Section 5: Tropical Cyclones
5.1 Introduction
5.2 Climatology and Structure
5.3 Angular Momentum and Thermal Wind
5.4 Theories for Genesis
5.5 Maximum Potential Intensity
5.6 Tracks
5.7 Hot Issues
Many Images in this lecture are courtesy of Kerry Emanuel
Resources:
http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html
http://wind.mit.edu/~emanuel/
5.1 Introduction: Definitions
The terms "hurricane" and "typhoon" are regionally specific names
for a strong "tropical cyclone". A tropical cyclone is the generic term
for a non-frontal synoptic scale low-pressure system over tropical or
sub-tropical waters with organized convection (i.e. thunderstorm
activity) and definite cyclonic surface wind circulation (Holland 1993).
5.1 Introduction: Definitions
Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt,
39 mph) are called “tropical depressions”. Once the tropical cyclone reaches winds of
at least 17 m/s (34 kt, 39 mph) they are typically called a “tropical storm” and assigned
a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called:
"hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the
dateline, or the South Pacific Ocean east160E)
"typhoon" (the Northwest Pacific Ocean west of the dateline)
"severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast
Indian Ocean east of 90E)
"severe cyclonic storm" (the North Indian Ocean)
"tropical cyclone" (the Southwest Indian Ocean)
(Neumann 1993)
5.1 Introduction: Definitions
2007
2008
2009
2010
2011
2012
Andrea
Arthur
Ana
Alex
Arlene
Alberto
Barry
Bertha
Bill
Bonnie
Bret
Beryl
Chantal
Cristobal
Claudette
Colin
Cindy
Chris
Dean
Dolly
Danny
Danielle
Don
Debby
Erin
Edouard
Erika
Earl
Emily
Ernesto
Felix
Fay
Fred
Fiona
Franklin
Florence
Gabrielle
Gustav
Grace
Gaston
Gert
Gordon
Humberto
Hanna
Henri
Hermine
Harvey
Helene
Ingrid
Ike
Ida
Igor
Irene
Isaac
Jerry
Josephine
Joaquin
Julia
Jose
Joyce
Karen
Kyle
Kate
Karl
Katia
Kirk
Lorenzo
Laura
Larry
Lisa
Lee
Leslie
Melissa
Marco
Mindy
Matthew
Maria
Michael
Noel
Nana
Nicholas
Nicole
Nate
Nadine
Olga
Omar
Odette
Otto
Ophelia
Oscar
Pablo
Paloma
Peter
Paula
Philippe
Patty
Rebekah
Rene
Rose
Richard
Rina
Rafael
Sebastien
Sally
Sam
Shary
Sean
Sandy
Tanya
Teddy
Teresa
Tomas
Tammy
Tony
Van
Vicky
Victor
Virginie
Vince
Valerie
Wendy
Wilfred
Wanda
Walter
Whitney
William
5.1 Introduction: Definitions
The USA utilizes the Saffir-Simpson hurricane intensity scale (Simpson and Riehl 1981)
for the Atlantic and Northeast Pacific basins to give an estimate of the potential flooding
and damage to property given a hurricane's estimated intensity:
Saffir-Simpson Scale
Minimum
central
pressure
SaffirSimpson
Categor
y
mph
m/s
kts
mb
ft
m
1
74-95
33-42
64-82
> 980
3-5
1.0-1.7
2
96-110
43-49
83-95
979-965
6-8
1.8-2.6
3
111-130
50-58
96-113
964-945
9-12
2.7-3.8
4
131-155
59-69
114-135
944-920
13-18
3.9-5.6
5
156+
70+
136+
< 920
19+
5.7+
Maximum sustained wind
speed
Storm surge
Note : Classification by central pressure was ended in the 1990s, and wind speed
alone is now used. These estimates of the central pressure that accompany each
category are for reference only.
5.1 Introduction: Tropical Cyclones are Important!
"The death toll in the infamous Bangladesh Cyclone of 1970 has had several
estimates, some wildly speculative, but it seems certain that at least 300,000
people died from the associated storm tide [surge] in the low-lying deltas."
