Extreme ice thicknesses from freezing rain
Final Report
ruler to measure the maximum dimension of the ice on the wires where they entered the depot
above the double window, 8 to 10 ft above ground.
The CRREL Ice Storm Team has deployed in a number of freezing rain storms beginning in the
mid 1990s to measure equivalent radial ice thicknesses and document the distribution and
severity of icing on trees and wires. The photographs in Figure 2.2-1 were taken in those storms
and are chosen to illustrate some of the variety of ice accretion shapes that occur. This figure
shows that because of the great variety of ice accretion shapes, the maximum dimension of the
accretion that is reported in papers that use ARA data is not a good measure of the load of ice on
the wire. The determination of the equivalent radial ice thickness t from field measurements of
ice samples in freezing rain storms is presented in Figure 2.2-2.
2.3
Application to weather data
The approach used to apply the CRREL and Simple ice accretion models to U.S. and Canadian
hourly weather data to determine equivalent radial ice thicknesses in past freezing rain storms is
described in Jones et al. (2002). A map of the weather stations used in this analysis in the lower
48 states and neighboring portions of Canada is shown in Figure 2.3-1. Stations in Alaska and
portions of northwest Canada are shown on the extreme ice maps for Alaska in the Appendix. As
is mentioned in the 2002 paper, we obtained damage information from Storm Data (NOAA
1959-present), Climatological Data: National Summary (NOAA 1950-1958), and newspaper
reports on freezing rain storms for which the modeled ice thicknesses were significant. From the
model results and this qualitative damage information we mapped the region of each storm in
which there was enough ice to damage trees, overhead wires (telegraph, phone, power, cable TV,
etc.), and communication towers. A map of the compiled damaging ice storm footprints is in
Figure 2.3-2.
The weather conditions during ice storms are not extraordinary. Winds are typically light to
moderate, precipitation rates are relatively low, and temperatures are at or below freezing but not
bitterly cold. Thus, extraordinary ice storms typically occur when conditions favorable for
freezing rain or drizzle persist for many days. This also means there is no natural maximum
equivalent radial ice thickness. Weather conditions that produce 1 in. of ice in two days will
produce 2 in. after four days, or 4 in. after eight days, with the same freezing rain rate and wind
speed.
2.4
Extreme value analysis
A discussion of extreme events and various approaches for the calculation of extreme values is in
Jones and White (2002). The maps of equivalent radial ice thickness (henceforth shortened to
"ice thickness") from freezing rain with concurrent gust speeds are organized by mean
recurrence interval (50, 100, 200 and 400 years) in the Appendix. For each recurrence interval,
there is a map of the lower 48 states, a map of Alaska, and detail maps of Lake Superior, the
Columbia River Gorge, and the Fraser Valley. For 50 and 100-yr mean recurrence intervals, ice
thicknesses are mapped in 0.25 in. increments. Each ice thickness zone includes values from
70% below the nominal value to 30% above. For example, the 0-in. zone includes ice thicknesses
up to 0.075 in.; the 1-in. zone includes ice thicknesses from 0.825 in. to 1.075 in. On the 200 and
400-yr maps, ice thickness zones are 0 in., 0.25 in., 0.5 in., 1 in., and continue in 0.5-in., rather
than 0.25-in. increments, reflecting the greater uncertainty in the values for these long mean
September 2004
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