Andrew Ackerman

The radiative forcings of aerosols represent a leading source of uncertainty in recent assessments of radiative forcings due to industrial activity. Although aerosol residence times are short (~1 week or less, compared to ~50 years for carbon dioxide molecules), and they only influence the radiation budget during the day, it is estimated that by increasing reflection of sunlight the cooling effects of aerosols may offset the radiative forcing (globally averaged). The traditional radiative forcings due to aerosols are 1) direct, by which aerosols directly scatter and absorb sunlight (cooling and heating effects, respectively), and 2) indirect, by which aerosols increase cloud albedo, thereby reflecting more sunlight back to space (a cooling effect). Globally integrated, these forcings are estimated to range between 1 and 2 W/m².

A primary objective of the Indian Ocean Experiment (INDOEX) was to quantify the indirect forcing of aerosols. Although a murky haze covered the Arabian Sea (this during the northeast monsoon, with predominant flow off the Indian subcontinent), few clouds were found. Perhaps the presence of dark haze and the lack of clouds are not merely coincidental, but are connected. We are evaluating this possibility, namely that the absorption of solar energy by the dark haze dries the air sufficiently to dissipate the clouds.

The predominant cloud type expected at those latitudes is trade cumulus, in which cloud coverage is largely determined by the coverage of stratiform "anvils" that are left over from cumulus convection. The lifetime of these anvils (and hence their time-averaged coverage) decreases as the relative humidity of the air in which they are embedded decreases. To evaluate our hypothesis, we run three-dimensional model cloud simulations (based on observations of trade-cumulus observed under clean conditions in the Atlantic) under clean and polluted conditions. For the indirect effect, cloud droplet concentrations are doubled from 250 to 500/cm³, resulting in a diurnally-averaged indirect radiative forcing of -12 W/m² at the top of the atmosphere (a cooling effect), half of which is due to simply distributing cloud water over a greater surface area, and half which is due to decreased precipitation. However, if we also include a dark haze as observed during INDOEX in 1999 (with aerosol-induced heating rates of 2 K/d at noon), the boundary layer dries significantly during the daytime, resulting in a noticeable reduction in cloud coverage during the afternoon (see figure). The reduction in cloud coverage overwhelms the indirect cooling effect and leads to a net radiative forcing of 9 W/ m² (a strong heating effect) for the conditions we have simulated. Because light-absorbing aerosols are found downwind of industrial continents, our results suggest the possibility that the global cooling effect of aerosols may be drastically overestimated.

Collaborators: Owen B. Toon, University of Colorado, David E. Stevens, Lawrence Livermore National Laboratory

Time-height contours of cloud fractional coverage (%) in 3-D model simulations of trade-cumulus in clean air (left) and in dirty air (right). Changes in droplet concentrations and the aerosol-induced heating in the dirty simulation are described in the text.