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Volume 11, issue 13 | Copyright

Special issue: MILAGRO/INTEX-B 2006

Special issue: Megacities: air quality and climate impacts from local to...

Atmos. Chem. Phys., 11, 6245-6263, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 01 Jul 2011

Research article | 01 Jul 2011

Simultaneous retrieval of aerosol and cloud properties during the MILAGRO field campaign

K. Knobelspiesse1, B. Cairns2, J. Redemann3,4, R. W. Bergstrom3, and A. Stohl5 K. Knobelspiesse et al.
  • 1NASA Postdoctoral Program Fellow, NASA Goddard Institute for Space Studies, New York, New York, USA
  • 2NASA Goddard Institute for Space Studies, New York, New York, USA
  • 3Bay Area Environmental Research Institute, Sonoma, California, USA
  • 4NASA Ames Research Center, Moffett Field, California, USA
  • 5Norwegian Institute for Air Research, Kjeller, Norway

Abstract. Estimation of Direct Climate Forcing (DCF) due to aerosols in cloudy areas has historically been a difficult task, mainly because of a lack of appropriate measurements. Recently, passive remote sensing instruments have been developed that have the potential to retrieve both cloud and aerosol properties using polarimetric, multiple view angle, and multi spectral observations, and therefore determine DCF from aerosols above clouds. One such instrument is the Research Scanning Polarimeter (RSP), an airborne prototype of a sensor on the NASA Glory satellite, which unfortunately failed to reach orbit during its launch in March of 2011. In the spring of 2006, the RSP was deployed on an aircraft based in Veracruz, Mexico, as part of the Megacity Initiative: Local and Global Research Observations (MILAGRO) field campaign. On 13 March, the RSP over flew an aerosol layer lofted above a low altitude marine stratocumulus cloud close to shore in the Gulf of Mexico. We investigate the feasibility of retrieving aerosol properties over clouds using these data. Our approach is to first determine cloud droplet size distribution using the angular location of the cloud bow and other features in the polarized reflectance. The selected cloud was then used in a multiple scattering radiative transfer model optimization to determine the aerosol optical properties and fine tune the cloud size distribution. In this scene, we were able to retrieve aerosol optical depth, the fine mode aerosol size distribution parameters and the cloud droplet size distribution parameters to a degree of accuracy required for climate modeling. This required assumptions about the aerosol vertical distribution and the optical properties of the coarse aerosol size mode. A sensitivity study was also performed to place this study in the context of future systematic scanning polarimeter observations, which found that the aerosol complex refractive index can also be observed accurately if the aerosol optical depth is larger than roughly 0.8 at a wavelength of (0.555 μm).

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