A Synoptic Classification of Inflow-Generating Precipitation in the Snowy Mountains, Australia

Abstract

Precipitation falling in the Snowy Mountains region of southeastern Australia provides fuel for hydroelectric power generation and environmental flows along major river systems, as well as critical water resources for agricultural irrigation. A synoptic climatology of daily precipitation that triggers a quantifiable increase in streamflow in the headwater catchments of the Snowy Mountains region is presented for the period 1958-2012. Here, previous synoptic-meteorological studies of the region are extended by using a longer-term, year-round precipitation and reanalysis dataset combined with a novel, automated synoptic-classification technique. A three-dimensional representation of synoptic circulation is developed by effectively combining meteorological variables through the depth of the troposphere. Eleven distinct synoptic types are identified, describing key circulation features and moisture pathways that deliver precipitation to the Snowy Mountains. Synoptic types with the highest precipitation totals are commonly associated with moisture pathways originating from the northeast and northwest of Australia. These systems generate the greatest precipitation totals across the westerly and high-elevation areas of the Snowy Mountains, but precipitation is reduced in the eastern-elevation areas in the lee of the mountain ranges. In eastern regions, synoptic types with onshore transport of humid air from the Tasman Sea are the major source of precipitation. Strong seasonality in synoptic types is evident, with frontal and cutoff-low types dominating in winter and inland heat troughs prevailing in summer. Interaction between tropical and extratropical systems is evident in all seasons.

Trends in synoptic circulation and precipitation in the Snowy Mountains region, Australia, in the period 1958–2012

“…Hoy et al 2014;Newton et al 2014a, b;Bettolli et al 2010;Stahl et al 2006). Theobald et al (2015) presents a novel, automated approach to classifying year-around synoptic systems across SEA, building upon previous studies which largely focussed on cool-season precipitation, ; Pook et al 2006;Wright 1989, Fiddes et al 2014. In addition to the widely studied cold front and closed low weather systems, Theobald et al (2015) found a strong emphasis on tropical moisture pathways from oceans to the north-east and north-west of Australia for delivering precipitation to the Snowy Mountains region.…”

“…Winter and spring are typically the wettest seasons, when precipitation is heavily influenced by the interaction between the prevailing mid-latitude westerly airflow and orographic effects (Chubb et al 2011;Ummenhofer et al 2009b;Cai and Cowan 2008;Pook et al 2006), and frontal and closed low systems dominate. However, Theobald et al (2015) revealed the importance of warm-season precipitation to the generation of inflows, with more than 40% of precipitation days ≥ 10 mm occurring during the warmer months. During these months, inland heat troughs prevail and high intensity precipitation associated with trajectories of warm, moist air from tropical latitudes is common.…”

“…This study focuses on investigating temporal trends of the synoptic types described in Theobald et al (2015), allowing recently observed precipitation declines to be placed in context of longer-term hydroclimate variability spanning the period 1958 -2012. In addition, this study sheds light on the impact of future precipitation regime changes on the hydroclimate of the Snowy Mountains due to climate change.…”

“…The synoptic types used in this study are an automated identification of daily synoptic-scale atmospheric circulation for all precipitation days above an inflow-generating threshold of ≥ 10 mm (hereafter referred to as "precipitation days"), for the Snowy Mountains region of SEA between 1958 and 2012 (Theobald et al 2015). For each day on which the 10 mm threshold was exceeded, daily reanalysis data from the European Centre for Medium-Range…”

“…A total of 3435 precipitation days when ≥ 10 mm of precipitation was recorded were classified (Theobald et al 2015). The 11 synoptic types include cold front (T1), pre-frontal types (T5, 8), closed lows (T2, 6), inland heat troughs (T3, 4,7) and offshore lows (T9), as well as circulation conducive to north west cloud bands ("NWCBs", T4, 8 and 11).…”