IT is easy to write off farmers’ theories that their neighbours always get more rain, or that they have consistently missed rain all season, as selective memory.
However, climatologists have said there have been a number of changing weather patterns on a macro scale, while local topography can also play an important role in determining where rain falls.
David Stephens, agricultural meteorologist with the Australian Export Grain Innovation Centre (AEGIC) said there had been a shift in pressure patterns which has resulted in interesting changes to rainfall distribution.
“We’ve mapped the changes and there are some clear trends emerging.
“There’s definitely more summer rainfall, while in the Mediterranean climate areas we are seeing a reduction in winter rainfall.”
He said growers who noticed they were missing out on autumn rain during rainfall events across the country, when once they might have got a fall, may be victim of the changing pressure systems.
“We seem to be seeing moisture from the north-west going a lot further north than it once did.
“And later in the season, we are seeing typical winter rain shower activity contracting further south.”
CSIRO climatologist Peter McIntosh said dryness could persist for months.
“It is not as simple as a lack of evaporation in the area, as the vast majority of evaporation occurs over the sea, but in instances where the air is close to saturated, a little evaporation can be enough to trigger rain.”
He said the effect of some areas missing out on rain may be caused by the topography of an area.
“If patterns come from a certain direction, then the topography can influence how it falls.”
Dr Stephens said the patterns established during the millennium drought of the 2000s remained in place still, even though the seasons have generally not been as bad.
“Using the models based on the patterns of those years, the skill has been quite good, the winters have been drier.
“On the other hand, summer rain is actually increasing overall, so the difference to annual rainfall hasn’t been that great, it is just the timing that is changing.”
He said southern Australia had been hit by a double-whammy effect with its in-crop rainfall.
“The pressure systems feeding the moisture in are becoming less intense and there is less moisture feeding in from the north-west.”
Dr McIntosh said it was important to note what climatic factors were causing the rain to fall.
“The sort of a system that has brought the rain influences where the falls are heaviest.
“An obvious example is on the east coast.
“Onshore flows will mean most of the rain is in coastal areas, whereas if the rain comes down from the north-west, then inland areas will do better.”
In regards to localised events, Dr Stephens said farmers’ hunches regarding storm activity probably had a basis in truth.
“Farmers talk about ‘thunderstorm lines’ which appear to get more storm rainfall than other places.
“There is meteorological research to show areas that are either at the edge of vegetation or in an area where more moist air will be drawn up, such as an elevated area facing the weather system, can get more rain out of some systems.
“It is a similar thing with rain shadows, valleys can have mountains blocking weather systems from getting in.”
Dr McIntosh said in flat inland Australia, even relatively minor topographic features could change rainfall patterns.
“You see in the Victorian Mallee that there are certain areas that anecdotally receive more storm rain, and there is just the slightest rise.”
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