forest canopies (on average from 0.95 to 0.7). But the differences between the used omega approaches increased. Especially the results from the longer period in summer 2007 demonstrate that the various modifications of the decoupling coefficient lead to a change in the long-term quantity of omega. This has, for example, consequences for the description of the coupling of heterogeneous landscapes.

A roving tower concept was used to compare a semi-arid grassland site in Inner Mongolia (China), which was fenced in 1979 and ungrazed thereafter (UG79) with differently grazed semi-arid steppe ecosystems. The study was conducted during three consecutive years characterised by contrasting precipitation. The different grazing intensities included continuously and moderately grazed (CG), winter grazed (WG), and heavily grazed (HG). Here, we compare the energy fluxes and surface parameters that characterise the differently managed plots. The main focus is on sensible heat flux (H), available energy (AE), surface temperature (T ( s )), and surface albedo (alpha). Systematic errors were excluded by a side-to-side intercomparison of the instruments, and systematic climatic differences were minimised by the close distance between the fixed and the roving eddy covariance tower. Statistically, AE and T ( s ) were always significantly different between two simultaneously measured grazing intensities. Whereas AE was higher at UG79 in all years (mean difference of about 19Wm(-2)), T ( s ) was typically lower at UG79 (mean differences of 0.4 degrees C to about 2 degrees C). The exception was the end of the vegetation period in 2004 when T ( s ) was 0.6 degrees C higher at UG79 compared to CG. At UG79 alpha was typically significantly lower, and H was typically significantly higher. Consequently, latent heat fluxes (both as energy balance residual and directly measured) do not differ much between the different grazing intensities. It is concluded, that (1) the roving tower concept is able to detect differences due to grazing, (2) differences between the sites can be attributed to real surface differences, and (3) differences due to grazing intensities are small compared to interannual differences in surface fluxes.

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 quantitative simulation of gross primary production (GPP) at various spatial and temporal scales has been a major challenge in quantifying the global carbon cycle. We developed a light use efficiency (LUE) daily GPP model from eddy covariance (EC) measurements. The model, called EC-LUE, is driven by only four variables: normalized difference vegetation index (NDVI), photosynthetically active radiation (PAR), air temperature, and the Bowen ratio of sensible to latent heat flux (used to calculate moisture stress). The EC-LUE model relies on two assumptions: First, that the fraction of absorbed PAR (fPAR) is a linear function of NDVI; Second, that the realized light use efficiency, calculated from a biome-independent invariant potential LUE, is controlled by air temperature or soil moisture, whichever is most limiting. The EC-LUE model was calibrated and validated using 24,349 daily GPP estimates derived from 28 eddy covariance flux towers from the AmeriFlux and EuroFlux networks, covering a variety of forests, grasslands and savannas. The model explained 85% and 77% of the observed variations of daily GPP for all the calibration and validation sites, respectively. A comparison with GPP calculated from the Moderate Resolution Imaging Spectroradiometer (MODIS) indicated that the EC-LUE model predicted GPP that better matched tower data across these sites. The realized LUE was predominantly controlled by moisture conditions throughout the growing season, and controlled by temperature only at the beginning and end of the growing season. The EC-LUE model is an alternative approach that makes it possible to map daily GPP over large areas because (1) the potential LUE is invariant across various land cover types and (2) all driving forces of the model can be derived from remote sensing data or existing climate observation networks.

The eddy-covariance method is the primary way of measuring turbulent fluxes directly. Many investigators have found that these flux measurements often do not satisfy a fundamental criterion—closure of the surface energy balance. This study investigates to what extent the eddy-covariance measurement technology can be made responsible for this deficiency, in particular the effects of instrumentation or of the post-field data processing. Therefore, current eddy-covariance sensors and several post-field data processing methods were compared. The differences in methodology resulted in deviations of 10% for the sensible heat flux and of 15% for the latent heat flux for an averaging time of 30 min. These disparities were mostly due to different sensor separation corrections and a linear detrending of the data. The impact of different instrumentation on the resulting heat flux estimates was significantly higher. Large deviations from the reference system of up to 50% were found for some sensor combinations. However, very good measurement quality was found for a CSAT3 sonic together with a KH20 krypton hygrometer and also for a UW sonic together with a KH20. If these systems are well calibrated and maintained, an accuracy of better than 5% can be achieved for 30-min values of sensible and latent heat flux measurements. The results from the sonic anemometers Gill Solent-HS, ATI-K, Metek USA-1, and R.M. Young 81000 showed more or less larger deviations from the reference system. The LI-COR LI-7500 open-path H2O/CO2 gas analyser in the test was one of the first serial numbers of this sensor type and had technical problems regarding direct solar radiation sensitivity and signal delay. These problems are known by the manufacturer and improvements of the sensor have since been made.

