Does aerosol pollution of orographic clouds result in precipitation suppression in the Australian Snowy Mountains?

There is ample evidence that pollution from biomass burning, agriculture and industry have the capacity to modify local and regional weather conditions. A claim to such an effect is that "...air pollution must be an important factor in determining precipitation amounts in the [Australian] Snowy Mountains" (Rosenfeld, 2000). This claim is made with reference to aerosol pollution sources that are quite remote from the region, based on satellite observations of cloud droplet effective radius without consideration of surface precipitation measurements, and unsurprisingly has sparked a degree of controversy.

A novel method is proposed to directly evaluate this hypothesis. A modern network of carefully quality-controlled precipitation gauges and satellite (MODIS) observations of cloud top effective radius (re) are used to investigate the link between remotely-sensed cloud microphysical characteristics and surface precipitation. Distributions of precipitation rate for cloudy scenes partitioned by the “critical” re=14 μm threshold are distinct and precipitation rate is positively correlated with cloud re, so there is a physical link between these parameters.

To provide a meteorological link between potential pollution sources and the Snowy Mountains, a climatology of back trajectories has been compiled using the HYSPLIT model. Trajectories which pass directly through “source” or nearby “control” regions are selected, and statistics for both cloud re and precipitation are compared. No significant relationship is found between the pollution sources considered and the Snowy Mountains for either parameter. While convincing examples may be found, the claim that a systematic reduction in precipitation amount has occurred as a result of aerosol pollution is not supported.