Aerosol indirect effect dictated by liquid clouds

Abstract

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.

Unveiling aerosol–cloud interactions – Part 1: Cloud contamination in satellite products enhances the aerosol indirect forcing estimate

“…Aerosols are hypothesised to cool the climate system due to their ability to enhance the reflection of clouds (Twomey, 1974;Albrecht, 1989), particularly ubiquitous warm-phase clouds located in the boundary layer (Christensen et al, 2016a). The uncertainty attached to this cooling effect is considered to be very large (IPCC, 2013) in both satellite and general circulation model (GCM) estimates.…”

“…This process uses aerosol properties that are extracted from reanalysis products at the location of the observed cloud (Bellouin et al, 2013;Amiri-Farahani et al, 2017). However, the aerosols simulated by the models may be strongly affected by wet deposition (Gryspeerdt et al, 2015), particularly in locations where the satellite observations cannot provide a constraint on the aerosol loading in cloudy areas (Christensen et al, 2016a).…”

“…To solve the collocation problem a pre-averaging methodology is commonly used (e.g. Sekiguchi et al, 2003;Quaas et al, 2008;Lebsock et al, 2008;Grandey and Stier, 2010;Chen et al, 2014;Gryspeerdt et al, 2016;Christensen et al, 2016a). Typically, the aerosol and cloud properties are pre-averaged over broad regions as a means to encapsulate both retrieval types within a (level-3-type) grid box.…”

Impacts of 3D Aerosol, Cloud, and Water Vapor Variations on the Recent Brightening during the South Asian Monsoon Season

“…The significance level of the AOD variation detected by MODIS was higher during the non-monsoon season (p < 0.01) than during the monsoon season (p < 0.22), primarily because of the disturbance of widespread clouds during the monsoon season [16]. The AOD detected by MODIS is inevitably affected by cloud contaminations caused by thin and mixed-phase clouds undetected by MODIS, broken cloud layers, and precipitating clouds [53,54]. Moreover, the accuracy and significance of MODIS observations are considerably lower during monsoon season with widespread clouds than during the pre-monsoon and dry seasons with insufficient clouds in South Asia [55].…”

“…Generally, a high concentration of water vapor favorably generates additional or robust convective clouds and results in decreased radiation at the surface, and vice versa [52]. Moreover, water vapor also regulates cloud amount by affecting the aerosol-cloud interaction, which generally shows a positive relationship under moist atmospheric conditions [53]. Therefore, the effect of water vapor is more complicated and uncertain in the all-sky than in clear-sky conditions, considering the direct and indirect effects of water vapor on solar radiation.…”