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Characterizing Clear-Sky Direct Radiative Forcing of Aerosols from Surface Broadband Solar Observations: a Long-Term Globally Distributed Validation Data Set
Paul W. Stackhouse, Jr., PI
Bryan A. Baum, Co-I
Donald R. Cahoon, Co-I
Jennifer R. Olson, Co-I
R. Bradley Pierce, Co-I
Roberta Dipasquale, Co-I
Charles H. Whitlock, Co-I
Martial P. A. Haeffelin, Co-I
Abstract: One strategy for assessing the radiative forcing of aerosols on the global climate during the last 20 years is to investigate relationships between aerosol variability and subsequent radiative forcing over a long time period. Concurrently, with the past 20 years of satellite obser- vations, are a large number of globally distributed surface solar insolation measurements. The purpose of this proposal is to derive a multi-year data set using these insolation measurements and a gridded meteorological product (e.g., European Centre for Mid-Range Weather Forecasting Reanalyses) to provide a measure of the variability in the clear-sky direct aerosol radiative forcing (ARF) and identify possible mechanisms for the observed trends and perturbations. This data set will provide a first-order global validation for future aerosol satellite retrieval algorithms and aerosol climate models.
Thus, we propose to produce a multi-year monthly averaged climatology of a clear-sky broadband total (direct + diffuse) solar residual flux (SRF). The monthly SRF is the average of the difference between the daily averaged observed clear-sky (no clouds) solar insolation and the solar insolation predicted by a radiative transfer model with no clouds and no aerosols (clean-sky flux). The variability of this residual flux is strongly related to the variability of the direct ARF in clear-skies within certain error bounds over a given location and/or region. To complement this data set, we also propose to characterize statistically, on a monthly basis at several different atmospheric levels, the sources of air masses that are likely to advect over the given surface site during the month using a transport model. The air masses will be classified according to the surface type of their origin and the likelihood that the air mass contains smoke particles. The most valuable contributions of this research toward the goal of assessing the radiative forcing of atmospheric aerosols are that the proposed data set will:
- provide an approximate measure of the variability including long-term trends and perturbations in clear-sky SRF for a large number of locations in climactically diverse areas where there is no other validation;
- facilitate a linkage between aerosol variability as inferred from more accurate measurement systems being developed and deployed in the present to the large amount historical broadband total solar measurements of the past;
- provide physical insight into the mechanisms responsible for the observed SRF variability, not only from biomass burning but from other sources like dust that can be advected over large distances; and
- use/improve the derivation of biomass burning source maps to identify areas affected by smoke where satellite retrievals are problematic.
A complete error analysis of the measurements, atmospheric inputs, and radiative transfer sensitivities will be performed by considering various case studies and newly developed surface sites that contain multiple measurements of the solar insolation (e.g., Baseline Surface Radiation Network) and aerosol measurements (e.g., Atmospheric Radiative Measurement/Clouds and Radiation Testbed, Surface Radiation Budget Network). The transport modeling will be also be tested by selecting case studies of large fire events, using AVHRR imagery and surface radiometry.
