Andreas Christen

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

Professor

About

196
Publications
57,549
Reads
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4,380
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 2013 - August 2017
University of British Columbia - Vancouver
Position
  • Professor (Associate)
July 2012 - June 2013
École Polytechnique Fédérale de Lausanne
Position
  • Visiting Professor
August 2006 - June 2013
University of British Columbia - Vancouver
Position
  • Professor (Assistant)
Education
January 2001 - March 2005
University of Basel
Field of study
  • Meteorology
August 1995 - December 2000
University of Basel
Field of study
  • Geography

Publications

Publications (196)
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...
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...
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
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, NWT, 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 continuously measured...
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...
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
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
Peatland ecosystems are generally carbon (C) sinks. However, the role of dissolved organic C (DOC) relative to gaseous fluxes of CO 2 and CH 4 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...
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...
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...
Book
Urban Climates is the first full synthesis of modern scientific and applied research on urban climates. The book begins with an outline of what constitutes an urban ecosystem. It develops a comprehensive terminology for the subject using scale and surface classification as key constructs. It explains the physical principles governing the creation o...
Article
Full-text available
Despite storing approximately half of the atmosphere’s carbon, estimates of fluxes between wetlands and atmosphere under current and future climates are associated with large uncertainties, and it remains a challenge to determine human impacts on the net greenhouse gas balance of wetlands at the global scale. In this study we demonstrate that the r...
Article
Full-text available
Many peatlands have been drained and harvested for peat mining, agriculture, and other purposes, which has turned them from carbon (C) sinks into C emitters. Rewetting of disturbed peatlands facilitates their ecological recovery and may help them revert to carbon dioxide (CO2) sinks. However, rewetting may also cause substantial emissions of the mo...
Article
Tree mortality due to the recent mountain pine beetle (MPB) (Dendroctonus ponderosae) outbreak in British Columbia (BC) is expected to impact evapotranspiration (E), gross primary productivity (GPP), and snow accumulation and melt, thereby influencing ecosystem hydrology. To quantify the impact on E and GPP, we have made eddy-covariance (EC) measur...
Article
Often the meteorological forcing data required for urban hydrological models are unavailable at the required temporal resolution or for the desired period. Although reanalysis data can provide this information, the spatial resolution is often coarse relative to cities, so downscaling is required prior to use as realistic forcing. In this study, WAT...
Article
Large-eddy simulations (LES) are used to gain insight into the effects of trees on turbulence, aerodynamic parameters, and momentum transfer rates characterizing the atmosphere within and above a real urban canopy. Several areas are considered that are part of a neighbourhood in the city of Vancouver, BC, Canada where a small fraction of trees are...
Article
Tree mortality due to the recent mountain pine beetle (MPB) (Dendroctonus ponderosae) outbreak in British Columbia (BC) is expected to impact evapotranspiration (E), gross primary productivity (GPP), and snow accumulation and melt, thereby influencing ecosystem hydrology. To quantify the impact on E and GPP, we have made eddy-covariance (EC) measur...
Article
Full-text available
A method for directly measuring carbon dioxide (CO2) emissions using a mobile sensor network in cities at fine spatial resolution was developed and tested. First, a compact, mobile system was built using an infrared gas analyzer combined with open-source hardware to control, georeference, and log measurements of CO2 mixing ratios on vehicles (car,...
Conference Paper
This paper presents a method of analysis for deriving temporally continuous urban surface temperature and heat island assessments from hemispherical measurements of upwelling thermal radiation. This method was developed to overcome geometric and temporal biases inherent in traditional thermal remote sensing of urban surface climates. Hemispherical...
Article
Retrogressive thaw slumps (RTS) are permafrost disturbances common on the Fosheim Peninsula, Ellesmere Island, Canada. During the 2013 growing season, three different RTS were studied to investigate the impact on vegetation composition, soil, and growing season net ecosystem exchange (NEE) of carbon dioxide (CO2) by comparing to the adjacent undist...
Conference Paper
Full-text available
Much of our understanding of land-atmosphere interactions is rooted in direct flux measurements by the eddy covariance (EC) method. The EC method of analyzing in-situ data from flux towers has become the standard tool for monitoring turbulent heat exchange, trace gas emissions, and the water balance over land surfaces, including cities. The EC meth...
Conference Paper
Full-text available
Diurnal and seasonal changes of δ 13 C and δ 18 O in carbon dioxide measured by eddy covariance over an urban surface The isotopic composition of the greenhouse gas CO 2 can provide valuable information about its sources. For example , linear mixing models have been used to identify the sources of enhanced CO 2 in the urban atmosphere using urban a...
Poster
Full-text available
Poster presentation for the American Geophysical Union fall meeting 2016
Article
Full-text available
Proactive management of invasive species in urban areas is critical to restricting their overall distribution. The objective of this work is to determine whether advanced remote sensing technologies can help to detect invasions effectively and efficiently in complex urban ecosystems such as parks. In Surrey, British Columbia, Canada, Himalayan blac...
Article
Full-text available
Single-point measurements from towers in cities cannot properly quantify the impact of all terms in the turbulent kinetic energy (TKE) budget and are often not representative of horizontally-averaged quantities over the entire urban domain. A series of large-eddy simulations (LES) is here performed to quantify the relevance of non-measurable terms,...
Article
Full-text available
A method for directly measuring carbon dioxide (CO2) emissions using a mobile sensor network in cities at fine spatial resolution was developed and tested. First, a compact, mobile system was built using an infrared gas analyzer combined with open-source hardware to control, georeference and log measurements of CO2 mixing ratios on vehicles (car, b...
Article
Full-text available
Soil carbon stored in high-latitude permafrost landscapes is threatened by warming and could contribute significant amounts of carbon to the atmosphere and hydrosphere as permafrost thaws. Thermokarst and permafrost disturbances, especially active layer detachments and retrogressive thaw slumps, are present across the Fosheim Peninsula, Ellesmere I...
Article
Full-text available
Rewetting can promote the ecological recovery of disturbed peatland ecosystems and may help to revert these ecosystems to carbon dioxide (CO2) sinks. However, rewetting of disturbed peatlands can also cause substantial emissions of methane (CH4) and possibly nitrous oxide (N2O). This study quantified summertime emissions of the three major long-liv...
Conference Paper
Full-text available
The Burns Bog Ecological Conservation Area, located in Delta, British Columbia, is one of the largest bogs in North America surrounded by urban development. Greenhouse gas exchanges in wetlands are an essential ecosystem service that mediate climate from a local scale. Bogs act as long-term carbon storages by sequestering carbon dioxide (CO2), but...
Article
Observations of carbon dioxide (CO2) mixing ratios in the urban boundary layer (UBL) are rare, even though there is potential for such measurements to be used to monitor city-scale net CO2 emissions. This work presents a unique dataset of CO2 mixing ratios observed in the UBL above Vancouver, British Columbia, Canada, by means of a tethered balloon...