Transcript LAKE-EFFECT SNOW

LAKE-EFFECT SNOW
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Greg Byrd
Cooperative Program for Operational Meteorology,
Education and Training
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Overview of the Lake-Effect Process
• Occurs to the lee of the Great Lakes during
the cool season
• Polar/arctic air travels across a lake, picks
up heat and moisture, and is destabilized
• Cloud formation is enhanced by thermal
and frictional convergence and upslope
along lee shore
Lake-Effect-type Phenomena in
Other Regions
• Lake-effect (Great Salt Lake)
• Lake-effect (Finger Lakes, NY)
• Bay-effect (Chesapeake, Delaware,
Massachusetts Bays)
• Ocean-effect (Gulf Stream, Sea of Japan)
Lake-Effect vs. Lake-Enhanced
• Lake-effect: precipitation which results
from cold polar air flowing over warm lake
water after passage of a synoptic cyclone
• Lake-enhanced: the additional precipitation
resulting from a boundary layer fetch over a
lake during a synoptic cyclone (e.g.,
overunning) event
LAKE-EFFECT
Occurs during the unstable season when mean lake
temperatures exceed mean land temperatures
Mean annual snowfall exceeds 100 inches in the
snowbelts to the lee of the lakes, and exceeds 200 inches
in the Tug Hill Plateau in New York, to the lee of Lake
Ontario and on the Keweenaw Peninsula of northern
Michigan, to the lee of Lake Superior.
Notable Snowfall Statistics
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5-10 in (13-25 cm) per hour documented
68 in (172 cm) at Adams, NY on 1/9/76
102 in (259 cm) at Oswego, NY 1/27-31/66
149 in (378 cm) at Hooker, NY in 1/77
466.9 in (1186 cm) at Hooker, NY, 1976-77
Conceptual Model of Lake-Effect
Heat and moisture from lake + frictional convergence +
upslope flow = clouds and lake-effect precipitation
Formation Regions
from LaDue (1996)
Downwind of concave coastline, bays, etc.
Ingredients Determining Lake-Effect
Characteristics
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Instability
Fetch
Wind shear
Upstream moisture
Upstream lakes
Synoptic (large)-scale forcing
Orography/topography
Snow/ice cover on the lake
Instability
• Depth of instability: relates to depth of
mixed layer. Difficult to get heavy snow if
depth of mixed layer < 1.0-1.5 km
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• Degree of instability: Tlake-T850>13 C
gives absolute instability/vigorous heat and
moisture transport (10 C/synoptic forcing)
Intense single band
deep, but marginal
instability; winds
aligned with long
axis of lake.
Weak multiple
bands shallow, but
strong instability;
winds normal to
long axis of lake.
from Reinking et al. (1993)
Fetch
• Distance air travels over water--relates to
wind direction (850 mb)
• Small changes in wind direction can
significantly change the fetch
e.g., Lake Erie: 250 deg wind--225 mi fetch
230 deg wind-- 80 mi fetch
Favorable Fetches for Lake-Effect
Snow
from LaDue (1996)
Wind Shear
• Directional turning: significantly impacts
character
sfc-700 mb dir chg
0-30 deg
30-60 deg
>60 deg
character
strong, well organized
bands
weaker bands
nothing/poss. flurries
• Wind speed: strong winds may carry bands far
inland; but bands may be “sheared off” if wind is
too strong
Upstream Moisture
• Impacts precipitation potential
• Low RH: difficult to get condensation,
clouds, and precipitation
• High RH: more precipitation
Upstream Lakes
• Impact snowfall to the lee of downwind
lakes, e.g.,
In northwest flow, Lake Huron snow
bands re-form/intensify over Lake
Ontario and Lake Erie.
• Upstream lakes may not always bring an
increase in snowfall to downwind lakes!
Simulation of the Effects of
Upwind Lakes--NW Flow
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from Byrd et al. (1995)
As expected, the upstream lake (Huron) increases
precipitation downwind of Lake Ontario in northwest flow.
Simulation of the Effects of
Upwind Lakes--West Flow Case
from Byrd et al. (1995)
Precipitation downwind of Lake Ontario actually increases
when upwind lakes are frozen.
Synoptic-Scale Forcing
• Cyclonic vorticity advection aloft may
enhance lake-effect by lifting the capping
inversion
• Cold advection may enhance lake-effect by
increasing the instability
Favorable Synoptic Setting
Niziol (1987)
Sfc-850: Broad WSW trof extension from parent low over Maritimes.
500: Closed low south of James Bay; Deep (>3 km) layer of instability.
Result is prolonged, unidirectional fetch over long axes of Erie, Ontario.
