Reduction of Tropical Cloudiness by Soot
Research Staff: Andrew Ackerman
A primary objective of the Indian Ocean Experiment (INDOEX) was to quantify the indirect forcing of aerosols on climate (through their effects on clouds). Conventionally, aerosols are expected to increase cloud albedo and coverage by increasing droplet number concentration and decreasing precipitation, thereby cooling the earth-atmosphere system by exerting a negative radiative forcing at the top of the atmosphere.
A dark, thick haze due to aerosol pollution was found during INDOEX (Feb-March of 1998 and 1999) to cover much of the Indian Ocean north of the Inter-Tropical Convergence Zone. However, very sparse cloud cover was found over the Indian Ocean during INDOEX. The few boundary-layer clouds observed in the northern hemisphere were typically embedded in the haze layer.
In unpolluted conditions, cloud fractional coverage in trade cumulus clouds varies diurnally, with an afternoon minimum due to solar heating by clouds and water vapor. In the simplest analysis, solar heating reduces relative humidity, thereby accelerating the evaporation of the remnants of convection that comprise most of the cloud coverage, which appear as stratiform anvils just below the trade inversion. Such a diurnal variation is evident in our large-eddy simulations of trade cumulus shown in Figure 1, in which the ensemble of unpolluted simulations (labeled "baseline") responds to solar radiation (compared to those without, labeled "No sun") through a reduction of cloud coverage, liquid water path, and trade-inversion altitude.
Progressively adding a solar-absorbing component (soot) to the haze, to represent the amount of aerosol-induced solar heating observed during INDOEX during 1998 and 1999, significantly amplifies the afternoon clearing of the trade cumulus clouds, thereby allowing more sunlight into the atmosphere and ocean, which opposes the conventional effects of aerosols on clouds. In our simulations, this cloud-burning effect of the soot exerts a radiative forcing (diurnally averaged) of 3 to 7.5 Wm-2, or 2 to 5 times the globally-averaged forcing due to increases in CO2 since pre-industrial times. Clearly, estimates of the net indirect effect of aerosols must take into account not only the conventional effects but also this cloud-burning effect.
Collaborators: O. B. Toon, University of Colorado; D. E. Stevens, Lawrence Livermore National Laboratory; A. Heymsfield, National Center for Atmospheric Research; V. Ramanathan, Scripps Institution of Oceanography; J. Welton, University of Maryland-Baltimore County
Point of Contact: Andrew Ackerman, 650/604-3522, firstname.lastname@example.org
Evolution of simulated domain averages of (A) cloud fractional coverage (defined as the fraction of columns with optical depth > 2.5), (B) liquid water path, and (C) altitude of the trade inversion (defined as the mean height of the 6.5 g/kg total water mixing ratio surface). Results are shown as centered 6-h running averages to smooth over the convective noise. Cloud droplet concentrations are fixed at 250 cm-3 for the simulations shown. For the baseline (gray area) the haze is non-absorbing and the concentration is 1200 cm-3; for the INDOEX 1998 and 1999 cases (dotted and dashed lines) the haze absorbs solar radiation (as measured in the field experiment) and the concentrations are 1200 and 2400 cm-3, respectively. For the modified baseline (solid line), solar radiation is omitted. For the baseline an ensemble of four simulations was run; the gray area represents the range of values realized in the ensemble.