Download Chap12

Chap. 12
Freezing Precipitation and Ice Storms
Learning objectives
1. Explain the processes by which freezing
precipitation forms
2. Recognize on a sounding the atmospheric thermal
structure that supports formation of the different
types of freezing (or frozen) precipitation
3. Identify the synoptic patterns of fronts and pressure
centers that are conducive to freezing precipitation
events in the U.S.
4. Sketch on a map the North American distribution of
freezing precipitation
5. Explain the hazards, both at the surface and aloft,
associated with freezing precipitation
What is freezing precipitation* (FP)?
• Defined as: Rain or drizzle (supercooled – to be
defined) that freezes on surfaces and causes an
ice glaze
• Between 1949-2000, FP storms have produced
$16B in damage, averaging $300M per year
• FP aloft – aircraft icing
• FP at the surface – ice storms
– Structural damage or ice accumulation of
0.25 inch
*Freezing precipitation is different from “frozen” precipitation such as
sleet and snow. This is already frozen when it reaches the surface.
Supercooled water drops
• Existence of water (drops) in a liquid state at
T < 0 C
• Dependence on size
– Supercooled water is more likely for small drops
• Ice nuclei (IN) are required to freeze
supercooled drops
– IN are aerosol particles that activate (initiate)
freezing
– IN are typically not active for T > -7 C
Question for discussion: Can freshwater lakes,
ponds, and puddles be supercooled?
Vertical structure of the
atmosphere during FP events
• Source of the drops in freezing rain: melted snow
flakes
– Melting requires an atmospheric layer > 0 °C (Fig. 12.1)
– Freezing rain is associated with nimbostratus clouds
• Source of drizzle in freezing drizzle: warm rain
process (ice phase is not involved), growth by
condensation on cloud condensation nuclei
– Source near the top of shallow (~1 km deep) clouds (Fig.
12.2)
– Drizzle drops grow as they fall downward thru the cloud
– Freezing drizzle is often associated with stratocumulus
clouds
Production of freezing/frozen precipitation
• Precipitation forms as snow, which melts
at lower levels and then refreezes when
the supercooled drop strikes the surface.
– Typical for freezing rain.
– The surface temperature is < 0 °C
• For freezing drizzle, drops form in liquid
form (warm rain process) and then freeze.
– This is more likely to be small drops, such as
drizzle. (Smaller drops remain supercooled
for a longer time period.)
• Generation of freezing rain: see Fig. 12.1
Temperature profiles associated with freezing rain
Four soundings at
various locations
across a warm front
within a midlatitude
cyclone.
Fig. 12.1: This shows generation of snow, ice pellets, freezing rain, and rain.
The temperature profile is important. What about mixtures of rain & snow?
Temperature profiles associated with freezing rain
Four soundings at
various locations
across a warm front
within a midlatitude
cyclone.
Fig. 12.1: This shows generation of snow, ice pellets, freezing rain, and rain.
The temperature profile is important. What about mixtures of rain & snow?
Freezing drizzle
• Light precipitation (small drops in the form
of drizzle, 0.2 mm in diameter) that freezes
on surfaces and causes an ice glaze
• Total accumulations much lower than FR
• Aloft – aircraft icing – is a potentially big
danger
• Forms in relatively shallow clouds with
cloud tops warmer than -10 °C
Fig. 12.2. Situation for production of freezing drizzle. These clouds are
relatively shallow and also present icing hazards to aircraft
(Important to
prevent
freezing)
Smallest droplets here
Hazard
zone
Drop
growth
All supercooled water
Largest droplets fall from cloud
Drizzle (0.2-0.5 mm diameter)
The drizzle does not freeze in the cloud because ice
nuclei are not active or common for T > -7 C
Fig. 11.3. In-cloud and subcloud icing present hazards to aircraft
1) Added weight
2) Change in airflow
around wings
See Storms and Society, page 212. Roselawn aircraft accident due to freezing drizzle.
Large-scale weather patterns associated with freezing precip.
anticyclone
Cyclone alone
Relatively narrow
bands of freezing
rain are produced for
each case.
Cyclone/Arctic high
Need to get cold air Cyclone + arctic anticylone
southward via the
push by the
anticyclone
Which of these is
most common for
freezing rain in the
SE US?
Fig. 12.4
West flank of anticyclone
This one!
The anticyclone
pushes very cold air
into the Southeast.
A cyclone develops
and advects warm
moist air over the
stationary front.
Very high ice
accumulations can
result (>2 inches).
