Helmut Mayer

Modelling radiation fluxes in simple and complex environments: basics of the RayMan model

Short- and long-wave radiation flux densities absorbed by people have a significant influence on their energy balance. The heat effect of the absorbed radiation flux densities is parameterised by the mean radiant temperature. This paper presents the physical basis of the RayMan model, which simulates the short- and long-wave radiation flux densities from the three-dimensional surroundings in simple and complex environments. RayMan has the character of a freely available radiation and human-bioclimate model. The aim of the RayMan model is to calculate radiation flux densities, sunshine duration, shadow spaces and thermo-physiologically relevant assessment indices using only a limited number of meteorological and other input data. A comparison between measured and simulated values for global radiation and mean radiant temperature shows that the simulated data closely resemble measured data.

Air pollution in cities

Abstract Air quality in cities is the result of a complex interaction between natural and anthropogenic environmental conditions. Air pollution in cities is a serious environmental problem – especially in the developing countries. The air pollution path of the urban atmosphere consists of emission and transmission of air pollutants resulting in the ambient air pollution. Each part of the path is influenced by different factors. Emissions from motor traffic are a very important source group throughout the world. During transmission, air pollutants are dispersed, diluted and subjected to photochemical reactions. Ambient air pollution shows temporal and spatial variability. As an example of the temporal variability of urban air pollutants caused by motor traffic, typical average annual, weekly and diurnal cycles of NO, NO 2 , O 3 and O x are presented for an official urban air-quality station in Stuttgart, southern Germany. They are supplemented by weekly and diurnal cycles of selected percentile values of NO, NO 2 , and O 3 . Time series of these air pollutants give information on their trends. Results are discussed with regard to air pollution conditions in other cities. Possibilities for the assessment of air pollution in cities are shown. In addition, a qualitative overview of the air quality of the worlds megacities is given.

Girdling affects ectomycorrhizal fungal (EMF) diversity and reveals functional differences in EMF community composition in a beech forest.

ABSTRACT The relationships between plant carbon resources, soil carbon and nitrogen content, and ectomycorrhizal fungal (EMF) diversity in a monospecific, old-growth beech (Fagus sylvatica) forest were investigated by manipulating carbon flux by girdling. We hypothesized that disruption of the carbon supply would not affect diversity and EMF species numbers if EM fungi can be supplied by plant internal carbohydrate resources or would result in selective disappearance of EMF taxa because of differences in carbon demand of different fungi. Tree carbohydrate status, root demography, EMF colonization, and EMF taxon abundance were measured repeatedly during 1 year after girdling. Girdling did not affect root colonization but decreased EMF species richness of an estimated 79 to 90 taxa to about 40 taxa. Cenococcum geophilum, Lactarius blennius, and Tomentella lapida were dominant, colonizing about 70% of the root tips, and remained unaffected by girdling. Mainly cryptic EMF species disappeared. Therefore, the Shannon-Wiener index (H′) decreased but evenness was unaffected. H′ was positively correlated with glucose, fructose, and starch concentrations of fine roots and also with the ratio of dissolved organic carbon to dissolved organic nitrogen (DOC/DON), suggesting that both H′ and DOC/DON were governed by changes in belowground carbon allocation. Our results suggest that beech maintains numerous rare EMF species by recent photosynthate. These EM fungi may constitute biological insurance for adaptation to changing environmental conditions. The preservation of taxa previously not known to colonize beech may, thus, form an important reservoir for future forest development.

Modification of Human-Biometeorologically Significant Radiant Flux Densities by Shading as Local Method to Mitigate Heat Stress in Summer within Urban Street Canyons

Increasing heat will be a significant problem for Central European cities in the future. Shading devices are discussed as a method to mitigate heat stress on citizens. To analyze the physical processes, which are characteristic of shading in terms of urban human-biometeorology, experimental investigations on the thermal effects of shading by a building and shading by tree canopies were conducted in Freiburg (Southwest Germany) during typical Central European summer weather. Urban human-biometeorology stands for the variables air temperature , mean radiant temperature , and physiologically equivalent temperature PET, that is the human-biometeorological concept to assess the thermal environment which was applied. The measuring setup consists of specific human-biometeorological stations, which enable the direct or indirect determination of , , and PET. With respect to both shading devices, the reduction did not exceed 2°C, while PET as a measure for human heat stress was lowered by two thermal sensation steps according to the ASHRAE scale. As has the role of a key variable for outdoor thermal comfort during Central European summer weather, all radiant flux densities relevant to the determination of were directly measured and analyzed in detail. The results show the crucial significance of the horizontal radiant flux densities for and consequently PET.

