Andreas Christen

Using Gradient Boosting for gap-filling to analyze temperature and humidity patterns in an urban weather station network in Freiburg, Germany Weather station networks (WSNs) are essential to investigate urban–rural and intra-urban variability of urban weather and climates. However, missing d…

In a paper just published in "Urban Climate" we describe and assess a new machine-learning algorithm for gap-filling urban weather station networks. We use this to construct maps of annual climate statistics - code and paper are open access #I4C #urbisphere www.sciencedirect.com/science/arti...

Aerial view of the experimental forest near Hartheim with the 30-metre-high crown access tower in September 2023. Dead Scots pines and gaps in the canopy are visible throughout the forest. Photo: Fabio Scarpa

Years of drought turn forest into source of CO₂ - Press release on our research with @simonhaber.bsky.social and @chriswernerlab.bsky.social at @icos-ri.eu site DE-Har, showing tree mortality following droughts has fundamentally shifted forest structure & functioning uni-freiburg.de/en/years-of-...

Great presence of @icos-ri.eu at the International Conference on Urban Climate (#ICUC12) today in Rotterdam. Presenting latest science on modelling, observing, and attributing urban greenhouse gas emission reductions using ICOS infrastructure in Paris, Munich, Zurich Berlin, Heidelberg & Heraklion.

Today was another unusually hot day in Freiburg, with temperatures above 38°C. Our urban weather station network operated by @uni-freiburg.de and funded by @erc.europa.eu reveals intra-urban differences and where it was coolest (in a forest). Explore the map weather.uni-freiburg.de/map/freiburg...

Stations colored in red show a tropical night, i.e. that air temperature at those weather stations did no drop below 20°C all night on Jun 21, 2025.

During the current #UKHeatWave we measured for the first time a #TropicalNight across our Bristol street-level weather network. Stations outside the city and on hills cooled down to < 20°C while the urban heat island kept the city core above 20°C last night. weather.uni-freiburg.de/map/bristol/...

Recurrent hot droughts cause persistent legacy effects in a temperate Scots Pine forest A series of hot drought events caused persistent legacy effects in a Scots Pine forest, with severe negative impacts on ecosystem carbon fluxes.

By combining ecophysiological, satellite, and ‪‬eddy covariance data we show how our @icos-ri.eu forest ecosystem changes in function and structure following hot-drought-induced tree mortality. New article by
@simonhaber.bsky.social as part of @ecosense-sfb-1537.bsky.social
doi.org/10.1111/plb....

The city centre of Bristol has the highest nocturnal air temperatures measures, 5K warmer than a rural site to the west of the city at same elevation. Note also that the station in the NW near the coast stays even warmer (12.2ºC), while the Avon Gorge is relatively cool (8.8ºC).

A weather station on a lamp-post measures at 3 m in a typical street canyon, here in the Windmill Hill neighborhood. Data are transmitted every 5 minutes over the cell-phone network and shared as open-data as part of several UK and EU research projects enhancing urban-scale weather and climate modelling.

New weather stations on lamp-posts across Bristol, UK reveal interacting urban, orographic and coastal effects in real time. Last night, we observed an urban heat island of 5ºC. Multi-institutional effort between urbisphere ‪@erc.europa.eu‬, @urbanair.bsky.social‬ and ASSURE: doi.org/10.5281/zeno...

Celebrating our successful new excellence cluster #FutureForests @uni-freiburg.de. Thanks @dfg.de, this will enable us to substantially invest in exciting and much-needed research for the next 7 years - predicting and transforming forest systems in a changing climate uni-freiburg.de/futureforest...

Using high-resolution weather models and a network of 25 LiDAR-ceilometers over Berlin, Germany, we explore how large cities modify the atmospheric boundary layer. New preprint on collaborative work by @uniofreading.bsky.social‬, @metoffice.bsky.social‬ and @uni-freiburg.de
doi.org/10.5194/egus...

