Table of Contents


Common Remarks

Explosive midlatitude cyclogenesis over the Australian region occurs predominantly over southern and eastern parts of the continent and adjacent waters during the late autumn and winter months. During the summer months explosive cyclogenesis occurs over the western Australian region, as shown below.

Over southeastern parts of Australia and adjacent waters synoptic systems which undergo explosive cyclogenesis include "East Coast Lows" or "Tasman Sea Lows" Over Western Australia ex-tropical cyclones can interact with prefrontal westerlies and midlatitude troughs and undergo explosive cyclogenesis during summer months.

In this conceptual model study we will principally discuss explosive cyclogenesis (EXCY) over southeastern parts of Australia, with particular emphasis on the "East Coast Lows".

The nature of this EXCY is different to that observed over Europe and North America. This is due to the low latitude location of Australia, between 15 and 40 degrees latitude south, which is similar to the latitudes of southern Europe and northern Africa. This means that EXCY over Australia rarely involves a significant contribution from warm fronts.

In addition, there is a vast ocean adjacent to the poleward boundary of the landmass so that cold fronts and associated cyclogenesis does not involve the cold airmasses common to northern Europe and North America.

In order to classify EXCY in the Australian region a number of criteria have been used in the past. These include the normalised central pressure deepening rate, relative central pressure, the 24 hour changes in geostrophic vorticity and also a significant wave height threshold.

The Normalised Central Pressure Deepening Rate is expressed by the formula

NDRC = (Δpc/24h)*(sin 60°/|sin φ|) = 1

where Δpc is the change in central pressure and j is latitude. For latitude j = 25°, Δpc = 11.7 hPa and for j = 40°, Δpc = 17.8 hPa, which represent the normalized deepening rates for a "bomb" (explosive cyclogenesis)

This method has the limitation when applied to rapidly moving low pressure systems that migrate across the climatological MSLP pattern, in particular a strong meridional gradient in background MSLP. Both the Relative Central Pressure Method and the 24 hour Vorticity Change Method seek to address this limitation.

The Relative Central Pressure Method takes into account the climatological mean sea level pressure at the location of the developing cyclone. In analogy with the Normalised Central Pressure Deepening Rate the formula for the Relative Central Pressure Method is

NDRR = (Δpr/24h)*(sin 60°/|sin φ|) = 1

where Δpr is the difference between the central pressure of a cyclone and the climatological pressure at the cyclone location at that time of the year.

The 24 hour Change in Geostrophic Vorticity Method defines explosive cyclogenesis to occur if a 24 hour decrease in geostrophic vorticity of more than 7.8 x 10-5 s-1 per day is associated with the developing cyclone.

The Significant Wave Height Threshold of at least 5m sustained for at least 6 hours as measured by the BHP oil and gas platforms in eastern Bass Strait defines low pressure systems that produce "storm conditions" in Bass Strait.

Figure 1: Diagram from Liam and Simmonds 2002.

Mean southern hemisphere explosive cyclone system density during the months JJA (left), SON (center), DJF (right). Statistics for the months of MAM are not shown as during these months explosive cyclones occur well to the south of the Australian continent. The contour interval is 1x10-5 explosive cyclones (degree latitude) -2. Continental Australia is outlined in red. Location of Tasman Sea annotated by "T". The statistics are evaluated using the "relative central pressure method".

I. Cloud Structure In Satellite Images

Learn about how to recognise and detect Explosive Cyclogenesis in satellite images.

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II. Meteorological Physical Background

Find out more about the meteorlogical and physical background of Explosive Cyclogenesis

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III. Key Parameters

Learn which key parameters to use for montoring Explosive Cyclogenesis

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IV. Typical Appearance In Vertical Cross Sections

Find out the typical appearance of Arctic Explosive Cyclogenesis in vertical cross section

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V. Weather Events

Explore the weather events associated with Explosive Cyclogenesis

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VI. References

Let these comprehensive documents in the references assist you in finding more about Explosive Cyclogenesis

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