The Impacts of the 9 April 2009 Dust and Smoke on Convection

Abstract

©2010 American Meteorological Society the relatively pristine environment. They argue that smaller droplets decrease the collision-coalescence efficiency, which effectively delays the evolution of precipitation-sized droplets and shifts the water mass upward in the cloud. They also argue that shifting the precipitation production upward results in more vigorous updrafts and stronger storms. Other work related to the impacts of aerosols on deep convection include Lindsey and Fromm (2008), who observed pyrocumulonimbus (pyroCb) clouds, or storms initiated by the intense heat of a wildfire. Using Geostationary Operational Environmental Satellite (GOES) imagery, they found that pyroCbs exhibited unusually small cloud-top ice crystals as determined from multispectral observations, which the researchers attributed to ingestion of wildfire-produced biomass aerosols. Convective clouds that were displaced from the wildfire aerosol source (but in an environment with the same general stability characteristics) displayed significantly larger cloud-top particle sizes. They hypothesized that the smoke served as a source of enhanced CCN, and like the Amazonian clouds discussed above, an unusually large number of cloud droplets nucleated near cloud base. These droplets were then lofted within the vigorous updrafts of the storm, where they froze homogeneously above the -40°C level and populated the storms’ anvils with tiny ice crystals (see the conceptual diagram in Fig. 1). The anvils of these pyroCbs persisted for longer than those of the regular convection, a good example of the cloud lifetime effect. Presumably, smaller cloud droplets decreased the precipitation efficiency of the storm, and more water mass was lofted to the anvil, where it resided longer before either sublimating or falling out. This process is also considered an aerosol indirect effect, where aerosols alter cloud properties (such as the lifetime of the cloud), which in turn affect incoming and/or outgoing radiation. In a 2005 article in the Journal of the Atmospheric Sciences, Heymsfield et al. propose an alternative mechanism that involves aerosol particles being entrained into convection at The Impacts of the 9 April 2009 Dust and Smoke on Convection