Transcript Slide 1

EXTRATROPICAL CYCLONES
(Winter Storms and Blizzards)
WINTER STORMS: CYCLONES, ANTICYCLONES, FRONTS
Outside the tropics, the winds are organized into large systems called low and high
pressure areas, that produce much of our weather. Highs and lows are typically about
1000 miles in diameter and travel from west to east at about 25 miles per hour. Earth's
rotation gives them the form of giant wind spirals. Air spirals counterclockwise in toward
lows (cyclones) and then is forced to rise. Therefore, lows bring mostly cloudy, wet
weather. Highs (anticyclones) have the opposite circulation. Air spirals clockwise and
outward from the center. Then air from above sinks to fill “the void”. Highs bring mostly
clear weather but if sunshine heats the ground enough thunderstorms are possible.
As winds spiral into low pressure areas, large regions of polar and of tropical air called
air masses are brought close together. Temperature contrasts become concentrated in
long narrow zones or fronts, which are basically boundaries that separate air masses. It
is important to identify the fronts and predict their motion because much of the major
weather changes and stormy weather outside the tropics forms along fronts.
The next slide describes the different air masses. Fronts are diagnosed in several
slides after that. Then 2-D and 3-D illustrations of a low pressure area with winds, air
masses (cold or warm), fronts and clouds.
WEB Sources: NOAA Summaries of Major Winter Storms
http://www.hpc.ncep.noaa.gov/winter_storm_summaries/winter_storm_summaries.shtml
Weather Archive – You make the maps
http://vortex.plymouth.edu/u-make.html
Air Masses
Air masses are large regions of air with reasonably uniform properties of temperature
and humidity that tend to form in high pressure areas. There are four air masses,
Continental Polar, cP. This cold, dry air mass forms in polar regions when the surface
(often snow covered) cools by radiating heat to space. The air also loses moisture by
deposition of frost on the ground.
Continental Tropical, cT. This hot, dry air mass forms over the subtropical deserts,
where air from the tropics sinks and the sun superheats the dry ground.
Maritime Tropical, mT. This humid, warm air mass forms over tropical and subtropical
oceans, acquiring moisture from the sea below or from abundant tropical showers and
heat from the warm tropical surface.
Maritime Polar, mP. This chilly, humid air mass forms over cold polar waters or when cP
air is charged with moisture from rain or snow under warm frontal surfaces.
At least 3 different air masses converge in most extratropical cyclones, - cP from the
northwest, mT from the south and mP from the northeast. The cold air masses form a
large dome thousands of miles wide and 5 - 10 km high. The air masses converge in
extratropical cyclones. When the warm air encounters the cold air dome at the surface it
forms fronts. The lighter warm air then either rises abruptly to form thunderstorms or
glides over the dome of denser cold air to produce widespread clouds and precipitation.
Air Masses
Continental
Polar Air, cP
Maritime Polar
Air, mP
Maritime Tropical
Air, cT
Continental
Tropical Air, cT
Winds near the Earth’s surface spiral counterclockwise and
inward around low pressure areas in the North Hemisphere.
Fronts are depicted with thick lines that are studded with triangles (for cold
fronts) and with semicircles for warm fronts that protrude in the direction the
front moves. For a stationary front alternating triangles and semicircles extend
in opposite directions. For an occluded front alternating triangles and
semicircles protrude in the direction of motion.
When a cold front passes by, T can drop more than 10 F in an hour. In the
typical cold front (below left) at two times 12 hours apart, the cold air (shaded)
blows from the ___ and the warm air blows from the __.
Temperature changes tend to be more gradual at warm fronts. In the typical
warm front (below right), the cold air is on the ___ side and blows from the
___, while the warm air is on the ____ side and blows from the ___.
Indicators for Finding Fronts
Fronts are located in low pressure troughs
Fronts separate regions with
different wind directions
Indicators for Finding Fronts
Fronts separate regions with different
weather. A line of thunderstorms often is
parallel to the cold front
Fronts often lie at the tropical side of
regions of large temperature gradients.
