Michigan Technological University
Department of Physics

is pleased to announce a colloquium

with

William Conant

Reconciling models and observations of aerosol direct and indirect effects

The relationships among aerosols, clouds, and climate constitute one of the greatest puzzles in the earth sciences. Major field campaigns conducted in the past decade have provided observations ideal for testing models of radiative transfer, aerosol-cloud interactions, and ultimately the direct (aerosol scattering and absorption) and indirect (aerosol-cloud interactions) climatic effects of aerosol. Black carbon (BC) has emerged as a critical player in aerosol-climate effects because its solar heating can have significant effects on both the dynamical and microphysical evolution of a cloud system.

Two field experiments, the Indian Ocean Experiment (INDOEX) and the Aerosol Characterization Experiment - Asia (ACE-Asia) have provided unique datasets that document direct radiative forcings in excess of –15 W m-2 over extensive ocean regions downwind of Asian aerosol sources. An analysis using spectral and broadband radiometers together with detailed physical and chemical characterization of the aerosol isolates the radiative impact of aerosol amidst variability and uncertainties in the radiative effects of water vapor and clouds. These data have allowed the construction and validation of a radiative transfer model that predicts regional values of direct forcing and the attribution of this forcing to various aerosol types (e.g. sulfate, organics, black carbon, and mineral dust).

The indirect effects contribute the largest uncertainty to the IPCC's assessment of the radiative forcing of climate by anthropogenic activities. The interactions between aerosol and clouds can be subtle - theoretical calculations are presented that identify cases in which radiative heating suppresses the activation of BC-containing cloud condensation nuclei (CCN); this effect can result, paradoxically, in an increase in cloud drop concentrations. Models of cloud activation are tested against detailed field measurements of the chemical and physical properties of aerosols and clouds collected during the NASA CRYSTAL-FACE campaign conducted this July. Excellent closure is obtained between the observed cloud drop concentration and that predicted by the activation model over conditions ranging from background to polluted. The next step in this ongoing analysis will be to identify the relative roles of CCN concentration, giant nuclei concentration, and aerosol composition in the initiation of precipitation in these warm tropical clouds. Ultimately these studies will allow us to better predict the influence of aerosol on the radiative and latent heating of cumulus cloud systems.

Thursday, November 7, 2002

4:00 p.m., Fisher 139

Refreshments will be served

MTU | Physics | Colloquium