Such storms produce
extensive damage.
Damage from ice storms
a)Weight of the ice*
b)Wind (see “Wind and Ice Storms” box on p. 213)
• Increased stress and oscillating motions
• Ice + wind can be devastating
*Quantitative question: What is the weight of ice on a
single electric line over a span of 50 meters? Assume an
ice accumulation of 1 cm, uniform around the cable.
Extreme Event
• 4-9 Jan 1998
• $3B damage
• http://severewx.atmos
.uiuc.edu/
Fun fact, p. 214
Warm Air Aloft!
Very shallow arctic air
mass exists below 900
mb.
The inversion strength
(difference in temperature
between the base and top)
is large – about 24 C in
this case.
The air mass above is
conditionally unstable and
would be supportive of
thunderstorms and heavy
rain (and hence large ice
accumulations).
Shallow cold air mass
Freezing rain associated with east coast events – 3 types
Fig. 12.5. Weather patterns associated with freezing rain within
the Appalachian Mountains
Freezing rain associated with east coast events – 3 types
Fig. 12.5b
Freezing rain associated with east coast events – 3 types
Fig. 12.5c
Fig.
Fig. 12.6.
11.6.
Climatology
Climatology of
of freezing
freezing rain.
rain.
Hours
Hours per
per year
year at
at sites
sites that
that receive
receive freezing
freezing rain,
rain, freezing
freezing drizzle,
drizzle, and
and ice
ice pellets.
pellets.
Infrequent over Southeast
Rare occurrence over wester U.S.
Fig. 12.7. Median number of
hours per year of freezing
drizzle.
Significant difference from
location of freezing rain
events. Why?
Determining Precipitation Type
• Rain (R, RA)- Rain is liquid precipitation
that reaches the surface in the form of
drops that are greater than 0.5 millimeters
in diameter.
• The intensity of rain is determined by the
accumulation over a given time.
• Categories of rain are light, moderate and
heavy.
Cold Rain Sounding
Determining Precipitation Type
• Snow (SN, SNW, S)- Snow is an aggregate of
ice crystals that form into flakes.
• Snow forms at temperatures below freezing. For
snow to reach the earth's surface the entire
temperature profile in the troposphere needs to
be at or below freezing.
• It can be slightly above freezing in some layers if
the layer is not warm or deep enough the melt
the snow flakes much.
• The intensity of snow is determined by the
accumulation over a given time. Categories of
snow are light, moderate and heavy.
Snow Sounding
Determining Precipitation Type
• Sleet / Ice Pellets (PE, PL, IP, SLT)- Sleet (Ice Pellets)
are frozen raindrops that strike the earth's surface.
• In a sleet situation the precipitation aloft when it is first
generated will be snow.
• The snow falls through a layer that is a little above
freezing and the snow partially melts.
• If the snow completely melts it will be more likely to
reach the earth's surface as supercooled water instead
of sleet.
• If the snow partially melts there will still be ice within the
falling drop for water to freeze on when the drop falls into
a subfreezing layer. The lowest layer of the troposphere
will be below freezing in a sleet situation and deep
enough to freeze drops completely.
Sleet Sounding
Determining Precipitation Type
• Graupel (GS)- Graupel forms in the same
way as hail except the diameter is less
than 5 millimeters. It usually grows by soft
hail processes.
• Drizzle (DZ, L)- Drizzle is liquid
precipitation that reaches the surface in
the form of drops that are less than 0.5
millimeters in diameter.
Determining Precipitation Type
• Freezing Drizzle (FZDZ, ZL)- Freezing Drizzle is
liquid precipitation that reaches the surface in
the form of drops that are less than 0.5
millimeters in diameter. The drops then freeze
on the earth's surface.
• Freezing Rain (FZRA, ZR)- Freezing Rain is
liquid precipitation that reaches the surface in
the form of drops that are greater than 0.5
millimeters in diameter. The drops then freeze
on the earth's surface.
Freezing Drizzle Sounding
Freezing Drizzle / Freezing Rain Sounding
Freezing Rain Sounding
Determining Precipitation Type
• Freezing Fog (FZFG)- Freezing fog is a fog
composed of supercooled water drops. These
drops freeze just after they wet the earth's
surface.
• Mixed Precipitation (MXD PCPN)- The
combination of two or more winter precipitation
types occurring at the same time or over a
period of time at the same place.
Homework
• Test Your Understanding
– 1, 2, 5, 6, 9, 10
• Test Your Problem Solving Skills
–3