The first aerosol indirect effect

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract The first aerosol indirect effect (1AIE) is investigated using a combination of in situ and remotely-sensed aircraft (NCAR C-130) observations acquired during VOCALS-REx over the Southeast Pacific stratocumulus cloud regime. Satellite analyses have previously identified a high albedo susceptibitility to changes in cloud microphysics 5 and aerosols over this region. The 1AIE was broken down into the product of two independently-estimated terms: the cloud aerosol interaction metric ACI τ = d ln τ d ln N a LWP , and the relative albedo (A) susceptibility S R−τ = dA 3d ln τ LWP , with τ and N a denoting retrieved cloud optical thickness and in-situ aerosol concentration, respectively and calculated for fixed intervals of liquid water path (LWP). ACI τ was estimated by combining 10 in-situ N a sampled below the cloud, with τ and LWP derived from, respectively, simultaneous upward-looking broadband irradiance and narrow field-of-view millimeter-wave radiometer measurements, collected at 1 Hz during four eight-hour daytime flights by the C-130 aircraft. ACI τ values were typically large, close to the physical upper limit (0.33), increasing with LWP. The high ACI τ values were in agreement with other in-situ 15 airborne studies in pristine marine stratocumulus and reflect the imposition of a LWP constraint and simultaneity of aerosol and cloud measurements. S R−τ increased with LWP and τ, reached a maximum S R−τ (0.086) for LWP (τ) of 58 g m −2 (13–14), decreasing slightly thereafter. The net first aerosol indirect effect thus increased over the LWP range of 30–80 g m −2. These values were consistent with satellite estimates 20 derived from instantaneous, collocated CERES albedo and MODIS-retrieved droplet number concentrations at 50 km resolution. The consistency of the airborne and satellite estimates (for airborne remotely sensed N d < 1100 cm −3), despite their independent approaches, differences in observational scales, and retrieval assumptions, is hypothesized to reflect the robust remote sensing conditions for these homogeneous clouds. 25 We recommend the Southeast Pacific for a regional assessment of the first aerosol indirect effect in climate models on this basis. Abstract Introduction Conclusions References Tables Figures Back Close Full …