Directly beneath an area of divergence downstream

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Unformatted text preview: ps 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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This note was uploaded on 03/18/2012 for the course UNDERSTAND 212 taught by Professor Qi during the Spring '12 term at Saint Louis.

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