heights are also different. On the other hand, in this study, the ∆Ts are much smaller, i.e., on the
order of 2C (or even less), thus creating much less intense convective heat transfer.
As in this study, Tanner and Green (1989) reported an average sensible heat flux value of 49 W/
2. His field test was conducted 1417 August, and four sonic anemometers were used (one for
m
each post, but each installed at a different level). For a comparable period from May to August
(Table 4 or eq 84), and for the same Re value of 2.95 105, a heat flux of about 142 W/m2 can be
found, which is about three times higher than the value calculated from eq 83 for tests over snow
and frozen and partially frozen soil covered with thin ice. This value, however, is in quite good
agreement with the average heat loss over Arctic leads of about 250 W/m2 reported by Andreas
(1977). The data given by Tanner and Green were not sufficient to give a statistical analysis,
however, Andreas's data were taken from a test with a much more homogeneous field than the one
in this study.
In a study on atmospheric turbulent flux over snow, Hicks and Martin (1972) measured eddy
fluxes of momentum, sensible heat, and water vapor over Lake Mendota, Wisconsin, and reported,
under stable atmospheric conditions, an average sensible heat flux of 9 W/m2 for a wind speed of
~2.7 m/s and ∆T from surface to air at 2 m of approximately 3C. This average heat flux was
computed from a total of four 1-hour measurements, and flux varies from 4.2 to 11.9 W/m2. In this
study, for the period with snow or thin ice-covered surface, the heat flux was found to be on the
order of 25 W/m2, about twice the maximum value reported by Hicks and Martin. Since, as
indicated, the test site is quite inhomogeneous due to the limited fetch length and other obstacles, it
was very difficult to measure the sensible heat flux properly under stable atmospheric conditions.
Stable conditions usually prevail during a clear night with calm wind, but under these conditions
frost forms on the sonic head, inhibiting its sensitivity of measurement. Therefore, there are hardly
any reliable means to measure this low heat flux. Nevertheless, the sensible heat flux under stable
conditions for the period from May 1991 to April 1992 (Table 4) has a value of ~13 W/m2, nearly
identical to the value reported by Hicks and Martin (1972). It should be noted that even for a period
of nearly a year, there was not sufficient data to make a valid statistical analysis (note the near-zero
value of the correlation coefficient).
For unstable atmospheric conditions and for a grass-covered field, the sensible heat flux is found
to be approximtely 90 W/m2 (based on eq 84) for a wind speed of 2 m/s. Based on five sets of field
measurements for a range of 0.02 < |z/L| < 0.6, Dyer (1967) reported an average sensible heat flux
of H = 212 ∆θ3/2 where ∆θ is the potential temperature difference between heights of 1 and 4 m. In
this study, the value of ∆θ is on the order of 3C. Therefore, based on Dyer's expression, the
sensible flux is about ~600 W/m2, which is about seven times greater than 90 W/m2.
In a study involving measurements of evaporation and heat transfer in the lower atmosphere by
an automatic eddy-correlation technique, Dyer (1961) conducted a great number of measurements
of 5-min duration covering a 26-day period from January to March over a level pasture with wind
speeds varying from 1.96 to 7.28 m/s, with a mean wind speed of 4.2 m/s. Because of the short
response time of the sensing equipment (~0.3 s), he indicated that, for conditions of moderate and
high instability, significant deficiencies in eddy-flux measurements will occur only with wind
speeds exceeding 10 m/s. He reported an average sensible heat flux of 155 W/m2 (during his test
the wind speeds were mostly in the vicinity of 4 m/s; in only seven of the 26 tests were the wind
speeds less than 4 m/s). Using 4 m/s as the mean wind speed in eq 84 (a correlation from this
study), a heat flux of 180 W/m2 results, which is in remarkably good agreement with the value of
155 W/m2 reported by Dyer. He further stated that with measurements at a height of 4 m, minor
surface irregularities of up to several tens of centimeters are clearly of no consequence, and, if
sufficient fetch is available, great irregularities could be readily accommodated by increasing the
height and period of observation. For stable conditions, Dyer (1961) reported a heat flux value of
~14 W/m2, which is nearly identical to the value reported by Hicks and Martin (1972).
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