Middle-Latitude Cyclones MET 240 Middle-Latitude Cyclonic Storms We?ve learned that lower pressure is often accompanied by precipitation But how does low pressure deepen into an extratropical or mid-latitude cyclonic storm? A group of scientists from Norway developed the theory describing the development of these storms just after World War I Known as polar front theory Recall the polar front as part of the General Circ Describes the birth, growth, and decay of these systems Used to describe any mid-latitude system Polar Front Theory Six steps in the idealized life cycle of a wave cyclone (just like a wave in the ocean) Stationary Front Frontal Wave Open Wave Maturing Phase Mature - Occluded Dissipated Stage Stationary Front Recall fronts lie in troughs of lower pressure Air of different temp on either side Winds parallel to the boundary ? but opposite directions Sets up cyclonic wind shear Pen example Frontal Wave Sometimes, a wavelike kink forms in the front Also known as an incipient cyclone Cold front pushes S, warm front N Lowest pressure in middle Precip where we would expect it Winds aloft act as steering flow, moves system E-NE Becomes open wave in 12-24 hrs Open Wave Warm sector lies between regions of precip Energy from 2 sources: Rising and sinking air: Density differences result in frontal forcing Latent heat release from condensation Central pressure lowers, increasing the speed of cyclonically flowing winds Spiraling inward Maturing Phase As open wave moves eastward, pressure decreases, winds increase Cold front catches up with warm front Area of warm sector decreases Mature - Occluded Eventually, cold front overtakes warm front and system is occluded Storm often most intense, precip widespread Triple point ? intersection of three fronts This point resembles the open wave stage Thus, a new wave (secondary low) can develop and intensify at the triple point Central pressure gradually increases Dissipated Stage Storm dissipates because cold air is on both sides of occlusion Warm sector far removed from center of storm Storm has lost a major energy source Entire cycle lasts a few days to over a week Polar Front Theory in Motion Click here for a demonstration ?Family? of Cyclones Lows are in various stages of development Storms can develop over central US then occlude off English coast a week later Few, if any, storms obey this model exactly, it is idealized Polar front developed into a series of loops At the apex of each loop is a cyclone Common Areas for Cyclone Formation Cyclogenesis ? any development or strengthening of a mid-latitude cyclone Gulf of Mexico, Atlantic east of the Carolinas, eastern slope of mountain ranges W-E flow crossing a N-S mountain will curve cyclonically on the lee side, leading to lee cyclogenesis Gulf stream in Atlantic can cause low pressure to suddenly spring up along the front ? nor?easters Explosive cyclogenesis or ?bomb? describes a storm that deepens > 24 mb in 24 hours Typical Paths of Cyclones and Anticyclones Lows named for regions in which they form Lows generally move E ? NE Highs generally move SE ? E What causes some storms to intensify while others decay rapidly???? Vertical Structure of Pressure Systems What controls surface pressure? Total mass of air above surface If there is a net removal of air in column, sfc pressure decreases Warming column causes it to expand ? less dense ? lower sfc pressure Cooling column causes an increase in sfc pressure ?Any process that warms the air can contribute to a decrease in surface air pressure, whereas any process that cools the air can contribute to an increase in surface air pressure? Convergence and Divergence Patterns Only at the surface do winds converge Convergence ? piling up of air, increases density in the column Will fill in the low Same can be applied to divergence ? spreading out of air If upper level pressure systems were always arranged this way, pressure systems would always die rapidly Idealized Model of Vertical Structure Behind cold front there is both cold air at sfc and aloft (sfc high) A cold column of air is ?compressed? ? meaning most air molecules are near the surface Thus an area of relatively low pressure will be present aloft above the cold surface air Idealized Model of Vertical Structure Upper low is located to the north and west of the surface low Above the closed surface low is a trough The surface low is located directly beneath an area of divergence downstream from the base of the trough Air converges at surface and diverges near tropopause A Model for Cyclone Development When upper level divergence is stronger than sfc convergence: Sfc pressures drop Low intensifies (deepens) When upper level divergence