Simple Model of Surface
Low Pressure Area
Stormy Sector
Dismal, Damp, Drenching
Cold, Dry Sector
Clear, Crisp and Cold
Warm Sector
Hazy, Hot and Humid
Cold Front – Cold Air Advances
Warm Front – Warm Air Advances
Model Extratropical Cyclone (Low) with Cold and Warm Fronts
In the drawing
of the low to the
left, the clouds
NE of the warm
front should be
much wider and
continuous, as
in the warm
front drawing
above.
http://rst.gsfc.nasa.gov/Sect14/Sect14_1c.html
An intense low gave Minnesota and Wisconsin up to a foot of snow while thunderstorms
raged along the cold front (blue line) that separates warm and cold air masses.
Infrared satellite images are temperature maps. Cold regions are usually clouds with high tops.
The warm clouds west of the cold front over Iowa and Missouri are cumulus and stratocumulus.
A higher resolution view of the same storm by the MODIS satellite shows the cold front,
individual cumulus in rows parallel to the wind and cirrus of the jet stream further west.
Fog
An idealized view of the clouds of the
Extratropical Cyclone. Numbers indicate
the likely Locations of the Paintings
Cirrus
Granada, Spain, 1030 UTC 06 Oct 2007
Cs with 22o and circumscribed halos over Paramus, NJ before rain
Cosmic waves in Altostratus. Precip begins shortly after cloud base blurs.
Nimbostratus in a blizzard with huge
snowflakes in Upper Saddle River, NJ
Ominous but Clearing Sky at end of an Extratropical Cyclone
Vincent van Gogh. Crows in Wheatfield, Auvers, 1890
Finally clearing up after a dreary month
3-D View of Fronts and Air Masses
mT
Weather of Extratropical Cyclones
Weather Sequences in
the Moving Cyclone
If you know the pattern
of clouds and weather
then the sequence at
any place follows
naturally by moving the
storm because whenever
a pattern in space moves
it becomes a sequence
in time.
H
L
H
L
NYC
DCA
L
H
H
H
Structure and Weather of the Extratropical Cyclone
The classic extratropical cyclone is about 1000 km across and extends from the ground
to the tropopause. It consists of three sectors separated by two fronts.
1. The wet sector, north and east of the center and the warm front.
2. The warm sector, southeast of the center.
3. The cold, dry sector, west of the center and northwest of the cold front.
Some storms have one or two additional sectors that are described later, namely,
4. The warm, dry sector, between the dry line and the cold front.
5. The dry slot, wedged between the cold, dry sector and the wet sector.
Each sector is filled by a giant conveyer belt of moving air. The warm conveyer belt fills
the warm sector. It consists of maritime Tropical (mT) air from the tropics that turns
eastward as it overruns the dome of polar air. It then produces the wide area of clouds
and precipitation in the wet sector. Poleward of the warm front, a cold, damp conveyer
belt from the east slithers under the warm conveyer belt. Precipitation falling from the
warm conveyer belt above soaks this air and transforms it to maritime Polar (mP). The
polar conveyer belt consists of cold, dry continental Polar (cP) air from the northwest. It
sinks as it wraps around the western side of the low to produce mostly crystal clear
skies.
The stormy sector contains the storm’s Dank, Dreary, and Drenching (DDD) weather.
The air is cold and skies are overcast because the warm conveyer belt rises aloft. The
stratiform cloud cover includes a wide area of continuous precipitation, sometimes
with heavy snow and blizzard conditions.
The form of precipitation in the stormy sector depends on the soundings and cross
sections. Snow almost always forms high in the clouds, but the entire sounding must
remain below freezing temperatures for the snow to reach the ground. This usually
happens far poleward of the warm front. Rain occurs where a layer of air near the
ground is warm and thick enough to melt snow. This usually happens near the surface
warm front. Most freezing rain and ice pellets are produced when the air above the
warm frontal surface is above freezing but the air near the ground is below freezing.
This occurs in storms with strong warm fronts and is usually sandwiched in a narrow
zone between snow and rain.
Wrap Around: As storms intensify, the cold, damp conveyer belt spirals upward as it
wraps around the center. This extends the stormy sector west of the surface low
center and gives the storm a cold core. Precipitation here becomes more showery.
