Ch. 8 Baroclinic Instability and Cyclogenesis (Based on Dr. Matt Eastin’s Lecture Notes) Baroclinic Instability and Cyclogenesis • Basic Idea • Evidence of Initiation from the Flow Patterns Special Types of Cyclogenesis • The “Bomb” • Polar Low • “Zipper” Low • Dryline Low • Thermal Low Advanced Synoptic M. D. Eastin Baroclinic Instability The Basic Idea: “Coin Model” • Consider a coin resting on its edge Center of (an “unstable” situation) Gravity • Its center of gravity (or mass) is located h some distance (h) above the surface • As long as h > 0, the coin has some “available potential energy” . If the coin is given a small push to one side, it will fall over and come to rest on its side (a “stable” situation) . The instability was “released” and “removed” • Its center of gravity was lowered and thus its potential energy was decreased • The coin’s motion represents kinetic energy that was converted from the available h ≈ 0 potential energy Advanced Synoptic M. D. Eastin Baroclinic Instability The Basic Idea: “Simple Atmosphere” Tropopause Light • Consider a stratified four-layer atmosphere with the most dense air near the surface at the pole and the P least dense near the tropopause above the equator (an “unstable” situation) • Each layer has a center of gravity ( ) located some distance above the surface Surface Heavy • Each layer has some available potential energy • The entire atmosphere also has a center of gravity ( ) Equator T Pole and some available potential energy . If the atmosphere is given a small “push” (e.g. a weak Light cyclone) then the layers will move until they have adjusted their centers of gravity to the configuration that provides lowest possible center of gravity for the atmosphere (the most “stable” situation) . The baroclinic instability was released and removed • Each layer’s motion represents a portion of the total Heavy atmospheric kinetic energy that was converted from Equator Pole the atmosphere’s available potential energy Advanced Synoptic M. D. Eastin Baroclinic Instability The Basic Idea: “Simple Atmosphere” Tropopause Light • Several “events” occurred during this process in our simple atmosphere that are commonly P observed in the real atmosphere: • Kinetic energy (or wind) was generated similar to the increase in winds as a weak low pressure system intensifies Surface Heavy • Warm (less dense) air was lifted over Equator T Pole cold (more dense) air in a manner very similar to fronts • There is a poleward transport of warm air Light and an equatorward transport of cold air similar to the typical temperature advection pattern around a low pressure system. Heavy Equator Pole Advanced Synoptic M. D. Eastin Baroclinic Instability Basic Idea: • Animation of this simple baroclinic instability release process: http://www.atmos.washington.edu/~hakim/542/cyclone.html Advanced Synoptic M. D. Eastin Baroclinic Instability The Basic Idea: Cold Air . Most mid-latitude cyclones and Cold Air anticyclones evolve via this baroclinic instability release • Intense (minimal) solar heating Warm Air Warm Air at the equator (pole) regularly develops the “unstable situation” with a strong north-south temperature gradient Cold Air • A little forcing (or “push”) along the gradient (a stationary front) and the baroclinic instability Warm Air process can spontaneously begin to generate a low pressure system Cold Front Warm Front • Recall: Polar Front Theory Warm Warm Air Air Cold Cold Air Air Advanced Synoptic M. D. Eastin Baroclinic Instability and Cyclogenesis Advanced Synoptic M. D. Eastin Baroclinic Instability and Cyclogenesis Evidence of Possible Initiation from Flow Patterns: Question: When is the atmosphere “ripe” for the baroclinic instability process to occur, and thus develop a strong cyclone? Answer #1: When a diffluent trough is Example of a Diffluent Trough above a weak surface low Diffluent troughs often have large PVA downstream (east) of the trough axis Recall: QG Omega Equation PVA → rising motion Note how the distance → surface pressure between the 6 height decrease contours increases downstream of the → cyclone formation trough axis Initiates WAA, CAA, and releases the baroclinic instability process Advanced Synoptic M. D. Eastin Baroclinic Instability and Cyclogenesis Evidence of Possible Initiation from Flow Patterns: Question: When is the atmosphere “ripe” for the baroclinic instability process to occur, and thus develop a strong cyclone? Answer #2: When a negatively tilted trough Example of a Negatively Tilted Trough is above a weak surface low Y Troughs with negative tilt often have large PVA downstream (or east) of the trough axis Recall: QG Omega Equation X PVA → rising motion Note how the slope → surface pressure of the trough axis decrease is