is less than sfc convergence: Sfc pressures rise System weakens Upper Level Waves - Longwaves Waves will show up in the pressure pattern of the upper levels, due to the unequal heating of the earth Areas of high and low pressure create the waving structure ? such as the upper level trough just discussed Wavelengths of many thousands of km ? longwaves 3-6 across globe Fewer waves, longer wavelength - zonal Upper Level Waves - Shortwaves Shortwaves ? small disturbances or ripples embedded in longwaves Shorter the wavelength ? faster it moves downstream (speed proportional to 700 mb flow) Longwaves can remain stationary or move very slowly eastward So shortwaves move through longwaves, and act to shape the longwaves A shortwave approaching a longwave trough will cause the trough to deepen, a shortwave approaching a longwave ridge will weaken Movement of Shortwaves and Longwaves Click to animate this figure Barotropic vs. Baroclinic Barotropic ? height lines are parallel to isotherms ? no temp advection occurs Baroclinic ? isotherms cross contour lines ? temp advection occurs Where do you see regions of baroclinicity? What impact do the shortwaves have on baroclinicity? Temperature Advection Transport of air of different temperature via winds blowing across isotherms Cold advection ? transport of colder air to a warmer region Warm advection ? transport of warmer air to a colder region Shortwaves disturb flow at mid-levels ? allow for intensification of surface system Baroclinic Wave Theory of Developing Cyclones For a surface cyclone to develop, proper conditions must be present both at the surface and at mid-levels How do conditions at these two levels interact to produce a storm? The theory of these interactions is known as the baroclinic wave theory of developing cyclones When a shortwave disturbs the flow at mid-levels, it creates an instability in the flow known as baroclinic instability What exactly happens here? Baroclinic Wave Theory of Developing Cyclones Longwave trough at 500 mb lies parallel to and above a surface stationary front Shortwave disturbs flow aloft, initiating temp advection Upper trough intensifies and provides vertical motions for surface cyclone to develop Surface storm occludes, cold pool of air above it Stage 1 ? Surface and Upr Lvl ?Setup? Low pressure can be initiated via an approaching shortwave aloft even if no clear stationary front is present Surface low pressure systems are initiated from the top down, not the bottom up!!! Stage 2 ? Shortwave Approaches As shortwave deepens the longwave trough, air converges upstream from the base of the trough (1) and diverges downstream (2) Divergence aloft initiates rising air that converges at the surface (like a vacuum cleaner) Surface air develops cyclonic spin ? sets fronts in motion Results in temp advection that is felt all the way up to 500 mb CON DIV Stage 2 Cold air advection toward base of trough Increases density of air in column below 500 mb ? lowering the height of the 500 mb surface here Recall low heights on a constant pressure surface are the same as lower than average pressures Thus, trough deepens Causes winds aloft to increase (larger pressure gradient) ? increasing advections CON DIV Stage 2 Warm air advection acts to build downstream ridge through raising 500 mb heights The overall effect of differential temperature advection is to intensify the wave As trough aloft deepens, region of divergence strengthens, deepening the surface cyclone Warm air advection ? rising (2) Cold air advection ? sinking (1) Latent heat release occurs ? warming air in the column and thus further deepening the surface low Stage 3 Eventually, warm air trapped aloft will curl around the north side of the low Low pressure will no longer deepen ? as it has moved out from region of upper level divergence Surface low cut off from warm air at surface Cold, dry air drawn into surface low Stage 3 An upper level pool of cold air lies above the surface low Upper low can break away entirely from mean flow ? cut-off low Upper low above surface low ? system is vertically stacked Little to no temp advection Winds weaken, pressure rises, low fills Baroclinic Wave Theory of Developing Cyclones Click here to animate this figure Summary of a Developing Wave Cyclone Must have upper-level support Must have an upper level trough of low pressure west of the surface low Short waves disturb flow aloft, causing regions of differential temp advection ? intensifying the upper level trough Zones of converging and diverging air associated with rising and sinking air are needed for storm?s growth Summary of a Developing Wave Cyclone
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