(ET) Streamline Configurations

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(ET) Streamline Configurations
H
H
Monsoon Trough
C
C
Confluent Westerlies
Tradewind Trough
H
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(ET) Streamline Configurations
H
H
Monsoon Trough
C
C
Confluent Westerlies
Tradewind Trough
H
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Diurnal Effects

• Temperature Range
• Clouds
• Rainfall

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Diurnal Effects

• Temperature Range
• On small islands and coastlines 3-5C with
  prevailing onshore flow.
• On inland locations or coasts 5-10C with
  prevailing land breeze.
• In the interior in the dry season gt 10C.

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Diurnal Effects

• Clouds
• Oceans
• Maximum (0400-0700L), minimum (1400L-1900L)
• Land
• Daytime maximum, nocturnal minimum

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Diurnal Effects

• Rainfall
• Nocturnal max over oceans, and small islands.
• Shower maximum over land in afternoon.
• Monsoon areas and areas in disturbances have
  night to early morning maximum.

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Tertiary Circulations

• Land and sea breezes
• Sea-breeze characteristics.
• Sets up a few hours after sunrise.
• Moves inland until late afternoon to evening.
• Strong breezes may extend 30 - 50 miles inland.
• Land-breeze characteristics.
• Generally shallower with weaker winds than sea
  breeze.
• Normally does not penetrate as far offshore as
  sea breeze penetrates onshore.
• Land breeze front often triggers convection,
  especially converging land breezes

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Rainfall - Special Cases
Prevailing flow onshore
Prevailing flow offshore
Daytime max on inland slopes
Afternoon max on coastline
Day
Sea Breeze
Nighttime max on coastline
Night
Land Breeze
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Sea Breeze
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Sea Breeze
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Land Breeze
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Tertiary Circulations

• Orographically-induced winds.
• Valley breeze.
• Slopes warm during day.
• Upslope wind.
• Clouds and convection over peaks during daytime.
• Mountain breeze.
• Slopes cool at night.
• Downslope wind.
• Clouds and convection in valleys at night.

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Tertiary Circulations

• Orographically-induced winds.
• Mountain gap winds.
• Funneling through passes when surface gradient
  across mountains is strong.
• Example Tehuantepecer winds off Mexico.

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Tehuantepecers
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Tehuantepecers
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Tehuantepecers
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Upper-Tropospheric Features

• Tropical Upper-Tropospheric Trough (TUTT)
• Forms in summer hemisphere over mid-oceans.
• Subtropical ridge - part that moves poleward over
  land masses (North America, eastern Asia).
• Subequatorial ridge - part that stays near
  equator over the ET.

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Upper-Tropospheric Features

• Tropical Upper-Tropospheric Trough (TUTT) -
  Continued -
• Level Most intense - 200 mb.
• Orientation ENE-WSW
• Season Late April - Mid November (most intense
  in August

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Upper-Tropospheric Features

• Tropical Upper-Tropospheric Trough (TUTT) -
  Continued -
• Position South of Surface Subtropical Ridge over
  trade winds (convergent weather associated with
  TUTT occurs in tradewinds)
• Major Convergent Weather Few degrees southeast
  of upper level center
• Weather in axis or Cyclonic Cell Center Few
  clouds, general sinking motion

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Upper-Tropospheric Features

• Tropical Upper-Tropospheric Trough (TUTT) -
  Continued -
• Temperature Cold trough (3-5 degrees colder than
  environment, Isold TCU and TSTMs in TUTT, and
  center of cells
• Axis Windshift Often an abrupt 180 degree turn
  at trough axis
• Cirrus Tracers Indicates gt 50 kts either side of
  TUTT or embedded cell.

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Overview

• Upper-Tropospheric cyclones (cold lows).
• Strongest 200 to 300 mb and weaken downward.
• TUTT cells are most common examples.
• Center
• Usually clear due to subsidence, though deep cold
  cells can have some core convection.
• Periphery
• Upper-level divergence, mostly SE side.
• Satellite Imagery
• Ring of clouds about 140 nm from center.
• Dark round area on water vapor image (best seen
  in water vapor!).

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TUTT Cell
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TUTT cell
lt------TUTT cell
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TUTT Cell------gt
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TUTT Cell------gt
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TUTT Cell ----------gt
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TUTT Cell -----------gt in Multispectral Imagery
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NOGAPS 200 mb temperatures
lt-----Trough
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Non-severe Weather Systems and Tertiary
Circulations

• Lines
• Tropical Waves
• Vortices
• Land and Sea Breezes
• Valley and Mountain Breezes

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Non-severe Lines

• Lines - synoptic scale convergence with length
  much greater than width.
• Squall lines
• Cold fronts
• Shear lines
• Surge Lines
• Near Equatorial Convergence Zones

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Shear Line
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Non-Severe Tropical Waves

• National Hurricane Center (NHC) definition - a
  trough or cyclonic curvature maximum in the
  trade-wind easterlies.
• Sometimes called Tropical Easterly Waves in the
  Atlantic.
• Originally, this term was based on older research
  on the tropics that was based on sparse surface
  and upper air data.

