46 



S. L. Dingman et al. 



TABLE 2-3 Annual Radiation Balance Compo- 

 nents over the Coastal Tundra at 

 Barrow (MJ m'^ yr'^) 



Component 



Land Lakes Average* 



Incoming shortwave radiation, Q 3200 



Net longwave radiation, 04 -0t -1000 



Net radiation, R^ 450 



Albedo, a 0.55 



♦Assuming about 50% of the surface is land and 50% is lakes. 

 Source: Maykut and Church (1973). 



A positive sign indicates a gain of radiant energy at the surface. The 

 energy unit used here is the joule (1 J = 0.239 cal), and balance compo- 

 nents are expressed in MJ m'^ per unit of time. Table 2-3 gives average 

 values of the components of the radiation balance over the coastal tundra. 



Insolation on the coastal tundra is markedly affected by cloud 

 cover, which averages 68% at Barrow (Figure 2-9). Because of this, most 

 of the shortwave radiation received is diffuse rather than direct (Maykut 

 and Church 1973). The cloud cover increases from 40'yo in winter to 85% 

 in summer, causing a pronounced skewness in the annual insolation 

 curve (Figure 2-10). 



An effective (weighted average) albedo is about 0.55 for the tundra 

 surface and 0.52 for lakes (Table 2-3). One of the most critical factors in 



100 



Clear 



Partly Cloudy 

 ^40-^/10 



Cloudy 



^40-% 



FIGURE 2-9. Cloud cover conditions (bars) and 

 transmissivity for coastal tundra. Transmissivity from 

 Maykut and Church (1973), cloud cover from University of 

 Alaska (1975). 



