Climate, Snow Cover, Microclimate, and Hydrology 49 



The net longwave flux is negative for each of the seasons and for the 

 year (Table 2-3). Positive net incoming longwave flux occurs only occa- 

 sionally when warm stratus clouds are advected over a cooler tundra sur- 

 face. The loss of longwave radiation from lakes is less than that from the 

 tundra during June, because of residual ice and the lower temperatures 

 of the lakes. However, the reverse is true from July to October as a result 

 of higher temperatures due to the greater absorption of insolation. 



Between October and April the radiation balance is dominated by 

 the negative net longwave radiation; near the end of May the balance be- 

 comes positive (Figure 2-11). After snowmelt, when albedo has decreased 

 dramatically, the net shortwave gain exceeds the net longwave loss by a 

 factor of four (Maykut and Church 1973). Maximum average net energy 

 receipt of about 12 MJ m"' day' occurs during the period 15-19 June. 

 After mid- August, the absorbed radiation gradually decreases with de- 

 creasing day length and solar altitude, becoming negative again in late 

 September-early October. 



Energy Balance 



The partitioning of the net radiation at the surface is described by 

 the energy-balance equation: 



R„ + H + L + G = 



where R„ = net radiation 



H = net exchange of heat with atmosphere by conduction/ 



convection (sensible heat flux) 

 L = net exchange of latent heat with atmosphere (vaporization 



and latent heat used in melting) 

 G = net exchange of heat with snowpack and/or soil. 



Estimates of average energy-balance components for the coastal 

 tundra at Barrow were developed from several sources, and are summar- 

 ized in Table 2-4. Differences in the various values probably reflect real 

 differences in the energy balance between 1957-58, 1962-66 and 1971-72, 

 but also include variations due to different measuring techniques as well 

 as procedures for the calculations. 



The energy used in melting snow is about 35 MJ m"^ yr'', assuming 

 that a snow water equivalent of 106 mm (see below) is melted annually. It 

 is generally assumed that there is no net heating or cooling of the ground, 

 so the average annual value of represents energy used in warming the 

 snowpack. This assumption is consistent with the findings of Kelley and 

 Weaver (1969) at Barrow; however, Lachenbruch and Marshall (1969) 



