On the nature of wintertime precipitation in the alpine regions of south-eastern Australia

Data from a precipitation gauge network in the Snowy Mountains of South-Eastern Australia has been analysed to produce a new climatology of wintertime precipitation and airmass history for the region in the period 1990–2009. Precipitation amounts on the western slopes and in the high elevations (> 1000 m) of the Snowy Mountains region have experienced a decline in precipitation of 19% for annual precipitation, and for wintertime precipitation the amount is about 22%, which is far greater than the decline experienced generally in south- eastern Australia during this period. The contrast in the decline east and west of the ranges suggests that factors influencing orographic precipitation are of particular importance. A synoptic decomposition of precipitation events has been performed, which demonstrates that about 57% of the wintertime precipitation may be attributed to storms associated with “cut-off lows” (equatorward of 45◦ S). A further 40% was found to be due to “embedded lows”, with the remainder due to Australian east coast lows and several other sporadically occurring events. The declining trend in wintertime precipitation over the past two decades is most clearly seen in the intensity of precipitation due to cut-off lows, and coincides with a decline in the number of systems associated with a cold frontal passage. Closer investigation of two typical wintertime storms in the nearby Brindabella ranges, using weather station and remotely-sensed data, reveals that the post-frontal period is characterised by shallow, supercooled, orographically forced cloud and sustained, low to moderate precipitation rates. High resolution numerical simulations with the WRF model match the observations remarkably well and have been used to provide a more complete picture of the nature of these storms. The transport of moisture to the Snowy Mountains during precipitation is investigated by coupling back trajectories with precipitation observations. Two principal “moisture corridors” are revealed, with the most frequent precipitation associated with westerly trajectories and orographic forcing, while heavy precipitation is associated with synoptic ascent ahead of a cold front, whereby trajectories are shown to come from the north. Finally, a method is proposed to directly evaluate claims that precipitation in the Snowy Mountains has been suppressed by pollution from urban and industrial sources. Back trajectories are used to provide a meteorological link between suspected pollution sources and a Snowy Mountains analysis region, and both satellite-derived cloud microphysics and surface precipitation observations are considered. It is shown that, while there is a clear link between the microphysics observations and surface precipitation rates, no convincing relationship exists between any of the pollution sources and Snowy Mountains cloud or precipitation observations.