Global solar radiation is one of the important factors in the formation of the radiation regime, weather, and climate. Its role in both net radiation and energy balance is crucial. Luckily, this component of the radiation balance is very easy to measure. From the reasons mentioned above, global solar radiation measurements are most often conducted in actinometric stations, not only in the Arctic. In spite of this, the network of stations is still insufficient for analysing the field of global solar radiation. Therefore, authors presenting the spatial distribution of this (or any other) component of radiation balance have to obtain additional information from calculations. There are two methods of determining the values of the global radiation (Marshunova and Chernigovskii 1971). The first method uses existing radiation data from some stations and established relationships between radiation and other meteorological elements such as cloudiness and sunshine duration. The second method, the so-callcd analytical method, uses knowledge concerning processes influencing the radiation both outside the atmosphere and on its way through atmosphere to the Earth's surface.
The most comprehensive and detailed information about global solar radiation for the Arctic in general or for specific parts of it is contained in the following works: Gavrilova 1963; Vowinckel and Orvig 1964b, 1970; Chernigovskii and Marshunova 1965; Marshunova and Chernigovskii 1971; Gorshkov 1980; Maxwell 1980; Atlas Arktiki 1985; McKay and Moms 1985 and Khrol 1992. These authors present various maps showing both monthly and annual mean sums of this element. Below, the main features of the distribution of global solar radiation in the Arctic for four months representing all seasons, and for the year as a whole, are described based on the recently published Atlas of the Energy Balance of the Northern Polar Area (Khrol 1992). The authors of these maps are Girdiuk and Marshunova.
In January, the polar night occurs in the greater part of the Arctic and the radiation flux is naturally zero. According to Gavrilova (1963), the zero isoline approximately follows the 7t°N latitude, while on Girdiuk and Marshunova*s map this isoline is shifted to about 68"N. It should be noted that all captions under figures presenting global radiation in the English translation of Gavrilova's book (Gavrilova 1966) are wrongly positioned. Most southern parts of the Arctic receive no more than 2 kJ/cm2,
In April, the mean sums of global solar radiation oscillate from 50 -53 kJ/cm2 in the soulhern part of central Canadian Arctic and in the southern inner fragment of Greenland to 25-29 kJ/cm2 over the Norwegian and Barents seas, where the cloudiness is highest. In the central part of the Arctic Ocean the incoming radiation changes from 35 kJ/cm2 (in the vicinity of the North Pole) to about 38 kJ/cm2 at the 80-85°N latitude (except in the part of the Arctic neighbouring the Norwegian and Barents seas) (Figure 3.2). Please note the similarities in pattern distribution of sunshine duration and global radiation in the Arctic (compare Figure 3.1 and Figure 3.2). The contribution of April to the annual influx of radiation is substantial at around 13-15% (Marshunova and Chernigovskii 1971).
In July, with the decreasing altitude of the sun, the global radiation fluxes are about 1.2-1.4 times lower than in June. Moreover they are reduced by the considerable increase of cloudiness in July. Clearly the highest sums of global radiation (84-85 kJ/cnv) are received in the northern half of Greenland. The secondary maximum occurs in the Canadian Arctic (except its western part) and in the Arctic Ocean neighbouring the Beaufort and Chukchi seas (59 -63 kJ/cm2). Almost the entire Atlantic region receives < 50 kJ/cm1. The absolute minimum (40^42 kJ/cm2) of incoming radiation occurs in the areas to the south and south-west of Spitsbergen (Figure 3.2), where mean cloudiness is the highest and clouds are most dense in the Arctic. The contribution of July to the annual flux of radiation is 17-19%.
In October, the pattern of global radiation distribution is very simple and depends mainly on the length of the days. Therefore, the ran of the isolines is more or less zonal. For example, in the region surrounding the North Pole, where the polar night has already begun, the zero isoline passes close to 83 N. The latitudinal band (73-75°N) receives about 4 kJ/cm2. The greatest sums of global radiation (> 10 kJ/cm2) are received in the southern parts of the Canadian Arctic and Greenland.
On an annual basis, the global radiation distribution pattern closely resembles the atmospheric circulation and cloudiness distribution patterns. The parts of the Arctic which have the greatest cyclonic activity and cloudiness (mainly Atlantic region) receive the lowest totals of global radiation (< 250 kJ/cm3). On the other hand, the southern part of the Canadian Arctic and Greenland (central part), where anticyclonic circulation prevails and the lowest cloudiness occurs, reccive more than 350 kJ/cm2 and even 400 kJ/cm2 (Figure 3.3a).
The values of global radiation in particular years may be different from the average conditions presented here. However, as reported Marshunova and Chernigovskii (1971), the mean deviations of the monthly sums of global radiation oscillate mainly between 8% and 12%. Only extreme deviations sometimes reach up to 30%. To reliably describe the radiation regime in the Arctic, at least five years of observations is needed (Marshunova and Chernigovskii 1971).
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