Aerosol Workshop — June 2-3, 1997
Session 4: Aerosol Effects on Clouds
(Facilitator: Steve Schwartz; Recorder: Tica Novakov)
Aerosol Effects on Clouds: An Overview
Peter Hobbs, University of Washington
In view of the potential importance of the indirect (cloud) effects of atmospheric aerosols in radiative forcing, it is essential that studies be carried out to quantify these effects.
This will require highly focused field studies to: 1) Improve understanding of the relevant cloud physical processes. 2) Validate the assumptions and algorithms used for deriving aerosol parameters from satellite measurements. 3) Provide inputs to GCM models. A field project that could provide important information on all three of these issues is outlined later in this report by Panel B (Indirect Aerosol Forcing).
Effect of Smoke Aerosol on Microphysics of Continental Clouds
Yoram Kaufman, Goddard Space Flight Center
Analysis of 1 km AVHRR data from South America is used to retrieve smoke optical thickness and cloud drop size and reflectance. The results are summarized on a one degree by one degree grid. Increase of smoke optical thickness from 0.2 to 0.8 decreased drop size from 14 to 8 microns, reduced reflectance from 0.71 to 0.68, but (increased) ?? 0.35 to 0.45. The effect on clouds depends on the latitude or on precipitable water vapor amount. This method increases to a maximum the range of effects that we observe, far above noise and calibration problems. It requires 1 km resolution data.
Recommendation: 1) Save and restore all "old" AVHRR 1 km data, 2) Apply this method on specific regions, detecting the correlation of aerosol optical depth, cloud droplet radius and cloud reflectance; examine effects of cloud height and meteorological conditions, 3) Measure the relationship between CCN, SSA, size distribution and optical thickness, 4) Generate models that simulate this interaction of aerosols and clouds.
Measuring Global Aerosol-Climate Effects
Qingyuan Han, University of Alabama
A global survey of the relationship of cloud albedo and liquid water path with droplet size using ISCCP data shows that cloud albedo increases with decreasing droplet size for most clouds over continental areas and for all optically thick clouds, but that cloud albedo decreases with decreasing droplet size for optically thin clouds over most oceans over tropical rainforest regions. For almost all clouds the liquid water path increases with increasing cloud droplet size. The pattern of cloud droplet column concentration shows more clearly the effect of aerosol concentration variation on clouds. It exhibits the expected increase of droplet column concentrations between ocean and continental clouds and in tropical areas during dry seasons where biomass burning is prevalent. Cloud susceptibility has been retrieved on a global scale and the result reveals that cloud susceptibilities around most continental areas are close to zero. For remote ocean areas, the susceptibility is very large, suggesting locations where the largest aerosol indirect effect may occur. Correlations of aerosol optical thickness over oceans with cloud droplet size, cloud column droplet concentration and cloud albedo show variations depending on region and season.
This study suggests that many aspects in the study of the aerosol indirect effect can be performed using satellite observations. It also illustrates that assumptions used in model studies need to be verified with observations on a global scale. Further progress in understanding the aerosol indirect effect can be made through close collaboration between model studies and satellite observations.
Measurement of Mineral and Anthropogenic/Biogenic Aerosols Over Northern North Pacific
Mitsuo Uematsu, University of Tokyo
One dust event in Japan revealed two peaks of particle size concentration. Chemical composition of the dust from the Asian continent was obtained by direct filter sampling and individual particle analysis with SEM and EDS. The dust event lasted about 20 hours. Small anthropogenic aerosols appeared first, then large mineral aerosols arrived.
It is difficult to observe aerosols from satellites over the northern North Pacific due to high cloud cover, but the region is interesting for aerosol studies because of the high biological activity and anthropogenic effects on aerosols. Pronounced seasonal variation of mineral and anthropogenic/biogenic aerosols were observed from a cargo ship traveling regularly between Japan and North America. The air masses sampled are affected by transport from the Asian continent and in some cases from North America.
Removal of mineral particles occurs in the marine atmosphere below cloud via wet scavenging, suggesting that mineral aerosols may not be important for the indirect effect. It is interesting to note that mineral particle was dominant as insoluble particle in the spring rains, but carbonaceous particles, with diameter less than a micron, were present heavily in the summer rains.