Download Lecture 18 Weather Systems zc

http://weather.msfc.nasa.gov/GOES/go
eseastconuswv.html
Weather Service began as part
of the War Department
September 1, 1872
Modern Day Satellite Photos
Geostationary Operational Environmental
Satellite (GOES) Program
Operated by National Weather Service (NWS)
http://www.cdc.noaa.gov/cgi-bin/db_search/SearchMenus.pl
Cloud charts, Thermometers, Wet Bulb shoelaces, Humidity charts. pinwheels
Density falls off with altitude; pressure is caused by impacts, dense air has more impacts
Buoyancy Lifting
Start
Local Heat
Atoms close together (dense, high pressure)
“fall into” the empty (low pressure) area
Air Expands
Dense Air
Jiggles
(heated atoms
Underneath
speed up
paddle board
forces warm
analogy)
air up
Pressure differences cause WIND
WIND
 Wind is caused by differences in pressure
 Pressure and Density of air are two aspects
of the same thing
 Atoms close together (in dense, high
pressure areas ) “fall into” the empty (low
pressure) area
The greater the contrast in pressure the faster the wind.
One atmosphere (pressure at surface) is about 1000 millibars
These red isobars are lines of equal pressure
L
H
Pressure Gradients
Coriolis turning
Initially wind flows from high to low
but Coriolis turns it nearly parallel
to lines of equal pressure (isobars)
Winds blowing parallel to isobars are called geostrophic winds
This occurs well above the surface where there is no friction
Notice pressure gradient force always from high to low pressure, but Coriolis perpendicular to actual flow direction
Winds blowing parallel to isobars are called geostrophic winds
Winds Aloft, maybe 3 km up
Wind speed feathers on the “from” end. Winds named for the “from” direction
Friction turns surface winds back toward the pressure gradient.
Near the surface, winds almost move from High to Low pressure
They spiral counterclockwise into a Low in Northern Hemisphere
Near the surface, winds almost move from High to Low pressure
They spiral counterclockwise into a Low in Northern Hemisphere
L
lowest local pressure
Blue arrows PGF
Red arrows Coriolis
Black arrows wind path
L
Polar Cell
Ferrel Cell
Hadley Cell
Since earth’s rotation turns air, flows are broken into 3 cells per hemisphere
Horizontal temperature differences
Temperature effects density and pressure:
P = r R’ T so T = P /r R’
If you heat something it expands and gets less dense
A 500 millibar pressure level is much higher in hot air.
Hotter air has lower density and greater volume
500 mb
700 mb
850 mb
1000 mb
Warm
Cold
Psurface
Polar Jet Formation
Steep gradients
of Pressure cause
higher velocity
geostrophic
winds.
This is the trigger
for jet stream
flow.
LOW _____________________HIGH
Polar air is
denser, so it
wedges
under the low
density warm
air. Rotation
causes an
eddy to form
For the Polar Jet, the eddy is
in the Ferrel cell on the
upper polar side, and so air
flows from the west to the
east, the “Westerlies”
Since the pressure
difference is great at the
boundary, the jet is a very
fast wind
Air Masses
 An air mass develops when the atmosphere
is located above a relatively uniform land or
water surface for several days.
 The lower atmosphere assimilates some of
the properties of the underlying surface.
Air Masses
 Large regions (1,000s km2) of the lower
troposphere with uniform characteristics
(temperature, moisture content) originally
defined by a source area.
 Labels refer to temperature (arctic vs.
polar vs. tropical) and source area
(continental vs. maritime). The source
area determines the moisture content.
 Labels around here: cT cP mT mP
http://www.met.tamu.edu/class/Metr
304/Dir-surface/surface.html~
Polar air enlarges and moves further south during
winter and retreats northward during summer.
Typical Air Mass Changes
Heating (cP air moving south) will lead to
instability (bouyancy) as warms.
Cooling (mT air moving
north) has the opposite effect,
because colder air cannot rise
Orographic Lifting forces maritime mP and
mT air upward over mountain ranges in the
western U.S., leading to condensation and
precipitation that converts the formerly humid
air to a much dryer air mass.
