Abstract
Abstract. The degree of non-linearity in DMS-cloud-climate interactions is assessed using the ECHAM5-HAMMOZ model by taking into account end-to-end aerosol chemistry-cloud microphysics link. The evaluation is made over the Southern oceans in austral summer, a region of minimal anthropogenic influence. In this study, we compare the DMS-derived changes in the aerosol and cloud microphysical properties between a baseline simulation with the ocean DMS emissions from a prescribed climatology, and a scenario where the DMS emissions are doubled. Our results show that doubling the DMS emissions in the current climate results in a non-linear response in atmospheric DMS burden and subsequently, in SO2 and H2SO4 burdens due to inadequate OH oxidation. The aerosol optical depth increases by only ~20 % in the 30° S–75° S belt in the SH summer months. This increases the vertically integrated cloud droplet number concentrations (CDNC) by 25 %. Since the vertically integrated liquid water vapor is constant in our model simulations, an increase in CDNC leads to a reduction in cloud droplet radius of 3.4 % over the Southern oceans in summer. The equivalent increase in cloud liquid water path is 10.7 %. The above changes in cloud microphysical properties result in a change in global annual mean radiative forcing at the TOA of −1.4 W m−2. The results suggest that the DMS-cloud microphysics link is highly non-linear. This has implications for future studies investigating the DMS-cloud climate feedbacks in a warming world and for studies evaluating geoengineering options to counteract warming by modulating low level marine clouds.
Highlights
Aerosols can influence the radiative balance of the Earth both directly and indirectly
Aerosols can act as cloud condensation nuclei, alter the microphysical properties of clouds and impose an indirect radiative forcing to the climate system (Carslaw et al (2010); Lohmann and Feichter (2005) and references therein)
While deriving motivation from the above results, the present study extends them by using a global aerosolchemistry-cloud microphysics-climate model (ECHAM5HAMMOZ) that accounts directly for the process based linkages between DMS emissions, aerosol formation, and cloud microphysics
Summary
Aerosols can influence the radiative balance of the Earth both directly and indirectly. Thomas et al.: Rate of non-linearity in DMS-cloud-climate interactions these models are constrained by satellite data, the forcing is estimated as −0.7±0.4 W m−2 (Quaas et al, 2009). Instead of imposing an increase of CDNC, Korhonen et al (2010) used a global aerosol transport model to quantify the change in droplet number concentrations resulting from an increase in sea salt emissions as prescribed by Salter et al (2008); they showed that the pathway from the emissions to CDNC formation was non linear because of the dilution and removal of particles from the atmosphere, and because the injection of a large number of accumulation mode particles suppressed cloud supersaturation. We will evaluate the degree of non-linearity in the downstream formation of sulphate, in aerosol optical depth (AOD), in cloud microphysical properties such as CDNC, CD effective radii, liquid water path, and in TOA radiative forcing. The DIAG3 measure is the mean percentage change in the aerosol parameters and cloud microphysics when the ocean DMS is doubled with respect to the present day DMS emissions
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