At the western edge of the cloud shield the polar conveyer belt undercuts the other
conveyer belts and lifts the cloud shield to altostratus. Tilted shreds of low scud
clouds, that form as the cP acquires moisture from the wet ground or the lingering
precipitation, race across the sky. The scud give the sky an ironically ominous
appearance, but it is usually a sign of imminent clearing. Clearing may be delayed for
a few hours during the peak of the day, if the sun can provide enough heat to launch
new cumulus or stratocumulus that may produce showers or flurries. But almost
Cross Sections and
Sequence of Precipitation
R
R
S
ZR - IP
S
Lake Effect Snow
Enormous snow totals both from individual storms and for the entire winter
occur on the downwind side (usually to the SE) of the Great Lakes. Some
individual storms have produced more than 60” of snow. The record for a single
event is 141” from 3-12 February 2007, which is shown on the first slide of the
Presentation.
The Lakes serve as sources of heat and moisture for the snow. When cold air
from northern Canada passes over the Lakes it is heated and charged with
vapor. Within a few miles cloud lines form (as when cold air pours over the
Atlantic Ocean and the Gulf of Mexico), and when the air is lifted as it passes
onto the land downwind from the lakes it drops much of this vapor as snow if it
is still cold enough. A forecast rule is that the air at 850 hPa must be at least
13C colder than the lake, provided the lake is not iced over. The greater the
temperature difference the greater the potential intensity of the snow.
Since the coldest air typically comes from the NW, the Lake Effect Snows most
often occur on the SE shores. Also, since the typical snow from extratropical
snowstorms occur with NE winds (Nor’easters), Lake Effect snows often occur
after the main storm has passed, and when skies are normally clear.
The prong of extra snow in West
Virginia occurs because the air is
force to rise over the Appalachians.
The snow maximum
east of Lake Ontario
occurs where the air
is force to rise over
the Adirondacks.
MODIS 04 FEB 2007 Image during the
Record Lake Effect Storm. Extremely
cold air poured over the Great Lakes,
after months of abnormally warm
weather kept the Lakes warm and open.
Cloud rows trace out the winds
as they cross the Great Lakes
Some winter storms produce
truly unBEARable weather. In
addition to snow, there can be
ice, which is rain that freezes on
contact with the ground. This is
called black ice and it is the
most dangerous, paralyzing form
of winter precipitation. It creates
a world without friction.
Earlier slides and lectures
illustrate the vertical temperature
profiles of winter storms that
produce freezing rain, ice
pellets, and snow.
http://severInewx.atmos.uiuc.ed
u/06/online.6.1.html
The Strange Tilt of Weather Systems
Outside the tropics, it is often observed that highs and lows and troughs and
ridges are not vertically aligned but tilt upward to the West. Recall the rules:
1: Lows and troughs always tilt upward toward the coldest air.
2: Highs and ridges always tilt upward toward the warmest air.
3. Systems with symmetrical Temperature patterns do not tilt with height.
The westward tilt with height of troughs
and ridges outside the tropics is due to
the fact that the west side is the cold
side of most extratropical cyclones and
the warm side of most extratropical high
pressure areas.
As a result of this slope, Jet Stream
winds typically blow from the SW directly
above Lows and from the NW directly
above highs. This in turn, moves Lows
from SW to NE and Highs from NW to
SE, as you can see in the next slide.
L
C
O
L
D
W
A
R
M
L
Extratropical Cyclones and Anticyclones
and Waves in the Jet Stream
H
L
The Jet Stream is fastest directly above fronts since they have the largest horizontal
temperature gradients. Thus, Extratropical Cyclones form beneath the Jet Stream and
are swept along it much as a railroad moves along tracks. As the extratropical cyclones
evolve, they modify both fronts and the jet stream. Thus the cyclone is much like a train
that modifies its own tracks.
Link between Surface Features and Jet Stream Aloft
Wind
Fast in Ridge
Slow in Trough
Convergence Aloft
Forces Air Down
East of Ridge
Divergence Aloft
Forces Air Up
East of Trough