A more specific analysis compares the day be-
(1986) shows that the thermal conductivity of a
fore the snowfall 15 December, Tp = 24.5C, with
snow layer varies as the bulk density of the snow.
Although snow rarely has a uniform density
a synoptically similar day 18 December, Tp =
25.5C. The temperatures in basin IV east of the
throughout its depth, we can conservatively esti-
bridge were 2C less on 15 December, and 2C
mate the bulk density of snow as 200 kg/m3, with
thermal conductivity of 0.12 W/ m C. The mean
greater on 18 December. These differences are 10
air temperature of 13C on 17 December would
times the mean temperature differences attribut-
produce an additional ice growth of 0.002 m, us-
able to difference in inversion structure with snow
ing either eq 2 or 4. The snow cover diminished
cover shown in Part II.
A heat island of approximately 4C has been
the rate of river ice growth by a factor of 10,
although mean daily air temperature did not ap-
found in the vicinity of a freezing, sparsely snow-
preciably change. This corresponds to a daily la-
covered river. This is a greater local heating than
tent heat release of 1.4 109 J/m of river length
associated with some urban heat islands. This heat
prior to the snowfall, and 8.1 107 J/m after,
island was recognized at the time of measure-
when distributed over the mean 150-m width of
ment, and preliminarily reported (Hogan and
the river.
Ferrick 1990). The magnitude of this observed
The heat transferred to the air above the ice
heat island was reassessed, considering the river
produces a change in enthalpy
plane reference of Part I and the variation of ver-
tical temperature structure in Part II. This has
= H2 - H1 = M Cp (T2 - T1)
1Q2
(5)
verified that ice growth under sparse snow liber-
ated sufficient heat to increase the temperature of
stagnant air 0.4C/hr. A 4C temperature increase
which can be used to estimate the maximum air
temperature change which could result from the
relative to surroundings was realized under a lo-
cal inversion that persists about 12 hours.
latent heat of freezing. Values for specific heat
and density of air are obtained from Keenan and
Kaye (1948) and List (1950). The mass of near
surface air M influenced by this heat release must
COLD AIR DRAINS
be estimated to evaluate the temperature differ-
ence (T2T1).
Observation points m and n, and several other
points where brooks or streams enter the lower
We showed in Part I that the temperature at h,
one river width from the river, was equivalent to
basin, were initially chosen to examine the fre-
the air temperature over the river. We showed in
quency of lesser temperature occurring at those
Part II that an inversion was generally present
places due to cold air drainage. On 13 of the 40
100 m above river elevation over sparse snow
days with lapse conditions, lesser temperatures
cover, and that an additional inversion was
were observed along the brook mouth at n than at
present just above the river elevation when more
the other points along the river. This was the
than 20 cm of snow covered the Connecticut River
most frequent occurrence of observed cold air
Valley. The points in basin IV, apparently unin-
drainage. Interestingly, the coldest air observed
fluenced by the latent heat release, are about five
along this observation path occurred at m, and at
river widths from the river and 20 m elevation
the adjacent Orford Green on 27 January 1994.
above the river. The volume of air influenced by
Cold air drainage may also account for lesser tem-
this latent heat source is about 1 km in width and
peratures observed near s on three days. Cold air
100 m in depth, that is 105 m3 per meter of river
drainage was not frequently apparent in these
length, over sparse snow cover. This volume di-
observations, but was coincident with increased
minishes to about 3 104 m3 per meter of river
local temperature difference when it was observed.
length beneath the lower inversion accompany-
The discussion of the variation of vertical tem-
ing more than 20 cm of snow cover. The addition
perature structure with respect to snow depth in
of 1.4 109 J/m day could account for a 10C
Part II alluded to the possibility of cold air drain-
stagnant air temperature increase, while the addi-
age down the narrow stream valley connecting
tion of 8 107 J/m day through the thicker snow
small basins II and III. The topographic cross sec-
cover could account for less than 2C of rise in a
tions of this valley and basin II show it to be quite
thinner stagnant air layer. The observed mean
narrow and steep sided, but much less so than the
temperature difference increased 4C after the
V-shaded valley of Yasuda et al. (1986). This val-
snowfall of 16 December.
ley is intermittently wooded, among small fields,
32
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