Download AOSC 200 Lesson 17

AOSC 200
Lesson 17
Birth of a an Extratropical Cyclone
LIFE CYCLE OF AN EXTRATROPICAL
CYCLONE
• THE LEFT HAND SIDE SHOWS HOW BJERKNES
DEPICTED THE LIFE CYCLE.
• FORM ALONG THE LINE BETWEEN THE POLAR
AIR MASS AND THE MARITIME TROPICAL AIR
MASS.
• SHEARING ACTION OF OPPOSING WINDS
PRODUCES CYCLONIC MOTION.
• UNDER SUITABLE CONDITIONS FRONTAL
SURFACE WILL ASSUME A WAVE SHAPE.
• OPEN WAVE DEVELOPS COLD AND WARM
FRONTS
• COLD FRONT CATCHES UP WITH WARM FRONT
• CYCLONE DISSIPATES
Fig. 10.6
Box 10.1
Box 10.1
Cyclones over the Rockies
• In order for Cyclones formed over the
Pacific to each the mid-west they have to go
over the Rockies.
• This squeezes the cyclone down, which
increases the radius of rotation
• This decreases the rate of rotation
(conservation of angular momentum)
• The cyclone appears to weaken.
• East of the Rockies the cyclone expands
and regains its full rate of rotation
Fig. 10.11
Formation of Cyclones
• .CLOSE RELATIONSHIP BETWEEN SURFACE
DISTURBANCES AND THE FLOW IN THE JET
STREAM.
• .FOR A MID-LATITUDE CYCLONE TO
FORM:CYCLONIC FLOW MUST BE
ESTABLISHED
• .INWARD FLOW OF AIR NEAR SURFACE MUST
BE SUPPORTED BY OUTFLOW ALOFT.
• DIVERGENCE AND CONVERGENCE ALOFT
• TOTAL SPIN / CYCLONIC HEIGHT = CONSTANT
• .VORTICITY - TENDENCY OF AIR TO ROTATE IN
A WHIRLPOOL LIKE VORTEX – SPIN AROUND A
VERTICAL AXIS
Fig. 10.13
TRAVELING CYCLONES AND
ANTI-CYCLONES
• .FLOW ALOFT DETERMINES HOW RAPIDLY THE
PRESSURE SYSTEMS ADVANCE AND THE
DIRECTION THEY WILL TAKE.
• .CYCLONES TRAVEL AT ABOUT ONE HALF OF
THE FLOW VELOCITY AT 500 MB, ABOUT 20 TO
50 KM PER HOUR.
• .TEND TO TRAVEL IN AN EASTERLY DIRECTION
AT FIRST, BUT THEN FOLLOW A NORTHEASTERLY PATH.
• .MOST PACIFIC LOWS DO NOT CROSS THE
ROCKIES, BUT MAY RE-DEVELOP ON THE LEE
SIDE OF THE MOUNTAINS.
Visible image of super thunderstorm from GEO satellite
Fig. 11.2a
IR image of the same super thunderstorm
Fig. 11.2b
Satellite derived image of the lifted index. Gray/white – no
data. Yellow/red lifted index less than -4 – possibility of
severe weather
Fig. 11.4
THUNDERSTORM
• IS A CLOUD OR CLUSTER OF CLOUDS THAT
PRODUCES THUNDER, LIGHTNING, HEAVY
RAIN, AND SOMETIMES HAIL AND TORNADOS
• CAN DIVIDE THUNDERSTORMS INTO TWO
MAIN TYPES
ISOLATED THUNDERSTORMS PRODUCES
WITHIN A
WARM HUMID AIR MASS
SEVERE THUNDERSTORMS PRODUCED BY
FORCEFUL LIFTING
•
IN THE USA, AIRMASS THUNDERSTORMS
GENERALLY OCCUR IN WARM MOIST AIR - mT
• LIFTING CAN BE BY FRONTS OR
OROGRAPHICALLY
Life cycle of an ordinary thunderstorm cell
Fig. 11.7
THUNDERSTORM
CUMULUS STAGE
• CUMULUS STAGE
• REQUIRES CONTINUOUS SOURCE OF
WARM MOIST AIR
• EACH NEW SURGE OF WARM AIR RISES
HIGHER THAN THE LAST
• STRONG UPDRAFTS
• FALLING PRECIPITATION DRAGS AIR
DOWN - DOWNDRAFT
• ENTRAINMENT
THUNDERSTORM
MATURE STAGE
• SHARP COOL GUSTS AT SURFACE
SIGNAL DOWNDRAFTS
• UPDRAFTS EXIST SIDE BY SIDE WITH
DOWNDRAFTS
• IF CLOUD TOP REACHES TROPOPAUSE
UPDRAFTS SPREAD LATERALLY - ANVIL
SHAPE
• TOP OF ICE LADEN CIRRUS CLOUDS
• GUSTY WINDS, LIGHTNING, HEAVY
PRECIPITATION, HAIL
THUNDERSTORM
DISSIPATING STAGE
• DOWNDRAFT AND ENTRAINMENT
DOMINATE
• NO UPDRAFT
• THUNDERSTORM LOSES ENERGY
SOURCE
THUNDERSTORM GROWTH AND
DEVELOPMENT
• AIR NEEDS TO BE UNSTABLE
• DRYLINE - LINE BETWEEN cT AND mT AIR
MASSES-LEADS TO UNSTABLE AIR
• LIFTED INDEX - DIFFERENCE BETWEEN THE
ACTUAL TEMPERATURE AT 500 MB, AND
TEMPERATURE OF AIR PARCEL AFTER
ADIABATIC EXPANSION TO 500 MB
• LIFTED INDEX IS NEGATIVE, AIR IS UNSTABLE
• VERTICAL WIND SHEAR CAN ‘SPIN UP’
THUNDERSTORM
• SEVERE THUNDERSTORMS - mT MEETS Cp.
• GREATEST CONTRAST - SPRING AND EARLY
SUMMER
• CAPPING INVERSION
An ordinary airmass thunderstorm
Fig. 11.8