Andreas Christen

Andreas Christen
University of Freiburg | Albert-Ludwigs-Universität Freiburg · Environmental Meteorology

Professor

About

227
Publications
83,592
Reads
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6,621
Citations
Introduction
I study the exchange of greenhouse gases, pollutants, energy and water between atmosphere and complex land surfaces (e.g. cities, wetlands, tundra, forests, agriculture). Our group's goal is to develop, advance and test new measurement methods, approaches and develop models. These tools are applied to quantify, analyze and attribute the exchange of gases, water and energy in complex landscapes.
Additional affiliations
July 2012 - June 2013
Swiss Federal Institute of Technology in Lausanne
Position
  • Visiting Professor
January 2001 - December 2003
ETH Zurich
Position
  • Research Assistant
January 2001 - March 2005
University of Basel
Position
  • Research Assistant
Education
January 2001 - March 2005
University of Basel
Field of study
  • Meteorology
April 1995 - December 2000
University of Basel
Field of study
  • Geography

Publications

Publications (227)
Article
Full-text available
We present an approach to generate location-specific forecasts of indoor temperature (Ti) and thermal comfort and issue indoor heat warnings for occupational settings. Indoor forecasts are generated using standard outdoor weather forecasting products and an artificial neural network (ANN) trained on-site using local indoor measurements from a low-c...
Preprint
Full-text available
As the frequency and intensity of heat waves will continue to increase in the future, accurate and high-resolution mapping and forecasting of human outdoor thermal comfort in urban environments is of great importance. This study presents a machine learning based outdoor thermal comfort model with a good trade-off between computational cost, complex...
Article
Full-text available
Indoor and outdoor heat stress, which can appear during warm periods of the year, often has a negative impact on health and reduces productivity at work and study. Intense heat waves (HWs) are causing increasing rates of morbidity and mortality. This study aimed to analyze the coupling and delay of indoor and outdoor heat stress during HW events, u...
Article
Full-text available
Water potential is a crucial parameter for assessing tree water status and hydraulic strategies. However, methods for measuring water potential, such as the Scholander pressure chamber, are destructive, discontinuous and difficult to perform in tall forests. Consequently, important dynamics in water potentials, particularly during short‐term drough...
Preprint
Full-text available
A relaxed eddy accumulation (REA) system was developed and tested that enables conditional sampling of air for subsequent 14CO2 analysis. It allows an observation-based partitioning of total CO2 fluxes measured in urban environments by eddy covariance into fossil and non-fossil components. The purpose of this article is to describe the REA system,...
Article
Full-text available
Urban observation networks are becoming denser, more diverse, and more mobile, while being required to provide results in near time. The Synergy Grant “urbisphere” funded by the European Research Council (ERC) has multiple simultaneous field campaigns in cities of different sizes, collecting data to improve weather and climate models and services,...
Article
This study presents an operational citywide monitoring network designed to measure meteorological and human biometeorological variables at a high spatio-temporal resolution. The network is based on an in-house developed, generic data logging and monitoring platform, with 13 stations strategically placed at the pedestrian level on public street ligh...
Article
Full-text available
This study proposes a machine learning (ML) framework generating spatially-distributed mean wind fields at a given height above ground within arbitrary urban canopy geometries. The framework is based on the Random Forest formulation and is trained using building resolving large-eddy simulations of flow over a range of realistic urban environments....
Conference Paper
Full-text available
Re-occurring dry and hot summers with high irradiance have caused irreversible damages in forest ecosystems across Central Europe, including the ICOS ecosystem associate site Hartheim (DE-Har). The site experienced irreversible damages to a Scots pine (Pinus sylvestris) plantation with a mortality of >50% of all P. sylvestris trees following the 20...
Preprint
Full-text available
Water potential is a crucial parameter for assessing tree water status and hydraulic strategies. However, methods for measuring water potential, such as the Scholander pressure chamber, are destructive and discontinuous, and difficult to perform in tall forests. Consequently, important dynamics in water potentials, particularly during short-term dr...
