HR: 17:05h
AN: A14B-05    [Abstracts]
TI: Error in Empirical Measures and Calculated Radiative Forcing of the First Indirect Effect
AU: * McComiskey, A
EM: Allison.Mccomiskey@noaa.gov
AF: Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder/ NOAA Earth System Research Laboratory, Global Monitoring Division, 325 Broadway, Boulder, CO 80305, United States
AU: Feingold, G
EM: Graham.Feingold@noaa.gov
AF: NOAA Earth System Research Laboratory, Chemical Sciences Division, 325 Broadway, Boulder, CO 80305, United States
AU: Frisch, S
EM: Shelby.Frisch@noaa.gov
AF: Cooperative Institute for Research in the Atmosphere, Colorado State University/ NOAA Earth System Research Laboratory, Chemical Sciences Division, 325 Broadway, Boulder, CO 80305, United States
AB: Aerosol effects on cloud albedo are the most uncertain climate forcing addressed in the recent Intergovernmental Panel on Climate Change report. The first aerosol indirect effect, or Twomey effect, relates a change in aerosol concentration to changes in cloud microphysical properties, and thus cloud albedo, that modify the radiative balance for clouds of similar liquid water content. These relationships are commonly expressed as a variable, IE, that is physically limited to values between 0 and 0.33. A survey of recently published observations of IE shows that values span almost this entire range. To explore the error that uncertainty in measures of IE may produce in climate models, we have used a radiative transfer model to calculate radiative forcings for a range of cloud properties and aerosol concentrations associated with anthropogenic activities. For each 0.05 change in IE, the error in radiative forcing at the top of the atmosphere can range from -3 to -10 W m-2 depending on the level of aerosol perturbation and cloud liquid water path. Because there is so much uncertainty in IE with considerable impact on radiative forcing, we have undertaken an extensive analysis of aerosol-cloud interactions based on surface-based in-situ and remote sensing data. Primary questions addressed are the extent to which the range in reported values is physical, rather than due to measurement artifacts, and what are the most accurate and robust approaches for observing aerosol indirect effects.
DE: 0321 Cloud/radiation interaction
DE: 1626 Global climate models (3337, 4928)
DE: 3311 Clouds and aerosols
SC: Atmospheric Sciences [A]
MN: 2007 Fall Meeting