Australia is a continent surrounded by large ocean areas. The origin of all weather is
differential heating of the earths' surface via solar radiation. The warmer sub-tropical
and tropical oceans store vast quantities of this heat energy as they warm by direct
sunlight.
The Indian Ocean to the west of Australia encompasses some of the warmest ocean areas in
the world. This warm ocean area evaporates large quantities of water, in the form of water
vapour, into the surrounding lower and middle atmosphere. This water vapour is a major
source of potential heat energy ready to fuel the southern Australian weather systems.
At the same time, this large area of warm and moist air is forming during the summer
months. A broad band of eastward flowing air, the westerly wind belt, lies to the south of
the continent of Australia. This broad belt of westerly wind increases in strength with
height due to the thermal contrast between polar and tropical areas of the earth. They
reach speeds of up to 100 to 200kts at high altitudes and form the strong westerly wind
jetstream belts of the earth. As these winds attempt to balance the earths' temperature
differences, the jetstreams bend, making a northwest to southwest curving westerly wind
belt around the globe. These curves are called Rossby Waves. A curve northward towards the
tropics is called a long-wave low pressure trough; while a curve towards the poles is
called a long-wave high pressure ridge.
As Autumn approaches, this westerly wind belt moves northward and starts to react with the
moisture to the west of the Australian continent. If a seasonal long wave trough is just
to the west of Western Australia, the northwesterly wind ahead of the trough will feed
this moisture southeastwards, ahead of short-wave frontal features. This southeastward
moving cloudband (called a northwest cloudband) will rise ahead of the front. The
cloudband will then thicken and often produce good rains to the cropping and grazing areas
of southern Australia.
The generation of these cloud-bands is dependant on sea-surface temperatures and gradients
in the general northeast Indian Ocean area. The effect of these cloudbands is dependant on
the presence and strength of a long-wave low pressure trough in the middle and upper
atmosphere,over or just west of the Australian continent.
I use surface and upper level measurements over northern and Western Australia, and the
Indian Ocean area; and also sea-surface temperature measurements over similar ocean areas
as input variables to my rainfall forecasting models.
These input variables cover a large range of climatic factors. The Models I use are
shown to be quite accurate for forecasting growing season rainfall in southern Australia.
Correlations of 0.7 to 0.9 (on a scale of 0 to 1) were commonly found on test growing
season rainfall data samples in southern Australia (Holton, 1999).
Some southern farmers say that we receive good rains from northwest cloudbands, but also
we receive a large proportion of our rainfall from post-frontal shower activity. This is
quite true. However, it should be noted that this post-frontal shower activity is also
dependant on the same upper level wind-flow patterns. And this shower activity is often
strengthened by inflows of potential heat energy from northwest cloud-bands. The surface
pressure systems of the southern Australian growing season are in fact formed and steered
by the upper level wave patterns, and by the orientation and strength of the upper
jet-streams.
I also use similar upper level Rossby wave patterns and sea surface temperature fields to
complete ten-day weather forecasting for farmers in south-eastern Australia.
The motto for weather forecasting in southern Australia is mainly, Look up and look west.
References: Holton. I. 1999. "Prediction of growing season rainfall and crop yields
in southern Australia". Aust. Met. Magazine.
Ian Holton, Holton Weather Forecasting Pty Ltd, PO Box 728, Nairne. SA. 5252
Phone 83886700 Fax 83886788 E-mail holton7@senet.com.au
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