370 Tropical
2016
Large Scale Conditions
• Hadley Circulation
• Trade winds
• ITCZ
Hadley Cell
Correction to most figures
• The upward motion is very concentrated
near the ITCZ
• The downward motion is more spread out in
latitude (roughly 15° and northward) and
weaker.
ITCZ is generally not on the
equator
• Moves northward during northern summer
ITCZ made up of cloud clusters
Sinking motion in the subtropics
creates deserts in those latitudes
(25-35 degrees from equator)
World’s Major Deserts
Trade Wind Inversions
a.k.a. subtropical inversions
Strongest and lowest over eastern
subtropical oceans
Trade Wind or Subtropical Inversion
• The height of the base of this inversion varies from
about 500 m at the eastern extremities of the
subtropical highs to about 2000 m at the western and
equatorial extremities.
• In the equatorial trough zone and over the western
portions of the trade-wind belt, the inversion does not
exist as a mean condition, although it appears in certain
weather patterns.
• The inversion is generally strongest when the height of
its base is lowest, and vice versa. The thickness of the
inversion layer varies from tens of m to more than
1000 m.
• On the average its thickness is about 400 m.
The resulting greater subsidence on the eastern
sides depressing heights on the eastern sides of
the ocean relative to the central portions
Why trade wind inversions
But the subsidence is not zonally uniform:
greater in the eastern side of the oceans
Why does subsidence create
inversions?
• Descent is greater aloft and decreases
towards the surface. More subsidence
warming aloft
• Well-mixed marine layer near the surface.
Tropical Analyses
• See both midlatitude style, isobaric analysis,
and streamline/isotach analyses.
• Why the latter is most popular?
Review
• A streamline is a line everywhere parallel
to the instantaneous flow.
• An isotach is a line of constant wind speed.
Why streamline/isotachs better
than isobars/heights in the
tropics?
• Flow not geostrophic within roughly 5
degrees of the equator (so connection
between pressure and winds are not
necessarily straightforward)
• Lot of pibals (just winds) in the tropics
compared to radiosondes.
• Pressure variations normally weak in the
equator (large noise to signal)
Thus, why not analyze winds
instead of pressure?
Streamline/isotach analysis is
used but there are alternatives
(isogon, isotach)
Not intuitive
Streamline/isotach Analysis
• Helps to remember component wind
circulations we studied earlier, and how
they combine:
–
–
–
–
Translation
Divergence/convegence
Rotation
Deformation
Streamline/isotach analysis
• Winds should go to zero at singular points:
– Centers of rotation
– Centers of convergence/divergence
– Centers of deformation
Tips on streamline/isotach
analysis (you will do this)
• Do streamlines first if possible.
• You will see a lot of familiar patterns
(circulation, deformation, convergence, etc.)
• Winds should go to zero at singular points:
such as centers of rotation,
convergence/divergence, and deformation.
Easterly Waves: The Major
Synoptic Disturbances in the
Tropics
•Westward moving synoptic waves characterize the tropics
• They are tropospheric waves that modulate clouds/rainfall and
move at about 5-10 m/s and have wavelengths of 2000-4000 km.
•Assignment: METED African Easterly Wave module
West
East
Trough tilts westward with height
Many easterly waves originate
over Africa and are known as
African Easterly Waves
Composite African Wave Structures (Reed et
al., 1977)
Observations of African Easterly Waves
Carlson 1969ab
Carried out case studies of several AEWs
Peak amplitudes at 600-700mb and at
surface
Eastward tilt with height from the surface to
the level of the AEJ
Synoptic variations in cloud cover
Peak of cloudiness close to AEW trough
Observations of African Easterly Waves
Burpee (1970)
Eastward tilt beneath the AEJ – Westward tilt
above the AEJ
Northerlies dry and warm
Southerlies wet and cold
African Easterly Waves Are
Closely Associated with and
Propagate in a Midtropospheric
Easterly Jet
• Maximum amplitude centered
around 650-700 hPa near 15N
• Associated with large temperature
difference between the hot Sahara
and cool Gulf of Guinea
HOT
Cool
The Mean State over West Africa: The African
Easterly Jet (AEJ)
Burpee, R.W. 1972 The origin and structure of easterly waves in the lower troposphere of North Africa,
J. Atmos. Sci. 29, 77-90
Why is the AEJ there?
Hint: Surface temps and thermal wind equation!
• Strong baroclinic zone 10o-20oN
Zonal Variations in the Mean State
Mean 700hPa U wind, 16th July – 15th August 2000
Berry and Thorncroft 2005
Origin of African Easterly
Waves
• A joint barotropic/baroclinic instability on the
African easterly jet since there are large
vertical and horizontal shears.
