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Andreas Christen @envmet.bsky.social · 3d

Is satellite land surface temperature (LST) a good proxy for urban heat stress? In our new article led by Ferdinand Briegel @kit.edu and @uni-freiburg.de in RSE we conclude "no" - comparing nearly 100 LST scenes to detailed 1-m heat stress modelling at pedestrian level. doi.org/10.1016/j.rs...

Maps of averaged LST (a) and UTCI (b) for 46 timesteps in summer in Freiburg, Germany. The difference between LST and UTCI is shown in (c), and the land cover classes are shown in (d).

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Andreas Christen @envmet.bsky.social · 29d

Summer 2025 was unusually warm and wet. At our at Hartheim @icos-ri.eu station of @uni-freiburg.de in the Upper Rhine Valley JJA was 1°C warmer and +25% precipitation vs. 1991–2020 (June +2.7°C). Only 4 years since 1978 were both wetter and warmer, but 2025 stands out as most extreme yet.

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Reposted by Andreas Christen

Universität Freiburg @uni-freiburg.de · Aug 20

#Hitzebelastung der Zukunft prognostiziert: Forschende von #UniversitätFreiburg und Karlsruher Institut für Technologie
@kit.edu haben KI-Modell entwickelt, das städtischen Hitzestress für ganze Stadtgebiete über mehrere Jahrzehnte hinweg simulieren kann. ufr.link/hitzeprognose
@envmet.bsky.social

Portrait von Prof. Dr. Andreas Christen,
Umweltmeteorologe, Universität Freiburg vor dem Hintergrund einer Heatmap der Stadt Freiburg.

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Andreas Christen @envmet.bsky.social · Jul 17

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...

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…

www.sciencedirect.com

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Andreas Christen @envmet.bsky.social · Jul 14

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-...

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

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Andreas Christen @envmet.bsky.social · Jul 8

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.

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Andreas Christen @envmet.bsky.social · Jul 2

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...

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Andreas Christen @envmet.bsky.social · Jun 21

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/...

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.

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Andreas Christen @envmet.bsky.social · Jun 17

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....

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.

doi.org

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Andreas Christen @envmet.bsky.social · Jun 2

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...

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.

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Andreas Christen @envmet.bsky.social · May 24

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...

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Andreas Christen @envmet.bsky.social · May 21

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...

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Reposted by Andreas Christen

Simon Haberstroh @simonhaber.bsky.social · Apr 30

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

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Andreas Christen @envmet.bsky.social · Apr 25

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/

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Andreas Christen @envmet.bsky.social · Apr 23

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...

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.

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Reposted by Andreas Christen

ICOS RI @icos-ri.eu · Apr 8

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

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Reposted by Andreas Christen

ICOS RI @icos-ri.eu · Apr 7

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

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Andreas Christen @envmet.bsky.social · Mar 26

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...

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.

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Andreas Christen @envmet.bsky.social · Mar 24

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 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.

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Andreas Christen @envmet.bsky.social · Feb 25

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.

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Andreas Christen @envmet.bsky.social · Feb 17

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...

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Andreas Christen @envmet.bsky.social · Feb 6

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).

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

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Reposted by Andreas Christen

Michael E. Mann @michaelemann.bsky.social · Feb 4

“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...

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

www.theguardian.com

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Andreas Christen @envmet.bsky.social · Jan 23

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....

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Andreas Christen @envmet.bsky.social · Jan 17

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...

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