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Unformatted text preview: 12.1 Polar Front Theory Polar front and wave cyclones Cyclone life cycle Cyclone formation and movement 12.2 Cyclone Structure and Development Vertical positioning of troughs aloft and lows at surface Upperlevel waves Polar jet streams Overview We've learned that low pressure is often accompanied by bad weather The lowest pressure occurs at the center of an extratropical or midlatitude cyclone A group of scientists from Norway developed the theory describing the development of cyclones It is Known as polar front theory The theory was originally applied to cyclones developed on the polar front, and later was modified to describe midlatitude cyclones 12.1 Polar Front Theory Six steps in the idealized life cycle of a wave cyclone
Stationary Front Frontal Wave Open Wave Maturing Phase Mature Occluded Dissipated Stage Development of Midlatitude Cyclone (a) (b) (c) Nearly straight flow aloft surface stationary front Upper trough provides vertical motion supporting surface cyclone Surface cyclone occludes and dissipates without diverging flow aloft Formation of Cyclones and Anticyclones Lows are named for regions in which they form, especially in winter Lows often form in the eastern slop of Rockies Lows also form in Gulf of Mexico and Alaska. Highs form Southern Canada. Typical Paths of MidLatitude Cyclones Surface lows often move in the same direction as the 500 mb winds, at about half the speed Anticyclone SE; Cyclone NE Surface lows move at 16 knots summer, 27 knots winter (what is knot?) 500 mb is about 5,500m (3.5 mile) above sea level 300 mb is roughly 9000m (5 miles). Faster winter speed due to stronger upper level flow in winter 1 knot is 1 nautical mile per hour, = 1.1 (statue) mile per hour Wrap Up Polar Front Theory Midlatitude cyclones begin from polar front when local low pressure area develops Cyclones life cycle spans six stages from stationary front to matured low pressure center. Cyclones or storms in U.S. form in some preferred regions and typically move eastward or northeastward Cyclones move at an average of 16 kts summer, 27 kts winter 12.2 Cyclone Structure and Development What controls surface pressure total mass of air column above surface? If there is a net removal of air from the column, Warming the column causes it to expand "spill out" or removal of air lowers surface pressure Recall surface pressure decreases; and vice versa Click here to activate th Cooling the column eventually increase in surface pressure Convergence and Divergence Convergence (Con) is local piling up of air, it increases air density in the column Divergence (Div) spreading out of air
http://www.islandnet.com/ ~see/weather/elements/wh atgoesup3.htm Con/Div has lots to do with storm development If upperlevel pressure systems were always arranged this way, pressure systems would always die rapidly Convergence and Divergence cont'd Con/Div can occur due to both change in wind direction and in speed Either way air piles up or spreads out Think of cars on a highway Merge: convergence Branch: divergence Idealized Vertical Structure for Cyclone Behind cold front is cold air both at surface and aloft 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 Upper low is located to the north and west of the surface low Above the closed surface low is a trough Idealized Structure cont'd
The surface low is located directly beneath an area of divergence downstream from the base of the trough When upper level divergence is stronger than surface convergence: surface pressure drops Low intensifies (deepens) If upper level divergence is less than surface convergence: Surface pressure rises System weakens All align Up Together Surface low is ahead of upperlevel trough where divergence exists The surface convergence and upperlevel divergence is coupled by upward motion Surface high is behind the upper level trough and thus associated with downward motion
C D UpperLevel Waves Longwaves Waves always show up in the pressure pattern, due to the unequal heating of the earth Areas of high and low pressure create the waving structure such as the upperlevel trough just discussed Wavelengths of thousands of km are longwaves Longwaves can remain stationary, move very slowly eastward, or even move westward Longwaves control daysweek ahead Upper Level Waves Shortwaves Shortwaves small disturbances or ripples embedded in longwaves The shorter is the wavelength the faster it moves So shortwaves move through longwaves, and act to shape the longwaves A shortwave approaching a longwave trough causes the trough to deepen; a shortwave approaching a longwave ridge weakens the ridge Shortwaves determine weather hourday ahead Click to animate this figure Jet Stream and Cyclone Development
Jet stream is the axis of maximum winds aloft It provide convergence in entrance and divergence on exit Its divergence region, while overlapping with surface low, supports cyclone development This is partly the reason you hear jet stream on TV often ~ 9km high Jet Stream and Cyclone Cont'd As a polar jet along with its div/con areas swings over a developing midlatitude cyclone, area of divergence draws surface air up (see a) Its area of convergence allows cold air to sink. When the surface storm moves northeastward, it no longer has upperlevel supports and dies out (see b) Wrap Up Structure and Development Midlatitude cyclones are deep lows supported by favorable upperlevel flows Upperlevel divergence is a necessary ingredient for developing cyclone Surface low ahead of upperlevel trough where divergence exists tends to develop The jet stream accompanying divergence favors cyclone deepening Shortwave disturbances to upperlevel flows can trigger surface pressure changes If divergence aloft is greater than lowlevel convergence, the cyclone deepens pressure decreases; if the opposite is true, the cyclone dies out ...
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- Spring '12