• Barotropic instability from large horizontal
wind shear
• Baroclinic instability from large vertical wind
shear
• Release of latent heat in convection can help
initiate and support these waves.
Monsoons
A monsoon is a term from early
Arabs called the "Mausin," or "the
season of winds." This was in
reference to the seasonally shifting
winds in the Indian Ocean and
surrounding regions, including the
Arabian Sea.
Essentially a large-scale sea breeze
Monsoon Circulations
• Exist in tropics and subtropics
• Well known monsoons:
– India/China/SE Asia
– West Africa
– SW US
Indian Monsoon
SW US Monsoon
Upper Level Features over the
Tropics
200 hPa Easterly Jet
• Most prominent in July
• Extends from SE Asia to eastern Africa
• Results from temperature gradient between
the Himalayas and the Indian Ocean
Explanation: Thermal Wind
• Elevated Himalayas cause warming aloft in
summer (sensible and latent heat)
• Cooler over Indian Ocean
• With warming to north, easterly flow gets
stronger with height
Tropical Upper Tropospheric
Troughs (TUTTs)
TUTT
• Maintained by subsidence near and above
tropopause
• Can provide lift below to assist tropical
cyclone genesis and intensification
• About 20% of hurricanes/tropical storms
can be traced to TUTTS
Hurricanes and Tropical Storms
•
•
•
•
•
•
•
•
Some Tropical Terminology
Easterly Wave: A wavelike disturbance in the tropical easterly winds that usually
moves from east to west. Such waves can grow into tropical depressions.
Tropical wave: A kink or bend in the normally straight flow of surface air in the
tropics which forms a low pressure trough, or pressure boundary, and showers and
thunderstorms. Can develop into a tropical cyclone.
Tropical disturbance: Often the earliest stages of a tropical cyclone. Normally an
organized area of thunderstorms that forms in the tropics and persists for more than
24 hours. Low pressure might form at the surface, but winds remain below 30 mph.
No closed wind circulation.
Tropical depression (TD): A tropical cyclone with a closed circulation and
maximum sustained winds near the surface of less than 34 knots (39 mph.) Tropical
Depressions are listed only with a number, not a name.
Tropical cyclone: A low-pressure weather system in which the central core is
warmer than the surrounding atmosphere.. The term "tropical cyclone" is also used in
the Indian Ocean and around the Coral Sea off northeastern Australia to describe
storms called "hurricanes" and "typhoons" in other areas.
Tropical storm: Tropical cyclone with winds of 35-64 knots (39 to 74 mph). In most
of the world, a storm is given a name when it reaches tropical storm intensity.
Hurricane: A tropical cyclone with winds greater than 64 knots (74 mph). Normally
applied to such storms in the Atlantic Basin and the Pacific Ocean east of the
International Date Line.
Midlatitude Versus Tropical
Storms
• Tropical storms are very different from their
midlatitude cousins
• Smaller than midlatitudes cyclones
(typically 500-100 km)
Midlatitude
Tropical
Large temperature gradients
Weak temperature gradients
Large Wind Shears
Weak Wind Shears
Energy source: baroclinic instability
Latent heat release, air-sea
interactions
Mainly stratiform clouds
Highly convective
Tropical Storms and Hurricanes
Saffir-Simpson Scale
(remember: these are sustained winds)
Hurricane Climatology (Avoids
the Equator)
The Origin
• Most hurricanes and typhoons begin as
easterly waves
• Easterly waves to tropical disturbance to
tropical depression to tropical storm to
hurricane
Energy Source of Tropical
Storms/Hurricanes
• Latent heat release is critical, which
requires warm water.
• CISK: Conditional Instability of the
Second Kind
• WISHE: Wind Induced Surface Heat
Exchange
For tropical cyclogenesis to occur, favorable
precursor environmental conditions that must be
in place
• Warm ocean waters (of at least 26.5°C
[80°F]) throughout a sufficient depth
(unknown how deep, but at least on the order
of 50 m [150 ft]). Warm waters are necessary to
fuel the heat engine of the tropical cyclone.
• An atmosphere which cools fast enough with
height such that it is potentially unstable to
moist convection. It is the thunderstorm activity
which allows the heat stored in the ocean waters to
be liberated for the tropical cyclone development.
• Relatively moist layers near the midtroposphere (5 km [3 mi]). Dry mid levels are
not conducive for allowing the continuing
development of widespread thunderstorm activity.
