Thomas H. Chubb

On the Decline of Wintertime Precipitation in the Snowy Mountains of Southeastern Australia

AbstractData from a precipitation gauge network in the Snowy Mountains of southeastern Australia have been analyzed 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 in excess of the general decline in southeastern Australia. 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 “cutoff 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 th...

Evaluation of the AWAP daily precipitation spatial analysis with an independent gauge network in the Snowy Mountains

The Bureau of Meteorologys Australian Water Availability Project (AWAP) daily precipitation analysis provides high resolution rainfall data by interpolating rainfall gauge data, but when evaluated against a spatially dense independent gauge network in the Snowy Mountains large systematic biases are identified. Direct comparisons with the gauge data in May–September between 2007 and 2014 reveal average root mean square errors of about 4.5 mm, which is slightly greater than the average daily precipitation amount, and the errors are larger for higher elevation gauges. A standard Barnes objective analysis is performed on the combined set of independent gauges and Bureau of Meteorology gauges in the region to examine the spatial characteristics of the differences. The largest differences are found on the western (windward) slopes, where the Barnes analysis is up to double the value of the AWAP analysis. These differences are attributed to a) the lack of Bureau of Meteorology gauges in the area to empirically represent the precipitation climatology, and b) the inability of the AWAP analysis to account for the steep topography exposed to the prevailing winds. At high elevation (>1400 m) the Barnes analysis suggests that the precipitation amount is about fifteen percent greater than that of the AWAP analysis, where the difficulties of measuring frozen precipitation likely have a large impact.

Estimation of Wind-Induced Losses from a Precipitation Gauge Network in the Australian Snowy Mountains

AbstractWind-induced losses, or undercatch, can have a substantial impact on precipitation gauge observations, especially in alpine environments that receive a substantial amount of frozen precipitation and may be exposed to high winds. A network of NOAH II all-weather gauges installed in the Snowy Mountains since 2006 provides an opportunity to evaluate the magnitude of undercatch in an Australian alpine environment. Data from two intercomparison sites were used with NOAH II gauges with different configurations of wind fences installed: unfenced, WMO standard double fence intercomparison reference (full DFIR) fences, and an experimental half-sized double fence (half DFIR). It was found that average ambient temperature over 6-h periods was sufficient to classify the precipitation phase as snow, mixed precipitation, or rain in a statistically robust way. Empirical catch ratio relationships (i.e., the quotient of observations from two gauges), based on wind speed, ambient temperature, and measured precipita...

Sources and pathways of dust during the Australian “Millennium Drought” decade

From the late 1990s to mid-2010, Australia was affected by a prolonged period of drought, the “Millennium Drought,” during which numerous severe dust storms crossed the continent. We inspect this period to produce the first continental-scale climatology of air-parcel trajectories that is specific to dust and use it to gain new insights into dust transport dynamics over the eastern half of Australia. The analysis is based upon dust arrival times from airport meteorological observations made at nine mostly coastal cities for 2000–2009. The Hybrid Single-Particle Lagrangian Integrated Trajectory model was used to calculate 1.26 million backward trajectories from receptor cities, with only those trajectories associated with a dust storm observation considered in the analysis of dust transport. To tie dust trajectories from receptors to likely emission sources, trajectories were linked to six known major dust source regions in and around the Lake Eyre Basin. The Lake Eyre North ephemeral lake system, alluvial-dominated Channel Country, and agricultural Mallee-Riverina regions emerge as important sources for the period, providing variable contributions to different parts of the seaboard as controlled by different front-related wind systems. Our study also provides new detail regarding dust pathways from continental Australia. For the Millennium Drought we identify that the broadly established Southeast Dust Path may be more accurately subdivided into three active pathways, driven by prefrontal northerly winds and a variation in the influence of frontal westerlies. The detail of these pathways has implications for dust delivery from specific Australian sources to different marine environments.