HURRICANE DEVELOPMENT
What conditions are required?
•Water temperatures > 26.5°C•Convergence of surface winds•Upper air divergence•Organized mass of thunderstorms•Coriolis force (none at equator)
What causes surface winds to converge?
• Low pressure over ITCZ
• East side of tropical wave(in North Atlantic, these waves areoften remnants of convective stormsfrom continental Africa)
• Front from middle latitudes
Trade wind inversions can prevent storm development even if surface conditions are ideal !
Upper level conditions:
1. Low shear(shear disrupts convection and dispersesheat and moisture; El Niño causes shear overNorth Atlantic, enhances activity over Pacific)
2. Upper level divergence
Strong, upper level divergence
Hurricane Mitchhttp://profhorn.aos.wisc.edu/wxwise/AckermanKnox/chap8/mitch.fli
Organized Convection Theory
1. Thunderstorms form over tropical wave,near ITCZ, or at remnant of midlatitude front (latent heat confined to limited area)
2. If air is much cooler aloft, strong convection occurs
3. Upper air rapidly warms due to condensation
4. Air pressure rises aloft, enhancingdivergence
5. Surface pressure drops
6. Surface air spins counter-clockwise andmoves more quickly near the centre(conservation of angular momentum)
7. Rough seas enhance convection due to friction and increase surface area
Positive feedback; strength only limited by latent and sensible heat