Abstract
Abstract. Increased aerosol concentrations can raise planetary albedo not only by reflecting sunlight and increasing cloud albedo, but also by changing cloud amount. However, detecting aerosol effect on cloud amount has been elusive to both observations and modeling due to potential buffering mechanisms and convolution of meteorology. Here through a natural experiment provided by long-term degassing of a low-lying volcano and use of A-Train satellite observations, we show modifications of trade cumulus cloud fields including decreased droplet size, decreased precipitation efficiency and increased cloud amount are associated with volcanic aerosols. In addition we find significantly higher cloud tops for polluted clouds. We demonstrate that the observed microphysical and macrophysical changes cannot be explained by synoptic meteorology or the orographic effect of the Hawaiian Islands. The "total shortwave aerosol forcin", resulting from direct and indirect forcings including both cloud albedo and cloud amount, is almost an order of magnitude higher than aerosol direct forcing alone. Furthermore, the precipitation reduction associated with enhanced aerosol leads to large changes in the energetics of air-sea exchange and trade wind boundary layer. Our results represent the first observational evidence of large-scale increase of cloud amount due to aerosols in a trade cumulus regime, which can be used to constrain the representation of aerosol-cloud interactions in climate models. The findings also have implications for volcano-climate interactions and climate mitigation research.
Highlights
Aerosols directly modify planetary albedo by absorbing and scattering solar radiation often referred to as the “aerosol direct effect” (McCormic and Ludwig, 1967)
In this study we explore aerosol effects on trade84c0uas evidenced in (a) MODerate resolution Imaging Spectroradiometer (MODIS) aerosol optical depth and (b) OMI SO2 maps averaged over June, July, August 2008
We use level-2 MODIS cloud fates is clearly visible in the Moderate Resolution Imaging 37 retrievals (Platnick et al, 2003) to assess the impact of added Spectroradiometer (MODIS) aerosol optical depth (Remer et volcanic sulfate aerosols, active cloud condensation nuclei al., 2005) map on 17 July 2008 (Fig. 2a)
Summary
Aerosols directly modify planetary albedo by absorbing and scattering solar radiation often referred to as the “aerosol direct effect” (McCormic and Ludwig, 1967). Increased aerosol concentration can enhance cloud macro-physical properties such as cloud amount (referred to as “cloud amount effect” here) and cloud organization (Albrecht, 1989; Stevens and Feingold, 2009; Feingold et al, 2010). The almost Gaussian shaped aerosol plume emanating from the volcano source provides strong testimony to the steady wind and diffusive nature of the mixing in this region (Fig. 1) These homogeneous background conditions coupled with the volcanic aerosol source are used to identify aerosol effects by comparing the plume-affected area and nearby non-affected area. We have a controlled natural experiment in which non-absorbing sulfate aerosols are introduced to an otherwise pristine trade wind boundary layer, where the direct and indirect effects can be tracked and quantified. We will analyze the impact of this far-field orographic effect (referred to as “wake effect” in this paper for convenience) on our results and show it does not affect our results
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
