The effect of energy and moisture contents of the air mass on evaporation/condensation has
generally been overlooked, until Nyberg (1965) and Rylov (1969) made gravimetric measurements
of evaporation/condensation at the snow surface and reported that the daily vapor flux is governed
by the moisture content of the air mass. Hanaford and Howard (1975) dealt with an unusual
snowmelt event due to an extreme warm upper air temperature. Granger (1977) and Granger and
Male (1978) reported that the major melt-producing flux is due to radiation, and the advancement
of the melt is governed by the energy content of the air mass. McKay and Thurtell (1978)
confirmed the generalized results reported by Sverdrup (1936) and indicated the increase in the
extent of evaporation during the transition from a warm to a cold air mass.
In general, it is fair to state that from the scatter and the variability of the results so far reported,
based on a number of investigations under a variety of environmental and site conditions, the
measurements of reliable and consistent sensible and latent heat exchanges have proven to be
difficult even at well-instrumented sites. Furthermore, techniques or methods for extrapolating
results from a small local measurement to larger areas, after taking into account the variations of
elevation, latitude, state of the air mass, or topographical characteristics, have not been successfully
developed.
Accurate information on sensible and latent heat and net radiative heat flux along with the heat
transfer processes within the snow medium would be needed to calculate the snow surface temper-
ature. This is largely due to the fact that the snow surface itself is so ill-defined that any intrusive
measurement device, regardless of how small the sensing probe is, will disturb the surface struc-
ture, so it is uncertain whether the device is measuring the temperature of the snow grain or the air
temperature of the void space enclosed by the snow grains. Male and Granger (1981) stated that
investigations of large-scale or air mass influences on turbulent energy exchanges are likely to yield
more practical results than investigations that are confined to a few meters above the snow surfaces.
They also claim that research on air mass scale should give better insight into the factors governing
changes in sensible and latent heat flux, because such studies are aimed at the cause rather than at
an effect.
VIII. CONCLUSIONS
As indicated in section V, Experimental, the field site was chosen primarily for its convenience.
The site is small, it is elevated on the west and north, and it definitely does not meet the fetch length
requirement for eddy-correlation-type measurements. However, as indicated in section VI, Results
and Discussion, the plots u* vs. u2m (Fig. 4) and σw′ vs. u2m (Fig. 5a) are nearly identical to the
results reported by Kai (1982), who used more sophisticated and modern sonic anemometers
(three-dimensional) and a greater circular test field (160 m diameter). These surprisingly good
agreements give much credence to our sensible heat flux results.
As indicated in section VII, Comparison of Sensible Heat Flux, a number of investigators have
attempted to measure the sensible heat flux over a variety of terrains and environmental conditions
but, due to the nature of the constantly changing atmosphere, there is really no way to apply the
results obtained under a specific set of conditions to any specific combination of conditions. In
other words, it is not possible to derive formulas to predict the sensible heat flux for areas other
than the place or area where the measurement was made.
Though the sensible heat flux results obtained thus far from this study are rather preliminary and
limited, the expressions derived as shown in eq 83 and 84 can be easily used to estimate the sensible
heat flux as long as the mean wind speed and the mean air temperature at measurement height z are
given. Since the snow surface temperature (in fact, any surface temperature) can hardly be deter-
mined without the introduction of errors, there is no easy way to evaluate the temperature differ-
ence between surface and the height of the measurement. However, by looking at the definition of
38
Previous Page
