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Sunlight-absorbing aerosol amplifies the seasonal cycle in low cloud fraction over the southeast Atlantic
  • Jianhao Zhang,
  • Paquita Zuidema
Jianhao Zhang
National Oceanic and Atmospheric Administration

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Paquita Zuidema
University of Miami/RSMAS
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Many studies examining shortwave-absorbing aerosol-cloud interactions over the southeast Atlantic apply a seasonal averaging. This disregards a meteorology that raises the mean altitude of the smoke layer from July to October. This study details the month-by-month changes in cloud properties and the large-scale environment as a function of the biomass-burning aerosol loading from July to October, based on measurements from Ascension Island (8°S, 14.5°W), satellite retrievals and reanalysis. In July and August, variability in the smoke loading predominantly occurs in the boundary layer. During both months, the low-cloud fraction is less and is increasingly cumuliform when more smoke is present, with the exception of a late morning boundary layer deepening that encourages a short-lived cloud development. September marks a transition month during which mid-latitude disturbances can intrude into the Atlantic subtropics, constraining the land-based anticyclonic circulation transporting free-tropospheric aerosol to closer to the coast, and resulting deeper, drier, and cooler boundary layers with strongly reduced cloud cover near the main stratocumulus deck. The October meteorology is more singularly dependent on the strength of the free-tropospheric winds advecting aerosol offshore. Low-level clouds increase and are more stratiform, when the smoke loadings are higher. The increased cloud-top moisture and cloud droplet number concentrations can help sustain a thinner stratiform cloud layer through microphysical interactions. Overall the monthly changes in the large-scale circulation and aerosol/moisture vertical structure act to amplify the seasonal cycle in low-cloud amount and morphology, raising a climate importance as cloudiness changes dominate the top-of-atmosphere radiation budget.