Mineral dust aerosols impact (force) the earth's climate by cooling and warming the surface and air. In the infrared (IR) dust generally has positive top of the atmosphere (TOA) forcing over ocean and land because it absorbs outgoing surface radiance. Over deserts the IR TOA forcing is significant because the TOA forcing in solar bands is negligible. However, details of the IR forcing remain uncertain. The results from combined ground-based and satellite-based measurements of a March 2006 Saharan dust event are reported to determine the IR aerosol optical thickness (AOT), mineralogy, temperature and TOA IR forcing. Measurements of the AOT based on high-spectral-resolution measurements in Senegal are reported along with direct surface IR radiative forcing. These show the dust IR AOT has unique IR spectra, but the AOT at 10 mm is only a third that at 550 nm. From a comparison of the measured and modeled AOT spectra based on Mie theory, the mineralogy of the dust and its size distribution are estimated along with the composition (including both external and internal mixtures). Spectral analyses suggest that the locations and shapes of the resonant extinction peaks for clays and quartz may not be precisely represented by Mie theory, possibly due to dust asphericity, and corrections in the extinction spectra are needed. For the satellite-based work, MODIS visible images and AOT maps over the Atlantic Ocean are compared with a new AIRS-based IR AOT algorithm. This simple algorithm has a good correlation with the MODIS 550 nm AOT, but the IR AOT algorithm works day and night and over ocean and land. The ratio of the IR AOT to the MODIS AOT may reveal dust size distribution properties. A radiative transfer model based on the measured AIRS IR radiances and level 2 parameters is used to determine the IR TOA forcing from the dust, and maps of the forcing over ocean indicate that when temperature inversions are present the IR forcing is negative. There is evidence that dust virtual source scattering from neighboring surface regions occurs and this reduces the TOA forcing by scattering additional surface irradiance towards the TOA.
|Commitee:||Mahowald, Natalie, Michaelsen, Joel, Redemann, Jens, Roberts, Dar|
|School:||University of California, Santa Barbara|
|School Location:||United States -- California|
|Source:||DAI-B 71/02, Dissertation Abstracts International|
|Subjects:||Physical geography, Atmospheric sciences, Remote sensing|
|Keywords:||Aerosols, Dust, Forcing, Infrared, Remote sensing|
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