Identification of a new dust-stratocumulus indirect effect over the tropical North Atlantic

Lumpy stratocumulus clouds. Photo courtesy of NOAA.

Lumpy stratocumulus clouds. Photo courtesy of NOAA.

TitleIdentification of a new dust-stratocumulus indirect effect over the tropical North Atlantic
Publication TypeJournal Article
Year of Publication2014
AuthorsDoherty O.M, Evan AT
JournalGeophysical Research Letters
Volume41
Pagination6935-6942
Date Published2014/10
Type of ArticleArticle
ISBN Number0094-8276
Accession NumberWOS:000344913800052
Keywordsafrican dust; cloud; marine stratocumulus; microphysics; mineral aerosols; observations; ocean; saharan air layer; satellite; smoke; variability
Abstract

Over the tropical North Atlantic, during boreal summer, both stratocumulus clouds and mineral aerosols are ubiquitous. We find that low cloud fraction increases in response to high mineral aerosols loadings by 3% to 10% over much of the tropical North Atlantic, in International Satellite Cloud Climatology Project (ISSCP) and Pathfinder Atmosphere Extended (PATMOSx) observations. Using the single-column mode of the Community Earth System Model (CESM), we estimate that this indirect effect nets a surface cooling of approximately -3 Wm(-2) to -12 Wm(-2) per unit of dust optical depth (DAOD) increase in mineral aerosols, similar to observed radiative forcings of -4 Wm(-2) to -6 Wm(-2) per unit of DAOD in ISSCP and PATMOSx. Increases in stratocumulus clouds are linked to increases in atmospheric stability, reductions in boundary layer height, and moistening of the lower atmosphere in response to increased dust load. Mineral dust is shown to behave similarly to other absorbing aerosols in indirectly forcing a response in stratocumulus clouds.

DOI10.1002/2014gl060897
Impact: 

Here we confirm previously observed coincidence of stratocumulus clouds and mineral dust over the tropical North Atlantic [Carlson and Prospero, 1972; Ott et al., 1991; Clarke et al., 1996; Powell et al., 1997; Chazette et al., 2001; Ben-Ami et al., 2009; Wang et al., 2011] and the previously identified statistical link between dust load and stratocumulus clouds [Mahowald and Kiehl, 2003; Kaufman et al., 2005]. More generally, we show that dust can behave like other absorbing aerosols in indirectly forcing a response in stratocumulus clouds. In doing so, this work supports the findings of Brioude et al. [2009] and Wilcox [2010, 2012] who found absorbing aerosols above low clouds acted to increase the abundance of low clouds, as well as Johnson et al. [2004] who found an increase in low clouds when absorbing aerosols top a stratocumulus-capped boundary layer. Overall we demonstrate observational and modeling evidence for a robust stratocumulus indirect effect in response to mineral dust outbreaks and quantify the radiative forcing. This so-called dust ScIE has potentially large implications for regional climate.

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