Response of the NCAR Community Climate Model to the Radiative Forcing by the Naturally Occurring Tropospheric Aerosol

Abstract

We insert the effect of naturally occurring tropospheric aerosols on solar radiation into the NCAR Community Climate Model (CCM). The effect of the aerosol depends on concentration and type (continental, maritime), surface albedo, solar zenith angle and cloud cover. The experiments were performed for perpetual July boundary conditions. Globally averaged, the aerosol reduces the solar radiative flux absorbed at the top of the model atmosphere by 3.0 W m⁻², at the surface by 4.4 W m⁻² and in the lower troposphere it increases the flux absorbed by 1.4 W m⁻². Owing to fixed sea surface temperatures the climate simulated by the CCM is hardly affected by these perturbations. The global surface temperature change is −0.08 K (−0.27 K for continents), zonal surface temperature changes are limited to a few tenths of a degree and regional surface temperature changes rarely surpass −1 K. Between 30°S and 60°N the aerosol suppresses convective activity by reducing solar beating for land surfaces. As a result the upper troposphere, which for these regions and time of year is heated largely through moist convective processes, cools more so than the surface and lower troposphere which are directly affected by the interaction of the aerosol with solar radiation. Small but significant changes in climate are obtained for isolated portions of the globe. The most notable changes occurred for a region of Africa just north of the equator where the aerosol pushed the model towards “desertification.” That is, for this region the radiative forcing due to the aerosol gave rise to changes in convection and wind fields which in turn led to a significant reduction in precipitation. The processes involved in the changes wore like thaw discussed by Charney et al. for the role of surface albedo in desertification.