A. Collection and analysis of data
Sonic data were collected with a 21X Micrologger based on a sampling rate of 10 per second
and an averaging period of 10 minutes (mostly). Six thousand readings were used to evaluate each
data point shown in the following figures.
Instantaneous vertical wind speed, fluctuating temperatures, and the horizontal wind speed
measured by the hot-film anemometer were collected at 10 samples per second but were discarded
after the 10-minute average values were evaluated. The micrologger was programmed to receive
and compute the reference temperature; the fine thermocouple reading at the measurement height;
the average vertical wind speed w ; the mean of the fluctuating temperature T ; the mean of the
horizontal wind speed u ; the standard deviations of the vertical wind speed σw′ = w′2 , the
vertical temperature σT = T ′2 , and the horizontal wind speed σ u′ = u′2 ; and the covari-
′
ances of w′ T ′ and u′ w′ .
Two typical sets of collected and computed values are shown in Table 1 and Table 2. Table 1
shows file number 9107101440, which covered a period of about 8 hr with 10-min data average
output. Table 2 shows file number 9107160815, which covered a period from 19:40 p.m. EST on 15
July, 1991, to 8:10 a.m. the next morning. The first and last 10-minute data were not used for
subsequent analysis, to ensure elimination of unsteady effects. Table 1b and Table 2b show the
friction velocity u*, the Obukhov length L, the ratio of σw′ / u*, the stability criterion z/L, and Ta,
the mean absolute air temperature. The values of T , u , σw′ , w′ T ′ , and u′ w′ are used to define the
values of u*, L, and σw′ / u*. Looking at Table 1b, and based on the values of z/L, the atmosphere
was clearly unstable during the entire measurement period. The mean wind speed u varied from a
minimum of 1.46 to a maximum of 4.31 m/s. Since we did not have the means to determine and
record the wind direction during the measurements, the random obstacles surrounding the test site
caused a great variation of turbulence characteristics affecting the value of σw′ = w′2 , which is
a measure of the intensity of the vertical fluctuating velocity. Indirectly it also affected the sensible
heat flux (i.e., ρ cp w′T ′ and the value of u*, which is calculated from (- u′w′) .
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Contrary to the results listed in Table 1b, the data shown in Table 2b are rather erratic. Since this
set of data was collected over a period from the evening to the next morning, the atmosphere was in
either a stable or a slightly unstable state. This phenomenon was evidenced by the values of w′ T ′ ,
which are mostly near zero and associated with alternate positive and negative signs (a negative
sign indicates the heat is extracted from the air and the atmosphere is stable). In cases when u′ w′
had positive values, the data were eliminated along with the cases when the value of L was 0,
infinity, or a ratio of 0/0. It should also be noted that the values of u , σw′ , σ u′ along with w′ T ′ and
u′ w′ , are much smaller in magnitude when compared with those shown in Table 1b. This is
because the atmosphere is near neutral stability, and the transports of heat and momentum are
mostly dominated by molecular processes.
In all, 47 sets of sonic heat flux measurements were made over the period from May 1991 to
April 1992. Most of the measurements were conducted from middle (or early) morning and ended
in late afternoon, depending on the weather conditions. Since the sonic anemometer cannot be used
under wet conditions, no tests were conducted on snowy, rainy, or windy days to ensure that no
damage would be done to the sonic probe and the fine-hair thermocouple.
′
σ u with mean wind speed u2m , as well as the variations of σ w and σ u with u* (to avoid over-
′
′
′
crowding the data points and the difficulty in using different notations for different sets of data).
Only six sets of data (91061345, 9106271405, 9107011334, 9107031408, 9107101440, and
9107301320) were plotted and only full dot notation was used. These plots were compared with
those reported by Kai (1982) in his study of statistical characteristics of turbulence and the budget
of turbulent energy in the surface boundary layer. Kai's measurement was conducted at a site that is
circular in shape with a radius of 80 m and covered by 0.5-m-tall grass. The 30-m tower was erected
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