chapter_13 - Middle Latitude Cyclones Middle This chapter...

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Unformatted text preview: Middle Latitude Cyclones Middle This chapter discusses: 1. The location, vertical structure, and developmental The stages of middle latitude cyclones stages 1. How upper level convergent winds, abrupt How topographic features, and planetary longwaves may enhance cyclonic development at the surface enhance Polar Front Theory Polar Figure 13.1A Figure 13.1C Figure 13.1B Figure 13.1D Figure 13.1E Figure 13.1F One explanation for development of middle latitude cyclones begins with a stationary front with warm and cold winds in opposite directions (e.g. wind shear). cold A wavelike kink, such as a low pressure system, then wavelike creates a frontal wave, or incipient cyclone. creates As the storm develops into an open wave, a broad band As of precipitation forms ahead of the warm sector. of Wave Cyclone Development Wave A series, or family, series, of cyclones, at various stages of development, may extend across North America. North Energy driving Energy their development originates from kinetic sources such as rising warm air, sinking cold air, and converging air, as well as latent heat of condensation. of Figure 13.2 Cyclone & Anticyclone Paths Cyclone Figure 13.3A Figure 13.3B Many well known paths for low and high pressure systems extend Many across North America, and their interaction helps develop the open wave cyclone. wave Cyclongenesis describes the strengthening of these storms into huge Cyclongenesis unstable waves. unstable Convergence & Divergence Convergence Deepening of cyclones Deepening into explosive cyclogenesis is prohibited when low pressure aloft is directly above the surface low. above In this scenario, the In convergence at the surface low builds up air pressure and fills in the low. the The same stacking of The high pressure, with divergence at the surface, will weaken the anticyclone. anticyclone. Figure 13.4 Storm Vertical Structure Storm Divergence of air aloft occurs as Divergence isobars intervals widen. isobars Low pressure systems deepen and Low intensify (e.g. cyclogenesis) when upper-level divergence is stronger than the surface convergence, which requires a vertical staggering of surface and upper lows. of Figure 13.5 Upper Level Waves Upper Earth's poles are Earth's encircled by 3 to 6 longwaves, or Rosby waves, directing upper level winds around lows at the 500 mb surface. surface. Small disturbances in Small these waves can trigger storms. trigger Figure 13.6 Shortwave Disturbance Shortwave Shortwave ripples Shortwave within the Rosby waves move faster, and propogate downwind into the Rosby troughs and cause them to deepen. cause Barotropic conditions, Barotropic where isobars and isotherms are parallel, then degenerate into a baroclinic state where the lines cross and cold or warm air is advected downwind. advected Figure 13.7A Cyclone Development: Upper Winds Cyclone Figure 13.8A Figure 13.8C Figure 13.8B Atmospheric conditions at the surface and aloft affect cyclogenesis. cyclogenesis. An upper level shortwave can trigger baroclinic An instability, which converges flow aloft, raises high pressure, and supports cold air advection. pressure, Downwind, divergent flow aloft deepens the surface Downwind, low, and warm advection increases rising air flow. low, Eventually the system occludes. Jet Streak Influence Jet Divergence aloft is Divergence enhanced by the polar jet stream, where the jet maximum, or jet streak, forms in the tightly packed pressure gradients. pressure Figure 13.9 Jet Convergence & Divergence Jet Figure 13.10A Figure 13.10B The polar jet forces air convergence aloft upstream of the deepening The open wave cyclone, and then divergence downstream. open When these winds are gone, the cyclone degrades. Summary of Cyclone Weather Summary Figure 13.11 Upper and Upper surface maps illustrate the role of convergence and divergence aloft, and the pattern of clouds, precipitation, and temperatures on the ground. the Conveyor Belt Model Conveyor This model This describes rising and sinking air along three conveyor belts, warm conveyor belt rises with water vapor above the cold conveyor belt which also rises and turns. rises Finally the dry Finally conveyor belt descends brining clearer weather behind the storm. behind Figure 13.12 Comma Clouds Comma Rising and Rising turning moist air, illustrated in the conveyor belt model, condenses into a large commalarge shaped cloud shaped typical of the open wave cyclone. cyclone. This March This 1993 storm wreaked havoc along the East Coast. Coast. Figure 13.13 3/93 Storm Size & Pressure 3/93 Figure 13.14 For the storm of the century, the low pressure center reached 980 For mb, and the storm extended across several southeastern states. mb, 3/93 Storm Temperature Advection 3/93 Figure 13.15 Upper level winds flowed along a deep trough with steep baroclinic Upper cold and warm air advection. cold Storm of Century Path Storm Low Low pressure values and location are charted with time to illustrate the storm track and intensity, moving from Texas to Maine in 2 days. days. Figure 13.16 Vorticity & Cyclone Spin Vorticity Figure 13.18 Figure 13.19 Vorticity describes the spin of an air parcel, which is positive in Vorticity counterclockwise cyclonic flow. counterclockwise Due to the conservation of angular momentum, vorticity Due increases with a decrease in parcel radius (e.g. stretching due to divergence aloft) and increase in earth's latitude. divergence Size of Polar Lows Size Cyclones that Cyclones develop above the polar front, called polar lows, are smaller in size than midthan latitude latitude cyclones. cyclones. They form They during the winter, have warm central cores, strong winds, and generate snow. generate Figure 13.17 Sources of Vorticity Sources Figure 13.20 Figure 13.21 Curvature of upper level isobars and wind, as well as strong Curvature changes in wind speed, or shear, generate the spin of relative vorticity. vorticity. Additional earth vorticity is generated by the earth's spin, Additional and together they comprise absolute vorticity. and Trough to Ridge Vorticity Change Trough Figure 13.22 Anticyclonic spin around a ridge reduces absolute vorticity, but the Anticyclonic convergence and cyclonic spin in the trough enhance the relative and hence absolute vorticity, as illustrated in this upper level diagram. diagram. Vorticity & Vertical Motion Vorticity The 500 mb map vorticity maximum is The a signal that to its east, air is diverging aloft. aloft. If there is also convergence below, then an open wave cyclone will likely deepen. deepen. Hence, 500 mb charts are useful to find the “vort max” and predict potential storms. storms. Figure 13.23 Imaging Vorticity Centers Imaging GOES West GOES satellite infrared imagery of water vapor are useful in identifying swilling vorticities, seen off Pacific Northwest coast. coast. Figure 13.24 Vertical Storm Profile Vertical Surface, 500 mb, and 200 mb Surface, charts are used to illustrate the structure of the February 1983 open wave cyclone exploding over North Carolina. Carolina. The 500 mb chart shows a The shortwave dashed line moving into the longwave trough and baroclinic cold air advection. baroclinic Figure 13.25 February 1983 Vorticity February Lines of equal Lines voriticity are plotted on the 500 mb chart for the February 1983 open wave cyclone that buried the east coast in snowfall. snowfall. Note that the vort Note max is west of the storm center, strengthening the cyclogenesis. cyclogenesis. Figure 13.26 ...
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