This altitude is where planetary waves, also known as "Rossby waves", are triggered by rising warm air. These waves cause changes in the atmosphere over thousands of kilometres via high-altitude westerly winds.

"The waves are efficiently excited by deep [high-altitude] heating of the atmosphere, but not by shallow [low-altitude] heating because, near the surface, the westerly wind jet is weak and friction is strong," says Shan-Ping Xie of the University of Hawaii, US, who collaborated with Minobe.

'Better forecasting'

Together with colleagues, Minobe and Xie used high-resolution satellite data gathered by NASA to show that a narrow rain band frequently hovers directly above the warm, northernmost flank of the Gulf Stream.

In this region, the warm temperatures of the Gulf Stream contrast sharply with the cold waters of the North Atlantic. The resolution of computer models currently used to predict the weather is too low to show this sharp temperature gradient, which extends over just tens of kilometres – current models cannot resolve anything smaller than about 150 kilometres.

Yet, according to Minobe's research, this sharp gradient produces upwelling wind currents that carry the warmth of the Gulf Stream deep into the troposphere.

To test this theory, the team used a higher resolution model. When the temperature gradient was sharp, the computer model was able to reproduce the rain seen in the NASA images. When the researchers smoothed the gradient over larger distances, it could not.

Current predictions are "not a exact science", says Adam Scaife of the Hadley Centre, the UK Met Office's research centre. And, he says it remains to be seen whether the next generation of supercomputers, expected for 2009, can handle the sheer volume of calculations needed to run such high-resolution models frequently. "If this process were properly represented in high-resolution models, we could see a jump in seasonal forecasting skill," he told New Scientist.  Source

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