Simulation of Optical Properties and Direct and Indirect Radiative Effects of Smoke Aerosols Over Marine Stratocumulus Clouds During Summer 2008 in California With the Regional Climate Model RegCM

Marc Mallet (1) , Fabien Solmon (2, 3) , Laurent Roblou (3) , F. Peers (4) , Solène Turquety (5) , Arnaud Waquet (5) , H. Jethva (6, 7) , O. Torres (7)

1 CNRM - Centre national de recherches météorologiques
2 ICTP - Abdus Salam International Center for Theoretical Physics
3 LAERO - Laboratoire d'aérologie
4 EMPS - College of Engineering, Mathematics and Physical Sciences [Exeter]
5 LOA - Laboratoire d’Optique Atmosphérique - UMR 8518
6 USRA - Universities Space Research Association [Washington]
7 GSFC - NASA Goddard Space Flight Center

The regional climate model RegCM has been modified to better account for the climatic effects of biomass-burning particles. Smoke aerosols are represented by new tracers with consistent radiative and hygroscopic properties to simulate the direct radiative forcing (DRF), and a new parameterization has been integrated for relating the droplet number concentration to the aerosol concentration for marine stratocumulus clouds (Sc). RegCM has been tested during the summer of 2008 over California, when extreme concentration of smoke, together with the presence of Sc, is observed. This work indicates that significant aerosol optical depth (AOD) ( 1-2 at 550 nm) is related to the intense 2008 fires. Compared to Ozone Monitoring Instrument (OMI) and Moderate Resolution Imaging Spectroradiometer, the regional pattern of RegCM AOD is well represented although the magnitude is lower than satellite observations. Comparisons with Polarization and Directionality of Earth Reflectances (POLDER) above-clouds aerosol optical depth (ACAOD) show the ability of RegCM to simulate realistic ACAOD during the transport of smoke above the Pacific Ocean. The simulated single scattering albedo is 0.90 (at 550 nm) near biomass-burning sources, consistent with OMI and POLDER, and smoke leads to shortwave heating rates 1.5-2°K d^-1. RegCM is not able to correctly resolve the daily patterns in cloud properties notably due to its coarse horizontal resolutions. However, the changes in the sign of the DRF at top of atmosphere (TOA) (negative to positive) from clear-sky to all-sky conditions is well simulated. Finally, the "aerosol-cloud" parameterization allows simulating an increase of the cloud optical depth for significant concentrations, leading to large perturbations of radiative fluxes at TOA.

Mots clés: aerosol; radiative direct forcing; indirect forcing

Domaines: Planète et Univers [physics]

Marc Mallet, Fabien Solmon, Laurent Roblou, F. Peers, Solène Turquety, et al.. Simulation of Optical Properties and Direct and Indirect Radiative Effects of Smoke Aerosols Over Marine Stratocumulus Clouds During Summer 2008 in California With the Regional Climate Model RegCM. Journal of Geophysical Research: Atmospheres, 2017, 122, pp.10,312-10,337. ⟨10.1002/2017JD026905⟩. ⟨insu-03668334⟩