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Tropical Waves
John Cangialosi and Lixion Avila
National Hurricane Center
WMO Region IV
Tropical Cyclone Workshop
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Outline
• Basic definition
• Schematic diagrams/Interactions
• Operational products/forecasts
• Tools for tracking
• Exercise
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What are tropical waves?
* Perturbations / disturbances in the tropical easterlies that typically move from east to west.
* Often seen as inverted troughs of low pressure (inverted-V pattern in satellite imagery). Significant rain producers.
* Convection typically on the east side. Subsidence/clearing on the west side.
* Convection highly modulated by atmospheric moisture, upper level features, topography, etc.
* Develop into tropical cyclones.
* Around 60 tracked per year (little annual variability)
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How/where they form
• Generated by an instability (baroclinic-barotropic)of the African easterly jet
• Jet arises as a result of reversed lower tropospheric temperature gradient over west-central
north Africa due to extremely warm temperatures over the Sahara Desert and substantially
cooler temperatures along the coast of Guinea.
near the Gulf of
Guinea ~ 22 C
Sahara Desert
~ 45 C
African Easterly Jet
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Typical Synoptic Setup
Tropical waves/African easterly waves move westward within the
trade wind flow south of the Bermuda-Azores high
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Frequency by month
Tropical wave activity in terms of numbers is highest June-August
Tropical Waves tracked by NHC
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Schematic diagrams
Classic inverted V-shape near the eastern Caribbean
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Schematic diagram
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Do waves tilt with height?
At what pressure level is the maximum
amplitude?
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Schematic diagram
• Notice eastward slope will height
• Maximum amplitude is around 700 mb
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Stratocumulus
* wavelength of about 2000 km
* period of 3 to 4 days.
* typically move westward at 10 to 15 kt
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Waves in Westerly Shear
Clouds/rain displaced to the east
of the wave axis
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HH
Clouds/rain displaced to the west
of the wave axis
Waves in Easterly Shear
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What happens when tropical waves interact with
upper-level lows and troughs?
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Although interaction with upper-level
lows are unfavorable for tropical
cyclogenesis, it can often induce heavy
rainfall.
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Sep 9 12z
Sep 10 00z
Sea-breeze convergence, upslope
flows, and afternoon heating can
cause convection to become chaotic
and difficult to predict.
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Saharan Air Layer
Very dry/warm air in the low-mid levels
of the atmosphere limits convection.
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Saharan Air Layer
Tool to help track the strength and position of the SAL
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NHC Products
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TAFB products: Surface Analysis
Analyze current positions
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TAFB products: Tropical Weather
Discussion
Discussion of
tropical waves
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TAFB products: graphical forecast
Predict future positions: 24h, 48h, and 72h
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Tropical Weather Outlook
Approximately 60 % of
Atlantic tropical cyclones
and 85 % of major hurricanes
originate from tropical
waves
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What is more important for tropical
cyclogenesis?
(a) the wave structure
(b) the environment
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Developing vs. Non-developing
Hopsch, Thorncroft, and Tyle (2009)
Relative Vorticity Relative Vorticity
Relative Humidity Relative Humidity
Very little different in structure between
developing and non-developing waves
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Tools to track tropical waves
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Satellite Imagery
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Satellite Hovmoller Diagrams
Time
Aug 27
Aug 28
Aug 29
Aug 30
Aug 31
Aug 26
Sep 1
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27
28
29
30
31
1
2
3
4GregGreg
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Surface Observations
Up to 4 mb pressure
falls associated with a
strong wave
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Scatterometer
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Total Precipitable Water
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Wave Splitting
The northern portion of the wave often fractures but
the southern extension continues moving westward
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Upper-Air Time Sections
Dakar
Sal
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Models
Relative vorticity Streamlines
Moisture fields
Sea Level Pressure
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EXERCISEEXERCISE
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Dakar
Dakar
Barbados
Barbados
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July 13, 1981 1600 UTC July 14, 1981 1600 UTC July 15, 1981 1600 UTC July 16, 1981 1600 UTC
July 17, 1981 1600 UTC July 18, 1981 1600 UTC July 19, 1981 1600 UTC July 20, 1981 1600 UTC
July 21, 1981 1600 UTC July 22, 1981 1600 UTC July 23, 1981 1600 UTC July 24, 1981 1600 UTC
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Barbados Dakar15 14 13
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July 13, 1981 1600 UTCJuly 14, 1981 1600 UTCJuly 15, 1981 1600 UTCJuly 16, 1981 1600 UTCJuly 17, 1981 1600 UTCJuly 18, 1981 1600 UTCJuly 19, 1981 1600 UTCJuly 20, 1981 1600 UTCJuly 21, 1981 1600 UTCJuly 22, 1981 1600 UTCJuly 23, 1981 1600 UTCJuly 24, 1981 1600 UTCJuly 25, 1981 1600 UTC
20 W30 W40 W50 W60 W70 W