Impacts of street design parameters on human-biometeorological variables

Abstract in Undetermined This study deals with a current problem of urban human-biometeorology on the micro-scale, which becomes more important due to the future increase of severe summer heat in Central Europe. The impact of street design parameters on the thermal comfort of citizens is analysed in an experimental way for typical summer conditions in Central Europe. The investigation is focused on the behaviour of mean radiant temperature Tmrt and physiologically equivalent temperature PET as the most important human-biometeorological variables for thermal comfort during these atmospheric conditions. To get quantitative results on how they depend on small-scale characteristics of urban street canyons, an investigation design is applied which is based on measurements of relevant meteorological variables - like air temperature Ta - by specific humanbiometeorological measuring systems. They were conducted in selected street canyons within different urban quarters of Freiburg, the warmest city in Germany, from 2007-2009. Tmrt and PET were calculated from the measured meteorological variables by well-tested approaches. The geometry of urban street canyons is characterised by (i) the sky view factor SVF determined from fish-eye photos, (ii) the ratio of building height H to street width W, (iii) the orientation to the sun, and (iv) the fraction of ctc (coverage by the street tree canopy). To eliminate the influence of slightly different weather conditions even on typical summer days, the results are not presented in form of absolute values for the human-biometeorological variables, but in form of ratios for the measured radiative flux densities and in form of differences for the measured and calculated temperatures. As the results for Tmrt and PET should primarily quantify universal patterns of the impact of street design parameters on human thermal comfort, they are only presented as mean values for the period 10-16 CET. The main results obtained from different analyses are: (i) SVF for the southern half of the upper hemisphere (SVF90-270) is more suitable to characterise the sites with respect to the thermal perception of citizens than SVF for the whole upper hemisphere (SVF1-360), (ii) in contrast to wide E-W oriented street canyons, narrow E-W oriented street canyons have larger spatial differences in Ta, Tmrt and PET, (iii) with respect to the orientation, these differences are larger in E-W than in N-S oriented street canyons, and (iv) an increase of ctc by 10 % leads to a decrease of Ta by only 0.2 °C, but to a reduction of Tmrt by 3.6 °C and of PET by 1.4 °C. (Less)

Wind effects on trees

The interactions between airflow and trees and forest stands are diverse. They include the reduction in nearsurface wind speed and production of turbulence by trees. Near-surface wind conditions affect physiological processes in trees, tree growth, and survival (Ennos 1997; Eugster 2008). Turbulent components of the flow field dominate the tree response behaviour (Mayer 1987; Gardiner 1995; Schindler et al. 2010) and drive the scalar exchange at the forest–atmosphere interface (Finnigan 2000). Wind–tree interactions take place at a wide range of temporal and spatial scales (de Langre 2008). Aerodynamic drag at all surfaces of the aerial parts of trees—from individual leaves (Vogel 1989) to whole tree crowns (Kane et al. 2008)—perturbs the airflow inside forest canopies (Shaw et al. 1974; Baldocchi and Meyers 1988; Turnipseed et al. 2003). Therefore, detailed information on forest structure is an essential precondition for understanding wind–tree interactions and the successful application of flow models to tall canopies. In flow models, forest structure is often represented by the mean vertical profile of the plant area density. Queck et al. (2011, this issue) present a method that can be used to record detailed 3D stand structure from terrestrial laser scanning. They investigated the relationship between wind speed, aerodynamic drag, and plant area density and show how 3D laser scanner data can be used to derive turbulence parameters for flow models. The structure of windward forest edges (Mitscherlich 1973; Dupont and Brunet 2008a, b) as well as the stand structure (Gardiner et al. 1997; Marcolla et al. 2003; Dupont and Brunet 2008b, c; Queck and Bernhofer 2010) affects the flow field within and above forests. In the nearedge region, pronounced gradients of flow quantities provoke high wind load on trees (Stacey et al. 1994; Peltola 1996; Gardiner et al. 1997), which may trigger damage in strong wind conditions. Although Gardiner and Stacey (1996) as well as Dupont and Brunet (2008a) report that tapered forest edges reduce wind loading and related bending moments of trees near the edge, it is still not completely clear, how changes in edge and stand structure affect the flow field at the canopy near forest edges, and whether these changes can contribute to the mitigation of damage in high wind conditions. In a detailed wind tunnel study, Ruck et al. (2011, this issue) investigated the effect of changes in taper angle of windward forest edges on the flow field for different stand densities. Their results demonstrate the impact of edge shape and stand density on quantities of the flow field near the canopy top, the region most relevant for tree failure in strong wind conditions. The aerial parts of trees start to vibrate in response to wind excitation (Sellier and Fourcaud 2005; Rodriguez et al. 2008). In forests, dynamic tree responses are not only observed at the single tree level (Mayer 1987; Gardiner 1995; Peltola 1996; Flesch and Wilson 1999; Rudnicki et al. 2008; Schindler et al. 2010) but also at the tree group level (Rudnicki et al. 2001, 2003; Schindler et al. 2011, this issue). At both levels, the resulting tree response patterns are complex and complicated. So far, even in horizontally homogeneous terrain, not all physical and biological This article belongs to the special issue ‘Wind Effects on Trees’.