• A minimum distance of at least 500 km [300
mi] from the equator. For tropical cyclogenesis
to occur, there is a requirement for non-negligible
amounts of the Coriolis force to provide for near
gradient wind balance to occur. Without the
Coriolis force, the low pressure of the disturbance
cannot be maintained.
• A pre-existing near-surface disturbance with
sufficient vorticity and convergence. Tropical
cyclones cannot be generated spontaneously. To
develop, they require a weakly organized system with
sizable spin and low level inflow.
• Low values (less than about 10 m/s of vertical wind
shear between the surface and the upper
troposphere. Vertical wind shear is the magnitude of
wind change with height. Large values of vertical wind
shear disrupt the incipient tropical cyclone and can
prevent genesis, or, if a tropical cyclone has already
formed, large vertical shear can weaken or destroy the
tropical cyclone by interfering with the organization of
deep convection around the cyclone center.
Hurricane Prediction: A Mixed
Report
• During the past thirty years there has been
substantial in improvement is hurricane
track forecasts as computer models
improved and more data became available
to describe their environment.
• Over the same period only minimal
improvement is hurricane intensity
forecasts.
Katrina
• But the intensity was not well forecast until the day
before….
• Friday’s intensity prediction (storm hit Monday AM)
HURRICANE KATRINA DISCUSSION NUMBER 12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT FRI AUG 26 2005
FORECAST POSITIONS AND MAX WINDS
INITIAL
26/1500Z 25.1N 82.2W 70 KT
12HR VT 27/0000Z 25.2N 83.1W 75 KT
24HR VT 27/1200Z 25.5N 84.3W 80 KT
36HR VT 28/0000Z 26.2N 85.2W 85 KT
48HR VT 28/1200Z 27.1N 85.9W 90 KT
72HR VT 29/1200Z 29.5N 86.3W 100 KT
96HR VT 30/1200Z 34.5N 83.5W 35 KT...INLAND
120HR VT 31/1200Z 40.5N 77.0W 25 KT...DISSIPATING INLAND
•
•
•
•
URGENT - WEATHER MESSAGE
NATIONAL WEATHER SERVICE NEW ORLEANS LA
1011 AM CDT SUN AUG 28 2005
...DEVASTATING DAMAGE EXPECTED...
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
.HURRICANE KATRINA...A MOST POWERFUL HURRICANE WITH UNPRECEDENTED
STRENGTH...RIVALING THE INTENSITY OF HURRICANE CAMILLE OF 1969.
MOST OF THE AREA WILL BE UNINHABITABLE FOR WEEKS...PERHAPS LONGER. AT
LEAST ONE HALF OF WELL CONSTRUCTED HOMES WILL HAVE ROOF AND WALL
FAILURE. THE MAJORITY OF INDUSTRIAL BUILDINGS WILL BECOME NON FUNCTIONAL.
PARTIAL TO COMPLETE WALL AND ROOF FAILURE IS EXPECTED. ALL WOOD
FRAMED LOW RISING APARTMENT BUILDINGS WILL BE DESTROYED. CONCRETE
BLOCK LOW RISE APARTMENTS WILL SUSTAIN MAJOR DAMAGE...INCLUDING SOME
WALL AND ROOF FAILURE.
HIGH RISE OFFICE AND APARTMENT BUILDINGS WILL SWAY DANGEROUSLY...A
FEW TO THE POINT OF TOTAL COLLAPSE. ALL WINDOWS WILL BLOW OUT.
AIRBORNE DEBRIS WILL BE WIDESPREAD...AND MAY INCLUDE HEAVY ITEMS SUCH
AS HOUSEHOLD APPLIANCES AND EVEN LIGHT VEHICLES. SPORT UTILITY
VEHICLES AND LIGHT TRUCKS WILL BE MOVED. THE BLOWN DEBRIS WILL CREATE
ADDITIONAL DESTRUCTION. PERSONS...PETS...AND LIVESTOCK EXPOSED TO THE
WINDS WILL FACE CERTAIN DEATH IF STRUCK.
POWER OUTAGES WILL LAST FOR WEEKS...AS MOST POWER POLES WILL BE DOWN
AND TRANSFORMERS DESTROYED. WATER SHORTAGES WILL MAKE HUMAN SUFFERING
INCREDIBLE BY MODERN STANDARDS.
THE VAST MAJORITY OF NATIVE TREES WILL BE SNAPPED OR UPROOTED. ONLY
THE HEARTIEST WILL REMAIN STANDING...BUT BE TOTALLY DEFOLIATED. FEW
CROPS WILL REMAIN. LIVESTOCK LEFT EXPOSED TO THE WINDS WILL BE
KILLED.