Can you spot the dead Pine trees? If you are interested to learn more about legacy effects in this ecosystem, join me for my talk @egu.eu this afternoon: meetingorganizer.copernicus.org/EGU25/EGU25-...
@ecosense-sfb-1537.bsky.social
@envmet.bsky.social @chriswernerlab.bsky.social

Impressive time-lapse of leaf emergence during the last weeks recorded by phenocams at our research forest @uni-freiburg.de. In the @dfg.de funded @ecosense-sfb-1537.bsky.social we study forest ecosystem functioning, and species interactions with novel sensor technologies uni-freiburg.de/ecosense/

Six Doppler Wind LIDARS (DWLs) operated across Paris concurrently are used to retrieve horizontal wind speed and direction through the ABL at 18–25 m vertical and 1–60 min temporal resolution.

The six DWLs were deployed in a multi-institutional effort along a 40 km transect through the centre of Paris (France) in 2023 - 2024.

New preprint on our harmonised wind dataset from 6 doppler wind lidars operated along a transect through Paris, France. This data supports urban weather and climate modeling in multiple @erc.europa.eu, @wmo-global.bsky.social, @icos-ri.eu and @horizoneu.bsky.social projects: doi.org/10.5194/essd...

At the #UrbanGHGsummit in Geneva, greenhouse gas emission experts from #ICOSCities project are joining forces to curb urban emissions. Leena Järvi, Olivier Laurent, and Andreas Christen are presenting the latest findings from the project. @helsinki.fi @envmet.bsky.social @lsce-ipsl.bsky.social

Andreas Christen @envmet.bsky.social from University of Freiburg presenting an overview of the principles, applications and limitations of eddy covariance flux measurements in cities. This is part of the Urban Good Practice Guidelines and will be discussed more this week
#UrbanGHGsummit
#ICOSCities

This study quantified surface energy balance and evapotranspiration in a Sphagnum-dominated ombrotrophic bog located near their southern limit in western North America on the Coast of British Columbia. We use ecohydrologicical and eddy-covariance measurements on a flux tower to assess the bog’s sensitivity to future climatic conditions, particularly to increasing severity and duration of drought conditions.

Daily evapotranspiration from the bog 2014 - 2022 adjusted for lack of energy balance closure from all years versus equilibrium evapotranspiration Eeq. and color coded by month of year. The dashed line indicates the 1:1-line.

Our new paper with @ubcgeography.bsky.social in Agric. For. Meteorol. investigates climatic sensitivity of evapotranspiration in an ombrotrophic bog on Canada's West Coast. The bog's water balance will be most sensitive to future increases in summer vapour pressure deficit. doi.org/10.1016/j.ag...

An eddy-covariance system on a tall tower over Zurich, Switzerland. The system is equipped with an inlet to a multi-species fast gas analyzer measures simultaneously emissions and uptake of carbon dioxide (CO2), carbon monoxide (CO), nitrogen monoxide and -dioxide (NOx), methane (CH4) and nitrous oxide (N2O). The information on the correlation and flux ratios between species is used to attribute CO2, CO and NOx to major emission sectors (road transport, stationary combustion and biospheric sources / sinks)

Correlation matrix of 30-min fluxes of measured species. The lower-left half is the summer months (Aug-Oct) and upper-right half is the winter months (Nov-Mar). Clearly, we see higher correlation between CO2 and combustion air pollutants CO / NOx, weaker correlation with non-CO2 greenhouse gases CH4 and N2O. Expectedly correlations are stronger in winter between CO2 and air pollutants, when biospheric CO2 fluxes are weaker.

New preprint at @egu.eu on greenhouse gas emission measurements from cities as part of @icos-ri.eu. Our group @uni-freiburg.de and partners propose a new method for attributing emissions to sources using multi-species eddy fluxes on a tall tower over Zurich. egusphere.copernicus.org/preprints/20...

An arcus cloud passed over Freiburg earlier this evening. It was part of a convective system embedded in a cold front. A sharp increase in wind, a drop in air temperature and an unusual pressure jump +2 hPa in just 5 minutes measured @uni-freiburg.de weather station.

We advertise a new Full Professorship (W3) in Fire Ecology @uni-freiburg.de. Do you work on effects of fires on vegetation biodiversity, functionality, dynamics and fire mitigation? Consider applying to join our vibrant Faculty of Environment and Natural Resources. uni-freiburg.de/en/job/00004...

