Impact of biomass burning aerosols on precipitation in the Amazon: A modeling case study

Abstract

[1] A study of the potential role of aerosols in modifying clouds and precipitation is presented using a numerical atmospheric model. Measurements of cloud condensation nuclei (CCN) and cloud size distribution properties taken in the southwestern Amazon region during the transition from dry to wet seasons were used as guidelines to define the microphysical parameters for the simulations. Numerical simulations were carried out using the Brazilian Development on Regional Atmospheric Modeling System, and the results presented considerable sensitivity to changes in these parameters. High CCN concentrations, typical of polluted days, were found to result in increases or decreases in total precipitation, depending on the level of pollution used as a reference, showing a complexity that parallels the aerosol-precipitation interaction. Our results show that on the grids evaluated, higher CCN concentrations reduced low-to-moderate rainfall rates and increased high rainfall rates. The principal consequence of the increased pollution was a change from a warm to a cold rain process, which affected the maximum and overall mean accumulated precipitation. Under polluted conditions, cloud cover diminished, allowing greater amounts of solar radiation to reach the surface. Aerosol absorption of radiation in the lower layers of the atmosphere delayed convective evolution but produced higher maximum rainfall rates due to increased instability. In addition, the intensity of the surface sensible heat flux, as well as that of the latent heat flux, was reduced by the lower temperature difference between surface and air, producing greater energy stores at the surface.

Climate diagnostics of three major drought events in the Amazon and illustrations of their seasonal precipitation predictions

“…The possible physical mechanisms leading to the effect of aerosol in cloud formation are summarized as follows. The expressive increase of biomass burning aerosol, which acts as cloud condensation nuclei for cloud droplet formation, can reduce the precipitation formation process efficiency (Rosenfeld, 1999;Ramanathan et al, 2001;Khain et al, 2005;Martins et al, 2009). Even for a moist saturated atmosphere, due to the abundant amount of aerosol, size droplets may not grow enough to precipitate in the form of rain (Twomey, 1974;Silva Dias et al, 2002;Koren et al, 2004).…”

Carbon monoxide and related trace gases and aerosols over the Amazon Basin during the wet and dry seasons

“…Finally, advection in the northeastern trade winds, especially in the early part of the year, brings in substantial amounts of dust from Africa and marine aerosol from the Atlantic Ocean Talbot et al, 1990;Swap et al, 1992;Formenti et al, 2001;Ben-Ami et al, 2009Baars et al, 2011). Changing aerosol burdens over Amazonia as a result of human activities are thought to have significant consequences for ecology and climate, e.g., by perturbing the flux of photosynthetically active radiation (Schafer et al, 2002;Oliveira et al, 2007), reducing the amount of convective clouds (Koren et al, 2004), and affecting the intensity and distribution of precipitation (Rosenfeld, 1999;Andreae et al, 2004;Lin et al, 2006;Jiang et al, 2008;Martins et al, 2009).…”

Coupled Aerosol-Cloud Systems over Northern Vietnam during 7-SEAS/BASELInE: A Radar and Modeling Perspective

“…The modified gamma distribution (Ulbrich, 1983) is commonly used in cloud studies since it adequately represents many cloud/rain drop size distributions (CDSD) measured by insitu probes and disdrometers (Miles et al, 2000;Brandes et al, 2003). Furthermore, this distribution function permits variability in CDSDs due to aerosols acting as CCN that affect cloud droplet sizes, number concentrations, and distribution spectral widths (e.g., Liu et al, 2008;Martins and Silva Dias, 2009).…”