Laboratoire d'Aérologie

Clouds exert multifaceted radiative effects on Earth's energy budget, acting as both insulators and reflectors that profoundly influence regional and global climate dynamics. Since 2006, spaceborne active sounders have monitored clouds with unprecedented vertical and horizontal resolution. Yet comparing cloud data from different lidars remains problematic - variations in wavelength, pulse energy, detector type, and observation times create discontinuities that complicate our understanding of long-term cloud behavior.This study presents a methodology to reconcile cloud observations from multiple spaceborne lidar platforms: CALIPSO (2006–2023), ALADIN/Aeolus (2018–2023), IceSat-2 (2018–present), ACDL/Daqi-1 (2022–present), and ATLID/EarthCARE (2024–present). We have already demonstrated this approach works for CALIOP and ALADIN (Feofilov et al., 2024); here we apply it to bridge CALIOP and ATLID. The approachWe use the Scattering Ratio at 532 nm (SR532) as our common language across all lidars. For measurements at other wavelengths, we convert the retrieved optical properties to SR532 and ATB532 (Attenuated Total Backscatter at 532 nm), enabling direct comparison. Since different signal-to-noise ratios between instruments can affect cloud detection near the detection threshold, we pay close attention to these differences.When satellites don't share the same viewing times - even with nearly identical equator crossings - we apply a diurnal cycle correction using climatology derived from CATS measurements as in (Feofilov and Stubenrauch, 2019; Feofilov et al., 2014). Since the satellites fly in opposite directions, they observe extratropical zones at different local times, and we must account for this.For missions that overlap in time, we fine-tune our cloud detection parameters until the datasets transition seamlessly. We then scrutinize collocated data across latitudes, altitudes, and seasons, hunting for differences and correcting for them where we find instrument sensitivity or noise effects.When instruments don't overlap that is the case for CALIOP and ATLID, we use a different strategy: we identify geographical zones characterized by minimal interannual variability and trends. These "stable" zones become our reference for intercalibration, allowing us to anchor ATLID to CALIOP without a shared observational period.What we getWe take ATLID's complete baseline, apply the wavelength conversion, perform diurnal cycle corrections, run our detection algorithm with the thresholds we've defined, generate global cloud distributions for the entire mission, and discuss its key properties with respect to CALIOP.

Benzene, toluene, ethylbenzene, and xylene (BTEX) are hazardous air pollutants with high toxicity and a strong potential for secondary pollutant formation. However, their occurrence and behavior in the Arctic remain poorly understood. During the Alaskan Layered Pollution and Chemical Analysis (ALPACA) field campaign in Fairbanks, Alaska in January-February 2022. Surface observations in downtown Fairbanks revealed two major pollution periods, with extremely cold (down to -35°C) and warmer temperatures (around 0°C), respectively. BTEX concentrations reached 4–12 times higher than those reported in the US and European countries under dark, cold Arctic winter conditions at breathing level, posing a significant health risk. We simulated BTEX atmospheric distributions in the Fairbanks region using the FLEXible PARTicle-Weather Research and Forecasting (FLEXPART-WRF) Lagrangian particle dispersion model and anthropogenic emissions at the surface and aloft. Due to limited photochemical loss in to the dark polar winter conditions, we treat BTEX as an unreactive tracer in the model. The control run with the emission inventory developed by Alaska Department of Environmental Conservation (ADEC) substantially underestimates BTEX concentrations compared to observations during both polluted periods, indicating deficiencies in winter emissions and near-surface mixing. Enhancing cold-start gasoline vehicle emissions by a factor of 2 during very low-temperatures substantially improved model results during the cold polluted period, while introducing a relative humidity dependence for mobile emissions improved simulated BTEX during the warm, humid pollution period. Addition of emissions of residential heating oil aromatics, not taken into account in the ADEC inventory, also reduced normalized mean biases by 5-10%.The improved model simulation was used to investigate contributing source sectors. While mobile traffic emissions were identified as the dominant source of BTEX across the Fairbanks North Star Borough, residential heating and non-point sources contributed substantially in downtown Fairbanks. Replacing residential wood burning in the inventory with oil heating during severe pollution periods, in line with air quality control guidelines, was found to effectively reduce BTEX concentrations, particularly benzene by up to 30%. While persistent surface-based temperature inversions largely confined BTEX below ~20 m, upward transport, induced by wind shear, during severe episodes, sometimes lofted near-surface pollutants to higher altitudes, potentially contributing to regional pollution and background Arctic haze.The findings of this study emphasise the need to accurately account for temperature and humidity dependent vehicle emissions, residential oil heating emissions, and winter boundary-layer dynamics for improved simulations of air quality in cold wintertime environments, not only in the Arctic but also in mid-latitudes.

