Geography 103
Weather
Lecture No. 10: Tropical Weather (Quiz
3, final)
1. General characteristics of the tropics
(1). Weak
pressure and temperature gradients.
(2). Streamline map
A. Streamline shows the direction of air flow, surface convergence and
divergence.
B. Col or saddle: Point of divergence and convergence.
2. Major synoptic weather systems
(1). ITCZ
(Intertropical Convergence Zone).
(2). Easterly waves.
(3). Hurricanes.
(4). Easterly jet.
3.
The ITCZ (Intertropical Convergence Zone)
(1). Definition
The convergence zone between the trade winds (easterlies) from both hemisphere.
(2). Locations
A. The position varies greatly over Asia.
January: 10 oS (North Australia).
July : 30 oN
(central China)
B. The position is rather invariant in Central and South America
January: 3 oS.
July: 10 oN.
(3). Weather
A. Depends on the strength of convergence.
B. Variable wind directions.
C. May cause heavy rains: Accounts for 80% of rains annually in the tropics.
D. Doldrum (the equatorial low): The mean annual
location of the ITCZ.
E. May develop into hurricanes.
4. The Easterly waves
(1). Definition
Cyclonic
perturbation in the prevailing easterlies (base currents in the tropics).
(2). Moves from east to
west: Unlike the middle-latitude cyclones and fronts.
(3). Weather
A. Ahead of an easterly wave (front portion or to the west of the wave)
(A). Surface divergence and subsidence
North winds: Air currents move toward the larger surface area (air spreads).
(B). Fair weather; at most Sc.
B. Behind an easterly wave (rear or eastern portion)
(A). South winds: Air moves toward the smaller surface area.
(B). Cloudy and stormy.
C. May develop into hurricanes.
5. Tropical cyclones
(1). Lows in the tropics: No
fronts, single air mass (mT).
(2). Classification
A. Tropical disturbance
(A). Mass of thunderstorms associated with ITCZ and easterly waves.
(B). A very weak tropical low with a single closed isobar about its low
pressure center
on a surface weather map.
(C). Wind speed < 20 knots.
B. Tropical depression
(A). Wind speed: 20-34 knots
(B). A weak tropical low which is denoted by several closed isobars about its
center
on a surface weather map..
C. Tropical storm
(A). A strong tropical low with packed isobars
(B). Wind speed: 35 to 64 knots.
D. Hurricane (typhoon)
An intense tropical low with wind speed exceeding 64
knots (74 mph).
6. Hurricanes
(1). Definition
A. Wind speed > 65 knots (74 mph).
B. Central pressure < 990 mb
Low air density is caused by excessive heating.
(2). Structure
A. Diameter: 600 Km (100 to 1000 km).
B. Cyclonic circulation reaches 200 mb.
C. Wind speed may reach 100 to 110 knots.
D. Three regions
(A). Outer region: high clouds (Ci).
(B). Hurricane-force region: Cb, Nb,
squalls
(C). Eye wall
a. All forces acting on an air parcel are balanced.
(Pressure gradient force + Centripetal force) = Coriolis
force.
b. Most stormy: Heaviest clouds (Cb), strongest
winds, heaviest rains.
(D). Eye:
a. Diameter: 10 to 50 km.
b. Rising air currents at eye walls cause compensating downward motion in the
center of eye due to friction or drag force.
c. Light winds, lower humidity, high temperature, subsidence, clear sky or
scattered
clouds.
d. Upper troposphere (200 mb): Air temperature may be
16 oC warmer than the
normal temperature at that location.
(3). Conditions for
hurricane formation
A. Large warm ocean surface with water temperature = 27 oC
(81 oF).
B. Coriolis force exceeds a critical minimum value:
Hurricanes form between 5 oN to 20 oN (or 5 oS
to 20 oS).
C. Weak vertical wind shear (weak upper wind)
(200 mb - 850 mb) wind
speed < 10 knots.
D. Upper
divergence
(A). Western flank of an upper anticyclone.
(B). Eastern flank (ahead) of an extended or polar trough (A trough runs from
high to
low latitudes).
E. Pre-existing disturbances: Easterly waves and the ITCZ.
(4). Frequency
A. High frequency in summer and fall.
B. Highest frequency in western North Pacific.
Western North pacific may have typhoons in mid-winter (December - February) due
to
warm ocean surface temperature in this season.