Article
Full-text available
For next-generation weather and climate numerical models to resolve cities, both higher spatial resolution and sub-grid parameterizations of urban canopy-atmosphere processes are required. Key is to better understand intra-urban variability and urban-rural differences in atmospheric boundary layer (ABL) dynamics. This includes upwind-downwind effec...
Article
Full-text available
Global climate change threatens ecosystem functioning worldwide. Forest ecosystems are particularly important for carbon sequestration, thereby buffering climate change and providing socio-economic services. However, recurrent stresses, such as heat waves, droughts and floods can affect forests with potential cascading effects on their carbon sink...
Preprint
Full-text available
Urban observation networks are becoming denser, more diverse, and more mobile, while being required to provide results in near-time. The Synergy Grant urbisphere funded by the European Research Council (ERC) has multiple simultaneous field campaigns in cities of different sizes collecting data, for improving weather and climate models and services,...
Article
Full-text available
Taylor’s Frozen Turbulence Hypothesis (TH) is a critical assumption in turbulent theory and practice which allows time series of point measurements of turbulent variables to be translated to the spatial domain via the mean wind. Using a 3D array of fibre-optic distributed temperature sensing in the atmospheric surface layer over an idealized desert...
Presentation
The role of vegetation in urban climate has been in the spotlight in recent years, as it can play significant roles in carbon sequestration through photosynthesis as well as in the urban energy balance, mainly through evapotranspiration and shading. Based on these, the green infrastructure of cities is considered as a potential solution to lower th...
Article
Full-text available
As the frequency and intensity of heatwaves will continue to increase in the future, accurate and high-resolution mapping and forecasting of human outdoor thermal comfort in urban environments are of great importance. This study presents a machine-learning-based outdoor thermal comfort model with a good trade-off between computational cost, complex...
Article
Full-text available
To adapt forest ecosystems and forest management to climate change, it is essential to know which forest regions and which tree species are resilient to climate variability and which ones are possibly affected most by past and anticipated future changes. In this contribution, for the main forest regions of Türkiye and six tree species, recent clima...
Article
Full-text available
Climate models predict meteorological variables for outdoor spaces. Nevertheless, most people work indoors and are affected by heat indoors. We present an approach to transfer climate projections from outdoors to climate projections of indoor air temperature (Ti) and thermal comfort based on a combination of indoor sensors, artificial neural networ...
Article
Full-text available
Urban surface albedo is an essential biophysical variable in the surface energy balance across all scales, from micro-scale (materials) to the globe, changing with land covers and three-dimensional structures over urban areas. Urban albedos are dynamic over space and time but have not yet been quantified over global scales due to the lack of high-r...
Article
Full-text available
A temporal upscaling study was conducted to estimate net ecosystem exchange (NEE) of carbon dioxide and net methane exchange (NME) for a Low-Center Polygon (LCP) ecosystem in the Mackenzie River Delta, for each of eleven growing seasons (2009 to 2019). We used regression models to create a time series of flux drivers from in-situ weather observatio...
Article
Full-text available
Different urban microscale models exist to model street-level mean radiation temperature (Tmrt). However, these models are computationally expensive, albeit to varying degrees. We present a computational shortcut using a convolutional encoder-decoder network (U-Net) to predict pedestrian level (1.1 m a.g.l.) Tmrt at a building-resolved scale (1 × 1...
Article
Full-text available
A total of 20 urban neighbourhood-scale eddy covariance flux tower datasets are made openly available after being harmonized to create a 50 site–year collection with broad diversity in climate and urban surface characteristics. Variables needed as inputs for land surface models (incoming radiation, temperature, humidity, air pressure, wind and prec...
Poster
Full-text available
The Central European 2018 hot drought had severe impacts on many forest ecosystems, including a Scots pine (P. sylvestris) plan- tation at the DE-Har ecosystem site in the Upper Rhine Valley. The co-occurrence of unfavourable site-specific conditions with high air temperatures and dry conditions resulted in massive accelerated tree mortality of P....
