HR: 09:30h
AN: A21E-07    [Abstracts]
TI: Polarimetric remote sensing of aerosol and cloud microphysics from the NASA Glory Aerosol Polarimetry Sensor (APS)
AU: * Cairns, B
EM: bc25@columbia.edu
AF: Columbia University, 2880 Broadway, New York, NY 10025 United States
AU: Chowdhary, J
EM: jc84@columbia.edu
AF: Columbia University, 2880 Broadway, New York, NY 10025 United States
AU: Knobelspiesse, K
EM: kdk2103@columbia.edu
AF: Columbia University, 2880 Broadway, New York, NY 10025 United States
AU: Sato, M
EM: pdmrs@kyoto.giss.nasa.gov
AF: SGT Inc., 2880 Broadway, New York, NY 10025 United States
AU: Mishchenko, M
EM: mmishchenko@giss.nasa.gov
AF: NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025 United States
AU: Travis, L
EM: ltravis@giss.nasa.gov
AF: NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025 United States
AB: Tropospheric aerosols play a crucial role in climate and can cause a climate forcing directly by absorbing and reflecting sunlight, thereby cooling or heating the atmosphere, and indirectly by modifying cloud properties. The indirect aerosol effect may include increased cloud brightness, as aerosols lead to a larger number of smaller cloud droplets (the so-called Twomey effect), and increased cloud cover, as smaller droplets inhibit rainfall and increase cloud lifetime. Both forcings are poorly understood and may represent the largest source of uncertainty about future climate change. In this paper we present results from various field experiments demonstrating the contribution that the multi-angle multi-spectral photopolarimetric remote sensing measurements of the NASA Glory APS will make to the determination of the direct and indirect radiative effects of aerosols. Remote sensing of aerosols from satellites is plagued by the need to make prior assumptions about the composition and size of the aerosols that are present, whether this is to calculate the phase functions of the aerosols for passive remote sensing, or the extinction to backscatter ratio for elastic backscatter lidar measurements. Measurements made by the Research Scanning Polarimeter (RSP) have demonstrated that many of these assumptions can be eliminated using polarimetric remote sensing and that it is possible to retrieve the optical depth, single scattering albedo, refractive index and the location and width of a bimodal size distribution. Moreover, polarimetric remote sensing provides this capability over both land and water surfaces. Measurements from the CLAMS and IHOP field experiments and over smoke from fires in Southern California have been used to demonstrate these capabilities and the ability to estimate the height of the aerosol layer if sufficient aerosol is present. In passive remote sensing of clouds it is generally the case that for water clouds the effective variance of the droplet size distribution and in the case of ice clouds the particle shape distribution must be assumed globally constant. Any errors in these assumptions can cause significant errors in the evaluation of the aerosol indirect effect. We find that polarimetric measurements allow for the accurate retrieval of both the effective radius (agreeing with in situ measurements to within the uncertainty caused by spatial variability) and the effective variance at cloud top for water clouds and an accurate retrieval of the effective radius and a reasonable particle shape distribution in the case of ice clouds. Furthermore, polarimetric measurements allow the thickness of a cloud to be estimated which, together with accurate size and optical depth estimates, enables us to determine the number concentration of droplets, or ice particles, in clouds. The determination of this quantity is of particular interest for understanding and prognosing the indirect of aerosols on clouds and has hitherto only been remotely estimated using active sensors. These capabilities have been demonstrated and validated using measurements taken during the CSTRIPE and CRYSTAL-FACE field experiments.
UR: http://www.giss.nasa.gov/data/rsp_air
DE: 0305 Aerosols and particles (0345, 4801, 4906)
DE: 0321 Cloud/radiation interaction
DE: 1640 Remote sensing (1855)
DE: 3311 Clouds and aerosols
DE: 3360 Remote sensing
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005