Transcript Chap13-1-LeS

Chap. 12
Lake-effect snow storms
Lake effect snow bands over the Great Lakes on 9 Jan 2011
Learning objectives
1. Explain the physical and dynamical processes
responsible for lake-effect snow.
2. Describe the large-scale weather pattern that is
most conducive to lake-effect snow in the Great
Lakes region.
3. Provide explanations for the regional and seasonal
climatology of lake-effect snow.
4. Distinguish the different types of organization of
lake-effect snow.
5. Explain how lake-enhanced snowfall can occure in
an extratropical cyclone.
6. Summarize the key challenges faced by weather
forecasters to lake-effect snow distribution.
What are lake-effect snowstorms?
• Localized mesoscale systems produced by the
flux (vertical transfer) of heat and moisture from
the lake surface (very cold air moving over
water)
• Potentially heavy, frequent, and sustained snow
• Very localized heavy snow – the snow does not
extend very far inland (see Fig. 13.1)
• Over Lake Ontario, the bands are quite narrow
when the low-level flow is parallel to the major
axis of the lake. (Also, see the previous satellite
image)
Where do lake-effect snow events occur?
Lake with most widespread impact?
Lakes with highly localized impacts
Lake-effect snow areas that impact society
Fig. 13.1 Additional wintertime precipitation estimated for lake-effect snow
Large-scale
weather patterns
associated that
promote lake-effect
snow events
• Flow of cold air (polar
continental or arctic in
origin) from the
northwest
• Post cold frontal air
mass (cP or A) as
shown in Fig. 12.2
• Northwesterly flow
between a receding
cyclone and
approaching
anticyclone
Fig. 13.2. Typical weather pattern for the lake-effect
snow storm
The physics of lake effect snow storm development
Physical processes shown in Fig. 13.3:
a. Approaching air is cold (and dry): -5 to -25 °C. (colder air is more effective)
b. Air accelerates over the lake: decreased friction over the smooth water surface
c. Warm air and water vapor is added from the lake surface (surface fluxes)
d. The air mass destabilizes at low levels, clouds form, deepen and develop snow on
the far side.
e. Enhanced convergence on the downwind shore decelerates the air, produces
convergence (and upward motion), and further enhances the snow fall.
Compare this GOES
visible image with Fig.
13.3 in the previous
frame.
• Cold, dry, clear mass
on west side
• Development of clouds
over the lake after heat
and water vapor are
added
• Snow over the eastern
portion of the lake and
over the eastern shore.
Fig. 13.4: A pictorial representation
of the lake-effect snow over Lake
Michigan, as shown in the GOES
satellite visible image.
Cold,
dry
fluxes
Cloud development
Enhanced convergence
Climatology of lake-effect snow storms
(upwind)
(downwind shore)
Lake effect snows begin in late
November and reaches a peak
in January.
Fig. 13.5. Mean monthly temperature difference between Milwaukee, WI on the
west shore of Lake Michigan and the lake temperature on the east shore near
Muskegon, MI. The period of lake-effect snow storms is highlighted in orange.
An extreme
event:
1996 Veteran’s
Day snowstorm
in Cleveland, OH
Long air trajectory over Lake Erie
160,000 power outages
Lightning and thunder occurred
Early season: warm water, cold air
Fig. 13.6. Lake temperatures and
ice concentrations during the
2003-2004 winter season.
A) Early, 12/12/08
B) Late, 3/13/09
A
Note the cooling of water
temperature and ice formation.
B
The lakes are typically coldest
during February.
The lake effect snows end
when a lake develops a
complete ice cover (panel B,
lower left image). Note the
sparse ice cover Lakes
Michigan and Ontario. Explain.
Fig. 13.7. Mean snowfall in December and February. Why is
there a difference?
Lake Superior
Lake
Michigan
Topography also influences the lake
effect snow storms. Additional lifting
of the air mass is produced by the
“upslope” flow
Tug Hill
Plateau
Where are the topo influences?
Fig. 12.8
Fig. 13.9. Effect of air residence time. Longer trajectories over
a lake surface allow for a greater addition of heat and moisture
to the cold air mass, and hence more snow in general.
Organization of lake-effect snowfall
Wind parallel rolls (narrow bands)
Fig. 13.10. Circulation and associated clouds associated with wind parallel rolls.
Clouds form where air rises and dissipate where air sinks. When this mechanism
is important, this patterns leads to cloud streets and narrow bands of snow.
Fig. 13.11. GOES visible image of narrow wind parallel bands (rolls) over
Lakes Superior and Huron
2004 November 30
NASA satellite
Fig. 13.12.
Radar image of wind
parallel snow bands
over the eastern side of
Lake Michigan