Orography/Topography
• Lake-effect increases with elevation to the lee of
the lakes (e.g., Tug Hill Plateau)
• Annual snowfall increases by 8-12 inches per 100
ft increase in elevation
Snow/Ice Cover on the Great Lakes
• Diminishes or ends the lake-effect season
• Lake Erie season often ends late January or
early February
• Lake Ontario season continues into March
as it doesn’t freeze completely
• A frozen lake doesn’t preclude a significant
lake-effect event
Types of Lake-Effect Snowbands
• Single Bands
• Multiple Bands
• Multiple-Lake Bands
Single Band Development
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• Absolutely unstable lapse rate (Tlake-T850>13C)
=> vigorous vertical transport of heat/moisture
• Cu and precipitation formation release latent heat.
 cases
This is significant for warm (T>-10C)
• Sensible and latent heating warm the air, causing
meso-low formation. The deeper the clouds, the
stronger the low (warm core)
• Horizontal thermal convergence into the low
below cloud base and diffluence aloft leads to
strong mesoscale ascent within the snowband
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Intense single
snowbands exhibit
strong confluence in the
lower part of the mixed
layer and diffluence near
the top of the mixed
layer.
Single Bands (cont.)
The Role of Frictional Convergence
e.g. lake effect snowbands are much more
likely to occur along the south shore of Lake
Ontario
Single Bands (cont.)
Thermal
Convergence
The warm core of
the snowband is
stretched out over
land by the
prevailing
synoptic wind.
Pronounced
convergence
occurs near the
edge of the
elongated band,
with the strongest
pressure gradient
on the south side
of the band.
Multiple Snowbands
(1-20 km wide)
from NWS Marquette (1996)
• Weaker than single bands,shallower mixed layer
• Horizontal roll convection
• Occur when mean boundary layer wind is more normal to the
long axis of the lake
• Little thermodynamic difference with environment
• Oriented parallel to the mean boundary layer wind direction
Multiple-Lake Bands
May be single or multiple, commonly initiate off
Lake Huron/Georgian Bay in northwest flow and
re-intensify over the lower lakes (Erie, Ontario).
Satellite Applications
Data from different GOES channels can be used in
combination with other data sources for diagnostic
studies and nowcasting/short-term forecasting
applications.
Water Vapor Imagery
Water vapor (6.7 micron) imagery may be used to
infer large scale flow patterns and track important
features such as upper-level short waves
VIS and IR Imagery
IR
VIS
The IR (10.7 micron) imagery shows the higher,
colder cloud tops resulting from forcing due to the
upper level trough, and also shows where lakeeffect convection may be occurring. Visible
imagery can readily depict lake-effect snowbands
during daylight hours.
3.9 micron imagery
During the day, ice crystal clouds are poorer
reflectors than clouds composed of water droplets.
Therefore, ice clouds appear darker, and liquid
water clouds appear brighter. The location where
clouds become glaciated or anvils are observed is
an area where heavy snowfall may be occurring.
Reflected Energy Product
• Differencing the 10.7 and daytime 3.9 micron
radiances gives a reflected energy product that
more clearly distinguishes ice and liquid water
clouds. (Caution: bare ground may be difficult to
distinguish from low water clouds, and snow
cover may appear similar to thin ice clouds.)
IR Cloud Top Temperatures
The 10.7 micron cloud top temperatures can be used
to infer lake-effect intensity. Temperatures colder
than -15 C imply efficient precipitation
production, and the coldest cloud tops infer areas
of relatively deep convection.
Distinguishing Snow Cover
Reflected Energy
Visible
Snow cover (outlined in yellow) shows up as white
ground in the visible, but very dark in the reflected
energy product.
Distinguishing Ice Clouds
RE Product
VIS
Opaque ice clouds (outlined in yellow) appear bright
in the visible and dark in the 3.9 micron and
reflected energy products. The existence of cold IR
cold cloud tops confirms the existence of ice clouds.
IR
Distinguishing Open Water
3.9
IR
Open water (outlined in yellow) appears dark in the
visible, relatively warm in the IR, and poorly
reflective in the 3.9 channel imagery.
VIS
Distinguishing Liquid Water
Clouds
RE Product
Liquid water clouds (outlined in yellow) appear
bright in the visible and are highly reflective in
both the 3.9 channel and reflected product
imagery.
VIS
Storm-scale Structure
(Integrated data sources)
Surface winds plotted with visible satellite imagery
shows convergence with major lake-effect bands.
The expansion of anvil clouds implies divergence
at the top of the bands.
Storm-scale Structure (cont.)
At the east end of Lake Ontario, the 3.9 micron data
shows darkness indicating ice crystal clouds,
where the 10.7 micron imagery indicates cloud
tops <-30 C. Doppler radar shows divergence
where the satellite shows glaciated cloud tops.
