8.2 Radiative impacts of absorbing aerosols on tropical maritime boundary layer and trade wind cumuli (2005

Radiative impacts of absorbing aerosols on tropical maritime boundary layer and trade wind cumuli

Hailong Wang, University of Illinois, Urbana, IL; and G. M. McFarquhar and W. W. Grabowski

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.