Mikroklima in Buchenbeständen – Teil I: Photosynthetisch aktive Strahlung

Zusammenfassung:Forstmeteorologische Untersuchungen werden im südwestlichen Teil der Schwäbischen Alb (Südwestdeutschland) durchgeführt, um die Einflüsse von Exposition und Überschirmung auf das Mikroklima in verschiedenen Rotbuchenbeständen (Fagus sylvatica L.) zu analysieren. Die Untersuchungen sind Bestandteil interdisziplinärer Arbeiten über die Auswirkungen von Klima und Bewirtschaftung auf Strukturen und Prozesse in buchendominierten Laubwäldern. Grundlage der Untersuchungen bilden Bodenstationen zur kontinuierlichen Erfassung forstmeteorologischer Variablen. Sie wurden in einem engen Tal am NO- und dem gegenüberliegenden SW-Hang auf jeweils einer unbehandelten Kontrollfläche und einer starken Schirmhiebfläche eingerichtet. Mit Sensoren an zwei Messtürmen auf den beiden unbehandelten Kontrollflächen werden zusätzlich die klimatischen Bedingungen über den Buchenbeständen und in ihren Kronenräumen erfasst. Durch die Anlage der Versuchsflächen ist es möglich, nicht nur die expositionsbedingten Einflüsse, sondern über die Variabilität des Pflanzenflächenindexes PAI auch die Auswirkungen von waldbaulicher Behandlung und Phänophasen auf das Mikroklima in den verschiedenen Buchenbeständen zu untersuchen. Dieser Artikel konzentriert sich auf die an den Bodenstationen und als Referenz über den Beständen gemessene photosynthetisch aktive Strahlung PAR. Sie wird mit horizontal orientierten Sensoren erfasst. Über den Beständen lassen sich Unterschiede zur hangparallelen PAR mit Modellrechnungen für die Globalstrahlung G abschätzen, weil PAR/G über den Beständen nahezu konstant ist (1,937 μmol J–1). Als Ergebnisse werden für PAR und die PAR-Interzeption im Kronenschirm im Jahr 2001 diskutiert: mittlere Tagesgänge für je einen Monat in der laubfreien und belaubten Phase, Tages- und Monatsmittelwerte über das gesamte Jahr sowie Mittelwerte für die laubfreie und belaubte Phase. Sie zeigen den Einfluss der Exposition beider Hänge in der laubfreien Phase auf PAR an den Bodenstationen und die PAR-Interzeption auf. Er trat in der belaubten Phase deutlich gegenüber dem Einfluss des voll entwickelten Kronenschirms zurück. Resultate aus Regressionsanalysen für den gesamten Untersuchungszeitraum deuten mit ansteigendem PAI eine lineare Abnahme von PAR/G unter dem Kronenschirm sowie eine logarithmische Zunahme der PAR-Interzeption im Kronenschirm und eine exponentielle Abnahme der PAR-Transmission durch den Kronenschirm der Buchenbestände an. Auch beim Extinktionskoeffizienten k von PAR besteht die Tendenz einer logarithmischen Abnahme mit ansteigendem PAI.Summary:Meteorological investigations were carried out in forests in the southwestern part of the Swabian Alb (SW Germany) to analyze the influences of exposition and silvicultural treatment on the microclimate within various stands of beech (Fagus sylvatica L.). The measurements were taken as part of an interdisciplinary project on the effects of climate and forest management on structures and processes within beech dominated deciduous forests. Near-surface stations for continuous recording of meteorological variables in forests provide the basis of the investigations. Two each were installed on two opposite slopes (NE and SW) of a narrow valley: one within a control plot (no silvicultural thinning) and one within a silviculturally treated plot (strong shelterwood felling). Meteorological towers on each control plot provided data for the analysis of climatic conditions above the beech stands and within their canopies. The experimental design is well-suited for investigation of not only the influence of exposition, but also the effects of forest management and phenophases on the microclimate within differently handled beech stands. The plant area index PAI is used to characterize the canopy. The present article deals with the photosynthetically active radiation PAR at the near-surface stations and above the stands as a reference. PAR is measured horizontally. Above the stands, differences to PAR received parallel to the slopes can be estimated by use of model calculations for the incoming solar radiation G due to the fact that PAR/G is almost constant (1.937 μmol J–1) above the stands. For PAR and the PAR-interception within the canopy in the year 2001, the following results are discussed: mean diurnal courses in one month within the leafless period (February) and the vegetation period (August), daily mean values over the whole year and monthly mean values. For the leafless period the results show the influence of the exposition of both slopes on PAR at the near-surface stations and the PAR-interception. In the vegetation period, however, the influence of the fully developed canopy was more significant than exposition. With increasing PAI, results from regression analysis for the entire investigation period indicate a linear decrease of PAR/G below the canopy as well as an exponential increase of PAR-interception within the canopy and a logarithmic decrease of the PAR-transmission through the canopy. The extinction coefficient k for PAR shows a tendency towards logarithmic decrease with increasing PAI.

