Temperature Inversions

Surface-based temperature inversions in the troposphere are one of the main features of the Arctic climate, particularly in the low-sun (or no-sun) periods. This differs from normal tropospheric conditions, in which temperature decreases with height from the surface. Because of the very high frequency of the temperature inversions in the Arctic in the annual march, the term 'semi-permanent inversion' is often used. Outside the Polar regions, the semi-permanent inversions occur only in the subtropical belt. In the latter areas, however, the inversions are solely dynamic and are separated from the surface by a highly unstable layer. The polar inversions are generally caused by the negative net radiation balance at the surface. The presence of the Arctic temperature inversions is closely related to the snow and ice surfaces dominant in the area studied. However, in the Arctic there are also upper tropospheric inversions, which arc a result either of subsidence of the air in anticyclones or of a warm air advcction over underlying cold air masses. One can agree with the view expressed by Vowinckel and Orvig (1970) that the Arctic inversions are a significantly more complex phenomenon than the subtropical type. They distinguished three categories of temperature inversions in the Arctic: surface, subsidence, and advection. The latter two can occur at any height and arc characterised by significantly smaller gradients.

Investigations of the thermal structure of the troposphere in the Arctic began at the beginning of the 20,h century. In 1906 balloon investigation of the upper levels was sponsored by the Prince of Monaco (Hergessell 1906). Belmont (1958) gave an excellent review of the history of investigations of the thermal structure of the troposphere in the Arctic carried out up to about

1957. Thus, there is no need to reiterate this information. Of the more important works which appeared in 1960s and 1970s one should mention Dolgin 1962; Gaigerov 1964; Stepanova 1965; Vowinckel 1965; Billeo 1966; Vowinckel and Orvig 1967, 1970; Dolgin and Gavrilova 1974. In the 1980s and especially in the 1990s investigations of the Arctic were intensified (Maxwell 1982; Kahl 1990; Nagurnyi etal. 1991; Timerev and L'gorova 1991; Bradley et al. 1992; Kahl et al. 1992a, b; Scrreze et al. 1992; Zaitscva et at. 1996). These efforts were undertaken in order to establish and describe the climatology of the temperature inversion. Such knowledge allows researchers to ascertain whether there have been changes in recent years when global warming and greenhouse gas build-up is present.

The characteristics of Arctic temperature inversions are presented here mainly according to the results published recently by Zaitseva et al. (1996). They analysed radiosonde data on air temperature (from surface to 3-5 km) over the western Arctic Ocean, which were made during the U.S. Air Force Ptarmigan weather reconnaissancc missions (1950-1961) and at the Soviet North Pole drifting stations (1950-1954). For more detailed investigation they have chosen two areas representing contrasting conditions: one situated near the North Pole (1079 soundings) and second in the Beaufort Sea (2040 soundings). The results of inversion frequency, height of inversion base, inversion thickness, and temperature difference across inversion are presented in Figure 4.14. It can be seen from this Figure that temperature inversion occurs with a very high frequency (93%). As would be expected, the highest inversion frequency occurs in winter (98-99%). For clarity, seasons are defined normally (Dec.-Feb., March-May etc.), not as has been done by Zaitseva et al. (1996) and for the characteristics of mean conditions median values were used. Most winter inversions (about 75%) begin at the surface. In the Western Arctic they are most common in February (100%). Zaitseva et al. (1996) did not find any significant regional differences in winter inversion frequency. This is probably connected with similar weather and surface conditions. The change from winter to spring conditions is very abrupt in the April-May period (similar results were obtained by Vowinckel and Orvig 1967), The frequency of surface-based inversions in May decreased to about 45% and 36% near the North Pole and the Beaufort Sea, respectively.

In summer, the lowest total frequency of inversions (88%), but the highest frequency of the upper inversions (52%) is observed (see Figures 4.14a and b). A significant part of surface-based inversion during this period is connected with the loss of energy used for the melting of snow and ice. In some stations, the secondary maximum of surface-based inversions can be seen (see Figure 4 in Bradley et al. 1992). The summertime minimum of surface-based inversions (36%) is, according to Zaitseva et al. (1996), connected with the highesl cyclonic activity at that period, which causes an intense mixing of the lower atmosphere and results in high cloudiness. In the annual march, the lowest frequency of surface-based temperature inversions occurs in August near the North Pole and in September in the Beaufort Sea, i.e., during the periods occurring just after the end of the summer melting. These results arc generally in line with those published by Bradley et al. (1992).

North Pole Beaufort Sea

North Pole Beaufort Sea

Figure 4.14. Monthly frequencies of (a, b) inversion, (e, d) cumulative frequency distribution of the height of the inversion base, (e, f) its thickness, and (g, h) temperature difference across the inversion for the North Pole (a, c, e, g) and the Beaufort Sea (b, d, f, h) sectors of the Arctic (after Zaitseva el at. 1996). Heights of the wide and narrow parts of the bars in Figures a and b denote the frequency of occurrence of surface-based and upper tropospheric inversions, respectively, The combined height of bars is equal to the total inversion frequency.