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Non-Severe Tropical Waves

• When NHC started using METSAT imagery in 1967,
  they saw new aspects of the so-called wave that
  didnt fit the theoretical model.
• Not as common as previously thought many systems
  called waves were actually vortices
• This model is falling out of favor with many in
  the tropical community.

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The Tropical Easterly Wave
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Non-Severe Tropical Waves

• Characteristics
• Many stem from upper-level cyclones (cold lows).
• Characteristics of Atlantic (easterly) waves.
• Form over Ethiopian Highlands June to October.
• Move across baroclinic zone south of Sahara -
  often form squall lines.
• Dampen under STR axis in eastern Atlantic and
  strengthen near Lesser Antilles.

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Inverted Vee or Screaming Eagle - really a
circulation
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Screaming Eagle
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Non-Severe Vortices

• Fair weather vortices - (Equatorial anticyclone,
  heat low, and TUTT)
• Bad weather vortices - (Tropical cyclones,
  monsoon depression, west African cyclones, and
  mid Tropospheric cyclones).

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Fair Weather Vortices

• Equatorial anticyclones
• Found where monsoon trough is more than 10 from
  equator
• Surge from opposite hemisphere crosses equator
  and turns anticyclonically
• Curved band of cloud seen at leading edge of
  anticyclone

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Fair Weather Vortices

• Heat lows/ heat troughs
• Low-level air rises over hot land with subsidence
  aloft.
• Lowest pressure coincides with highest
  temperature.
• Examples
• Sahara and SW Asia in summer (may be part of
  monsoon trough).
• Southern South America, southern and eastern
  Africa all year.

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Bad Weather Vortices

• Mid-tropospheric cyclones.
• Subtropical cyclones.
• Cut-off portion of deep mid-latitude trough in
  the tropics in winter.
• Example "Kona" storms in Hawaii, cause SW winds
  and heavy rains.
• Arabian Sea cyclones
• Major rainmaker on west coast of India in SW
  monsoon.
• Cloud pattern resembles a typhoon.

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The Central American Monsoons

• Regional Effects
• Region normally dominated by NE trade winds.
• Channeling causes 3 quasi stationary cyclones on
  Pacific side of Central America which anchor
  monsoon trough.
• Northern Hemisphere easterlies to north and
  Southern Hemisphere westerlies to south maintain
  cyclones.

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The Central American Monsoons

• Panama Bay cyclone
• Strongest anchoring cyclone of the three.
• Reinforced by warm ocean currents.
• Southwesterly flow channeled cyclonically by
  Andes year-round.
• Persistent feature May through January.

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The Central American Monsoons

• Lake Nicaragua cyclone
• Anchored between Nicaragua and Costa Rica.
• Weaker than Panama bay cyclone, persists May -
  November.
• Gulf of Tehuantepec (Guatemalan) cyclone.
• Weakest of the three.
• Maximum strength in October
• Merges with NECZ to west, which is anchored over
  SST thermal maximum.

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The Central American Monsoons

• American Monsoons don't meet Ramage's Criteria
  because
• Semi-annually reversing North-South pressure
  gradients do not form.
• Instead, Central America has a "transitional"
  monsoon.
• Oscillates north - south semiannually.
• Location determined by strengths of NE trades and
  Southern Hemisphere southwesterlies.

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The Central American Monsoons

• Monsoon Surges
• Often caused by low-latitude (10 - 20N) tropical
  cyclones.
• Acceleration of southwesterlies from south of
  monsoon trough.
• NE flow aloft back into Southern Hemisphere.

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The Central American Monsoons

• Monsoon Surges (Cont)
• Atemporalado Index
• Atemporalado term used to describe a winter
  rain event in Central America due to a vogorous
  cold front or shear line that crosses far enough
  south.
• Often results in Tehuanapecers.

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The Central American Monsoons

• Monsoon Surges (Cont)
• Rule of Thumb
• Take SLP difference (DP) between Merida Mexico
  and Houston TX.
• If DP lt 12 MB, no SURGE
• If DP 12-14 MB, marginal SURGE
• If DP 15-19 MB, Nominal SURGE
• If DP gt20 MB, STRONG SURGE

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Surge
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Severe Weather In The Tropics

• Severe Thunderstorms
• Non-Convective Winds

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Thunderstorms

• More common in tropics than in high latitudes
• 82 of thunderstorms are over South America,
  Africa, and Indonesia
• 18 of thunderstorms are over water
• Most are not severe by mid-latitude (midwest)
  standards

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Severe Thunderstorms

• Hail
• Rare in tropics
• The typical WBZ in tropics above 12,000 ft and is
  usually over 15,000 ft
• Tornadoes and waterspouts
• Most common in the U.S. and Australia.
• Location
• Most severe storms occur when continental polar
  air penetrates the tropics and squall lines or
  shear lines develop

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Severe Thunderstorms
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Bad Weather Vortices

• Monsoon depressions
• Hybrid mid-latitude/tropical cyclone over Bay of
  Bengal in summer.
• West African cyclones.
• Mid-level cyclones that form south of heat trough
  in western Africa.
• Causes dense stratiform clouds near coast,
  convective clouds and squall lines north.
• Move west and weaken over Atlantic.

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Monsoon Depression
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Monsoon Depression
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Monsoon Depression
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Monsoon Depression
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Monsoon Depression
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