Cirrus clouds have a major impact on the Earth radiative balance. They reflect solar radiation while trapping infrared energy, yielding a small but uncertain net warming of about +2 W/m².

In the tropics, cirrus cover exceeds 60%. This includes both thick convective anvils and thin in-situ cirrus. Their estimated net forcing (~–8 W/m² and +1 W/m², respectively) is comparable to that of anthropogenic greenhouse gases (+3 W/m²). The response of cirrus to climate change remains debated, with significant implications for climate sensitivity (see, e.g., the ''iris hypothesis'', Lindzen et al., 2001).  They also exert an indirect radiative effect by modulating the water vapor amount in the upper troposphere (UT) and stratosphere.

Besides their global forcing, tropical cirrus locally heat the UT, influencing large-scale circulations, transport to the stratosphere, and cloud lifetime. The magnitude of cirrus radiative heating varies widely among models (e.g., Wright et al., 2020). Existing observation-based datasets often underestimate thin cirrus cover, which significantly affects radiative transfer (Yang et al., 2010, L’Ecuyer et al., 2008, 2019). 

Thanks to their sensitivity to subvisible clouds, lidars are uniquely suited to probing cirrus. The new spaceborne ATLID and balloonborne BeCOOL lidars offer an excellent opportunity to improve estimates of tropical cirrus radiative effects.

ATLID, onboard ESA EarthCARE satellite, retrieves vertical profiles of extinction and backscatter at 355 nm. Combined with a cloud radar and radiometer, ATLID observations enable radiative heating rate estimates as an operational product. While ATLID is only sensitive to cirrus with optical depth > 0.005, the radar–lidar synergy enables measurements in optically thicker cloud layers.

The 808 nm lidar BeCOOL, developed in the frame of the Strateole 2 project (ST2), demonstrated exceptional sensitivity to ultrathin cirrus during the 2021–2022 balloon campaign—measuring down to optical depths of 2 × 10⁻⁵ (Lesigne et al., 2024), two orders of magnitude below CALIOP’s detection threshold. The cover by ultra-thin cirrus amounts to 37%, implying potentially large radiative impacts. Slowly drifting onboard the balloon, BeCOOL also has the unique ability to resolve the life cycle of cirrus (Lesigne et al., 2025).

In this context, the crux of this PhD project will be to quantify the radiative effects of tropical cirrus from EarthCARE and BeCOOL observations. We will

    1. Quantify both cirrus radiative forcing and heating rates, assessing their space and time variability.

    2. Evaluate the influence of data resolution and fine-scale structures (filaments, anvils, gravity waves) on the estimated radiative effects.

The student will use radiative transfer (RT) calculations performed offline using observed cloud fields as input. Specifically, vertical profiles of extinction/backscatter from ATLID and BeCOOL will be combined with in-situ microphysics data (Krämer et al., 2023) to estimate profiles of cirrus radiative properties. Those will be used as inputs to a RT model (RRTM). The conducted RT simulations will be validated through comparison with operational products (Yamauchi et al., 2024) and radiometer data for ATLID and with balloon-based estimates of infrared fluxes for BeCOOL.

Besides quantifying the overall cirrus radiative effect, the new dataset will enable the student to investigate the contribution of various factors to the radiative effects: different cloud types (subvisible cirrus, anvils), environmental conditions (background water vapor concentrations, cloud scene) and cloud life-cycle stage. They will also assess the consequences on RT results of including fine-scale structures uniquely resolved by BeCOOL, and of modifying the water partitioning between cirrus ice and supersaturated vapor phase. The results will be instrumental in assessing the radiative impacts of tropical cirrus over their whole range of optical depth, thus paving the way for an improved representation of these clouds in climate models.

Additionally, a side objective of the PhD will be to conduct a comparative analysis of BeCOOL and ATLID observations, taking advantage of expected colocations during the upcoming flights in the 2026-2027 phase of ST2. Combined with a previous comparison of BeCOOL and CALIOP, this will contribute to linking the CALIOP and EarthCare long-term time series of tropical cirrus.

The candidat will be jointly advised by A. Podglajen (LMD) and F. Ravetta (LATMOS, BeCOOL  PI, HDR). It is intended that A. P. will submit his HDR and become the official supervisor over the course of the PhD. The two researchers have advised together PhD candidate T. Lesigne (defense in Dec. 2025), who will be working as a research engineer on BeCOOL during the project, and provide support in handling the data. 

The PhD candidate will also interact with a network of local (A. Hertzog, C. Stubenrauch) and international collaborators (E. Jensen, M.Kramer).

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - francois.ravetta@latmos.ipsl.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online before March 13th, 2026 Midnight Paris time!