Published works

Negatively tilted upper-tropospheric troughs are a synoptic weather pattern that have been associated with the development of thunderstorms and severe weather, including extreme fire weather and tornadoes. While various case studies and some preliminary climatological analysis have been conducted in the past, a thorough investigation of the development of these synoptic features during extreme weather events has not yet been done in Australia or elsewhere. This study aimed to identify how often negatively tilted troughs occur and how often they are associated with extreme storms and fire weather.

The objectives of the project were to:

• identify the extent to which negatively tilted troughs are associated with recent extreme weather events across southeast Australia;
• assess the relationship between the occurrence of negatively tilted troughs and on-ground conditions such as elevated fire danger, high winds and/or dangerous thunderstorms;
• assess seasonality of negatively tilted troughs and any potential link with large-scale climate drivers such as the Southern Annular Mode (SAM) and Indian Ocean Dipole (IOD).

The major components of the project were:

• a review of previously documented cases of negatively tilted upper-tropospheric troughs associated with severe weather;
• a detailed case study of the role of a negatively tilted upper-tropospheric trough in the extreme fire weather during the January 2003 Canberra bushfires;
• a detailed case study of the role of a negatively tilted upper-tropospheric trough in the development of a tornado outbreak in Perth in July 2014;
• a climatology of negatively tilted upper-tropospheric troughs in southern Australia for the period 1990-2018 and their relationship with severe weather reported in the Bureau of Meteorology Severe Storms Archive.

Review of previously documented cases

• A wide range of locations, dates, impacts and morphologies of negatively tilted upper-tropospheric troughs was observed, establishing a very broad scope for the climatological analysis.

January 2003 Canberra fires

• This event showed similar characteristics to the negatively tilted upper-tropospheric troughs associated with three other severe fire weather events in southeast Australia.
• The negatively tilted upper-tropospheric trough occurred a long way to the south but strong evidence was found that an associated upper-tropospheric dry air intrusion could have reached well over the continent and played a role in the sudden drying event observed in Canberra on the day.
• Interactions between the negatively tilted upper-tropospheric trough and a weaker trough to the northwest were likely the origin of a broad dry band positioned over Bass Strait during the fire event and which was connected at low levels to the dry intrusion and dry air over the continent.

July 2014 Perth tornadoes

• Strong upper-tropospheric divergence associated with a negatively tilted upper-tropospheric trough and its interaction with another trough to the northeast resulted in ascending air over southwest Western Australia.
• This vertical circulation reinforced strong dynamic forcing at the surface favourable for the formation of tornadoes.
• Gravity wave processes in the upper-tropospheric jet at the apex of the negatively tilted upper-tropospheric trough could have played a role in coupling lower- and upper-tropospheric circulation in the frontal zone.

Climatology

• Several distinct patterns of negatively tilted upper-tropospheric trough development in southern Australia were evident.
• Negatively tilted upper-tropospheric trough were present in the region close to 7 % of the time, or roughly 2.5 times per month.
• There was a strong semi-annual cycle peaking in June-July and November-January.
• Over 40 % of the 862 negatively tilted upper-tropospheric trough events identified over the 1990-2018 period in the BARRA dataset were associated with a report in the Bureau of Meteorology Severe Storms Archive
• Severe weather was nearly twice as likely to occur when a negatively tilted upper-tropospheric trough was present.
• The two peaks in the semi-annual cycle appear to be the combination of anti-phased annual cycles in different regimes of northern and southern subregions, related to the annual migration of the subtropical ridge.
• Statistically significant preferences for negatively tilted upper-tropospheric trough occurrence was observed during the negative phase of the Southern Annular Mode (SAM) and during the negative and neutral phases of the Indian Ocean Dipole (IOD).
• There was an indication of a reduction in negatively tilted upper-tropospheric trough occurrence during El Niño, however, these differences were not statistically significant.
• Other biases towards negatively tilted upper-tropospheric trough occurrence were associated with more complicated combinations of season with the states of multiple regional climate drivers.

While modern numerical weather prediction (NWP) systems are increasingly capable of resolving the surface weather parameters associated with severe weather events, there are known deficiencies. An example of this is the over-estimation of low winds and under-estimation of high winds common to numerous models. Better understanding of the role of features such as NTT in generating surface weather helps interpretation and refinement of the NWP output, particularly for users of weather data such as emergency services agencies.

Furthermore, as products from NWP such as sub-seasonal to seasonal outlooks become more refined, there is value in recognising the occurrence of potential impacts of NTT that may be resolved at the synoptic or sub-synoptic scale in such model configurations even if the potential severity of the weather parameters are note explicitly resolved. For example, if a sub-seasonal to seasonal model ensemble shows a high probability of a NTT occurrence in a particular region a certain number of weeks in advance, that is useful information for planning and preparedness for emergency services managers.

Thus, this sort of synoptic climatological study in combination with detailed analyses of particular events continues to be of great relevance to natural hazard prediction and management.

Future work

• The material in the case studies and climatology sections of this report would each form the basis for manuscripts to be published in peer-reviewed literature for the benefit of enhanced scrutiny, credibility and broad dissemination of the results.
• Climatological analysis of numerical severe weather indicators using BARRA, in combination with the NTT climatology compiled in this study, would establish more specific relationships between particular severe weather phenomena with the distinct NTT morphologies and locations observed.
• Similar climatological analysis could be applied to regional climate projections to assess projected changes to NTT occurrence in the future.
• A climatology of vertical motions across the entire BARRA-R domain was compiled during the project. The climatology is to be shared with the meteorological community and would make a valuable subject of further publications in peer-reviewed literature.