Download AA11

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)
Spring: Too cold,
ITCZ too far south
The First South Atlantic Hurricane
Tropical Wave
What causes surface winds to converge?
•
Low pressure over ITCZ
•
East side of tropical wave
(in North Atlantic, these waves are
often remnants of convective storms
from 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 disperses
heat and moisture; El Niño causes shear over
North Atlantic, enhances activity over Pacific)
2. Upper level divergence
Strong, upper level divergence
Hurricane Mitch
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, enhancing
divergence
5. Surface pressure drops
6. Surface air spins counter-clockwise and
moves 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