served that a few millimeters of snow on ice pre-
covered ground to the terrain related microcli-
mate concepts of Geiger (1965) poses several ques-
sents a uniform radiative plane representative of
tions relative to a priori estimates of the
a snow surface.
temperature structure of overlying air.
The reference plane is a layer of air just above
the lowest elevation in the observation area, and
it can radiate to a larger sector of sky than a point
along a slope or in a smaller basin. Cold air does
A RIVER PLANE
not drain from the reference plane, but the air
temperature can continue to diminish through
We propose that the air over a still river pro-
exchange with the surface and radiation to the
vides a relatively uniform surface reference plane
sky. Comparison of nearby surface temperatures
that can be used to compare air temperatures oc-
in other terrain settings with those of the river
curring over differing surfaces near the river on
plane can initiate understanding of physical pro-
most winter days.
cesses responsible for surface temperature varia-
The Connecticut River is impounded at Wilder
tion. On a few days each winter, the river may
Dam. The impoundment is at 117 m (384 ft) above
present a surface of open water, an alternation of
mean sea level and this level remains constant
ice and water, or a thickening snow-free ice plane.
along the watercourse of Figure 3. The water in
These conditions provide an opportunity to study
this impoundment is replaced over a three-day
some additional mechanisms responsible for lo-
period by bottom water from McIndoe Falls Dam
cal air temperature modification, and will be dis-
approximately 30 km upstream of Figure 3. The
cussed in subsequent sections.
river is from 100 to 200 m wide along this section
The winter surface is variable with properties
of its course; the lower basin (terrain less than 25 m
greatly dependent on the thickness and continu-
above water level) along the river varies in width.
ity of snow cover. Quantitative snow measure-
The frequent recharge of the river prevents
ments were made at Z, and qualitative snow
stratification or stagnation of its water, which
observations were made at several other points
might cause a variation in heat source strength
each day. Snow depths were estimated with re-
among stagnated pools. The river is frozen and
spect to fixed objects as done in Hokkaido, and
snow covered on most winter days. This combi-
surface cover was visually estimated from the
nation of spatially uniform subsurface tempera-
relative area of bare spots. We propose, after com-
ture with a level snow surface free of obstacles
parison of the quantitative and qualitative obser-
provides an approximation of a "reference" radi-
vations, that the observation at Z numerically
ating snow surface with a relatively constant
characterizes the snow cover along the Connecti-
cut River Valley during the core of winter. This
roughness length (Stull 1988) that is 50 km in
north-south extent. Perovich* has recently ob-
characterization is shown in Table 1. We will
stratify data, using the categories in Table 1, to
examine processes that produce variation in win-
ter morning air temperature in the study area.
* Personal communication, D. Perovich, CRREL, 1994
Table 1. Definition of snow depth categories.
< 1 cm
This is reported for some early winter observations and some days following thaws when
snow is predominantly absent from the basin. It is also the characteristic of a light dusting of
bare ground by flurries.
< 3 cm
This is associated with sparse, or discontinuous remnant, snow cover in the Connecticut
River Valley. The <1 cm category is included within this category for analysis.
< 10 cm
This is associated with general snow cover, but bare or sparse spots occur in fields and on
southern exposures. Grasses and field crop stubble may protrude through 10 cm of snow.
< 20 cm
This category was extracted from the data set to test the statistical robustness of some
< 30 cm
This amount is associated with nearly total and continuous snow cover.
> 30 cm
This amount is associated with total and continuous snow cover at all locations.