Long-range rainfall forecasting in southern Australia:
Use of Indian Ocean data and upper level flow patterns:


By Ian Holton of Holton Weather Forecasting



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

 

Examples:

NORTH-WEST CLOUD-BAND SITUATION 21st to 22nd May 1999


Picture 1: 8am CST 21st May 1999:
(i) North-west Cloud-band runs from NE Indian Ocean Onslow Area to Cold Front over Eastern NSW & Victoria.
(ii) Another Cold front can be seen approaching SW tip of WA.

 

Picture 2: 4pm CST 21st May 1999:
(i) North-west cloud band breaks into two as new front over SW of WA drags the moisture towards the frontal zone.
(ii) Rain starts in western SA.

 

Picture 3: 4am CST 22nd May 1999:
(i) Rain becomes general over most of SA as North-west cloud-band moisture is lifted by the approaching cold front, now over the Great Australian Bight Area.

 

Picture 4: 10am CST 22nd May 1999:
(i) The cold front intensifies as the cold frontal airmass reacts with the warm moist North-west cloud-band airmass.
(ii) Potential energy is released by condensation and glaciation as the warm & moist Indian Ocean airmass is uplifted by the approaching frontal zone.
(iii) This helps cause an intense low pressure area to form on the frontal zone just south of SA.
(iv) Good rains and some thunderstorms result.
(v) This rain and thunderstorms move slowly across Victoria, southern NSW & Tasmania in the following few days.

Images courtesy of James Cook University

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Updated 28th June 2000