C. No hurricanes over South America: small and cool ocean.
(5). Life cycle
A. Incipient (Beginning) stage
A cold - core wave perturbation stage.
(A). Ascending air is colder than the surrounding air.
(B). Evaporation process dominates: a cooling process.
B. Mature stage
(A). Hurricane eye forms: Upper air warming by 5 oC.
(B). Warm-core: Release of latent heat plus subsidence in the eye.
(C). Energy source: latent heat (condensation to form clouds).
C. Degeneration stage:
Over continent or high-latitude ocean:
(A). Increase in friction force after landfall.
(B). Reduced moisture (latent heat)supply.
(C). Intrusion of cold air destroys the warm-core structure
Air pressure increases.
D. Rejuvenation
When hurricanes move into a baroclinic zone (Area of
strong instability or temperature gradient).
(6). Tracks (Paths)
A. Hurricanes tend to be steered by a subtropical High and polar trough:
(A). Tropics: Hurricanes travel from east to west (the easterlies or trade
winds).
(B). Temperate: Hurricanes travel from west to east
(the westerlies).
(C). Subtropic: Hurricanes recurve
(change direction almost 180 degrees).
(D). Polar trough: Hurricanes tend to travel into the front part of a polar
trough.
B. Hurricanes tend to follow warm water currents.
C. Weak upper wind
Hurricanes drifted aimlessly (Hurricane Ginger in 1971)
(7). Difference between
tropical and extratropical cyclones
A. Hurricanes occur over warm water surface only.
B. Hurricanes recurve when moving into the westerlies.
C. Hurricanes occur within a single air mass (No fronts).
(8). Saffir-Simpson’s
scale
A. 5 categories: 1 to 5.
Based on air pressure, sustained wind speed, storm surge, and
damage.
(A). Category 1
a. Air pressure: 980 mb or higher.
b. Wind speed: 74 - 95 mph.
(B). Category 5
a. Air pressure < 920 mb.
b. Wind speed > 155 mph.
B. 1900-1996 in USA mainland
(A). Category 5: 3 hurricanes
a. Hurricane Camille in 1969
b. Gilbert in 1988
(B). Category 4: 13 hurricanes
a. The great Galveston hurricane of 1900: 600 deaths.
b. Hurricane Andrew in 1992.
(9). Storm surge
A. A rise in water level induced by the hurricane.
B. Causes
(A). Strong winds pile up water along the coast and inland.
(B). Low air pressure: surge is 1 cm/mb.
a. Average surge: 1-2 m along the coast.
b.Extreme: 7 m (Hurricane Camille along the coast of
Mississippi in 1969).
C.
Severe storm surge: In phase with high tide (bays and inlets).
D.
Intense storm surge on the right sides of hurricanes
(A). Wind speed is added by storm speed on the right-hand side.
(B). Wind pushes water onshore on the right-hand side and offshore on the
left-hand side.
(10). Tornadoes:
Hurricane-spawned tornadoes
A. Most frequent in the right-forward quadrant.
B. Away from the eyewall: Relatively weak winds.
C. Shorter life spans than tornadoes in the central United States.
(11). Hurricane advisories
A. Hurricane Watch
An approaching hurricane will reach land in more than
24 hours by prediction.
B. Hurricane warning
(A). It is expected that a hurricane will make landfall over the United States
within
24 hours.
(B). Probability forecast (example)
a. Warning area covered: 500 km in length.
b. A 25% probability of for a hit near the center of the warning area.
c. A 5% chance along the margins.
d. Overall hit rate 20%.
There is one in five chance of the hurricane making landfall somewhere in the
warning area.
(12). Hurricane Forecasts
Computer models.
(13). Hurricane naming
A. A storm gets a name when reaching tropical storm strength.
B. 1953-1977: Female names.
C. 1978 and later in the eastern Pacific:
Alternately assigned female and male names.
D. 1979 and later for the North Atlantic hurricanes.
E. Infamous storms (great damage): The name is retired for at least 10 years
(Andrew
in 1992 and Hugo in 1989, for example).
7. The tropical easterly jet
(1). Over
the Indian Ocean and SE Asia.
(2). Summer
only at 100 mb.
(3). Strong air temperature
gradient with warmer air to the north (over the Tibet, land surface)
at about the 500-mb level.