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Effects of Absorbing Aerosols on Cloud Absorption and Forcing

March 25th, 2008 by rplath · No Comments

It would be assumed that in the exploration of the albedo and forcing ratios of pure clouds, one would attain values equal to 1.0. This being the case, how would an observer go about explaining an albedo ranging from .7 to .8 or a cloud forcing ratio consisting of 1.5? These two aberrations have been discussed and observed throughout the duration of multiple studies. Solar absorbing aerosols within cloud droplets have been the accused perpetrators of these results since 1969, yet, no finite conclusion has been accepted. Carynelisa Erlick and Dana Schlesinger study the effect of supermicron dust and soot particles in the American Meteorology Society Journal article entitled “Another Look at the Influence of Absorbing Aerosols in Drops on Cloud Absorption: Large Aerosols.” The purpose of this study is to uncover whether of not absorbing aerosols are really to blame for lower cloud albedos and heightened forcing ratios (the difference between the radiation budget for cloudy and clear days).

While there are several other explanations for these anomalies, along with arguments stating the justification of these issues has nothing to do with absorbing aerosols, a lack of research and limitations involved in previous studies have attributed to the debate of their causations. In the study conducted by Erlick and Schlesinger, large drop clouds (CL) and smaller drop clouds (CS) are modeled with the Intercomparison of Radiation Codes in Climate Models and set to a constant thickness and cloud extinction optical depth. These clouds are then placed at either a low height (between 800 and 900 mb) or a higher height (200 and 800 mb). Low and high CS and CL clouds are also observed at zenith angles of thirty and sixty degrees. Dispersed throughout these clouds are drops containing supermicron absorbing aerosols comprised of dust, soot and a combination of the two. It is improtant to note that within this experiment, the dispersion of aerosols is added to the clean clouds, rather than clean cloud drops being displaced by the contaminated droplets. It is also improtant to understand that while soot and mineral aerosols are observed at high elevations, the microphysical interaction with clouds has yet to be well assessed. The Maxwell-Garnett model (in which the distrubution and size distrubution of the included aerosols is random), the Sihvola and Sharma approximation (in which the distribution of included aerosols is random and the size distribution dictates inclusions larger than dipoels) and a core plus model are used in the calculation of the single scattering behaviors of droplets enclosing absorbing aerosols. An algorithm of Freidenreich and Ramaswamy is also used in the calculation of the solar cloud forcing ratio, scattering behaviors and the configuration of droplet dispersion.

Six scenarios are evaluated regarding the aerosol size distribution within the CL and CS clouds, all of which are analyzed with respect to a cloud of pure water (or without a core). The evaluated concentration categories are as follows: 1) All mineral dust, 2) Soot up to 1.250 micrometers, 3) Soot up to 1.584, 4) Soot up to 2.008 micrometers, 5) 97% dust and 3% soot and 6) 70% dust and 30% soot. Upon the completion of the multiple calculations and collection of vast data samples, arrangements into tables and figures are performed. This allows for easy comparisons to the reference cloud consisting of pure water droplets. The albedos of CL clouds appear to not have exceeded .7 to .8, which corresponds to the initial observation inquiry. Cloud forcing ratios are all significantly greater than 1.0, the accepted pure cloud forcing ratio. The CL clouds containing 70% dust and 30% soot have exhibited particularly high forcing values of 1.41 to 2.60. These results are also in accordance with the second aberration stated earlier. It is noted that the high (200 to 180 mb) clouds did not result in values which vary much from that of a pure cloud. This result was attributed, in this case, to the assumption that high clouds do not amplify absorption of solar radiation as efficiently as lower level clouds. The same general trend of heightened cloud forcing values and depleted albedos are evident among the CS clouds, as in CL clouds.

Through this study, Erlick and Schlesinger are able to show that significant enhancement of cloud forcing ratios can be achieved through the distribution of drops containing supermicron cores. It is indicated in the conclusion of the study that two commonitions may have influenced the archived results. Only one supermicron aerosol distribution method was used for lack of additional data, and second the two clean cloud size distributions used for comparison were static. While this study is fairly inconclusive, it is an important step in expressing the potential for absorbing aerosols in cloud droplets to be held accountable for the lower albedos and higher cloud forcing ratios observed.

While this article is very annotative, it would aid the reader if the methods used in the calculations were more clearly explained. Understandably so, the explanations of different methodologies would most likely result in a novel by themselves, the assistance in the comprehension of the ways in which the distributions were portrayed would be greatly improved under the assumption that the reader is unfamiliar with the named formulas. Overall the article was indepth and the results of the calculations were portrayed in such a way that made them easily understandable. While the issue regarding the effect of absorbing aerosols on cloud absorption and forcing ratios is far from being conclusive, the study conducted by Carynelisa Erlick and Dana Schlesinger is a step in the right direction to uncovering an accepted cause for cloud observations that deviate from one’s expected values.

Tags: aerosols · clouds · modeling · MTR3440 · Uncategorized

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