Tropical Cyclone Forecasting
Hadley Cell Circulation and the ITCZ
Classifications:
• Tropical disturbance—a disorganized group of
thunderstorms with weak pressure gradients and
little or no rotation.
– 90% (or more) die out before becoming organizing into
more powerful systems.
• Tropical Depression—more organized, wind speeds
less than 60 km/hr
• Tropical Storm—wind speed b/w 60 and 120 km/hr
• Hurricane—wind speeds exceed 120 km/hr.
Lower tropospheric easterly wave in the trade winds
Formation of TC:
• Tropical storms results from
easterly waves that are
troughs of low pressure
initiating CON off the coast of
Africa.
• Tropical wave may or may not
develop into a tropical storm.
• When a hurricane poses a
direct threat to an area, a
hurricane watch is issued,
typically 24-48 hours before
the storm arrives.
• When it appears that the
storm will strike an area w/in
24 h, a hurricane warning is
issued.
Typical Summertime Wind Pattern over the North Atlantic
Different names for the same thing.
• Hurricane—Atlantic and Eastern Pacific
• Typhoon—Extreme western Pacific (typically
larger and stronger than Atlantic hurricanes.
• Cyclone—Indian Ocean and near Australia
Necessary Ingredients:
• Warm water SST at least 81ºF
– Outside of area b/w 20N and 20S the water is usually
too cold.
– Tropical water is warmest in late spring to early fall
explaining the seasonal variation of hurricane
occurrence
• Hurricane Season—June to Novmember
• Coriolis force
– Must be strong enough to prevent filling of the central
low pressure center.
– Lack of coriolis force near the equator prevents
hurricane formation b/w 0 and 5 degrees latitude.
• 2/3 hurricanes form b/w 10 and 20 degrees from the
equator.
Necessary Ingredients:
•Unstable conditions throughout the troposphere
– Along the eastern margins of the oceans, upwelling and
cold currents lead to a statically stable environment.
– As you move west, water temps increase to make
hurricane formation more prevalent.
•Weak Vertical Wind Shear
– Strong Vertical Wind Shear disrupts the vertical
transport of LH, the primary source of hurricane’s
energy, which is supplied by evaporation from the
ocean’s sfc.
Triggering Mechanisms for Hurricane Formation
Hurricane Characteristics:
•
•
•
•
Size—several 100 km (avg 600 km in diameter)
Shape—generally circular with cyclonic bands (in NH)
Lifespan—several days to slightly more than a week.
Structure:
– Very low central pressure (very strong pressure gradient)
– Consists of large number of T-storms arranged in spiral
or pinwheel formation
– Bands of thick clouds and heavy t-storms spiral CCW
around the center of the storm (in NH)
– Bands are separated by areas of weaker uplift and less
intense precip.
– Wind speed and rainfall rate both inc. toward the center,
w/ max 10-20 km away from
Hurricane Characteristics:
• The eye:
– Avg 25 km in diameter can get up to as much as 60 km
– Relatively clear skies, scattered showers
– Slowly descending air
– Calm winds
• Tornadoes in the right forward quadrant as they make
landfall
• Most damage is due to storm surge.
• Atlantic Hurricanes—El Nino years
• Hurricanes are named using alternating Male and Female
names, once a storm reaches Cat. 3 or higher, its name is
retired.
2 main theories on why hurricanes
can exist for long periods of time.
• Conditional Instability of the Second Kind (CISK)
– Hurricanes are maintained by a positive feedback:
• Warm, moist air rises, then forms an anvil where it
begins to sink. This sinking air CON near the sfc due
to the lower pressure created by the LH released from
condensation. This air then rises again to perpetuate
the cycle.
• Wind Induced Surface Heat Energy (WISHE)
– Basically the same process as above, only described in
terms of heat.
– The hurricane removes heat from the ocean surface,
converts it to kinetic energy (wind), and then loses the
heat above the clouds due to radiational cooling.
Illustration of Wind-Induced Surface Heat Exchange
Primary Factors Contributing to Height of Storm Surge
Example
of Storm
Surge
During TC
Landfall
Trajectories of air parcels moving through a modeled
hurricane – over eight days
Typical Cross Section through a Hurricane
Tracks of Some Typical Atlantic Hurricanes
Tracks of Some Unusual Atlantic Hurricanes
Profile of a Hurricane
Conservation of Angular Momentum in a Hurricane
Hurricane Seasons
Satellite Image of Hurricane Georges (1998)
Radar View of Hurricane Georges (1998) off Florida Keys
Radar Estimate Rainfall from Hurricane Georges near
Mobile, Alabama
Double Eye Wall Structure: Hurricane Gilbert (1988)
Dropsondes