Cloud-Radiation Field Changes due to the Direct Effect of Smoke Aerosols in Southeast Mexico

In general, aerosols affect climate mainly by directly absorbing and scattering input solar radiation and indirectly through their rule as cloud condensation nuclei. Smoke aerosols from biomass burning are considered to be the second most important source of anthropogenic particles (sulfate aerosols being the first) that are influencing the climate. Numerical simulations are carried on MM5 (using the CCM2 radiative scheme) by introducing the smoke aerosol spectral optical properties in the southeastern Mexican region. The neighborhood of this region is the most important source of biomass burning aerosols in Central America during the dry season (February-June). The particles are considered to be homogeneous in composition and the optical properties are calculated using Mie theory and the Remer et al. (1998) smoke model. Simulations are performed for March 17-20 and April 18-20, 2003. These two periods resulted to be a relative maximum in the number of fires detected in the studied region according to different algorithms based on satellite imagery. GDAS data are used to initialize the MM5 model. The goal is to study the changes in the cloud-radiation field due to the aerosol direct effect varying the smoke aerosol optical properties, especially the optical depth. Preliminary results support the argument that not only the aerosol effect is important but also the cloud changes due to the radiative differences caused by the aerosol direct effect itself. These cloud effects followed very different ways sometimes depending on atmospheric conditions of course, but also on other characteristics such as orography or land surface features. The simulations indicate a wide range on the surface radiative forcing varying from -40 W/m² for smoke particles with an optical depth of 0.2 (at 670 nm), to -140 W/m² for particles with an optical depth of 0.8.