(Holland 1993)
The largest damage caused by a tropical cyclone as estimated by monetary
amounts has been Hurricane Katrina (2005) as it struck the Bahamas, Florida
and Louisiana: US $40.6 Billion in insured losses, and an estimated $81
billion in total losses. However, if one normalizes hurricane damage by
inflation, wealth changes and coastal county population increases, then
Katrina is only the third worst, after the 1926 Great Miami Hurricane and the
lethal 1900 Galveston Hurricane. If the 1926 storm hit in 2005, it is estimated
that it would cause over $140 billion in damages, and the 1900 storm about
$92 billion (Pielke, Gratz, Landsea, Collins, Saunders, Musulin 2006).
5.2 Climatology and Structure: Regions of Formation
• low latitudes, 5-20o (not at the equator): f important
• none in SE Pacific or S. Atlantic: SSTs important
• more appear in the Pacific than in the Atlantic: SSTs important
5.2 Climatology and Structure: Seasonality
Most Tropical Cyclones occur in Summer and early Fall
5.2 Climatology and Structure: Tracks
Tropical cyclones generally recurve polewards from their easterly track
5.2 Climatology and Structure: Conditions for occurrence
Gray (1979)
1. Warm ocean waters (of at least 26.5°C [80°F], some say 26oC)
throughout a sufficient depth (unknown how deep, but at least on the
order of 50 m [150 ft]), and area.
Why is this important?
Warm waters are necessary to fuel the heat engine of the tropical
cyclone. Warm SSTs allows cumulus convection to reach the
tropopause and give maximum warming in the troposphere.
5.2 Climatology and Structure: Conditions for occurrence
2. A minimum distance of at least 500 km [300 mi] from the equator.
Why is this important?
Must have background rotation (f > 0) large enough to convert inflow to
tangential flow (cf conservation of angular momentum).
Consider vorticity equation.
5.2 Climatology and Structure: Conditions for occurrence
Gray (1979)
3. Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind
shear between the surface and the upper troposphere.
Why is this important?
From thickness considerations the
minimum surface pressure is related to
the mean temperature of the column
above. If shear tilts the column of warm
air over then the surface pressure will
rise.
Other possible reasons include
“ventilation” effects or more simply
understood “shearing out”.
Some shear may actually be beneficial!
5.2 Climatology and Structure: Conditions for occurrence
Gray (1979)
4. A pre-existing near-surface disturbance with sufficient vorticity.
Why is this important?
Tropical cyclones cannot be generated spontaneously. They must be
triggered.
What can serve as a seedling?
Easterly wave, MCSs, Upper-level midlatitude troughs.
Problem: these are often cold core!
5.2 Climatology and Structure: Conditions for occurrence
Gray (1979)
4. A pre-existing near-surface disturbance with sufficient vorticity.
Why is this important?
Tropical cyclones cannot be generated spontaneously. They must be
triggered.
What can serve as a seedling?
Easterly wave, MCSs, Upper-level midlatitude troughs.
Problem: these are often cold core!
Why is this important?
5.2 Climatology and Structure: Conditions for occurrence
In Summary:
Environment:
Need significant f, SSTs and low shear.
Seedling:
Need finite amplitude disturbance.
5.2 Climatology and Structure: Conditions for occurrence
More Conditions from Gray (1979)
5. Decrease of equivalent potential temperature with height.
Large CAPE? Controversial. See WISHE theory later!
6. Relatively moist layers near the mid-troposphere (5 km [3 mi]).
Weak or no downdrafts. See boundary layer discussion later.
5 and 6 are mutually exclusive!
5.2 Climatology and Structure: Observed Structure
5.2 Climatology and Structure: Observed Structure
5.2 Climatology and Structure: Observed Structure
5.2 Climatology and Structure: Observed Structure
5.2 Climatology and Structure: Observed Structure
5.2 Climatology and Structure: Observed Structure
5.2 Climatology and Structure: Observed Structure
5.2 Climatology and Structure: Observed Structure
The Thetae Structure
Key is the strong surface radial gradient of thetae
Mature hurricanes are nearly moist adiabatic in the eye wall.
Thus the strong surface radial gradient in thetae leads to the large gradient in T in
the troposphere.
Hydrostatically this leads to the large surface pressure gradient and associated
large surface winds.
Compare with large-scale circulations discussed earlier.
For large-scale circulations variations in thetae were linked to variations in SST
For TC circulations variations in thetae are linked to variations in moisture content
of the boundary layer.
Surface fluxes are crucial!
5.3 Angular Momentum and Thermal Wind
See Notes