Radiative Forcing of a Tropical Direct Circulation by Soil Dust Aerosols

Abstract

The effect of soil dust aerosols upon the tropical climate is estimated by forcing a simple model of a tropical direct circulation. The model consists of a region vertically mixed by deep convection and a nonconvecting region, for which budgets of dry static energy and moisture are constructed. Dynamical effects are included implicitly, by prohibiting horizontal temperature contrasts above the boundary layer. Dust aerosols absorb sunlight to a greater extent than industrial sulfate and sea-salt aerosols. In a companion study, where the climate response to dust is calculated using an atmospheric general circulation model, the global-average dust radiative forcing is negligible at the top of the dust layer, in comparison to the large reduction of the net flux at the surface. Thus, dust aerosols redistribute radiative heating from the surface into the dust layer, unlike industrial sulfates and sea salt, which through reflection reduce the total radiative energy gained by the column. The simple model is perturbed by a reduction in the net radiative flux at the surface. Forcing at the top of the dust layer is idealized to be zero. Cooling occurs at the surface of the nonconvecting region, but surface temperature within the convecting region is only slightly perturbed. It is shown that the disproportionately small response within the convecting region is a consequence of the trivial radiative forcing at the top of the dust layer, and the occurrence of deep convection, which prevents the surface temperature from changing without a corresponding change of the emitting temperature in the upper troposphere. Additional experiments, where the absorptivity of the dust particles is varied, indicate that the anomalous surface temperature is most sensitive to the radiative forcing at the top of the dust layer. The reduction of the surface net radiation is less important per se but introduces an asymmetry in the response between the convecting and nonconvecting regions through the radiative forcing within the dust layer, which is the difference between the forcing at the surface and the layer top. This heating can offset radiative cooling above the boundary layer, reducing the strength of the circulation that links the nonconvecting and convecting regions. The weakened circulation requires cooling of the nonconvecting region relative to the convecting region in order to maintain the export of energy from the latter to the former. It is suggested that the “semi-indirect” effect of aerosols, wherein cloud cover is changed in response to aerosol heating, is sensitive to the vertical extent and magnitude of the aerosol forcing. The experiments suggest that dust optical properties (to which the top of the atmosphere forcing is sensitive) should be allowed to vary with the mineral composition of the source region in a computation of the climate response. More extensive measurements of the dust optical properties, along with the vertical distribution of the dust layer, are needed to reduce the uncertainty of the climate response to dust aerosols.