40
30
20
10
0
5
10
15
Wind Speed (m/s)
Figure 1. Wind profile for a 10-m/s wind at 10 m above ground
assuming the 1/7 power law (ASCE 1994).
ture rises above freezing after the freezing rain
steps. In the first time step (e.g., 14001410) these
ends. The occurrence of T > 0C after freezing rain
weather elements take on the values reported for
ends thus defines the end of an event. Many freez-
that hour (1400), and in the last time step (1450
ing-rain storms end with warming temperatures;
1500) they are equal to the values reported for the
however, after a well-documented ice storm in
next hour (1500). In each intermediate time step,
Hanover, N.H., the temperature remained cold,
one-fifth of the difference between the hourly val-
and accreted ice remained on wires for more than
ues is added to the value for the previous time step.
30 days (Ackley and Itagaki 1970).
This scheme weights the reported values: each
applies for 20 minutes, and the interpolated values
apply for 10 minutes. Precipitation rate and solar
radiation are reported as a total for the hour, and
3. ICE ACCRETION SHAPE
I assume them to be constant during each hour. In
In the models, the structure on which the ice is
the simple model, the wind speed is also assumed
accreting is a horizontal circular cylinder, which is
to be constant for each hour.
referred to as a wire in the following. Extrapola-
A missing precipitation rate for an hour in
tion of the models' results to other cylinder cross
which precipitation is reported by the present
sections is discussed in section 6.
weather code is assigned 0.01 mm/hr in ZRAIN.
Ice accreted on horizontal cylinders (wires and
This gives continuity to the freezing-rain event
tree branches) during freezing-rain storms in New
without arbitrarily adding significant mass to the
York, Arkansas, and Vermont is shown in Figure
accretion. If wind speed, temperature, dew-point
2 and on the cover. The shape of the ice accretions
temperature, or air pressure is missing in an hour
varies from a crescent on one side of a branch (Fig.
it is assumed to be the same as in the previous
hour.
2d), to a round accretion with a few icicles (Fig. 2a),
The models contain no procedure for melting
to a heavily icicled barbed-wire fence (Fig. 2b) and
accreted ice. I assume, in extracting freezing-rain
power line wire (cover), to an elongated accretion
events from the weather data, that accreted ice
shape, apparently fused icicles, on a wire. The
melts as soon as, and only when, the air tempera-
shape of accreted ice depends on the diameter of
3
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