Type of Document Dissertation Author Shaheen, Gauher Author's Email Address email@example.com URN etd-07232011-143307 Title Untangling the Satellite Estimated Aerosol Indirect Effect Degree Doctor of Philosophy Department Earth, Ocean & Atmospheric Science, Department of Advisory Committee
Advisor Name Title Guosheng Liu Committee Chair Paul H Ruscher Committee Member Robert G. Ellingson Committee Member Xiaolei Zou Committee Member Ruby Krishnamurti University Representative Keywords
- climate change
- anthropogenic forcing
- aerosol indirect effect
- cloud physics
Date of Defense 2011-06-20 Availability unrestricted AbstractAnthropogenic aerosols are known to alter clouds and their optical properties by serving as cloud condensation nuclei. An increase in aerosol concentration causing smaller but more cloud droplets for a fixed liquid water content, thereby resulting in an increased cloud optical thickness and increased reflectivity of solar insolation, is called the first aerosol indirect effect. This effect leads to radiative cooling of the Earth and has the potential to counteract the greenhouse effect to some extent. However, an adequate quantification of this process remains elusive and leaves many outstanding questions. Many studies have been undertaken to better understand the uncertainties in the aerosol indirect effect as the aerosol-cloud interactions are considered to be one of the most important and least known forcings in the climate system but the complexity of this problem seems to grow with each new study.
The objective of this study is to investigate and determine the problems in estimating the aerosol first indirect effect. The most common approach to estimate the indirect effect is to take into account only one or two controlling parameters, and emphasizing a result of a particular process, with all other processes being held constant. This type of simple approach often fails to extract the real aerosol indirect effect since many influencing factors that contribute to the change of cloud optical properties are interrelated in the nature. Furthermore, an incomplete specification of physical processes involved and not considering them simultaneously will produce inaccurate estimate of aerosol indirect effect. Therefore, in this study we have developed a methodology to untangle the aerosol effects from the interrelated factors, and then using satellite data, performed a comprehensive analysis of the aerosol indirect effect due to warm clouds over two oceanic regions. In this relatively idealized framework, the following four parameters can simultaneously contribute to the aerosol-related cloud radiative forcing: the change in spectral shape of the size distribution of cloud droplets (dispersion effect), the change in cloud droplet number concentration (Twomey effect), the change in cloud adiabaticity, and the change in cloud geometric depth, with variations in aerosol concentration. Furthermore, we have compared two different meteorological regimes, one (USWC, Pacific U.S. west coastal region) dominated by mainly stratiform clouds and the other (INDO, Indian Ocean region) by convective clouds. A detailed comparison of the mechanisms of aerosol-cloud microphysics/dynamics interactions in the two regions with different cloud types enables us to understand the dependence of the contributing parameters on cloud types.
Based on our framework and observational data primarily from satellite, our data analyses have resulted in the conclusion that of the four contributing factors, the number
effect and cloud depth effects are the most important in both regions. Dispersion effect, even though it offsets the effect in increased droplet concentration, is small in magnitude and is not a significant contributor for either stratiform or convective clouds. The effect of cloud adiabaticity, which is indicative of entrainment mixing processes, is found to be dependent on cloud type, although its magnitude is relatively small and its sign is opposite for the two contrasting cloud types. These results cannot be derived from simply correlating cloud optical depth with aerosol concentration like done in earlier studies. It can only be derived under the multiple parameter analysis framework developed in this study.
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