IR satellite cloud top temperatures may be correlated
with radar reflectivities and surface obs to infer
snowfall intensity. The IR data is especially
helpful over regions lacking adequate radar
coverage.
FORECASTING LAKE-EFFECT
Lake-effect snowstorms are difficult to observe
and forecast for the following reasons:
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They are shallow systems (depth often < 3 km); and
the lowest elevation radar scans overshoot the tops.
The onset, intensity, orientation, and exact location
are very sensitive to wind shear/direction and thermal
stratification in the lower troposphere.
Lake-effect difficult to distinguish from orographic
influences in some locations (e.g., Gt. Salt Lake)
Conventional rawindsondes measure profiles at times
and locations which are not optimum for monitoring
the atmosphere over the lakes.
Operational models do not have sufficient resolution
to resolve the scales of lake-effect snowbands.
Lake-Effect Decision Tree from Niziol (1987)
1) Is Tlake - T 850 13 deg C or more?
YES
NO
Lake-effect not likely.
2) Is the direction in the b.l. and 850 mb between: a) Lake Erie...230 to 340 deg?
b) Lake Ontario....230 to 80 deg?
YES
NO
Lake-effect not likely over W. and Central NY.
3) Is the directional shear between b.l. and 700 mb <30 deg?
YES
NO
Is the shear between 30-60 deg?
NO
YES
Instability exists, but shear detrimental to formation.
Bands spread out, are less intense and may be cut off.
4) Lake-effect snow likely. PVA will enhance snowfall, esp. in SW flow. Inversion
height/strength limits snowfall rate. Large fetches and instability will allow
snowfall >1 in/hr. Locator charts will pinpoint area.
5) Lake-effect snow possible. Directional shear makes location difficult to pinpoint and
will limt intensity. Inversion height/strength limits snowfall. PVA will enhance activity.
Locator chart only gives ballpark estimate of location.
Use of Numerical Models
Current operational
models do not adequately
resolve lake-effect.
Eta precip fcst
Higher Resolution Mesoscale Models
• Higher resolution models (5-15 km grid
spacing) will resolve some lake-effect
circulations
• Proper initialization and parameterization
are major challenges
• Initial results show considerable promise
10-km Penn State/NCAR (MM5) Model
Forecast Comparison with Radar
18-hr fcst VT 18 UTC
BGM radar 18 UTC
Integrated Sensor Approach
• Satellite data may be used in tandem with radar
data to infer precipitation intensity estimates at
locations outside of radar range
• Important for diagnosis and nowcasting
BUFKIT AUTOMATED GUIDANCE PACKAGE
Guidance product from NWS Buffalo which incorporates
hourly model sounding data
Concluding Remarks
• Lake-effect processes are fairly well
understood, but forecasting remains a
challenge
• New observing systems (e.g., WSR-88D,
GOES) and mesoscale models (e.g., MM5)
will greatly enhance the ability to forecast
lake-effect
References
Byrd, G. P. , R. A. Anstett, J. E. Heim, and D. M. Usinski, 1991: Mobile sounding
observations of lake-effect snowbands in western and central New York. Mon. Wea.
Rev., 119, 2323-2332.
Byrd, G. P. and R. S. Penc, 1992: The Lake Ontario snow event of 11-14 January 1990.
Proc. Fifth Conf. on Mesoscale Processes, Atlanta, GA, Amer. Meteor. Soc, J59-J66.
Byrd, G. P., D.E. Bikos, D.L. Schleede, and R.J. Ballentine, 1995: The influence of upwind
lakes on snowfall to the lee of Lake Ontario. Preprints, 14th Conf. on Weather Analysis
and Forecasting, Dallas, TX, Amer. Meteor. Soc., 204-206.
Eichenlaub, V. L., 1979: Weather and Climate of the Great Lakes Region, University of
Notre Dame Press, 335 pp.
Kelly, R. D., 1984: Horizontal roll and boundary layer interrelationships observed over
Lake Michigan. J. Atmos. Sci., 41, 1816-1826.
LaDue, J., 1996: COMET course notes and satellite meteorology modules.
Niziol, T. A., 1987: Operational forecasting of lake-effect snow in western and central New
York. Wea. Forecasting, 1, 311-321.
Niziol, T.A., W.R. Snyder, and J. S. Waldstreicher, 1995: Winter weather forecasting
throughout the eastern United States. Part IV: Lake effect snow. Wea Forecasting, 10,
61-77.
NWS/Buffalo, various forecast products.
NWS/Marquette, 1996: Web homepage.
Reinking, R. et al., 1993: Lake Ontario winter storms (LOWS) project final report. NOAA
Tech. Memo. ERL WPL-216, 147 pp.