using a large array of thermometers and tether-
the surface. Inversions and diminished tempera-
sondes. They were able to successfully model tem-
tures were found over smaller nearby basins with
peratures over snow-covered slopes and basins
only a few centimeters of snow cover. We do not
through application of these observations. We
have sufficient data to propose a climatically con-
have been able to reproduce their observed re-
clusive value of temperature decrease with re-
sults over diverse terrain, using a moving probe.
spect to snow cover, but offer typical differences
of 2 to 4C in small basins relative to surround-
We found that temperature differences with
respect to the river plane reference temperature
ings when snow was sparse or nil. Differences of
as much as 18C were found with respect to ridge-
defined at the Piermont (P) bridge increased
greatly with both distance and elevation along
tops over thicker snow cover. There were several
the hilly terrain east of the river. Temperature
instances when air temperatures diminished more
differences did not increase monotonically with
in the valley although cold advection continued
elevation, even when temperature lapse was the
at the ridgetops, and many more instances when
dominant inferred vertical structure. Inversion or
subzero or diminished temperatures persisted for
isothermal structure was intermittently present
another morning in the valley after warm advec-
over small basins, even under general tempera-
tion began on the ridgetops. There is little doubt
ture lapse structure. The inversion strength in-
that snow cover increased the frequency of colder
creased as snow covered these small basins, and
air temperature in this way, but specifying a cli-
exceeded 40C/km when the valley was under
matic magnitude would be speculation.
general inversion structure. Nakamura and
The coldest air was most frequently measured
Magono (1982) and Magono et al. (1988) found
in basin II in the vicinity of W. Temperature ex-
low inversions formed first over basins, which
ploration around the area found temperature in-
prevented warmer tropospheric air above from
flections or inversions at this elevation along
exchanging to the basin surface. This allowed the
several slopes. In other seasons the top of valley
basin to cool to lesser temperature than surround-
fog was frequently observed just below the eleva-
ing slopes where exchange of warm air from above
tion of W. All of our tabular and graphical analy-
offset radiational heat loss from the snow surface.
ses show a discontinuity at this level. We propose
Magono's group worked in a relatively large
that an inversion persistently formed at about 100
basin over 100 cm or more of snow. We observed
m above river level, that is, at an elevation of 220
that temperature inversion formed over these
to 250 m. The coincidence of this inversion, with
small basins along slopes when snow cover was
basin II, allowed this basin to cool to lesser tem-
sparse or nil, and that as these small basins be-
peratures than nearby smaller or larger basins on
came snow covered, the coldest local air tempera-
many nights.
ture was observed in these basins. The lower
The geometric standard deviation of tempera-
portion of the larger Connecticut River Basin had
ture difference, with respect to the temperature
lapse structure just above the surface on most
over the river at bridge P, was systematically
days of sparse or thin snow cover even when
greater at all observing points when inversion
strong inversion was present 100 m above river
was present. Figure 12 shows that under these
elevation. A surface-based inversion was predomi-
inversion conditions, small basins have lesser geo-
nantly present in the lower 100 m of the basin
metric standard deviation of temperature differ-
when 20 cm or more of snow cover was present.
ence than slopes or ridge tops. We interpret this
Baker et al. (1990, 1991) concluded that 5 to 15
difference in GSD as a measure of the decoupling
cm of snow cover was sufficient to mask agricul-
of basins from exchange with the warmer tropo-
tural surfaces. They attributed an 8.4C tempera-
spheric air above. Ridges and slopes remain
ture diminution to snow cover. Our observations
warmer because of greater air exchange. We pro-
indicated that 20 cm of snow was necessary to
pose that the GSD of temperature difference pro-
provide a continuous snow surface that allowed
vides an objective analytical index of the relative
an early and uniform inversion to form over a
amount of air exchange that occurs at a selected
large basin in more complex topography. This
location. Combining the moving probe tempera-
uniformity was evidenced by smoke drift above
ture measuring technique with analysis of the
chimneys indicative of 1-m/s wind by Beaufort
geometric standard deviation of observed tem-
analogy, and in agreement with Magono's pro-
peratures might provide a relatively rapid and
posal that diminished surface (1.25-m) tempera-
objective insight to exchange and dispersion pa-
ture accompanies winds of 1 m/s about 9 m above
rameters in a data-sparse winter environment.
28
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