|Dewalle, David - PENN STATE UNIVERSITY|
Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: June 1, 2008
Publication Date: July 1, 2008
Citation: Dewalle, D., Rango, A. 2008. Snowpack energy exchange: Basic theory. In: Dewalle, D., Rango, A., editors. Principles of Snow Hydrology. Cambridge, NY: Cambridge University Press. p. 146-181. Interpretive Summary: No interpretative summary required.
Technical Abstract: The exchange of energy between the snowpack and its environment ultimately determines the rate of snowpack water losses due to melting and evaporation/sublimation. Energy exchange primarily occurs at the snowpack surface through exchange of shortwave and longwave radiation and turbulent or convective transfer of latent heat due to vapor exchange and sensible heat due to differences in temperature between the air and snow. Relatively small amounts of energy can also be added due to warm rainfall on the snowpack surface and soil heat conduction to the snowpack base. Changes in snowpack temperature and meltwater content also constitute a form of internal energy exchange. Melting usually represents the major pathway for dissipation of excess energy when the snowpack ripens and becomes isothermal at 0'C (see Chapter 3). The energy budget for the snowpack can be written as the algebraic sum of energy gains and losses as: Qi = Qns + Qnl + Qh + Qe + Qr + Qg + Qm <p>Equation 6.1 where Qns = net shortwave radiant energy exchange (> 0), Qnl = net longwave radiant energy exchange ('), Qh = convective exchange of sensible heat with the atmosphere ('), Qe = convective exchange of latent heat of vaporization and sublimation with the atmosphere ('), Qr = rainfall sensible and latent heat ('0), Qg = ground heat conduction ('), Qm = loss of latent heat of fusion due to meltwater leaving the snowpack (<0), Qi = change in snowpack internal sensible and latent heat storage ('). Each of these energy exchange terms is commonly written as an energy flux density expressed as energy exchange per unit surface area and per unit of time. Flux density is generally expressed as J s-1 m-2 or W m-2. In older literature, units of cal cm-2 min-1 or ly min-1 were used where 1 ly (Langley) = 1 cal cm-2. All terms except for Qns, Qr and Qm can represent either energy gains or losses to the snowpack depending upon the time interval involved, as indicated by the signs given in Equation 6.1. Several reviews of the snowpack energy budget have been previously given. Early comprehensive works on the snowpack energy budget by the U. S. Army Corps of Engineers (1956) and Kuz’min (1961) were followed by more recent review papers and book chapters by Male and Granger (1981), Male and Gray (1981), Morris (1989), and Nakawo et al. (199?), Singh and Singh (2001). Although the processes of snowpack energy exchange are reasonably well understood, computation of snowmelt and/or vapor losses are often made difficult by the lack of on-site snowpack and meteorological data. The physics behind computations of each energy budget component with example calculations are given in this chapter for a horizontal snowpack in open conditions. Approximate analysis of snowmelt and cold content using temperature indices are also included.