Article
Full-text available
The occurrence of hot drought, i.e. low water availability and simultaneous high air temperature, represents a severe threat to ecosystems. Here, we investigated how the 2018 hot drought in Central Europe caused a tipping point in tree and ecosystem functioning in a Scots pine (Pinus sylvestris L.) forest in southwest Germany. Measurements of stres...
Preprint
Full-text available
Twenty urban neighbourhood-scale eddy covariance flux tower datasets have been harmonized and quality controlled, producing a 50 site-year collection with broad diversity in climate and urban surface characteristics. Observations are gap-filled and prepended with 10 years of reanalysis-derived local data to enable use as spin up and forcing for lan...
Conference Paper
Full-text available
Taylor’s frozen turbulence hypothesis is the most critical assumption through which time-resolving sensors may be used to derive statistics of the turbulent spatial field. Namely, it relates temporal autocorrelation to spatial correlation via the mean wind speed and is invoked in almost all boundary layer field work. Nevertheless, the conditions an...
Article
Full-text available
The measures taken to contain the spread of COVID-19 in 2020 included restrictions of people's mobility and reductions in economic activities. These drastic changes in daily life, enforced through national lockdowns, led to abrupt reductions of anthropogenic CO2 emissions in urbanized areas all over the world. To examine the effect of social restri...
Article
Full-text available
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...
Article
Full-text available
The MoBiMet (Mobile Biometeorology System) is a low-cost device for thermal comfort monitoring, designed for long-term deployment in indoor or semi-outdoor occupational contexts. It measures air temperature, humidity, globe temperature, brightness temperature, light intensity, and wind, and is capable of calculating thermal indices (e.g., physiolog...
Article
Full-text available
Water storage plays an important role in mitigating heat and flooding in urban areas. Assessment of the water storage capacity of cities remains challenging due to the inherent heterogeneity of the urban surface. Traditionally, effective storage has been estimated from runoff. Here, we present a novel approach to estimate effective water storage ca...
Article
Full-text available
Growing season surface–atmosphere exchange of carbon dioxide and methane were quantified at Fish Island, a wetland site in the lower northeast Mackenzie River Delta, Northwest Territories, Canada. The terrain consists of low-center polygonal tundra and is subject to infrequent flooding in high water years. Carbon dioxide and methane fluxes were con...
Article
Full-text available
Plain Language Summary Turbulent flows are presumed to be a superposition of swirling motions of different sizes and activities labelled as ‘eddies’. Eddies constitute a form of ‘ordered’ motion, and describing their characteristics (e.g., the distribution of sizes and associated energy content for each size) is a lacunarity in theory and practical...
Article
Full-text available
To quantify the ecosystem services of trees in urban environments, it is necessary to assess received direct solar radiation of each tree. While the Sky View Factor (SVF) is suitable for assessing the total incoming short- and longwave radiation fluxes, its information is limited to specific points in space. For a spatial analysis, it is necessary...
Article
We propose a gap-filling model for carbon dioxide fluxes measured by eddy covariance (EC) that combines the flux variance similarity (FVS) partitioning approach with the artificial neural network (ANN) technique (FVS–ANN). 18 years of EC-measured net ecosystem production (NEP) of a Douglas-fir (Pseudotsuga menziesii) stand in British Columbia, Cana...
Preprint
Water storage plays an important role in mitigating heat and flooding in urban areas. Assessment of the capacity of cities to store water remains challenging due to the extreme heterogeneity of the urban surface. Traditionally, effective storage has been estimated from runoff. Here, we present a novel approach to estimate water storage capacity fro...
Article
Full-text available
The understanding of intra-urban air temperature variations is crucial to assess strategies for cities' adaptation to impacts of present and future anthropogenic climate change. Depending on extensive measurement networks, high-resolution air temperature measurements in urban environments are challenging due to high instrumentation and maintenance...
