Thesis of Mégane Ventura

Defense of thesis Mégane Ventura - laboratory LOA

Synergy of active and passive airborne observations for the evaluation of the radiative impacts of aerosols. Application to the AEROCLO-SA field campaign in Namibia.

Abstract :

Aerosols exert substantial influence on both local and global climates, as well as on cloud and precipitation patterns. Presented herein are original findings from the Aerosol Radiation and Clouds in Southern Africa (AEROCLO-sA) field campaign conducted in Namibia during August and September 2017. This region demonstrates a robust response to climate change and is associated with significant uncertainties in climate models. Substantial quantities of aerosols resulting from biomass burning, emitted by vegetation fires in Central Africa, are transported extensively over Namibian deserts and are also detected above stratocumulus clouds covering the South Atlantic Ocean along the Namibian coast. Absorbing aerosols above clouds are linked to a pronounced positive direct radiative forcing (warming), a phenomenon still underestimated in climate models (De Graaf et al., 2021). The absorption of solar energy by aerosols above clouds may induce warming within the aerosol layer. This warming has the potential to alter the thermodynamic properties of the atmosphere, impacting the vertical development of low-level clouds and influencing cloud top height and brightness. This aerosol semi-direct effect has been previously observed off the coasts of Angola and Namibia (Wilcox, 2012; Deaconu et al., 2019).
The airborne field campaign involved ten flights conducted with the French F-20 Falcon aircraft in the designated region of interest. Several instruments were employed, including the OSIRIS polarimeter, a prototype of the upcoming European Space Agency's 3MI spaceborne instrument (Chauvigné et al., 2021), the LNG lidar, an airborne photometer named PLASMA, as well as fluxmeters and radiosondes used for measuring thermodynamic quantities. Additionally, in situ measurements of aerosol particle size distribution complemented the instrumentation suite.
In order to quantify the aerosol's radiative impact on the Namibian regional radiative budget, we employ an innovative approach that combines data from the polarimeter and lidar to derive aerosol heating rates. This methodology is assessed during substantial transports of biomass burning particles. To calculate this parameter, we utilize a radiative transfer code and additional meteorological parameters provided by radiosondes. We will present the results obtained for multiple flights conducted during the campaign, where the aerosol load was notably high. Biomass burning aerosols were transported along the Namibian coast, and aerosol plumes were typically observed above stratocumulus clouds. We will present vertical profiles of heating rates computed in the solar and thermal parts of the spectrum using this technique. Our findings indicate particularly high heating rate values estimated above clouds due to aerosols, reaching up to 8 K per day in extreme cases, which has the potential to disrupt the dynamics of the sub-cloud layers. The radiative impact of water vapor present in these plumes is also addressed.
To validate and quantify this novel methodology, we used flux measurements acquired during loop descents performed during dedicated segments of the flights. This approach provides unique measurements of the flux distribution (upwelling and downwelling) and heating rates as function of altitude.
Finally, we will discuss the feasibility of applying this method to available passive and active spaceborne observations to provide initial estimates of heating rate profiles above clouds on a global scale.

Keywords : aerosols,remote sensing,radiative transfer,Heating rate and fluxes,Polarization,Lidar