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Katbatic winds : dreadful blizzards The Antarctic continent is extremely cold, it is also very windy. Going back in time, the King Baudouin base, which housed the first Belgian expedition (1957-58) under the command of Gaston de Gerlache, had to anchor a cable between the headquarters and the geomagnetism buildings so that the scientist working there could return safely to base when there was a blizzard blowing; yet the distance between the two structures was barely more than 150 metres. In the same sort of way, the Belgian geologist Tony Van Autenboer (the most experienced Belgian in terms of stays in Antarctica) experienced such a wild blizzard one evening during the 1961 Belgian expedition exploring the Sør Rondane mountains, that he took an hour to get from his tent to the expedition's Snocat, while just a few hours earlier, he had covered the distance in under five minutes! During the Australian expedition led by Douglas Mawson (1911-1914), which had the misfortune to set up in a really windy spot close to Cape Denison on the eastern coast of Antarctica, the meteorologists measured an annual average wind speed of 70 km/h, or twice what it is at other polar bases... With the extreme temperatures mentioned above, the winds that plague the continent are without doubt both the most specific and spectacular that the Antarctic climate has to throw at its visitors. Why are the winds so strong?
Because they belong to a category of winds called katabatic winds (from the Greek 'kata', meaning downwards); these winds are observed at every latitude of the globe as soon as the course of cooled air meets a significant slope, but nowhere are they as strong as they are in Antarctica. As we have already seen, the lower layers of the atmosphere that come into direct contact with the ice cap are extremely cold (caused by the reflective properties of the ice and the pureness of the air); and the colder the air is, the heavier it becomes. These masses of cold air contract and become even heavier when they start moving under the effect of the incline of the ice cap - remember that the average altitude of the continent is approximately 2,500 metres and the topography takes the ice-covered surfaces sloping down towards the perimeter, gently at first, then more steeply as the land reaches 100 or 200 kilometres from the coast. So the air mass slips very easily across the surface of the continent where there is any relief; and as the air moves, it has an abrasive effect on the ground, grating and sculpting it into protruding designs called sastrugis. In addition to the cooling of the air and the weight it gathers, comes a corollary effect that also has a role to play in generating katabatic winds: while the air is cooled near to the ground, the air higher up is warmed. In Antarctica, it is common at an altitude of 1000 metres to find the air much warmer than it is on the ground - sometimes by more than 30 degrees. This is what the meteorologists call thermal inversion. Thermal inversion acts like a lid (like the ice floes on top of the ocean), a sort of screed, blocking any rising air circulation. The layers of air trapped on the ground therefore have to respond to other influences and start moving in unusual directions, from downwards to upwards. The French meteorologist Jean-Claude André has a good description for the phenomenon of katabatic winds (1): "As long as the slope is not too steep or the pool of cold polar air is not too great, this movement has a major eastward component parallel to the contour lines of the ground, under the combined action of the force of gravity and the Coriolis effect; by contrast, when it approaches the coastline, where the ground falls away more steeply, the katabatic cold air begins to hurtle downwards more quickly, following the line of the greatest slope, i.e. from south to north. Under the action of the katabatic force of gravity, the movement of the air intensifies until it reaches its equalised velocity where the acceleration due to gravity is compensated by the air rubbing on the ground and the layers of air higher up. At the moment it reaches the ocean, or the immediate vicinity of the coastline, the wind loses its generating force (the slope of the terrain) and starts to conflict with air masses with different properties". The many research programmes conducted on the ground - particularly the programmes undertaken by the American and French teams in Adélie Land in 1985-86, under the auspices of the IAGO programme (2) - have increased our knowledge of how katabatic winds behave. We know that they are especially active in winter and that they can reach frightening speeds as they approach the coast under the effect of the steepness of the slopes, while in the centre of the continent, they are relatively moderate. The scientists also theorise that katabatic winds could cause the appearance of breaks in the sea ice around the coast: in reaching such speeds and discharging with force on to the ice floes, the winds could push the ice away and thus enable some stretches of coastal water to remain ice-free. The IAGO research has also made it possible to observe a unique phenomenon called "katabatic projection"; this is a wind that when it reaches the flat surface of the ice expanse, loses its velocity in a few minutes while at the same time, the atmospheric pressure increases just as suddenly. This phenomenon was already known in particular for the spectacular turbulence it produces along the coast, but this time, the scientists were able to observe it more closely and systematically. In fact, it was thanks to the work by IAGO and international scientific co-operation, that the first simulations of katabatic winds and hydraulic projection were carried out, most notably by a Belgian researcher from the G. Lemaître Institute of Astronomy and Geophysics at Louvain-la-Neuve, Hubert Gallée; he developed a mathematical model whose performance attracted the attention of the Department of Atmospheric Sciences at the University of Wyoming, in Laramie, which despite the fact its research faculty included the pioneer in katabatic wind simulation (T. Parish), wrote to the Belgian researcher to ask whether it could use his model. Notes: (1) "Des chercheurs dans le vent : vous avez dit blizzard?", La Recherche, N° 192, October 1987, Jean-Claude André. p 256, vol 18. (2) To understand katabatic winds better, a scientific programme was put in place in conjunction with the Institute of Geophysics at the University of Alaska and the Establishment for Meteorological Study and Research, with the support of many other laboratories, such as the Department of Meteorology at the University of Wisconsin, French Polar Expeditions, the National Institute of Universe Sciences, the Laboratory of Glaciology and Environmental Geophysics (Grenoble) and the National Science Foundation (Washington). This programme was called IAGO (Interaction-Atmosphère-Glace-Océan [Interaction-Atmosphere-Ice-Ocean]). Its most intensive phase took place during the southern summer of 1985-86. Three teams (one American and two French) consisting of some thirty scientists, were set down in Adélie Land from the beginning of November; the teams worked for over two months at three separate camps to gather a maximum amount of observations into katabatic winds.
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