Aerosol Direct Forcing--Observational Perspective Schwartz S. E. Workshop On Monitoring Global Aerosol Forcing of Climate: Evaluating requirements for satellite monitoring, ground-based monitoring, in-situ measurements and global modeling. GFDL-NOAA. Princeton NJ. September 13-14, 2000.

This presentation is directed toward defining a strategy for obtaining the time-dependent aerosol climate forcing from a combination of satellite monitoring, ground-based monitoring, in-situ measurements, and global modeling. Ultimately it is required to represent aerosol forcing in climate models with known and reasonable uncertainty. This capability is required to provide a secular representation of aerosol forcing over the industrial period, to quantitatively attribute aerosol forcing to aerosol type (natural, anthropogenic; by source category), and to provide the capability of representing aerosol forcing in climate models for a range of assumed future scenarios; this capability can be achieved only with evaluated aerosol models. Observations are required to provide the understanding necessary to represent aerosol forcing in climate models, to provide an observational basis for present aerosol direct forcing, and to provide an observational data base to permit evaluation of models. Additionally in-situ observations are required to provide ground truth for remote sensing, especially remote sensing from satellite platforms. Key aerosol properties that must be documented by observation are loading (mass concentration, column burden), composition (as a function of size; homogeneity vs. heterogeneity), physical properties (size distribution and its RHdependence, non-sphericity...), optical properties (extinction coefficient, single scattering albedo, phase function or asymmetry parameter...) including wavelength dependence. Vertical and geographical distribution are required for of all the above, not at all places and times, but at sufficient, and sufficiently diverse, locations and times as to provide required confidence in process-level understanding and in model-based representation of this understanding. Key measurement approaches are satellite remote sensing; ground-based remote sensing; ground-based in-situ measurement of composition, physical, optical properties; aircraft based remote sensing; and aircraft based in-situ measurements. Different approaches have different strengths and weaknesses necessitating the use of multiple approaches. Satellite measurements are global and long-term but are often indirect and require calibration, cross-calibration, and validation by in-situ measurements. In-situ measurements provide much more detailed information but are necessarily restricted in location, and highly detailed measurements are often possible only for limited time periods. New measurement techniques are reviewed from the perspective of their ability to meet the observational requirements.


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