Air Mass Interactions
cP
mT
• Weather in any region is influenced by the interactions
between air masses. Recall that the boundaries between
contrasting air masses are called FRONTS
cP
mT
Heavy rains can result from the
interaction between the continental
polar cP air mass and the maritime
tropical mT air mass
Mid-latitude Cyclones
ExtraTropical Cyclones
2,000 km
control
for three days
Winds circulate
around the Low
L
are up to
across &
the weather
to a week.
counter-clockwise (CCW)
in Northern Hemis.
Def: Synoptic Scale: space and
timescales of
mid-latitude depressions
i.e. several thousand kilometers and
timescales of several days
8.
Cold air sinks
HIGH
LOW
Polar Cell
7. Mid-Latitude Cyclones are
dominant where Ferrel
meets Polar
HIGH
5.
6. Gyre
3.
LOW
to E
4. Ferrel Cell
Mid-Latitude Cyclones north
Dry south
2.
0. Maximum
heat from
Sun
1. ITCZ
Dishpan Experiment
Why are the boundaries between cold and hot not straight?
Camera rotates with dish
Cold Center hot edge
Large scale waves
and eddies form
They flow CW & CCW
Rossby Waves
Cold mass is
denser and
forces its way
under the
warmer mass
Mid-Latitude Cyclones
 This clash between cP and mT air
masses is the most common source
of frontal systems in the U.S.
 Weather conditions, and cloud types,
change in a predictable sequence as
warm and cold fronts pass over an
area.
 A front is a transition from one air mass
air mass to another
http://www.met.tamu.edu/cl
ass/Metr304/Dirsurface/surface.html~
cP
mT
DBZ is Decibels received back
Map Symbols
These show surface positions
cP
mT
Warm Front
• Ahead of a warm front, warm, humid air is
transported upward over a distance of
approximately 1,000 km (625 miles).
• Rain may last longer than for a cold front
because the warm front moves slowly and
extends over a larger area.
Dangerous
storms if cold air
is very cold
Warm Front Clouds and Winds
• First sign of warm front is sequence of clouds
(cirrus, cirrostratus, altostratus).
• If winds are right, up to 12 hours after the cirrus,
the higher clouds will be replaced by lower
nimbostratus with associated light to moderate
precipitation.
• Temperatures and humidity rise and winds
typically shift direction (first from the south or
southeast, then from the southwest) with the
passage of the warm front.
Warm Front Changes
Expect increasing Temperature and Humidity as the warm front passes
Cold Front
changes
 Cold, dense continental polar air cP
replaces moist, warm maritime tropical air
mT across a cold front
 Expect decreasing temperature and
humidity and increasing atmospheric
pressure with the passage of the cold front.
Warm Air Rises
 Warm wet air has lowest density, so
it will always rise over cooler air.
 Both the cold and warm fronts are
inclined toward the warm air mass.
Approaching Cold Front
is behind Thunderstorms
Warm air is pushed up and over the advancing cold front, causing
relatively rapid cooling and condensation that results in the
development of tall cumulonimbus clouds. They host heavy but
relatively short-lived precipitation
Cold Front cloud bands are narrow
because cold front wedges are steep
Cold Fronts are narrow
because the edge of the
cold air mass is steep
http://www.emc.ncep.noaa.gov/mmb/gmanikin/nas125/tstorm/squall1.gif
A Cold Front Squall Line
Rapidly advancing cold
fronts may be marked by
the growth of a squall
line of thunderclouds
Cyclogenesis 1 – Stationary Front
A small scale wave forms
cP
mT
These persist if the air masses have equal pressure
Then many small waves form, and storms are very frequent
Mid-Latitude Cyclones start as Stationary Fronts.
Cyclogenesis 2 –
Warm and Cold Fronts
cP
mT
In the Ferrell cell, winds have a strong westerly component, and storms move East.
As they mature and move, Warm Fronts pass over, followed by Cold Fronts.
Cyclogenesis 3 –
Occluded Front forms
Where the cold front catches up, they are called Occluded Fronts. Note symbol.
cP
mT
Cold front catches warm, forcing warm air aloft. Broad precipitation area results.
