aosc 200 lesson 17. birth of a an extratropical cyclone
TRANSCRIPT
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
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
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
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 NORTH-EASTERLY PATH.
• .MOST PACIFIC LOWS DO NOT CROSS THE ROCKIES, BUT MAY RE-DEVELOP ON THE LEE SIDE OF THE MOUNTAINS.
Fig. 11.4
Satellite derived image of the lifted index. Gray/white – no data. Yellow/red lifted index less than -4 – possibility of severe weather
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
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