Priestley (1959), on the other hand, points out that for an air layer being heated by radiation the
sum of the radiant and turbulent fluxes is essentially constant with height; therefore, to estimate
energy transfer over snow, one must ensure not only that the measurements of temperature and
humidity are made above the level of the raised maximum temperature, but that a simultaneous
radiation measurement is also included. Priestley suggested that we must be content with the total
surface energy transfer rather than that of the individual surface fluxes.
Large-scale parameters, such as topography, altitude, season, and air mass, were considered in
Male and Granger's (1981) analysis of snow surface energy exchange. For brevity and comparason,
they expressed their summarized results in percentages of the total energy input, indicating not only
different values from year to year at the same site, but also making it impossible to state which
particular energy flux will dominate overwhelmingly or be negligible in any particular environ-
ment. They also indicate that not only topography but other factors such as the air mass and large-
scale circulation affect the process of energy transfer. Therefore, it is difficult or even dangerous to
classify sites in general as being either open, forested, or alpine, since physically similar sites with
different geographical placement will necessarily have different air mass histories. For snow-
covered prairies and large open sites, theoretical approximation expressions derived after taking
into account the influences of stability and the possibility of radiative heating of the air above the
snow can be used directly to estimate the energy fluxes at the surface. In a review study of
snowmelt in a prairie environment, Male and Granger (1979) developed a number of empirical
methods of estimating energy fluxes and their accuracies and noted that the air mass exerts a
controlling influence on the relative magnitude of the various energy fluxes.
In the case of forest cover, turbulent flux measurements have received little attention either
because wind, temperature, and humidity measurements have been difficult to obtain or because
these fluxes have been ignored in the assumption that reduced wind speed results in near-negligible
energy fluxes. This may be true in the region of northern Quebec, for Hendrie and Price (1978)
pointed out that radiation alone provides an efficient predictor of the snowmelt, but farther south
turbulent exchange is found to be more significant (at least during the melting season) because
snowmelt is usually initiated by the movement of a warm air mass into the region. To estimate the
advective component of snowmelt Maf under a forest canopy compared to the corresponding part of
the Ma of an open locality, Kuz'min (1961) developed
Maf = (0.44 0.43 p2)Ma
(95)
for coniferous and
Maf = (0.45 0.15 p2)Ma
(96)
for deciduous, where p is the crown density of the canopy. Doty and Johnson (1969) reported 30%
and 53% reductions in evaporation rates within aspen and conif erous stands, respectively, as
compared with an open site.
Application of standard aerodynamic formulas to predict turbulent fluxes over a forest canopy
has been reported by Federer and Leonard (1971) to be inappropriate, based on the fact that in
subcanopy flow the sources of mechanical turbulence and sensible heat flux are distributed verti-
cally through the canopy. Brutsaert (1979) formulates bulk mass and heat transfer coefficients for
canopy flow but, as pointed out by Federer and Leonard (1971), the transfer characteristics are
likely to be quite different.
In the case of an alpine environment, Obled and Harder (1979) reviewed studies of snowmelt in
a mountain environment and listed a number of factors affecting the turbulent transfer of heat and
water vapor over snow. Wind and temperature regimes in the mountain environment are complex
and make the analysis of the boundary layer on the basis of local slope winds difficult. Martin
(1975) provides a formulation for the katabatic wind profile and applies it to the calculation of the
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