Figure 4.14. Monthly frequencies of (a, b) inversion, (e, d) cumulative frequency distribution of the height of the inversion base, (e, f) its thickness, and (g, h) temperature difference across the inversion for the North Pole (a, c, e, g) and the Beaufort Sea (b, d, f, h) sectors of the Arctic (after Zaitseva el at. 1996). Heights of the wide and narrow parts of the bars in Figures a and b denote the frequency of occurrence of surface-based and upper tropospheric inversions, respectively, The combined height of bars is equal to the total inversion frequency.

The height of the inversion base (Figures 4.14c and d) is lowest during the winter (surface-based) and highest in summer near the North Pole (up to about 600 m). In the Beaufort Sea the maxima of the inversion base are observed in May (up to 600 m) and in September (up to 650 m). In summer, due to melting, a significant drop of the inversion base can be seen. It reaches an average level of about 125 m. The inversion base returns to the surface in October (the Beaufort Sea) and in November (near the Pole).

The average thickness of the inversion near the North Pole is about 100 m higher (900 m) than in the Beaufort Sea (about 800 m). In both areas the maximum thickness very often exceeds 1200 m and more rarely 1600 m. On particular days, however, the inversion depth can even reach more than 3000 m

(sec Table 5 in Bradley et al. 1992). The greatest average thickness of inversion is noted near the Pole in February and December, and in the Beaufort Sea, in January. In the latter area, the inversion depth in December and February is only slightly lower. In spring, as a surface mixing layer forms in response to increased solar radiation and extensive cloud cover, the lower parts of inversion layers are destroyed. As a result, a significant decrease of the inversion depth is noted. The mean inversion thickness near the Pole is at its lowest in summer and autumn and slightly exceeds 400 m. In the Beaufort Sea, the situation is very similar when mean seasonal conditions are taken into account, but significantly a lower inversion depth occurs in two autumn months - September and October. Bradley et al. (1992) have obtained the same results for Barter Island and Point Barrow stations. Of course, melting processes, which increase the inversion thickness in summer, cause this situation. An abrupt rise of the inversion thickness is noted from November to December (near the Pole) and a month earlier in the Beaufort Sea (see Figures 4.14e and 0-

Temperature changes across the inversion layer (called inversion intensity or strength) are highly correlated with inversion thickness and inversely related to surface temperature (see Figs 6 and 7 in Bradley et al. 1992). As can be seen from Figures 4.14g and h, the inversions are strongest in winter months when surface temperatures are the lowest. The average temperature difference at this time oscillates between 7°C and 8°C. The highest noted values (> 15°C) occur in both areas in February. On particular days, the temperature across the inversion can significantly exceed 30°C (see Table 5 in Bradley et al. 1992), For example, on Barter Island on January 25, 1983 it reached 35.7°C (6.7°C/100 m). Such a situation most often occurs when lower tropospheric wanning due to a subsidence (upper inversion) merges with the strong surface-based radiation inversion. The intensity of inversion significantly decreases in April and May. Near the North Pole, the weakest inversions (about 3^t°C) occur in the period June September. Then a gradual increase of intensity towards the winter maximum is observed. On the other hand, in the Beaufort Sea, a relative flat minimum (3-4°C) occurs from May to October. The mean monthly differences across the inversion in the mentioned periods very rarely exceed 8°C.

It has been shown that the surface-based inversions are most frequent and intensive during clear sky periods in winter. In turn, the upper inversions show an opposite pattern; they are most pronounced in summer and are connected with great amounts of cloud. The intensity and thickness of the upper inversions are considerably less: 12°C and 0.5-0.9 km on average (Vowinckel and Orvig 1970). The mean duration of this inversion is also significantly lower in the cold half-year, but greater in summer.

The lowest frequency of surface-based inversions in the Arctic occur in the southernmost part of the Atlantic region, oscillating between 20% and

30% in January and between 30% and 40% in July (sec Figure 30 in Dolgin and Gavrilova 1974). Thus the annual cycle of frequency inversions occurring in other parts of the Arctic is reversed in this area. This is connected with the strong cyclonic activity which is common here as well as with the character of the surface (open, relatively warm water). Dolgin and Gavrilova (1974) showed that the distribution of surface-based inversion closely correlates with the mean conditions of atmospheric air pressure and the characteristics of the surface. The highest inversion frequency in the Arctic occurs in the areas where anticyclones and snow and pack-ice dominate. On the other hand, both frequent cyclone occurrencc and a surface not covered by snow and sea ice significantly reduce the inversion frequency. However, we must add that the inversions are not only characteristic features in anticyclone systems, but in cyclones as well. The mean annual frequency of inversions in cyclones was found to be 69% (Dolgin I960; Gaigerov 1962). In the northern sections of the cyclones, the frequency of inversions was greater than in the southern parts.

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    What is temperature inversion?
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    Can temperature inversion occur in polar area?
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    Which conditions are important in establishing a temperature inversion?
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    HOW THERMAL INVERSION OCCURS IN ARCTICNIN?
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    Why is a surface temperature inversion associated with an Arctic high?
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    What is the main characterisitic of a temperature inversion?
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