The formal definition of TOA cloud radiative forcing can be written as: !",!" = !"# − !"# , where F clr is clear sky net radiative flux (i.e., !"# = !"# ↑ − !"# ↓ for atmospheric columns containing no clouds, where !"# ↑ is the TOA upward radiative flux, and !"# ↓ is the TOA downward radiative flux), and F obs is the TOA net flux that is observed for all sky conditions (excluding ice clouds in this study). F obs can be decomposed into !"# = (1 − !) !"# + ! !"# , where F cld is the component of the radiative flux contributed by clouds. F cld can be calculated using this equation. The equations for !",!" and !"# can be combined to yield the following: !",!" = ! (!"# − !"#), for which the derivative of the shortwave (SW) component of the TOA cloud radiative forcing can be written as in Equation (1). The longwave (LW) component of TOA cloud radiative forcing can be written similarly as: Applying aerosol optical depth (AOD) and/or retrieved cloud droplet number concentration (!) as opposed to aerosol index (AI): In this study, aerosol index (AI) is applied as the proxy for CCN to investigate aerosolcloud interactions. Here we also consider AOD and ! as alternative parameters to examine the extent to which similar responses would occur.

Abstract. The concept of geoengineering by marine cloud brightening is based on seeding marine stratocumulus clouds with sub-micrometer sea-salt particles to enhance the cloud droplet number concentration and cloud albedo, thereby producing a climate cooling effect. The efficacy of this as a strategy for global cooling rests on the extent to which aerosolperturbed marine clouds will respond with increased albedo. Ship tracks, quasi-linear cloud features prevalent in oceanic regions impacted by ship exhaust, are a well-known manifestation of the effect of aerosol injection on marine clouds. We present here an analysis of the albedo responses in ship tracks, based on in situ aircraft measurements and three years of satellite observations of 589 individual ship tracks. It is found that the sign (increase or decrease) and magnitude of the albedo response in ship tracks depends on the mesoscale cloud structure, the free tropospheric humidity, and cloud top height. In a closed cell structure (cloud cells ringed by a perimeter of clear air), nearly 30 % of ship tracks exhibited a decreased albedo. Detailed cloud responses must be accounted for in global studies of the potential efficacy of sea-spray geoengineering as a means to counteract global warming.

Aerosol–cloud–radiation interactions are widely held to be the largest single source of uncertainty in climate model projections of future radiative forcing due to increasing anthropogenic emissions. The underlying causes of this uncertainty among modeled predictions of climate are the gaps in our fundamental understanding of cloud processes. There has been significant progress with both observations and models in addressing these important questions but quantifying them correctly is nontrivial, thus limiting our ability to represent them in global climate models. The Eastern Pacific Emitted Aerosol Cloud Experiment (E-PEACE) 2011 was a targeted aircraft campaign with embedded modeling studies, using the Center for Interdisciplinary Remotely-Piloted Aircraft Studies (CIRPAS) Twin Otter aircraft and the research vessel Point Sur in July and August 2011 off the central coast of California, with a full payload of instruments to measure particle and cloud number, mass, composition, and water uptake distributions. EPEACE used three emitted particle sources to separate particle-induced feedbacks from dynamical variability, namely 1) shipboard smoke-generated particles with 0.05–1-μm diameters (which produced tracks measured by satellite and had drop composition characteristic of organic smoke), 2) combustion particles from container ships with 0.05–0.2-μm diameters (which were measured in a variety of conditions with droplets containing both organic and sulfate components), and 3) aircraft-based milled salt particles with 3–5-μm diameters (which showed enhanced drizzle rates in some clouds). The aircraft observations were consistent with past large-eddy simulations of deeper clouds in ship tracks and aerosol– cloud parcel modeling of cloud drop number and composition, providing quantitative constraints on aerosol effects on warm-cloud microphysics.

Anthropogenic aerosols have been shown to enhance the solar reflection from warm liquid clouds and mask part of the warming due to the buildup of greenhouse gases. However, very little is known about the effects of aerosol on mixed‐phase stratiform clouds as well as other cloud regimes including cumulus, altocumulus, nimbostratus, deep convection, and anvil cirrus. These additional cloud categories are ubiquitous and typically overlooked in satellite‐based assessments of the global aerosol indirect forcing. Here we provide their contribution to the aerosol indirect forcing estimate using satellite data collected from several colocated sensors in the A‐train for the period 2006–2010. Cloud type is determined according to the 2B‐CLDCLASS‐LIDAR CloudSat product, and the observations are matched to the radiative flux measurements from CERES (Clouds and the Earth's Radiant Energy System) and aerosol retrievals from MODIS (MODerate resolution Imaging Spectroradiometer). The oceanic mean aerosol indirect forcing is estimated to be −0.20 ± 0.31 W m−2 with warm low‐level cloud largely dictating the strength of the response (−0.36 ± 0.21 W m−2) due to their abundance and strong cloud albedo effect. Contributions from mixed‐phase low‐level cloud (0.01 ± 0.06 W m−2) and convective cloud (0.15 ± 0.23 W m−2) are positive and buffer the system due to strong aerosol‐cloud feedbacks that reduce the cloud albedo effect and/or lead to convective invigoration causing a countering positive longwave warming response. By combining all major cloud categories together, aerosol indirect forcing decreases and now contains positive values in the uncertainty estimate.

Abstract. Three-dimensional large-eddy simulations (LES) with detailed bin-resolved microphysics are performed to explore the diurnal variation of marine stratocumulus (MSc) clouds under clean and polluted conditions. The sensitivity of the aerosol-cloud-precipitation interactions to variation of sea surface temperature, free tropospheric humidity, largescale divergence rate, and wind speed is assessed. The comprehensive set of simulations corroborates previous studies that (1) with moderate/heavy drizzle, an increase in aerosol leads to an increase in cloud thickness; and (2) with non/light drizzle, an increase in aerosol results in a thinner cloud, due to the pronounced effect on entrainment. It is shown that for higher SST, stronger large-scale divergence, drier free troposphere, or lower wind speed, the cloud thins and precipitation decreases. The sign and magnitude of the Twomey effect, droplet dispersion effect, cloud thickness effect, and cloud optical depth susceptibility to aerosol perturbations (i.e., change in cloud optical depth to change in aerosol number concentration) are evaluated by LES experiments and compared with analytical formulations. The Twomey effect emerges as dominant in total cloud optical depth susceptibility to aerosol perturbations. The dispersion effect, that of aerosol perturbations on the cloud droplet size spectrum, is positive (i.e., increase in aerosol leads to spectral narrowing) and accounts for 3 % to 10 % of the total cloud optical depth susceptibility at nighttime, with greater influence in heavier drizzling clouds. The cloud thickness effect is negative (i.e., increase in aerosol leads to thinner cloud) for non/light drizzling cloud and positive for a moderate/heavy drizzling clouds; the cloud thickness effect contributes 5 % to 22 % Correspondence to: J. H. Seinfeld (seinfeld@caltech.edu) of the nighttime total cloud susceptibility. Overall, the total cloud optical depth susceptibility ranges from ∼0.28 to 0.53 at night; an increase in aerosol concentration enhances cloud optical depth, especially with heavier precipitation and in a more pristine environment. During the daytime, the range of magnitude for each effect is more variable owing to cloud thinning and decoupling. The good agreement between LES experiments and analytical formulations suggests that the latter may be useful in evaluations of the total cloud susceptibility. The ratio of the magnitude of the cloud thickness effect to that of the Twomey effect depends on cloud base height and cloud thickness in unperturbed (clean) clouds.