Chapter 16: Hurricanes
Weather in the tropics
Recall from the 3-cell model of air circulation, Fig 11.2 :
In the tropics:
• pressure gradient is small
• winds blow east to west
• air converges at ITCZ
• Coriolis force is zero at equator
• Air sinks at ~ 30° latitude (Subtropical high)
ccw
L
Middle latitudes
H
cw
Streamlines
H
cw
Tropics
L
ccw
So, in the tropics, an easterly wave, or tropical wave, which is a low looks like a ridge in
middle latitudes.
Get rain on east side of trough.
Hurricane Formation - Requirements
1.
2.
3.
4.
Warm water
Humid air
Light winds aloft
Convergence of winds at surface
• Remnant of front from mid-latitudes
• ITCZ (not at equator)
• Easterly wave
5. Weak trade wind inversion (falling air from subtropical high creates stable
atmosphere)
6. Diverging upper level winds
Two theories of hurricane driving mechanism
1. Organized Convection Theory
Cold air initially above organized thunderstorms creates unstable atmosphere.
Cumulus clouds grow and latent heat released warms upper air and creates H above L
at surface. Upper air diverges, deepening surface low. CCW flow at surface causes
rough seas, feeding storm.
2. Heat Engine Process
Heat taken in near ocean surface (sensible and latent) is converted into kinetic energy
of wind and lost to cold clouds at top of storm.
Tc
W = QH – QC
Qc
⇑
W
E=1-
TC
TH
E is efficiency
QH
⇑
E=
TH
W
QH
Stages of Development:
1.
2.
3.
4.
Tropical Disturbance (or wave) – Mass of thunderstorms with slight circulation
Tropical Depression - 20 knots wind (23 mph), several closed isobars around center
Tropical Storm - 35 knots wind (40 mph), deeper low, storm gets name!
Hurricane - 64 knots wind (74 mph) sustained
See table 16.2 Saffir-Simpson Scale
Hurricane
Mid-Latitude Cyclone
Energy from warm water
and latent heat
Energy from horizontal
temperature contrast
Weakens with height
Strengthens with height
Eye in center, falling air
warm, calm
Rising air in center
CCW winds around surface
low
CCW winds around surface
low