negative in the X-Y coordinate → cyclone formation system Initiates WAA, CAA, and the baroclinic process Advanced Synoptic M. D. Eastin Baroclinic Instability and Cyclogenesis Evidence of Possible Initiation from Flow Patterns: Question: When is the atmosphere “ripe” for the baroclinic instability process to occur, and thus develop a strong anticyclone? Answer #1: When a confluent trough is Example of a Confluent Trough above a weak surface high Confluent troughs often have large NVA upstream (west) of the trough axis Note how the distance between the 4 height Recall: QG Omega Equation contours decreases downstream of the NVA → sinking motion trough axis → surface pressure increase → anticyclone formation Initiates WAA, CAA, and the baroclinic process Advanced Synoptic M. D. Eastin Baroclinic Instability and Cyclogenesis Evidence of Possible Initiation from Flow Patterns: Question: When is the atmosphere “ripe” for the baroclinic instability process to occur, and thus develop a strong anticyclone? Answer #2: When a positively tilted trough Example of a Positively Tilted Trough is above a weak surface high Troughs with positive tilt often Y have large NVA upstream (or west) of the trough axis Note how the slope of the trough axis Recall: QG Omega Equation is positive in the X-Y coordinate NVA → sinking motion system → surface pressure increase → anticyclone formation X Initiates WAA, CAA, and the baroclinic process Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis The “Bomb”: Definition: A mid-latitude low pressure system where the central pressure drops 24 mb over the course of 24 hours (a rate of 1 mb/hr) Common Characteristics: Location of Bombs during three NH winters • Explosive cyclogenesis (nor’easters) • Occur over the ocean near strong Kuroshio gradients in sea-surface temperature Current (northern edge of Gulf Stream) • Occur primarily during the winter • Often triggered by a high pressure dropping down along the east coast with a cold, dry continental air • Form downstream of a diffluent trough • Develop strong warm and cold fronts • Extreme danger to shipping (“The Perfect Storm”) Gulf • Often produce coastal blizzards Stream From Sanders and Gyakum (1980) Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis The “Bomb”: Important Physical Processes: Diffluent Trough • Unusually strong WAA at upper levels (500-200 mb) downstream from the trough axis that helps provide a deep column of ascent • Very strong low-level WAA (CAA) downstream (upstream) Strong WAA From Bluestein (1993) Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis The “Bomb”: Important Physical Processes: • Strong surface heat and moisture fluxes from the warm ocean to the cold, dry continental air lead to strong low-level diabatic heating and deep convection Warm Core • SST gradient and surface heat fluxes can act to enhance low-level WAA • Many bombs develop low-level WAA “warm cores” due to strong WAA WAA and diabatic heating effects, allowing bombs to resemble tropical cyclones Note (many bombs exhibit “eyes”) The “eye” From Bluestein (1993) Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis 15-16 April 2007 “Bomb Cyclone” 15 April 1200Z MSLP = 993 mb 850 mb 300 mb Heights Heights Temps Winds Surface Press GOES - IR Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis 15-16 April 2007 “Bomb Cyclone” 16 April 0000Z MSLP = 979 mb 850 mb 300 mb Heights Heights Temps Winds Surface Press GOES - IR Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis 15-16 April 2007 “Bomb Cyclone” 16 April 1200Z MSLP = 968 mb 850 mb 300 mb Heights Heights Temps Winds Surface Press GOES - IR Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis 15-16 April 2007 “Bomb Cyclone” Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis The Polar Low: Common Characteristics: • Develop within a polar air mass without strong low-level WAA with links to the Tropics • Diameters < 1000 km • Exist for < 2 days • Occur primarily during the winter in the Northern Hemisphere over the polar oceans • Southern Hemisphere varieties are less frequent, less intense, and have shorter lifetimes From Bluestein (1993) Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis The Polar Low: Common Characteristics: • Triggered over strong surface thermal gradient (snow boundary, coastline, ice field edge, or gradient in sea-surface temperature) . Development due to differential surface heat and moisture fluxes across the boundary producing local diabatic heating, shallow baroclinic instability, weak “WAA”, and often deep convection • Some polar lows develop low-level warm cores and “eyes” Polar Low over the Barents Sea on 27 February 1987 Advanced Synoptic M. D. Eastin Special Types of Cyclogenesis The “Zipper” Low: Common Characteristics: •