Wintertime precipitation in the Australian Alpine Region: Insights from an airmass climatology

The WMO statement on the status of weather modification suggests that there is ample evidence that human activities such as 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, and perhaps unsurprisingly, has sparked a degree of controversy. It should be noted, however, that the post-frontal airmass typically originates in the Southern Ocean and is among the most pristine in the world, so any suppression effect will be most clearly seen in this region.

To address the question of whether precipitation has been suppressed by aerosol pollution can be identified in surface precipitation records, back trajectories arriving at Automatic Weather Station (AWS) sites within the Australian alpine region during precipitation events have been calculated. Back trajectories for air parcels arriving at three hour intervals were calculated using the Hybrid Single Particle Lagrangian Integrated Trajectory Model (HYSPLIT), driven by ERA-Interim reanalysis meteorological data. Trajectories are classified according to synoptic type and an airmass-precipitation climatology for each arrival point is generated by convolving the trajectories with sub-hourly precipitation records.

The precipitation associated with these trajectories is to first order dominated by the effect of orography, in particular showing signs of suppression where there is elevated terrain upstream. The use of multiple AWS locations in varied topographical conditions helps to overcome this feature, and provides valuable insight into the nature of orographic precipitation in South Eastern Australia. To date, results pertaining to precipitation suppression due to aerosol pollution remain inconclusive, but there is scope for refinement of techniques.