Performance of solar radiation models—a case study

Performance of cloud-based (Kasten) and sunshine-based (Angstrom–Prescott, Garg and Garg, Sivkov) empirical solar radiation models is presented for a lowland and mountain grassland sites. Data sets utilised for the study extended from 1991 to 1994. Adjustable parameters in the models were determined. Observed monthly mean values of solar radiation K↓ and those estimated using Kasten model agreed within 2.5% for the lowland site and 13% for the mountain site, while those estimated using Angstrom–Prescott relation agreed with observation within 2.5% for lowland site and 3.4% for the mountain site. The incorporation of air mass, latitude and water vapour terms into Angstrom–Prescott relation did not significantly improve estimates of K↓. In general, Angstrom–Prescott as well as Garg and Garg models yielded the least RMSE (<0.047) for the study sites.

Variability of PM10 concentrations dependent on meteorological conditions

The influence of atmospheric exchange conditions on near-surface PM 10 concentrations is analysed for four cities in the South-West of Germany in the period from 2001 to 2005. For each city, the PM 10 data originates from roadside stations and Urban Background (UB) stations, that is, two types of official urban air quality monitoring stations characterised by different emission situations. The atmospheric exchange conditions are represented by solar radiation, air temperature, wind speed, mixing-layer height, precipitation and backward-trajectories. The results of the statistical analysis indicate that

Hydrometeorological reconstruction of the 1824 flood event in the Neckar River basin (southwest Germany)

Abstract The integration of extreme historical floods in contemporary flood protection contributes towards improved risk management and safer handling of floods in the future. As a case study within the “Xfloods” project at the University of Freiburg (Germany), the discharges of the extreme flood in 1824 in the Neckar River basin (Baden-Württemberg/southwest Germany) were reconstructed using historical data. Quantitative and qualitative historical sources were applied to model the regional atmospheric circulation pattern, the weather conditions and the precipitation distribution associated with the event. Discharges were simulated using the water-balance model LARSIM (Large Area Runoff Simulation Model), the operational flood forecasting model in Baden-Württemberg. The developed methodology shows potential for wider use in assessing extreme historical floods and for application to contemporary flood management.