Article
It is critical to have long-term carbon dioxide (CO2) flux observations in forest ecosystems to understand how changing climate can affect forest carbon (C) stocks and CO2 exchange between forests and the atmosphere. In this study, fifteen years (2002–2016) of continuous eddy-covariance flux and climate measurements in an intermediate-aged Douglas-...
Article
Partitioning measured net ecosystem exchange of CO2 (NEE) into ecosystem respiration (Re) and gross primary production (GPP) is essential for understanding the biophysical controls on forest ecosystem carbon (C) dynamics. Obtaining Re and GPP accurately from NEE remains a challenge. In this study, we measured stable CO2 isotopologue signatures at e...
Article
Full-text available
Thermokarst features are widespread in ice-rich regions of the circumpolar Arctic. The rate of thermokarst lake formation and drainage is anticipated to accelerate as the climate warms. However, it is uncertain how these dynamic features impact the terrestrial Arctic carbon cycle. Methane (CH4) and carbon dioxide (CO2) fluxes were measured during p...
Preprint
Full-text available
This is a comment to the paper "Magnitude of urban heat islands largely explained by climate and population" by Manoli et al. (2019, Nature 573 p. 55-60; https://doi.org/10.1038/s41586-019-1512-9)
Technical Report
Full-text available
The COVID-19 lockdown has affected our lifestyles and work, forcing us to stay at home. This has strongly reduced road traffic and economic activities particularly in cities, and, consequently, cut down emissions of carbon dioxide (CO2) to the atmosphere. Whilst lockdown does not affect baseline emissions such as ecosystem respiration and CO2 exhal...
Article
Full-text available
High-frequency measurements are available at five heights within and above a row-gap trellised vineyard located on a 7◦ slope in the Southern Okanagan Valley, British Columbia, Canada. During a 3-week campaign in July 2016, approximately 17% of the nocturnal conditions exhibit drainage flow along the local slope. Drainage conditions are characteriz...
Article
Vegetation alters urban climates via transpirational cooling; however, unlike shorter vegetation, trees additionally provide shade and shelter. Urban canopy models (UCMs) are coupled with mesoscale models for assessment of neighbourhood-scale climate, but their representation of urban trees is limited. We present BEP-Tree, a multi-layer UCM that in...
Preprint
Full-text available
Abstract. Thermokarst features are widespread in ice-rich regions of the circumpolar Arctic. The rate of thermokarst lake formation and drainage is anticipated to accelerate as the climate warms. However, it is uncertain how these dynamic features impact the terrestrial Arctic carbon cycle. Methane (CH<sub>4</sub>) and carbon dioxide (CO<sub>2</sub...
Chapter
In diesem Beitrag wird die Differenzierung des Mikroklimas innerhalb von bebauten Flächen, also inner-halb von Straßenschluchten, Plätzen wie auch lokal-klimatische Unterschiede zwischen einzelnen Stadtteilen, Parks und städtischen Wasserflächen herausgearbeitet. Die Unterschiede in der Strahlung, der Lufttempera-tur und-feuchte, der thermischen Be...
Article
Full-text available
Tower-based measurements from within and above the urban canopy in two cities are used to evaluate several existing approaches that parametrize the vertical profiles of wind speed and temperature within the urban roughness sublayer (RSL). It is shown that current use of Monin-Obukhov similarity theory (MOST) in numerical weather prediction models c...
Poster
Full-text available
El principal efecto climático de las ciudades sobre el clima es la isla de calor, que hace referencia a mayores temperaturas en el centro que en la periferia. Dentro de este fenómeno se determinan tres tipos de islas de calor: superficial (la cual depende de cada cobertura), de canopeo (desde la superficie a la altura máxima de los edificios, relac...
Article
Douglas-fir (Pseudotsuga menziesii) forests in the Pacific Northwest are the most productive managed forests in North America. Nitrogen (N) fertilizers are generally applied in this region to increase the rate of tree growth and consequently carbon (C) sequestration. However, the long-term effects of N fertilization on C and water exchanges of Doug...