Better synoptic forecasts and
more information in the
environment of these storms have
aided prediction of their tracks
Substantial Promise
• The use of higher resolution prediction and
more detailed data around and in hurricanes
should result in better intensity predictions.
15-km grid spacing
1.67 km grid spacing
Katrina
48-h Before: Virtually a Perfect Forecast of Track
Katrina
• But the intensity was not well forecast until the day
before….
• Friday’s intensity prediction (storm hit Monday AM)
HURRICANE KATRINA DISCUSSION NUMBER 12
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
11 AM EDT FRI AUG 26 2005
FORECAST POSITIONS AND MAX WINDS
INITIAL
26/1500Z 25.1N 82.2W 70 KT
12HR VT 27/0000Z 25.2N 83.1W 75 KT
24HR VT 27/1200Z 25.5N 84.3W 80 KT
36HR VT 28/0000Z 26.2N 85.2W 85 KT
48HR VT 28/1200Z 27.1N 85.9W 90 KT
72HR VT 29/1200Z 29.5N 86.3W 100 KT
96HR VT 30/1200Z 34.5N 83.5W 35 KT...INLAND
120HR VT 31/1200Z 40.5N 77.0W 25 KT...DISSIPATING INLAND
•
•
•
•
URGENT - WEATHER MESSAGE
NATIONAL WEATHER SERVICE NEW ORLEANS LA
1011 AM CDT SUN AUG 28 2005
...DEVASTATING DAMAGE EXPECTED...
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
.HURRICANE KATRINA...A MOST POWERFUL HURRICANE WITH UNPRECEDENTED
STRENGTH...RIVALING THE INTENSITY OF HURRICANE CAMILLE OF 1969.
MOST OF THE AREA WILL BE UNINHABITABLE FOR WEEKS...PERHAPS LONGER. AT
LEAST ONE HALF OF WELL CONSTRUCTED HOMES WILL HAVE ROOF AND WALL
FAILURE. THE MAJORITY OF INDUSTRIAL BUILDINGS WILL BECOME NON FUNCTIONAL.
PARTIAL TO COMPLETE WALL AND ROOF FAILURE IS EXPECTED. ALL WOOD
FRAMED LOW RISING APARTMENT BUILDINGS WILL BE DESTROYED. CONCRETE
BLOCK LOW RISE APARTMENTS WILL SUSTAIN MAJOR DAMAGE...INCLUDING SOME
WALL AND ROOF FAILURE.
HIGH RISE OFFICE AND APARTMENT BUILDINGS WILL SWAY DANGEROUSLY...A
FEW TO THE POINT OF TOTAL COLLAPSE. ALL WINDOWS WILL BLOW OUT.
AIRBORNE DEBRIS WILL BE WIDESPREAD...AND MAY INCLUDE HEAVY ITEMS SUCH
AS HOUSEHOLD APPLIANCES AND EVEN LIGHT VEHICLES. SPORT UTILITY
VEHICLES AND LIGHT TRUCKS WILL BE MOVED. THE BLOWN DEBRIS WILL CREATE
ADDITIONAL DESTRUCTION. PERSONS...PETS...AND LIVESTOCK EXPOSED TO THE
WINDS WILL FACE CERTAIN DEATH IF STRUCK.
POWER OUTAGES WILL LAST FOR WEEKS...AS MOST POWER POLES WILL BE DOWN
AND TRANSFORMERS DESTROYED. WATER SHORTAGES WILL MAKE HUMAN SUFFERING
INCREDIBLE BY MODERN STANDARDS.
THE VAST MAJORITY OF NATIVE TREES WILL BE SNAPPED OR UPROOTED. ONLY
THE HEARTIEST WILL REMAIN STANDING...BUT BE TOTALLY DEFOLIATED. FEW
CROPS WILL REMAIN. LIVESTOCK LEFT EXPOSED TO THE WINDS WILL BE
KILLED.
The Biggest Threat from Tropical
Storms is NOT the Winds, but
the Storm Surge
New Research Shows the
Important of Warm, Dry, Dustladen air off the Sahara: The
Saharan Air Layer (SAL)
SAL
• The SAL can cover an area the size of the
continental U.S. and has been tracked as far
west as the Caribbean Sea and Central
America.
• SAL can damp down tropical storm activity
Who is responsibility for tropical
storm prediction in the U.S?
• National Hurricane Center (Miami):
– Atlantic, Carribbean, Gulf of Mexico, E.