Abstract. Observations from airborne field campaigns are used to study the interplay between boundary-layer thermals and clouds in the trades. The size distributions of thermal and cloud-base chords inferred from turbulence and horizontal lidar-radar measurements are robustly described by the sum of two exponentials. Analytical calculations and statistical simulations show that the merging of objects is sufficient to explain the two exponentials, representing, respectively, the populations of merged- and unmerged-object chords. They also show how circulations induced by convective objects facilitate the merging process. The observed day-to-day variability of these populations at cloud base can thus be tied to the variability of thermal merging across the depth of the subcloud layer. Clouds rooted in unmerged thermals are small and shallow while those rooted in merged thermals are wider and deeper. An intricate interplay between thermal- and cloud-merging arises: when thermal merging is weak, thermal number density is high and cloud bases merge easily, leading to strong mesoscale mass fluxes and “Gravel” shallow mesoscale organizations. In contrast, when thermal merging is strong, clouds are fed by sparser but wider thermals, leading to longer cloud lifetimes but weaker cloud merging, weaker mesoscale mass fluxes, and “Flower” mesoscale organizations. This interplay between thermal- and cloud-merging imposes an upper bound on cloud coverage and suggests a negative feedback on the growth of mesoscale circulations. Thermal merging also controls observed size distributions of thermals in deep convective regimes. The merging process thus appears to be a fundamental player in the mesoscale organization of convection.

Abstract. The climate-altering potential of wildfires through their emissions into the stratosphere has only recently been realized following the major wildfire outbreaks in Canada and Australia. The 2023 Canadian wildfire season stood out for its extended burned area and duration, by far exceeded the previous record-breaking events, including the Australian “Black Summer” in terms of the emitted power and pyroCb count with a total number of 142 Canadian pyroCb events over the season. The incessant fire activity all across Canada produced a succession of smoke injections into the lower stratosphere. Here, we use various satellite data sets, airborne and ground-based observations together with chemistry-transport model simulations to show that despite the exceptional vigor of the 2023 Canadian wildfires, the depth of their stratospheric impact was surprisingly shallow and limited to the lowermost stratosphere. Conversely, the incessant fire activity featuring a long succession of moderate-strength pyroCb events, combined with numerous episodes of synoptic-scale smoke uplift through the warm conveyor belt, led to unparalleled levels of pollution at commercial aircraft cruising altitudes throughout the season.

<strong class="journal-contentHeaderColor">Abstract.</strong> Citizen science is increasingly recognized as essential for engaging the public in participatory sustainability research and for addressing the complex challenges of the Anthropocene. However, fostering meaningful dialogue between science and society remains difficult, often hindered by limited opportunities for interaction and varying levels of scientific understanding. Identifying outreach formats that foster citizen engagement and initiate productive exchanges between scientists and the public is therefore a key challenge. Here we present a hands-on science outreach workshop, based on environmental magnetism methods and conducted at schools and science fairs, that encourages citizen participation in air monitoring projects. We conducted the workshop between 2018 and 2023, reaching 850 people at 9 scientific outreach events and 195 children at 3 elementary schools. The workshop prompted more than 150 people to participate in the associated NanoEnvi community-based participatory research project, which offers to host passive biosensors in their homes or at school for a year. The workshop includes three hands-on demonstrations and experiences. It proposed to discover the magnetic phenomena, to extract airborne magnetic particles from soils, and to measure air pollution trapped on bark like a scientist. The workshop was also accompanied by lectures and an exhibition. We observed that the workshop fostered two-way dialogue between researchers and a wide range of participants, creating opportunities for shared experimentation and knowledge co-production. We found that the positive emotions raised by experimenting and playing with magnetic phenomena during the workshop led to engagement in a participatory project on potential air pollution in urban surroundings. Our findings demonstrate that hands-on geoscience outreach activities have a positive impact on the science-society dialogue.

Abstract. We use an offline radiative transfer model driven by IAGOS aircraft observations to estimate the tropospheric ozone radiative forcing (RF) at decadal time scale (three time intervals with a common starting period in 1994–2004 and ending periods: 2011–2016, 2017–2019 and 2020–2023), over 11 selected Northern Hemispheric regions. We found a positive trend in the tropospheric ozone column (TOC) for the three time intervals, even if decadal trends are progressively reduced from 2011–2016 (ΔTOC +3.6 ± 2.0 DU, +14.8 ± 11.5 %, 2.5 ± 1.4 DU per decade) to 2017–2019 (ΔTOC +3.9 ± 3.9 DU, +17.5 ± 22.1 %, 2.0 ± 2.0 DU per decade) to 2020–2023 (ΔTOC +3.5 ± 2.8 DU, +13.8 ± 12.1 %, 1.5 ± 1.2 DU per decade). The progressively reduced average TOC decadal trends in 2020–2023 and 2017–2019, with respect to 2011–2016, originate from stagnation in ΔTOC in more recent time intervals (especially in the COVID and post-COVID crisis periods) and decreases of trends of the more radiative efficient upper tropospheric ozone. These average trend reductions are accompanied with reductions of the tropospheric ozone RF, from 2011–2016 (41.6 ± 24.0 mW m−2 per decade) to 2017–2019 (27.6 ± 36.7 mW m−2 per decade) to 2020–2023 (14.6 ± 25.6 mW m−2 per decade). The total tropospheric ozone RF sensitivity varies between 18.4 ± 7.4 mW m−2 DU−1, in 2011–2016, and 11.1 ± 10.4 mW m−2 DU−1, in 2020–2023. Our decadal RF estimates vary from being ∼ 25 %–90 % larger (2011–2016 and 2017–2019 ending periods) to ∼ 30 % smaller (2020–2023 ending period) than the most recent global average RF estimates with online modelling. Our study underlines the importance of the evolution of ozone vertical profiles for the tropospheric ozone RF.

