Cloud Albedo Perturbations on Climate
S. E. Schwartz and C. M. Benkovitz, Brookhaven National Laboratory
Research Objectives
In this project we evaluate the magnitude of the perturbation in clear-sky and cloud albedo due to anthropogenic emissions of SO2 associated with fossil-fuel combustion, as a function of time and location. We also systematically examine for evidence of such perturbation, principally in satellite measurements of albedo. This information will permit critical testing of two hypotheses: (1) sulfate aerosol arising from these emissions enhances clear-sky albedo by forming light-scattering aerosol particles, and cloud albedo by increasing the number density of cloud droplets, thus affecting the global radiative balance; and (2) this albedo enhancement causes a global annual average climate perturbation comparable to that of fossil-fuel CO2, but opposite in sign.
In addition, the distribution of the forcing, thought to be confined almost entirely to the Northern Hemisphere and to be rather nonuniform within this hemisphere, will be determined. If the distribution is found to be largely nonuniform, our analyses will also address the question of whether a climate response to this SO2 albedo perturbation may be more readily discernible than that due to CO2.
Computational Approach
The chemical transport and transformation model has been implemented using FORTRAN. The model solves material balances based on the continuity equations with the application of gradient-transport assumptions. To solve these equations, numerical approximations at any location (x, y, z) are considered mechanistically and dimensionally independent over short periods of time (time step is 1 h) according to the operator splitting technique, also known as the "locally one-dimensional" approximation. Spatial integrals of the transport are approximated using the area-preserving flux form developed by Bott. Dry deposition is represented as the lower boundary condition for the vertical transport. The integration procedure for each transformation term (gas- and aqueous-phase chemical conversions) treats loss terms as exponential decay terms. The C90 computer at NERSC has been used so far to run the model; work is being transferred to the J90.
Accomplishments
A three-dimensional Eulerian transport and transformation model driven by observation-derived synoptic meteorological data has been applied to calculate mixing ratios (MRs) of sulfate and SO2, and wet deposition of sulfate over the North Atlantic and adjacent continental regions for one-month periods in each of four seasons in 1986-87. Model performance is evaluated by comparison of grid-cell average (1.125°) modeled MRs for sulfate (24-h average) and SO2 (6 and 24-h average) in the lowest model level (surface to ca. 65 m) to surface MRs observed at monitoring stations in North America and Europe.
Sulfate column burden (vertical integral of the concentration) on July 9, 1986 at 00 UT for secondary sulfate from anthropogenic emissions west of 30°W. White denotes the lowest column burden, red the highest.
Significance
Global climate change due to enhanced concentrations of greenhouse gases is of major current concern. Much of the present debate on this issue surrounds the quantitative relation of tempreature change over the industrial era (1850 to present) to models of the forcing of climate change. The greatest uncertainty in this forcing is due to aerosols, light-scattering particles in the atmosphere.
This work is directed to developing chemical transport models for such aerosols and evaluating their performance by comparison with observations. Only with such models can one gain confidence in the estimates of this forcing. Work to date has focused on evaluation of model performance by comparison with observations at specific dates and locations.
Publications
C. M. Benkovitz and S. E. Schwartz, "Evaluation of modeled sulfate and SO2 over North America and Europe for four seasonal months in 1986-87," J. Geophys. Res. 102, 25305-25338 (1997).
C. M. Benkovitz, R. C. Easter, S. Nemesure, R. Wagener, and S. E. Schwartz, "Sulfate over the North Atlantic and adjacent continental regions: Evaluation for October and November 1986 using a three-dimensional model driven by observation-derived meteorology," J. Geophys. Res. 99, 20725-20756 (1994).