GACP Projects
A Longterm Testbed and Analysis of Radiative Forcing by Aerosols at Two Sites
Thomas P. Charlock, PI
Timothy L. Alberta, Co-I
Fred G. Rose, Co-I
Abstract: The radiative forcing of aerosols to the atmospheric column is the difference of flux with aerosol and flux without aerosol. As the entire column always contains some aerosols, there is strictly speaking no single direct measurement of radiative forcing by aerosols. Aerosol forcing must rather be inferred from theoretical radiative transfer calculations of fluxes under different conditions of aerosol loading. The theoretical aerosol forcing may then be validated by an analysis of separate measurements of fluxes and aerosol loadings. Such an analysis is proposed here. A rigorous testbed will be developed for the radiative transfer calculations, for the measurements of the fluxes, and for the measurements of aerosols and other atmospheric and surface properties. The testbed will be available to the Science Team on-line. Successive versions of the testbed will re-visit, as appropriate, the original time series with improvements. The testbed is intended as a guidepost for improving retrievals of aerosol radiative forcing over the globe with satellite and surface data.
The testbed will begin at the ARM Southern Great Plains (SGP) Central Facility with a nearly continuous record of:
- (1) measured surface broadband radiative fluxes under full-sky (total-sky, all-sky) conditions,
- (2) atmospheric soundings,
- (3) measured spectral optical depths of aerosol under clear-sky conditions,
- (4) computed vertical profiles of broadband SW and LW fluxes for clear skies, and
- (5) the height profile of aerosol optical properties used to calculate the fluxes.
For a subset of this domain, retrievals of cloud optical properties and measurements of TOA fluxes will be included; these will be obtained from:
- (a) GOES data processed for ARM during Intensive Observing Periods (IOPs),
- (b) CERES overpasses of TRMM, EOS-AM, and EOS-PM, and
- (c) those ARM ground-based MWR, MPL, and cloud radar measurements which coincide with (a) and (b).
For the subset with cloud measurements, computed fluxes for full-sky will be included. For the subset with TOA measurements, an experimental objective analysis algorithm will produce an alternate set of aerosol optical properties that are constrained with radiative transfer calculations.
Radiative fluxes will be computed with a delta-4-stream code, testbed inputs, and assumed aerosol optical properties. Aerosol radiative forcing will be analyzed. New models of aerosol optical properties will be developed to produce fluxes more consistent with the measurements. Aerosol retrievals and optical models by other groups (i.e., MODIS, MISR, and the Science Team supported by this NRA) will be included in the sequence of tests. Data from a second site, an ocean platform at the Chesapeake Lighthouse, will eventually be included.
