Abstract
Anthropogenic aerosols are hypothesized to enhance planetary albedo and offset some of the warming due to the buildup of greenhouse gases in Earth's atmosphere. Aerosols can enhance the coverage, reflectance, and lifetime of warm low-level clouds. However, the relationship between cloud lifetime and aerosol concentration has been challenging to measure from polar orbiting satellites. We estimate two timescales relating to the formation and persistence of low-level clouds over [Formula: see text] spatial domains using multiple years of geostationary satellite observations provided by the Clouds and Earth's Radiant Energy System (CERES) Synoptic (SYN) product. Lagrangian trajectories spanning several days along the classic stratus-to-cumulus transition zone are stratified by aerosol optical depth and meteorology. Clouds forming in relatively polluted trajectories tend to have lighter precipitation rates, longer average lifetime, and higher cloud albedo and cloud fraction compared with unpolluted trajectories. While liquid water path differences are found to be negligible, we find direct evidence of increased planetary albedo primarily through increased drop concentration ([Formula: see text]) and cloud fraction, with the caveat that the aerosol influence on cloud fraction is positive only for stable atmospheric conditions. While the increase in cloud fraction can be large typically in the beginning of trajectories, the Twomey effect accounts for the bulk (roughly 3/4) of the total aerosol indirect radiative forcing estimate.
Highlights
Anthropogenic aerosols are hypothesized to enhance planetary albedo and offset some of the warming due to the buildup of greenhouse gases in Earth’s atmosphere
The aim of this approach is to ensure the consistency in meteorological sampling between polluted and clean trajectories, and while the procedure results in the removal of roughly one-third of the tracks that deviate from the median wind, we retain several thousand trajectories to analyze aerosol–cloud responses
These results demonstrate that aerosols have a nonnegligible effect on cloud fraction in the stratus-tocumulus transition zone
Summary
Anthropogenic aerosols are hypothesized to enhance planetary albedo and offset some of the warming due to the buildup of greenhouse gases in Earth’s atmosphere. The A-Train can provide only single snapshot images of cloud systems once per day at 13:30 local time in the afternoon (and at 01:30 but without optical property retrievals), thereby constraining the observations to an Eulerian framework The connection of these variables to the dimension of time in a Lagrangian framework offers the capability to examine the aerosol influence on cloud development and lifetime. The Lagrangian framework has traditionally been used to study the time-dependent response of the marine stratus-tocumulus transition zone in observational [9,10,11] and modeling [12,13,14] studies This transition typically follows a three-day equatorward redundant trajectory where the boundary layer proceeds through several diurnal cycles [13], increasingly warmer sea surface temperatures and unstable atmospheric conditions, stronger surface moisture and sensible heat fluxes, and more frequent precipitation.
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