Pacific
• Joint Typhoon Warning Center (Hawaii):
– Joint between NOAA/Air Force/Navy
– Pacific and Indian Ocean
Tropical-Extratropical
Transition (ET)
Some Tropical Cyclone Move
Northward and Are Transformed into
Extratropical Storms
Extratropical Transition
• A significant number of tropical
cyclones move into the midlatitudes
and transform into extratropical
cyclones.
• The take on the structural
characteristics of midlatitude cyclones
and switch energy sources (latent
heating to baroclinic instability)
ET
• During ET a cyclone frequently acquires
increased forward motion and
sometimes intensify substantially, so
that such systems pose a serious threat
to land and maritime activities.
• Often poorly forecast by current
numerical models and associated with
periods of poor synoptic predictability
over a wide area downstream.
Satellite
Sequence
Superstorm Sandy
ET
• The largest number of ET
events occur in the western
North Pacific while the North
Atlantic basin contains the
largest percentage of tropical
cyclones that undergo ET with
45% of all tropical cyclones
undergoing ET.
Climo
Tracks
of ET
Systems
ET
• The low pressure system eventually loses its warm
core and becomes a cold-core system.
• A cyclone in extratropical transition will invariably
connect with nearby fronts and/or troughs and the
size of the system will usually appear to increase.
• After or during transition, the storm may restrengthen, deriving energy from primarily baroclinic
processes, aided by the release of latent heat.
• The cyclone will also distort in shape, becoming less
symmetric with time, but sometimes retains a tight,
tropical-like core.
Some Big Impacts of ET
• Severe flooding associated with the ET of Tropical Storm
Agnes [1972
• Hurricane Hazel (1954) resulted in 83 deaths in the Toronto
area of southern Ontario, Canada. In the northwest Pacific,
severe flooding and landslides have occurred in association
with ET.
• Tropical Storm Janis (1995) over Korea, in which at least 45
people died and 22, 000 people were left homeless.
http://oregonstate.edu/~readw/October1
962.html
ET and Predictability
• ET events are often not predicted well by
today’s synoptic models (e.g., GFS)
• The interaction of ET’s with the midlatitude
jet stream/baroclinic zone can result in the
production of Rossby wave trains/packets
that can have a large impact on downstream
flow.
• ET events are often associated with periods
of poor predictability over large areas
downstream of the transition.
MOTIVATION: IMPROVED WEATHER FORECASTS
Impacts on Numerical Model Performance The poleward
movement of a
tropical cyclone
often results in a
period of
reduced forecast
accuracy in
operational
numerical
Hurricane
Maria
global weather
prediction
models
TY Nabi
TY Saola
GFS ensemble membe
+00 0000 UTC 16 Sep 2003
500 hPa height (m) at a
240 m interval
Hurricane Isabel
GFS 500 hPa
ensemble
+108 h
VT 1200 UTC 20
Sep 03
El Nino Southern Oscillation
(ENSO)
• A 3-7 year quasi-periodic variation between:
– El Nino
– Neutral
– La Nina
• Has large influence both in the tropics and
midlatitudes.
• Main source of forecast skill beyond a few
weeks.
Walker Circulation
Walker Circulation
The Pacific Ocean is Like A Big
Bathtub
An Important Measure is the Surface
Temperature in the Tropical Pacific
Why do we care?
• The circulations in the midlatitudes are
substantially different in El Nino, Neutral,
and La Nina years.
• Since the temperature of the tropical Pacific
changes relatively slowly, this gives some
meteorologist some insights into the
weather over the next several months.
El Nino – weak Aleutian High
La Nina – strong Aleutian High
Great Web Site on ENSO: Climate
Prediction Center
http://www.cpc.ncep.noaa.gov/prod
ucts/precip/CWlink/MJO/enso.sht
ml
Madden Julian Oscillation
(MJO)
Definition
• The MJO is an eastward
moving disturbance of clouds, rainfall,
winds, and pressure that traverses the planet
in the tropics and returns to its initial
starting point in 30 to 60 days, on average.
MJO not only influences the
tropics, but has an impact on
midltitudes…including our
region!
Diurnal Circulations and Diurnal
Variability is Large in the
Tropics
• Not surprising
with large diurnal
heating
• Strong sea breeze
circulations
• Large diurnal
precipitation
variations
Large
diurnal
variations in
precipitation
on coasts of
big islands
Strong Sea
Breeze
Circulations
Sea Breeze Circulations
• Start at the coastline and build both offshore
and inland
• Typically have a scale of 50-100 km, but
can be more.
• Lends some predictability to tropical
weather.
Interactions of Sea/Land Breezes
with trade winds: Hawaii
example
The End