Abstract The term “Mediterranean tropical-like cyclone” or “medicane” has been used in different ways by different authors. Identification of medicanes has been based on features observed from satellite imagery or on diagnostics applied to numerical weather prediction model outputs. In the absence of an official definition, medicanes are generally considered to be cyclones over the Mediterranean sharing physical processes with tropical cyclones. Nevertheless, recent studies on the dynamics of several systems widely recognized as medicanes show different underlying development mechanisms. A commonly agreed definition is critical and necessary to assess their climatology in past and future climates, as well as to consistently identify such systems in weather forecasts. The scientific community working on Mediterranean cyclones hereby proposes a definition, which is based solely on Earth observations: “A medicane is a mesoscale cyclone that develops over the Mediterranean Sea and displays tropical-like cyclone characteristics: a warm core extending into the upper troposphere, an eye-like feature in its center with spiral cloud bands around, an almost windless center surrounded by nearly-symmetric sea surface wind circulation with maximum wind speed within a few tens of km from the center.” Significance Statement Nearly two decades have passed since the term “medicane,” short for “Mediterranean hurricane,” has become commonly accepted to represent Mediterranean vortices with satellite-observed characteristics typical of tropical cyclones. The absence of an official or commonly accepted definition has created some confusion within the scientific and operational community. To overcome this ambiguity, here, we propose a phenomenological definition that can serve as a reference for future scientific research and operational applications, to be applied to any cyclonic system in the Mediterranean that bears structural similarities to tropical cyclones. This simple definition will assure consistency and, at the same time, can be used with the outputs from numerical models, working as a reference for future scientific research or operational applications.

Abstract. Agroforestry systems — combining trees with crops and/or livestock — are increasingly promoted as sustainable and climate-resilient land-use strategies. Despite their widespread presence in the Sahel, experimental data on their potential as carbon sinks are scarce. This study presents a full-year, high-frequency dataset of CO2 fluxes in a Sahelian agro-silvo-pastoral parkland dominated by F. albida, located in Senegal’s groundnut basin. CO2 fluxes were continuously measured using automated static chambers, allowing the quantification of soil and crop respiration (Rch), gross primary production (GPPch), and net carbon exchange (FCO2ch) under both full sun and shaded (under tree canopies) environments. Seasonal patterns of CO2 fluxes were similar in both environments, with peaks during the rainy season. Rch and GPPch were significantly higher under tree canopies, indicating a ‘fertile island’ effect. CO2 flux variability was primarily driven by soil moisture and leaf area index. Chamber-based GPP estimates closely matched those from Eddy Covariance measurements. On an annual scale, F. albida trees contributed approximately 50 % of total ecosystem GPP, with a carbon use efficiency of 0.48. Net annual CO2 exchange was estimated at −1.4 ± 0.02 and −1.8 ± 0.01 Mg C-CO2 ha⁻¹ using chamber and Eddy Covariance methods, respectively. These findings underscore the role of F. albida-based agroforestry systems as effective carbon sinks in Sahelian landscapes, supporting their potential contribution to climate change mitigation.

Abstract. Tropospheric ozone trends from models and satellites are found to diverge. Ground-based (GB) observations are used to reference models and satellites, but GB data themselves might display station biases and discontinuities. Reprocessing with uniform procedures, the TOAR-II working group Harmonization and Evaluation of Ground-based Instruments for Free-Tropospheric Ozone Measurements (HEGIFTOM) homogenized public data from five networks: ozonesondes, In-service Aircraft for a Global Observing System (IAGOS) profiles, solar absorption Fourier transform infrared (FTIR) spectrometer measurements, lidar observations, and Dobson Umkehr data. Amounts and uncertainties for total tropospheric ozone (TrOC; surface to 300 hPa), as well as free- and lower-tropospheric ozone, are calculated for each network. We report trends (2000 to 2022) for these segments using quantile regression (QR) and multiple linear regression (MLR) for 55 datasets, including six multi-instrument stations. The findings are that (1) median TrOC trends computed with QR and MLR trends are essentially the same; (2) pole-to-pole, across all longitudes, TrOC trends fall within +3 to −3 ppbv per decade, equivalent to (−4 % to +8 %) per decade depending on site; (3) the greatest fractional increases occur over most tropical and subtropical sites, with decreases at northern high latitudes, but these patterns are not uniform; (4) post-COVID trends are smaller than pre-COVID trends for Northern Hemisphere mid-latitude sites. In summary, this analysis conducted in the frame of TOAR-II/HEGIFTOM shows that high-quality, multi-instrument, harmonized data over a wide range of ground sites provide clear standard references for TOAR-II models and evolving tropospheric ozone satellite products for 2000–2022.