Scientific Theme: Advanced Modeling and Observing Systems

Complementary Research: Innovative Research Program
brings together researchers from different disciplines. The ASG was originally designed as a downlooking
instrument for observations of surface reflectance properties. With the use of methods from the atmospheric
sciences, the instrument can be applied to a very different and important problem in aerosol and climate science.
Research Plan. Radiometric retrievals of atmospheric aerosol properties require observations under cloudless
conditions. In most cases, aerosol characteristics remain constant throughout a given day, though rapid changes are
also possible when dictated by variations in meteorology and source strength. In order to incorporate as much of this
variability as possible in our measurements, deployments will be made on clear sky days as often as possible
throughout the year. Full sky-radiance scan will consist of measurements obtained at approximately 100 scattering
angles, either in the principal plane, or at constant zenith viewing angle. This large number of sky-radiance samples
will allow for the identification and removal of views contaminated by sub-visual cirrus clouds and to correct for
angular pointing errors.
The robot will be programmed to produce up to 10 sky-radiance scans per hour, which is about an order of
magnitude greater than the typical sample rate from AERONET. The availability of several scans per hour will help
differentiate the effects of natural aerosol variation from noise introduced by scene conditions (e.g., sub-visual
cirrus). Standard data products available from the AERONET Cimel sunphotometers are produced from
measurements with less angular resolution (Figure 1b). These observations will provide detailed information on
aerosol properties at spectral and temporal scales that are not available with current instruments.
The retrieval of aerosol properties is sensitive to situational variables, such as aerosol amount, the surface albedo,
atmospheric pressure, and the presence of sub-visual cirrus cloud. In order to constrain these variables, we will make
measurement near the Boulder Baseline Surface Radiation Network (BSRN) site. Continuous measurements at the
site include solar downwelling global, direct, and diffuse radiation at the surface at the top of a 300-m tower as well
as solar upwelling global radiation at the top of the tower at 1- to 3-minute resolution. Good estimates of aerosol
optical depth will greatly reduce the need for precise absolute calibration of the ASD-FR. The BSRN direct and
diffuse radiation observations will also be used to test closure by comparing modeled and observed radiation with
the aerosol properties retrieved from the ASG.
Expected Outcome and Impact. These measurements will represent an unprecedented data set of spectrally resolved
aerosol properties and a proven concept for dramatically more accurate and comprehensive measurements of these
properties. This novel data set will include wavelength dependence of the asymmetry parameter and single
scattering albedo across the entire shortwave spectrum for a variety of aerosol types. Modeling studies that attempt
to calculate the radiative impacts of aerosol on climate have had to use a ―best guess‖ at the dependence. This
information in itself will have a significant impact on the uncertainties associated with modeling the impact of
aerosols on climate. Finally, we will be able to assess the requirements for developing a robust and autonomous
instrument using the ASD and goniometer configuration for continuous operation in the field.
Figure 1 (a, left) The Automated Spectro-Goniometer (ASG) deployed for measurements of directional reflectance
of snow. (b, right) Cimel almucantar (diamonds) and principal plane (squares) observations of radiance at 676 nm
obtained at the ARM program Southern Great Plain facility on 29 May 2003. These data were used as part of a
study to test for closure between in-situ and remotely sensed aerosol retrievals [Ricchiazzi et al., 2006].
144