Tuesday, September 22, 2015

Darwin's second sea-breeze

The passage of a sea-breeze front is a regular occurence in Darwin, Northern Territory, Australia. But, on some days, something like a “second sea-breeze” is observed during the evening, a phenomenon that has puzzled local forecasters for ages. In 2008, a study conducted by Gerald Thomsen and Roger Smith at the Meteorological Institute of the University of Munich, Germany, brought the solution.

Some sea-breeze theory

A sea-breeze is a local circulation, induced by differences in surface temperature between the land and the sea. While the sea surface temperature doesn't change much throughout a sunny day, the surface temperature of the land often increases very rapidly. As a result the air above the land also warms up and becomes less dense than the air above the sea. (1) This difference in density then produces a difference in air pressure at a certain altitude (2), with a higher pressure above land and a lower pressure above the sea. A pressure difference is at the origin of an air flow (= wind) from the higher pressure to the lower pressure. (3)

As a result of this wind at a certain altitude, more air particles are being advected towards the sea and at the surface, the pressure rises, while at land, the surface pressure decreases, simply because the air is escaping aloft. (4)
Again, this difference in air pressure at the surface produces a wind, blowing from the sea to the land. (5) This is the sea-breeze.
When the sea-breeze hits the coast, the temperature may drop by several degrees and typically the wind shifts and increases. At the same time, the humidity increases, as a more moist air mass is advected by the sea-breeze.

At Darwin though

Such a circulation typically starts up around noon or in the early afternoon, but at Darwin, a similar sudden jump of the wind, increasing wind speed and increasing humidity often have been observed in the evening.

The researchers from the University of Munich found at that this second sea-breeze isn't a sea-breeze at all.
From the early morning on sunny days on, a band of dry inland air, lying over the “Top End”, is being advected northwestwards towards the Tiwi Islands by the prevailing easterly to southeasterly winds. This dry airmass subsequently is moving southwestwards to Darwin with the sea-breeze. When it passes in the late evening, it is finally replaced by moist maritime air. At this moment, a jump in the wind direction and an increasing wind speed is mostly observed.
But, although it looks like a sea-breeze, it's certainly not the same thermally driven phenomenon.

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