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.

Anemometer and CO2 concentration data from temporary campaigns performed atsix CARBOEUROFLUX forest sites were used to estimate the importance of non-turbulentfluxes in nighttime conditions. While storage was observed to be significant only duringperiods of both low turbulence and low advection, the advective fluxes strongly influence thenocturnal CO 2 balance, with the exception of almost flat and highly homogeneous sites. Onthe basis of the main factors determining the onset of advective fluxes, the ‘advection velocity’,which takes net radiation and local topography into account, was introduced as a criterion tocharacterise the conditions of storage enrichment/depletion. Comparative analyses of the sixsites showed several common features of the advective fluxes but also some substantial dif-ferences. In particular, all sites where advection occurs show the onset of a boundary layercharacterised by a downslope flow, negative vertical velocities and negative vertical CO2concentration gradients during nighttime. As a consequence, vertical advection was observedto be positive at all sites, which corresponds to a removal of CO2 from the ecosystem. Themain differences between sites are the distance from the ridge, which influences the boundary-layer depth, and the sign of the mean horizontal CO2 concentration gradients, which isprobably determined by the source/sink distribution. As a consequence, both positive andnegative horizontal advective fluxes (corresponding respectively to CO 2 removal from theecosystem and to CO 2 supply to the ecosystem) were observed. Conclusive results on the importance of non-turbulent components in the mass balance require, however, further experimental investigations at sites with different topographies, slopes, different land covers, which would allow a more comprehensive analysis of the processes underlying the occurrence of advective fluxes. The quantification of these processes would help to better quantify nocturnal CO2 exchange rates.

The coupled atmospheric boundary layer model HIRVAC (High Resolution Vegetation Atmosphere Coupler) combines a 1.5 dimensional planetary boundary layer model (developed at the Humboldt University Berlin) and the mechanistic photosynthesis module PSN6 (University of Bayreuth) as well as a soil water module. This multi layer model includes a vertically structured vegetation and covers a range from 0-2 km height. The model layers are coupled via the turbulent exchange using a K-approach. Recent work aimed to include several new modules to enable model calculations for realistic conditions. Therefore rainfall and a multi layer interception and a soil water module were included. Further the light extinction through the canopy was upgraded by including sunfleck dynamics for calculating a more realistic radiation distribution within and below canopy. The upgraded model version was run for the Anchor Station in Tharandt where various measurements for comparison with model results are available. The obtained results demonstrate that periods of several days can be modelled successfully. A good model performance was found for both, soil water and interception. The modelled energy fluxes finally confirm the reliability of the overall model performance.

Within climate change research vegetation plays an important role in both indicating climate change by plant phenology and mitigating climate change by carbon sequestration. A 41-year study period of phenological observations at Tharandt International Phenological Garden and an 8-year study period of continuous flux measurements of carbon dioxide above Norway spruce (Picea abies L. [Karst.]) in the Tharandt forest were used to investigate long-term trends of phenological observations concurrent with climatic trends and to assess the importance of phenological changes for annual carbon gain of the forest. It could be shown that length of growing season determined from phenology was less variable than determined from temperature levels and significantly correlated with mean annual temperature. The slight increase in the length of growing season (LGS) (0.14 d/a) resulted from an earlier onset of spring (-0.32 d/a) and of autumn phases (-0.18 d/a). Obviously, this reflected regional climatic trends with significant temperature increase in spring and slight decrease in autumn. The pronounced effect in spring is also reflected by strongest correlation of annual carbon gain with the emergence of May shoot while the correlation with LGS was less significant. It was therefore concluded that earlier emergence of May shoot was more important for annual carbon gain (22.4 gC/m 2 /d gross uptake) than the increase in total LGS (17.1 gC/m 2 /d gross uptake). However, positive effects of premature phenological stages on carbon gain may be reduced by drought effects during summer as observed for the study year 2003.

The deposition of nitric oxides into a spruce stand at the Tharandt Forest, near Dresden, Saxony, was analysed. Eddy Covariance measurements, standard meteorological measurements and concentrations of nitric oxides of the anchor station nearby were available as model input. We used a resistance analogy model with a location-specific parametrisation for the resistances of transport to determine the deposition rates of NO and NO 2 for 1998 and 1999. The annual sums of deposition of NO 2 -N average between 0.96 and 1.16 kg/(ha*a) for 1998 and between 1.12 and 1.35 kg/(ha*a) for 1999. These results match well results of other studies from sites with similar characteristics. As expected, the comparison of the results of a diurnal-based model (DM) with a 30 min model (HHM) shows a better adaptation of the HHM. However, due to the better availability of daily data it is necessary to use the daily model for the estimation of long term entries beyond the period of parametrisation.