Wednesday, 12 January 2005: 8:45 AM
Radiative impacts of absorbing aerosols on tropical maritime boundary layer and trade wind cumuli
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Observations
during the Indian Ocean Experiment (INDOEX) show that trade wind cumuli
with a low coverage were embedded in the widespread anthropogenic haze
over northern Indian Ocean. The purpose of this study is to better characterize
the magnitude of semi-direct and indirect effects by conducting three dimensional
limited domain simulations of trade wind cumuli in the Indian Ocean region.
The Eulerian version of a non-hydrostatic Eulerian/semi-Lagrangian (EULAG)
anelastic fluid model developed at NCAR with warm-rain bulk microphysics
parameterization is used for this study. Model simulations are initialized
using dropsondes obtained during INDOEX and sea surface temperatures estimated
from the microwave imager (TMI) on board the Tropical Rainfall Measuring
Mission (TRMM) satellite. A control experiment (CON) without soot in the
environment and experiments with soot above (SAC), within (SWC) and below
(SBC) the cloud layer and throughout the model atmosphere (STA) are conducted
to study the effects of absorbing soot on cloud formation and development.
At 0.55µm wavelength, the single scattering albedo and extinction
coefficient of absorbing aerosols are computed as 0.889 and 0.124km-1
respectively. Aerosols are homogeneously distributed in their location.
For the run STA, compared to the run CON,
the daytime reduction of cloud fraction and liquid water path is 0.8% and
5.3gm-2 respectively. As a result, at the ocean surface (top
of model atmosphere) absorbing aerosols exert a daily total radiative forcing
of -15.1 Wm-2 (-1.0 Wm-2) and a semi-direct forcing
of 1.4 Wm-2 (1.5 Wm-2). Considering the simulated
total reduction of 2.3 Wm-2 in surface heat and moisture fluxes,
the absorbing aerosols in run STA have a net cooling effect on the ocean
surface and warming effect on the atmosphere. The downdrafts, updrafts,
total water mixing ratio, potential temperature and the diurnal evolution
of cloud fraction, liquid water path and cloud heights are sensitive not
only to the concentration and absorption properties of aerosol, but also
to its vertical distribution, as will be presented in the conference. The
maximum (minimum) daytime updrafts and downdrafts, which partly determine
the amount of water vapor in the lower boundary layer, occurred in run
SBC (SWC). As a result, the simulated surface moisture flux actually increases
in run SBC compared to the control run.Supplementary URL: http://www.atmos.uiuc.edu/~hailong/cloud.htm