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( and surface pressure falls) in the northerly flow to the east of the thermal ridge, and cool air advection( and pressure rises) to the west of the thermal ridge ahead of the front.
Today, this process is well-captured by NWP guidance- back then it shifted our attention to analysis of the pressure, wind and temperature fields over southern WA and tracking of the trough in the pre-frontal flow.
— Barry Hanstrum.
These modifications of fronts as they propagate over land have operational importance in forecasts for fire-fighting operations where the timing of wind changes is critical. In an analysis of the Alpine fires of January 2003, Mills( 2005a) observed that the surface structures of the cool changes were strongly affected by diabatic heating, causing them to vary strongly over relatively short distances.
The contribution from new technology
Naturally, field experiments and operational forecasting have been facilitated by major improvements in observations – the expanding network of automatic weather stations( AWS), wind profilers, radars( including Doppler) and multispectral high resolution satellite imagery. Mills and Morgan( 2006) were able to use clear-air radar reflectivity to study the complex convergence lines that result over Victoria from the confluence of diabatically modified post-frontal air with the cool air behind a coastal surging part of the same cool change. The increasing availability of Doppler data in the Bureau’ s radar network since 2003 is also assisting in the surveillance of complex wind changes.
A major technological advance in satellite observations came with the deployment active microwave sensors( scatterometers) capable of measuring the 10-metre wind speed and direction over the ocean at high spatial resolution. One of the earliest was the scatterometer on board the National Aeronautics and
Space Administration( NASA) Quick Scatterometer( QuikSCAT) satellite that operated from 1999 to 2009. Previously unobtainable detail of the structure of oceanic lows and fronts at 25 km spatial resolution could now be seen and had immediate impact on synoptic analysts. The data were soon added as inputs to the numerical weather prediction systems. Some examples from an Australian context are given by Leslie et al.( 2008). This capability has been continued in subsequent satellite missions.
Another revolution in technology since the 1970s has come with numerical weather analysis and prediction. Early NWP systems had coarse spatial resolution, smooth topography and very limited treatment of key physical processes such as atmospheric radiative transfer and the energy balance at the surface. Reeder and Smith( 1988) used a two-dimensional model to explore the capability of NWP to capture frontogenesis and frontal motion, finding that a resolution between 50 and 100 km was adequate but that a model with a 150 km grid spacing was generally inadequate. McInnes et al.( 1994) considered the performance of the Bureau’ s operational NWP system for cold fronts over south-eastern Australia and noted significant deficiencies such as slower than observed translation speed. The grid spacing at that time was 150 km. Mills( 2005b) applied a more advanced NWP system with a grid spacing around 10 km to study the synoptic situation associated with 1983 Ash Wednesday fires and showed how NWP systems can accurately predict many salient features of such severe weather situations.
More recently a very high-resolution NWP system has been used to study the 2009 Black Saturday fires in Victoria( Engel et al., 2013). The NWP system has also been coupled to a firebehaviour model to study the effect of the weather on the fires on Black Saturday and the subsequent feedback of the fires on the weather( Toivanen et al. 2019). The high horizontal and vertical resolution can capture and accurately predict mesoscale variations, topographical distortions and modifications of wind changes as they progress across the domain. Mills et al.( 2020) have prepared a wind change climatology for Victoria, diagnosing abrupt wind changes, whether they be frontal or not, as part of a broader fire weather climatology. It contains a good review of the significance of wind changes, to fire-fighting, and recent advances in analysis, understanding and prediction.
Current status – is the concept of fronts still useful?
One disadvantage of charts from early numerical weather prediction, for forecasters and the public, was the absence of fronts, and the smoothing out of discontinuities due to low resolution. Objective criteria have been proposed to diagnose fronts in NWP output based on thermodynamic or wind change thresholds( e. g. Hewson, 1998) and, while adopted in some research studies, are not commonly used.
Now there are visualisation systems to display, analyse and animate NWP output and to make them publicly available. In the case of a passage of a front users can focus directly on the weather relevant to them, rather than having to decode the weather that may be implicit in the symbol of a front.
Nevertheless, fronts remain part of the communication lexicon and are still drawn or depicted on charts even if they are often merely annotations added on the basis of a suitable diagnostic. They are still a powerful information-laden symbol. It is remarkable that the concept has endured for a century although understanding of them is much different today. While the focus is shifting to the detailed display of weather elements from NWP, fronts are still useful as an explanatory tool in communicating weather to users.
Why draw cold fronts?
• If we leave out any dynamic or theoretical aspects, they are a communication feature – they explain likely weather changes in terms of past observed conditions.
• The Norwegian model, and subsequent conceptual models, provided frameworks with which to interpret sparse observations.
• Satellite imagery extended this framework, but where was the surface front? There is often a disconnect between where Guymer’ s rules placed the analysed front and what the punter felt.