NTRS - NASA Technical Reports Server

Back to Results
Cloud Cover Increase with Increasing Aerosol Absorptivity: A Counterexample to the Conventional Semidirect Aerosol EffectWe reexamine the aerosol semidirect effect using a general circulation model and four cases of the single-scattering albedo of dust aerosols. Contrary to the expected decrease in low cloud cover due to heating by tropospheric aerosols, we find a significant increase with increasing absorptivity of soil dust particles in regions with high dust load, except during Northern Hemisphere winter. The strongest sensitivity of cloud cover to dust absorption is found over land during Northern Hemisphere summer. Here even medium and high cloud cover increase where the dust load is highest. The cloud cover change is directly linked to the change in relative humidity in the troposphere as a result of contrasting changes in specific humidity and temperature. More absorption by aerosols leads to larger diabatic heating and increased warming of the column, decreasing relative humidity. However, a corresponding increase in the specific humidity exceeds the temperature effect on relative humidity. The net effect is more low cloud cover with increasing aerosol absorption. The higher specific humidity where cloud cover strongly increases is attributed to an enhanced convergence of moisture driven by dust radiative heating. Although in some areas our model exhibits a reduction of low cloud cover due to aerosol heating consistent with the conventional description of the semidirect effect, we conclude that the link between aerosols and clouds is more varied, depending also on changes in the atmospheric circulation and the specific humidity induced by the aerosols. Other absorbing aerosols such as black carbon are expected to have a similar effect.
Document ID
Document Type
Reprint (Version printed in journal)
External Source(s)
Perlwitz, Jan
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Miller, Ron L.
(NASA Goddard Inst. for Space Studies New York, NY, United States)
Date Acquired
August 25, 2013
Publication Date
April 27, 2010
Publication Information
Publication: Journal of Geophysical Research - Atmospheres
Volume: 115
Subject Category
Report/Patent Number
ISSN 0148-0227
Funding Number(s)
Distribution Limits

Available Downloads

There are no available downloads for this record.
No Preview Available