The map of Freiburg, Germany shows the number of frost days with a minimum air temperature below 0°C in the city of Freiburg and its surroundings as measured by a street-level sensor network in 2022-2023. Credits: Marvin Plein and Andreas Christen, Chair of Environmental Meteorology, University of Freiburg, sensor network supported and funded by ERC Grant urbisphere

Urban Heat Islands are usually associated with heatwaves in summer. But during cold winter nights, they are equally present. Our street-level network @uni-freiburg.de reveals that frost is less likely to occur in the city of Freiburg (24 days per year) compared to the rural surrounding (50-60 days).

Scientists brace ‘for the worst’ as Trump purges climate mentions from websites Trump administration pulling references online ‘won’t make crisis’ stop affecting Americans’ lives, say experts

“Scientists brace ‘for the worst’ as Trump purges climate mentions from websites” by Oliver Milman for The Guardian. Read more here:

www.theguardian.com/us-news/2025...

Our new paper in "Ecohydrology" led by @simonhaber.bsky.social describes dynamics of short-term droughts on tree water potentials. We were using new continuous stem water sensors at our @icos-ri.eu site in Hartheim @uni-freiburg.de as part of @ecosense-sfb-1537.bsky.social doi.org/10.1002/eco....

New @eurogeosciences.bsky.social preprint on our @icos-ri.eu relaxed eddy accumulation system for measuring 14C fluxes. Developed by @uni-freiburg.de, @uniheidelberg.bsky.social and @maxplanck.de, it aims to partition city emissions into fossil fuel and biogenic contributions doi.org/10.5194/egus...

University of Basel ICOS tower to measure GHG fluxes from the city.

Students visit a black-globe weather station to measure human thermal comfort on a newly planned public plaza in Basel

Meteoblue street-level weather station in Basel to test different weather sensors.

Field trip with @uni-freiburg.de students to learn about urban climate activities in Basel - Meteoblue.com showcased their city climate services and city wide sensor networks. At @unibas.ch we learned how eddy covariance on towers is used to monitor GHG emission reductions.

Weather station part of the Freiburg Sensor Network to monitor outdoor thermal comfort - Here in the neighbourhood of Vauban in Freiburg, Germany.

The thermal comfort monitoring stations documented in the article in the city of Freiburg (blue symbols) are also complemented by regular commercial all-in one stations (yellow symbols) - together creating a network of 42 stations across the city and in its surrounding.

New article in "Sustainable Cities and Society" documents custom-built weather stations to monitor pedestrian-level thermal comfort on street lights. They are part of our city-wide sensor network @uni-freiburg.de to evaluate urban climate models - funded by ERC @ec.europa.eu doi.org/10.1016/j.sc...

Modular approach to near-time data management for multi-city atmospheric environmental observation campaigns Abstract. 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 Res...

Our new article in @eurogeosciences.bsky.social Geoscientific Instrumentation - How meteorological data streams from diverse instruments, scales, and campaigns are integrated in a data management approach to support urban climate modelling @suegrimmond.bsky.social gi.copernicus.org/articles/13/...

Honored to receive FLUXNET 2024 Paper of the Year award! Greatly appreciate the recognition of our flux work in BC Canada. A heartfelt thank you to everyone who supported me (Andy, Paul, and many more). #AGU2024 #AGU24 (pc: Sara Knox)

Our new paper in Urban Climate presents a machine-learning approach to simulate airflow in real urban canopies based on large eddy simulations. Result from recent #I4C project on AI applications in environmental research and collaboration between @unifreiburg and Columbia U. doi.org/10.1016/j.uc...

First field VOCs measurements using REA and chamber techniques 😄 Can't wait for the results! #ECOSESNSE project @ecosense-sfb-1537.bsky.social @chriswernerlab.bsky.social

Methodology to evaluate numerical weather predictions using large‐aperture scintillometry sensible heat fluxes: Demonstration in London To evaluate numerical weather prediction (NWP) models using observations, the horizontal and vertical resolution characteristics need to be considered to ensure variables are spatially and temporally...

Job: DWL/LAS analysis with NWP
jobs.reading.ac.uk/Job/JobDetai...
Recent papers relevant to this:
doi.org/10.1175/BAMS...
doi.org/10.1002/qj.4...
doi.org/10.1002/qj.4...

Work with the MO/ECMWF/ERC urbisphere /NERC ASSURE at University of Reading.
Closing date: 11/12/2024