Article
Full-text available
Peatland ecosystems are generally carbon (C) sinks. However, the role of dissolved organic C (DOC) relative to gaseous fluxes of CO2 and CH4 in the C balance of these ecosystems has not often been studied. Dissolved C fluxes are important for understanding C partitioning within the peatland and the potential C drainage from it. This research was co...
Article
This work explores the feasibility of real-time large-eddy simulations of flow over urban canopies at the neighborhood scale. The cumulant lattice Boltzmann method is employed using a single General Purpose Graphic Processing Unit (GPGPU). In order to demonstrate the validity and efficiency of this approach we simulate wind flow in a neighborhood o...
Article
Full-text available
A short, but severe, wildfire smoke episode in July 2015, with an aerosol optical depth (AOD) approaching 9, is shown to strongly impact radiation budgets across four distinct land-use types (forest, field, urban and wetland). At three of the sites, impacts on the energy balance are also apparent, while the event also appears to elicit an ecosystem...
Book
Dieser Beitrag beschreibt die Grundlagen und Besonderheiten der vertikalen Gliederung der Atmosphäre über Städten. Strömt die Atmosphäre über eine Stadt, bildet sich bodennah eine städtische Grenzschicht aus, die etwa 1-2 km in die Höhe reicht und im Abwindbereich über zehn bis hundert Kilometer messbar ist. Die städtische Grenzschicht ist im Gegen...
Book
In diesem Beitrag wird die Differenzierung des Mikroklimas innerhalb von bebauten Flächen, also innerhalb von Straßenschluchten, Plätzen wie auch lokal-klimatische Unterschiede zwischen einzelnen Stadtteilen, Parks und städtischen Wasserflächen herausgearbeitet. Die Unterschiede in der Strahlung, der Lufttemperatur und -feuchte, der thermischen Bel...
Conference Paper
Full-text available
We developed “Meteobike”, a light and low-cost urban traverse system based on the Raspberry-Pi. It can be assembled and programmed in undergraduate classes. The system can be mounted on bicycles to record geotagged air temperatures and humidity as one moves through cities and adjacent rural areas. In summer 2018, we used 25 systems to map and analy...
Article
Hydrological cycles of two suburban neighborhoods in Vancouver, BC, during initial urban development and subsequent urban densification (1920–2010) are examined using the Surface Urban Energy and Water Balance Scheme. The two neighborhoods have different surface characteristics (as determined from aerial photographs) which impact the hydrological p...
Article
Full-text available
A short, but severe, wildfire smoke episode in July 2015, with an aerosol optical depth (AOD) approaching nine, had a significant impact on air quality, radiation and energy budgets across four land use types, and elicited a clear ecosystem response with respect to carbon fluxes at a bog and a forested site. Greatest impacts on radiation and energy...
Article
Insect outbreaks can significantly influence carbon (C) and water balances of forests. Forest tent caterpillars (FTC) (Malacosoma disstria Hübner) are one of the most prominent insects found in aspen forests in Canada and have the potential to considerably influence regional C and water fluxes. In the summer of 2016, an FTC infestation occurred in...
Article
The most recent mountain pine beetle (MPB) (Dendroctonus ponderosae) outbreak in British Columbia (BC), which began in the late 1990s, killed ∼54% of the mature merchantable lodgepole pine and was expected to impact gross primary productivity (GPP), ecosystem respiration (R) and thus net ecosystem productivity (NEP) of infested stands. Eddy-covaria...
Article
Full-text available
Traditional methods for remote sensing of urban surface temperatures (Tsurf) are subject to a suite of temporal and geometric biases. The effect of these biases on our ability to characterize the true geometric and temporal nature of urban Tsurf is currently unknown, but is certainly nontrivial. To quantify and overcome these biases, we present a m...
Article
Full-text available
While approximately 338 million people in the Northern hemisphere live in regions that are regularly snow covered in winter, there is little hydro-climatologic knowledge in the cities impacted by snow. Using observations and modelling we have evaluated the energy and water exchanges of four cities that are exposed to wintertime snow. We show that t...