Longwave Net Radiation

Long-wave net radiation (so-callcd effective radiation) is a residual of two fluxes: terrestrial radiation (upward infrared radiation) and the "counter-radiation" of the atmosphere (downward infrared radiation). The main factors determining effective radiation are air temperature and humidity, temperature of the surface, stratification of the atmosphere, and cloudiness (cloud amount and type, height and physical properties of clouds). Counter-radiation plays a very important role in the Arctic, especially in the winter when insolation becomes negligible. Vowinckel and Orvig (1970), however, have also shown that this component is dominant in summer, too (Table 3.1),

Tahle 3.1. Per cent contribution by insolation and counter-radiation to total surface radiation income in June (after Vowinckel and Orvig 1970)

Type of radiation

Latitude (°N)

65

70

75

80

85

90

Long-wave (%)

60

64

68

69

69

69

Short-wave (%)

40

36

32

31

31

31

Long-wave net radiation and its elements are very rarely measured in Arctic actinometric stations, and, as Marshunova and Chernigovskii (1971) write, such observations were not made in the non-Soviet Arctic at all. Therefore, our knowledge about effective radiation comes mainly from computations.

Figure 3.6. Mean totals of effective radiation in the Arctic for March, June, and September (in kcal/cmVmonth) and for the year (in kcat/cmVyear) (after Marshunova and Chernigovskii 1971).

From November to March, the effective radiation over the open water surface in the Barents and Norwegian seas and on the eastern coast of Greenland is twice that in the coastal regions (4-5 kcal/cinVmonth [16.7 20.9 kJ/ cm3/month] and 2-2.25 kcal/cmVmonth [8.4-9.4 kJ/cmVmonth]). In March, the effective radiation near the North Pole is only a little lower than in the southern part of the Arctic (Figure 3.6). In the warm half-year, the effective radiation in the central part of the Arctic is twice as low as in winter, while in the rest of the Arctic the decrease is not so big (compare especially the continental parts of the Arctic). On an annual basis (Figure 3.6), the effective radiation is lowest in the vicinity of the North Pole (20 kcal/cm2 [83.6 kJ/cm2]). According to Marshunova and Chernigovskii (1971) this is a consequence of the low surface temperature in winter and high cloudiness in summer. In the rest of the Arctic, the effective radiation rarely exceeds 30 kcal/cm2 (125.4 kJ/cm2) reaching a maximum (> 40 kcal/cnr [167.2 kJ/cm3]) in a small area located to the west of Spitsbergen (all-year open water connected with the warm West Spitsbergen Current). Positive values of effective radiation mean here that the terrestrial radiation is greater than the counter-radiation of the atmosphere.

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