Talk Title: Interactions Between Water and Biomass Burning Aerosols
Abstract: Atmospheric aerosol particles act as cloud condensation nuclei, affording them the ability to influence cloud microphysics, planetary albedo, and precipitation. Many field, satellite remote sensing, and modeling studies have attempted to quantify the magnitude of aerosol-induced reduction in precipitation but with limited success; there is still no statistically robust evidence of a decrease in surface precipitation in association with pollution. This work presents a framework within which to study aerosol effects on precipitation, termed “precipitation susceptibility”. Models of varying complexity, aircraft measurements, and satellite observations from NASA’s A-Train constellation of satellites are used to determine what controls the precipitation susceptibility of warm clouds to aerosol perturbations. Connections between this work and biomass burning aerosols will be discussed, in addition to a discussion of research needs in this field of research.
Research Interests: Perhaps the first field evidence of rainfall suppression by biomass burning aerosols was provided by Warner (1968) in his pioneering study of the effect of sugarcane smoke on precipitation. However, since that time there has been significant controversy surrounding the effect of biomass burning on rainfall; there still is no statistically robust evidence of a decrease in surface precipitation in association with such pollution (Levin and Cotton, 2008). My interest in this special forum is related to the important issue of how biomass burning aerosols influence clouds and precipitation. I am actively studying aerosol-cloud-rain interactions using a synergistic and diverse set of tools including aircraft measurements, process-based cloud models, and satellite observations. Recent work (Sorooshian et al., 2009) suggests that using such tools together will afford critical insight into how biomass burning aerosols will influence cloud microphysics and rainfall. I hope to participate in this forum to actively discuss this issue and suggest future research directions.
Aerosols and their interactions with solar radiation, water, and clouds are identified as the largest single source of uncertainty in current estimates of the total anthropogenic radiative forcing (IPCC, 2007). Important issues that remain to be studied in greater detail include the nature of how biomass burning aerosols interact with water vapor and influence cloud properties and precipitation. A few questions of importance include:
What is the magnitude and nature of biomass burning aerosol interactions with water vapor?
How effective are these aerosols as cloud condensation nuclei and ice nuclei and how do these properties change as a function of aerosol age and transport?
What parts of the world are most vulnerable to the effects of biomass burning aerosols and how will this change with population growth?
Research needs: large-scale and coordinated measurement programs (surface and aircraft), satellite observations, laboratory measurements, and modeling. There are limitations to each of these tools of analysis and therefore probably the most effective way to proceed is to use them all together in a coordinated way. With regard to field measurements, long-term campaigns likely would be very useful. It would be great if there were more opportunities for young scientists who want to be involved with instrument development and field measurements.
More improvements to satellite remote sensing techniques are desirable to achieve more accurate measurements of parameters such as cloud liquid water path, droplet size, and proxy measurements of cloud condensation nuclei. There is also a need for improved methods of combining satellite data from multiple sensors to study the effects of biomass burning aerosols on clouds and the hydrologic cycle. For example, in certain parts of the world (i.e. West Africa outflow) biomass smoke plumes reside above clouds and often can be misinterpreted as influencing clouds at lower altitudes; the use of CALIPSO data is instrumental in addressing this issue.
References:
Levin, Z., and Cotton, W. R. (2008). Aerosol Pollution Impact on Precipitation: A Scientific Review. Springer Press.
Sorooshian, A., Feingold, G., Lebsock, M. D., Jiang, H., and Stephens, G. (2009). On the precipitation susceptibility of clouds to aerosol perturbations, Geophys. Res. Lett., 36, L13803, doi:10.1029/2009GL038993.
Warner, J. (1968). A reduction in rainfall associated with smoke from sugar-cane fires – an inadvertent weather modification?, J. Appl. Meteorol., 7, 247-251.