Occluded Fronts
• The cold front is faster than a warm front and will
eventually close the gap between the fronts, forcing the
intervening warm air upward generating additional rain
mT
Nimbostratus
cP
Rain then covers a wide area
Storm evolution
1. Surface map, so winds cross
Isobars and spiral into the LOW
Along the occluded front warm
moist air is force aloft, resulting in
a broad band of rain
2. Wide precipitation band
mT
mT
cP
cP
Thunderstorms


Thunderstorms form where warm,
humid air is forced upward to
altitudes of up to 15 km (20 for
supercells).
Condensation occurs as the air cools,
releasing latent heat and ensuring that
the rising air remains unstable (warmer
so less dense than surrounding air).
Rising Warm air overshoots into stratosphere
Prevailing Wind Aloft
Freezing Line
Condensation
Line
Surface
Type 1: Isolated Thunderstorms from unstable (hot, moist, low density) air
Droplets coalesce
15 minutes
Only updrafts.
No rain.
Rain, plus gusty winds
caused by downdrafts
Rain and Ice too heavy for updrafts
Lasts 15 to 30 minutes
All downdrafts
http://www.nssl.noaa.gov
Lightning
• Most dangerous
frequently encountered
weather hazard that
people commonly
experience each year.
• Second most frequent
weather killer in the
United States with
nearly 100 deaths and
500 injuries each year,
after floods and flash
floods
Lightning equalizes large charge differences between storm levels and the ground
Ice often positive
+ marks concentrations
of rising and falling ice
2. Synoptically Forced Thunderstorms
Most of the cloud formation associated with a
cold front is actually in the warm moist air mass
Supercells


Severe thunderstorms, or supercells, are
associated with frontal lifting along the cold
front between the continental polar and
maritime tropical air masses in mid-latitude
cyclones. The lower portion of these storms,
the mesocyclone, rotates. They often
contain severe hail and sometimes tornadoes.
Most common during spring and early
summer, when the contrast in temperatures
and moisture between air masses is greatest.
Tropopause
3. The strong
updrafts lift falling
ice and it gets
covered with
another layer of ice.
If this happens
enough times,
large hailstones are
the result.
Tropopause
Hail Formation
2. Strong updrafts pull
in more air from
below.
1. Condensation heats
the moist air, which
accelerates upwards
Supercell Hail
Tornadoes




Funnel clouds that rotate at speeds of
up to 500 km/hr beneath supercells.
Ranked from F0 (weakest) to F5
(strongest) using the Fujita Intensity
scale.
Most move to the east or northeast at
an average speed of approximately 50
km/hr.
Develop in association with
mesocyclones
Wind Shear (different wind
directions at different altitudes)
causes rolling
Updrafts can pick up the roll
Jet Stream winds result from large pressure gradients at the boundary
between mT and cP air, where a large difference in Tropopause height
exists.
The Jet Stream
N
The jet stream and tornadoes: pressure differences in the extreme
Tornados are narrow
areas of extremely
fast updrafts
A strong tornado often is
associated with rapid
removal of updraft air by
the Jet Stream aloft
Jet Stream Aloft
cP
mT
cT
Potential for wind shear with
hot dry unstable cT
Similar temp but different
density than mT which is
moist and therefore “light”
Dry, so dense
The Dry Line
Tornado Conditions
Multiple vortices in an F5
May 3, 1999
Stecker
Supercell tops overshoot into
stratosphere
http://www.nssl.noaa.gov/teams/swat/Cases/990503/A_images/stecker.gif
A Stecker, Oklahoma Home
“Only” F3
May 4th, 1999
A Perfect Hook
Hurricanes &Typhoons
(Tropical Cyclones)
When extremely hot ocean surface
temperatures (>26oC) cause hot, moist
surface air, huge clusters of
thunderstorms develop at sea. If uplift
gets extreme, these can organize into
a gigantic Low with spiral storm lines,
and winds exceeding 74 mph, a
Tropical Cyclone, aka Hurricane
Hurricanes are fueled by Latent Heat of Condensation release.
One day equals the energy production of US for a year
Jeanne
Hurricanes need hot
moist air as fuel. This
is why they weaken
over land
Katrina
Trapped in
House,
swept away
Storm Surge
Storm Surge
Freshwater (rain)
floods cause most
fatalities