Abstract
Biomass burning aerosols seasonally overlie the subtropical southeast Atlantic stratocumulus deck. Previous modeling and observational studies have postulated a semidirect effect whereby shortwave absorption by the aerosol warms and stabilizes the lower troposphere, thickening the low-level clouds. The focus herein is on the dynamical and moisture effects that may be convoluted with the semidirect effect. Almost-daily radiosonde data from remote St. Helena Island (15.9°S, 5.6°W), covering September-October 2000-11, are combined with daily spatial averages (encompassing the island) of the MODIS clear-sky fine-mode aerosol optical depth (τaf ). Increases in τaf are associated with increases in 750-500-hPa moisture content. The net maximum longwave cooling by moisture of almost 0.45Kday-1 reduces the aerosol layer warming from shortwave absorption. ERA-Interim spatial composites show that polluted conditions are associated with a strengthening of a deep land-based anticyclone over southern Africa, facilitating the westward offshore transport of both smoke and moisture at 600 hPa. The shallower surface-based South Atlantic anticyclone exhibits a less pronounced shift to the northeast, strengthening the low-level coastal jet exiting into the stratocumulus deck and cooling 1000-hPa potential temperatures. Warm continental outflow further increases the 800-hPa potential temperatures (θ800), reinforcing the lower tropospheric stability (θ800 - θ1000) over the stratocumulus deck. Enhanced southerly dry air advection also strengthens the cloud-top humidity inversion. The increased stability helps explain an observed decrease in cloud-top heights despite an anomalous reduction in subsidence. The changes to the horizontal dynamics enhance low-level cloudiness. These are separate but not necessarily distinct from an aerosol semidirect effect, encouraging care in attribution studies.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 1997-2024 |
| Number of pages | 28 |
| Journal | Journal of Climate |
| Volume | 28 |
| Issue number | 5 |
| DOIs | |
| State | Published - 2015 |
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Keywords
- Aerosols
- Anticyclones
- Clouds
- Dynamics
- Humidity
- Radiative forcing
ASJC Scopus subject areas
- Atmospheric Science
Cite this
The convolution of dynamics and moisture with the presence of shortwave absorbing aerosols over the southeast Atlantic. / Adebiyi, Adeyemi A.; Zuidema, Paquita; Abel, Steven J.
In: Journal of Climate, Vol. 28, No. 5, 2015, p. 1997-2024.Research output: Contribution to journal › Article
}
TY - JOUR
T1 - The convolution of dynamics and moisture with the presence of shortwave absorbing aerosols over the southeast Atlantic
AU - Adebiyi, Adeyemi A.
AU - Zuidema, Paquita
AU - Abel, Steven J.
PY - 2015
Y1 - 2015
N2 - Biomass burning aerosols seasonally overlie the subtropical southeast Atlantic stratocumulus deck. Previous modeling and observational studies have postulated a semidirect effect whereby shortwave absorption by the aerosol warms and stabilizes the lower troposphere, thickening the low-level clouds. The focus herein is on the dynamical and moisture effects that may be convoluted with the semidirect effect. Almost-daily radiosonde data from remote St. Helena Island (15.9°S, 5.6°W), covering September-October 2000-11, are combined with daily spatial averages (encompassing the island) of the MODIS clear-sky fine-mode aerosol optical depth (τaf ). Increases in τaf are associated with increases in 750-500-hPa moisture content. The net maximum longwave cooling by moisture of almost 0.45Kday-1 reduces the aerosol layer warming from shortwave absorption. ERA-Interim spatial composites show that polluted conditions are associated with a strengthening of a deep land-based anticyclone over southern Africa, facilitating the westward offshore transport of both smoke and moisture at 600 hPa. The shallower surface-based South Atlantic anticyclone exhibits a less pronounced shift to the northeast, strengthening the low-level coastal jet exiting into the stratocumulus deck and cooling 1000-hPa potential temperatures. Warm continental outflow further increases the 800-hPa potential temperatures (θ800), reinforcing the lower tropospheric stability (θ800 - θ1000) over the stratocumulus deck. Enhanced southerly dry air advection also strengthens the cloud-top humidity inversion. The increased stability helps explain an observed decrease in cloud-top heights despite an anomalous reduction in subsidence. The changes to the horizontal dynamics enhance low-level cloudiness. These are separate but not necessarily distinct from an aerosol semidirect effect, encouraging care in attribution studies.
AB - Biomass burning aerosols seasonally overlie the subtropical southeast Atlantic stratocumulus deck. Previous modeling and observational studies have postulated a semidirect effect whereby shortwave absorption by the aerosol warms and stabilizes the lower troposphere, thickening the low-level clouds. The focus herein is on the dynamical and moisture effects that may be convoluted with the semidirect effect. Almost-daily radiosonde data from remote St. Helena Island (15.9°S, 5.6°W), covering September-October 2000-11, are combined with daily spatial averages (encompassing the island) of the MODIS clear-sky fine-mode aerosol optical depth (τaf ). Increases in τaf are associated with increases in 750-500-hPa moisture content. The net maximum longwave cooling by moisture of almost 0.45Kday-1 reduces the aerosol layer warming from shortwave absorption. ERA-Interim spatial composites show that polluted conditions are associated with a strengthening of a deep land-based anticyclone over southern Africa, facilitating the westward offshore transport of both smoke and moisture at 600 hPa. The shallower surface-based South Atlantic anticyclone exhibits a less pronounced shift to the northeast, strengthening the low-level coastal jet exiting into the stratocumulus deck and cooling 1000-hPa potential temperatures. Warm continental outflow further increases the 800-hPa potential temperatures (θ800), reinforcing the lower tropospheric stability (θ800 - θ1000) over the stratocumulus deck. Enhanced southerly dry air advection also strengthens the cloud-top humidity inversion. The increased stability helps explain an observed decrease in cloud-top heights despite an anomalous reduction in subsidence. The changes to the horizontal dynamics enhance low-level cloudiness. These are separate but not necessarily distinct from an aerosol semidirect effect, encouraging care in attribution studies.
KW - Aerosols
KW - Anticyclones
KW - Clouds
KW - Dynamics
KW - Humidity
KW - Radiative forcing
UR - http://www.scopus.com/inward/record.url?scp=84961291041&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=84961291041&partnerID=8YFLogxK
U2 - 10.1175/JCLI-D-14-00352.1
DO - 10.1175/JCLI-D-14-00352.1
M3 - Article
AN - SCOPUS:84961291041
VL - 28
SP - 1997
EP - 2024
JO - Journal of Climate
JF - Journal of Climate
SN - 0894-8755
IS - 5
ER -