Local Circulation and Mesoscale Disturbances

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As mentioned at the beginning of previous section, local factors can sometimes significantly change the surface wind speed and direction. This change in many cases is so great that little or any connection with the large-

scale circulation exists. In addition, local circulation and other mcsoscale phenomena such as polar (also called Arctic) lows can also markedly change the parameters of wind. Thus to describe large-scale atmospheric circulation, we cannot use the observations of wind speeds and directions from the stations (especially land stations) where local influences are great. In such cases it is better to use geostrophic winds as an indicator of surface winds.

2.5.1 Local Winds

According to the nomenclature proposed by Barry (1981), three kinds of so-called "fall" winds can be distinguished: the bora, the foehn, and (mcsoscale) katabatic winds. Katabatic winds are treated here in the strict sense as local downslope gravity flows caused by nocturnal radiative cooling near the surface under calm clear-sky conditions. It seems to me that in the Arctic, where polar night and polar day occurs, distinguishing between the bora and the katabatic winds can be difficult. Thus, I propose to treat them as one type of wind, more or less cold, dry and gusty.

The bora winds are very well known from Greenland, where they are generally directed from the interior towards the coast. These downslope winds result from the presence of cold air over the ice cap, which subsequently flows down the slopes of ice cap under the influence of gravity. The speeds of these winds mainly depend on the steepness of the slope and on the pressure gradient between the summits of the ice cap and the coasts. On the ice cap it is mostly katabatic winds which occur (Putnins 1970). According to measurements taken by Loewc (1935), katabatic winds were recorded at Weststation and at Eismitte for three-quarters of the observation period. They are more common in the winter half-year (see the directions of winds in Greenland stations, Figure 2.5). The katabatic winds are better developed on clear days when the temperature at the centre of ice cap is below normal (greater radiation cooling). Bora winds are also present in other Arctic regions. They can occur everywhere else where a sufficient area of high elevation exists and allows the accumulation of air masses. Boras were observed, for example, in Novaya Zemlya (Vize 1925: Kanevskiy 1962; Shapaev 1959; Barry and Chorley 1992), Svalbard, Zemlya Frantsa Josifa, Ostrov Vrangelya (Shapaev 1959), on the coast of the Kara, Laptev, East Siberian, and Chukchi seas (Shapaev 1959), and on some islands of the Canadian Arctic Archipelago (Maxwell 1980).

The foehns also occur quite often in the Arctic. They are warm, dry, and gusty winds occurring on the Ice side of mountain ranges. The warmth and dryness of the air is due to adiabatic compression on descending the mountain slopes. In Greenland their advection is generally towards the sea on both coasts (Putnins 1970). The vertical extents of the foehns are not great. On the other hand, temperature increases can be very large. Exceptionally strong foehns can raise the temperature by more than 30 C (Schatz 1951). Their duration is not long and usually does not exceed two to five days. Generally speaking, foehns can also occur everywhere else in the Arctic, where flowing air masses have to cross sufficiently high mountain ridges. They were identified in, among other locations, Spitsbergen (e.g. Rempp and Wagner 1917; Pcreyma 1983; Wöjcik et al. 1983; Kalicki 1985; Marciniak et al. 1985; Gluza and Piasecki 1989), in Novaya Zemlya (Vize 1925; Shapaev 1959; Kanevskiy 1962; Kanevskiy and Davidovich 1968), in Zemlya Frantsa Josifa (Krenke and Markin 1973b), on the coasts of the Kara, Laptev, East Siberian, and Chukchi seas (Shapaev 1959); in Alaska (Glcdonova 1971), and in the Canadian Arctic (Defant 1951; Andrews 1964; Barry 1964; Müller and Roskin-Sharlin 1967; Jackson 1969; Gledonova 1971).

Other types of local winds, such as land and sea breezes, occur more rarely in the Arctic. Shapaev (1959) notes that breeze circulation occurs in July and August on the coasts of Arctic seas and on the coasts of largest islands. Jackson (1969) has noted the importance of the sea breeze influence for the high frequency of south-westerly winds during the summer at Tanquary Fiord in the Canadian Arctic. Sea breezes occur mainly in summer in the Arctic when there is a strong thermal gradient between the wann land and the relatively cold seawater. A detailed investigation of wind directions in Hornsund (Spitsbergen) shows that their frequency is greatest near noon and in the afternoon hours (Wöjcik and Kejna 1991), On the other hand, in winter the land breezes occur in places where warm open waters come into contact with the cold land coasts or pack ice (Maxwell 1980).

In the hilly and mountainous regions of the Arctic in the warm half-year, mountain (katabatic) and valley (anabatic) winds may also occur. On warm sunny days, the heated airs in a valley go up the axis of the valley. At night the process is reversed: the cold denser air from higher elevations flows down. When the glacier covers the upper part of the valley, then the glacier wind merges with the mountain wind. Glacier winds, which are special cases of mountain wind, occur both at night and during the day. During the day, however, the wind is weak and present only in a shallow layer over the glacier (usually < 2 m).

2.5.2 Polar Lows

As Turner et al, (1991) have stated, the investigation of polar lows docs not have a long history - only about 30 years. British meteorologists were the first to use the term "polar lows" to describe cold air depressions that affect the British Isles (Meteorological Office 1962). Busingerand Reed (1989) for mulated a rough definition of polar lows. This phenomenon may "denote any type of small synoptic or subsynoptic scale cyclonc that forms in a cold air mass polewards of major jet streams or frontal zones and whose main cloud mass is largely of convective origin." Typical horizontal resolutions of polar lows vary from a few hundred kilometres to more than 1000 km in diameter. Similarly, their intensities may range from moderate breezes to hurricane force winds (e.g. Reed 1979; Locatelli et al. 1982; Rasmussen 1981, 1983; Shapiro et al. 1987). They form over the oceans and decline rapidly on reaching land. In the Northern Hemisphere polar lows occur only in winter. More information about polar lows may be found in a book entitled Polar and Atvtic Lows (Twitchell et al. 1989).

From this description it may be ascertained that polar lows can significantly change the weather. In the Arctic, polar lows occur mainly in the Barents and Norwegian seas (Rasmussen 1985a, b; Shapiro el al. 1987), but have also been noted (although more rarely) in the Greenland Sea (Fett 1989), the Beaufort and Chukchi seas (Parker 1989), and in the Bering Sea (Businger 1987).

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