Icing - Freezing
Rain with Trace Frozen Category
- Largely consists of freezing rain, often mixed with a
trace of frozen during the 6 hour period. Infrequent exception is
significant snow associated with a deep near freezing isothermal layer.
70% of the total events
were predominantly freezing rain
15% were predominantly snow
15% rain w/trace snow
- Surface wet bulb temperatures define southern &
eastern most extent of icing area
- High potential for significant icing
- Freezing rain as a self limited process vs. sustained
low level cold advection
- Evaluate factors favoring formation of deep near
freezing isothermal layer
- Weak thermal advection (baggy highs, flat lows)
- Northwest quadrant of mature cyclone
- Moderate to heavy precipitation rates & surface
temperatures within several degrees of 0 C (precipitation banding)
- Near snow/rain boundary on the snow side
- In this predominant p-type category, deep near freezing
isothermal layers are most likely associated with 850/700 mb partial
thickness values from 1550-1557 m
profiles associated with the “Icing - Freezing Rain with a trace of frozen”
predominant type category
melting layer (>1560) above a relatively shallow surface based
sub-freezing layer (1291-1300) characterized nearly 40% of the soundings.
near freezing isothermal layer characterized 12% of the soundings
layer (1550’s) above a surface based sub-freezing layer (1293-1303)
characterized 48% of the soundings.
Prolonged Freezing Rain
an ongoing source of low level cold air advection typically associated
with a surface cold air high, ridging into the freezing rain area
an ongoing cold air advection source, freezing rain is a “self limiting
process” due to warming associated with the latent heat of freezing.
temperatures near 30 F, the ratio between liquid and ice accrual is no
longer 1:1. Once temperatures reach 32 F (point at which water freezes and
ice melts), there is no longer any significant ice accrual.
freezing rain storms may occur without a surface low via isentropic lift.
- “Icing -
Freezing Rain with a trace of frozen” is the predominant p-type category
most likely to be associated with prolonged freezing rain events.
Freezing Rain at Tree-top
level temperatures and dew points near or just above freezing can be
unrepresentative of colder temperatures and drier dew points at tree top
more research is needed, temperatures in the upper to mid 20s supporting
heavy icing in trees, can occur while ground level temperatures remain
“Icing - Freezing Rain with a trace of frozen” category is defined on the
predominant p-type category by the partial thickness values of 1292/1550 –
1303/1550 – 1299/1580* - 1290/1580*.
rain events infrequently can occur with the 850-700 mb thickness values
>1580 m. Until more cases have been documented, it is recommended
that users simply extrapolate the current category boundaries for “Icing”
from 1580 to 1590