13.3
Direct radiative effect of dust in a simple climate model: Climate response and sensitivity to dust optical properties
Karen M. Shell, SIO/Univ. of California, La Jolla, CA; and R. C. J. Somerville
Airborne mineral dust can influence the climate system by altering the radiative properties of the atmosphere, but little is known quantitatively about the mechanisms of dust-climate interactions. Dust optical properties and concentrations are uncertain, and thus the sign and magnitude of climate forcing and response are not well known. An idealized global model is developed to study processes and feedbacks within the dust-climate system. The model determines the longwave and shortwave forcing due to dust, as well as the resulting surface and atmospheric temperature change, based on the specified dust distribution, height, and optical properties.
The model produces radiative forcing values for the present day dust distribution which are comparable to those obtained in more complicated models, such as general circulation models. In addition, modeled radiative, temperature, and hydrological changes caused by stratospheric aerosols from a volcanic eruption (Mount Pinatubo, 1991) compare favorably to observations and general circulation model results for the eruption. Thus, the model, despite its simplicity, is able to produce realistic results.
Recent observations suggest that dust absorbs less solar radiation than previously thought. Using this new solar single scattering albedo value of 0.97, the modeled global average top-of-atmosphere (TOA) shortwave forcing is -0.73 W/m2 for the present day dust distribution. The TOA longwave forcing is 0.23 W/m2, the surface shortwave forcing is -1.3 W/m2, and the surface longwave forcing is 0.37 W/m2. Dust alters the model climate, resulting in a temperature decrease of about 0.1 K for both the surface and the atmosphere. However, the primary effect of dust is the reduction in latent and sensible heat transferred from the surface to the atmosphere by 1%. The reduction of these fluxes moderates the surface cooling. These results suggest that changes in the hydrologic cycle may be the largest effect of dust. Focusing on the TOA forcing alone, rather than the vertical distribution of forcing, neglects important climatic changes.
Finally, the model is used to explore the response of the climate to a range of dust concentrations, distributions, and optical properties in order to determine the dust sensitivity and highlight important feedbacks within the system. These results provide an estimate of the uncertainty in the dust climate forcing and response. In addition, these experiments identify the parameters responsible for the largest uncertainty so that future research can focus on the critical regions of parameter space.
.Session 13, Radiative Forcing of the Climate (parallel with Session 12)
Thursday, 13 January 2005, 8:30 AM-9:45 AM
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