Snow ablation and melt

The rate of snow melt is dependent on energy availability, which is mostly in the form of radiation [12]. Cold snowpacks have a negative energy balance, but warming causes the snowpack to become isothermal (0^oC) and additional energy results in positive energy balance and melt. Daily snow melt in forested areas is considerably less than melt in open areas, as forests protect the snow cover from solar radiation and wind. Canopy warming can increase longwave radiation, but the net effect of forest is reduction in melt. Rain falling on snowpack may accelerate its melt rate, but intense sunshine of late spring and summer is the principal melting energy source.

Most operational procedures for snow melt prediction rely on ambient air temperature as the index of the energy available for melt. The temperature index is usually used to characterize the level of the energy balance because it is superior to other simple methods for the full energy balance at the snow surface [5]. The most common expression relating snow melt to the temperature index is:

M = C_m(T_air - T_melt),

where M is the daily snow melt (mm/day), C_m is the melt rate factor (mm/^oC per day), T_air is the daily ambient temperature (^oC) and T_melt is the threshold melt temperature (^oC). The critical melt temperature is often set to 0 ^oC but can be optimized for a particular location.