1 afwa/xogm originally produced by: tsgt nast initial tropical training for satellite analysts...

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AFWA/XOGMAFWA/XOGM

Originally produced by:Originally produced by:TSgt NastTSgt Nast

Initial Tropical Training Initial Tropical Training

for Satellite Analystsfor Satellite Analysts

Tropical SectionTropical Section

Revisions by: Revisions by: Paul McCronePaul McCrone

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SourcesSourcesSourcesSources

AWS/TR-95/001AWS/TR-95/001AWS TR240AWS TR240NAVEDTRA 40970/40971NAVEDTRA 40970/40971JTWC Forecasters HandbookJTWC Forecasters HandbookNEPRF TR-85-01NEPRF TR-85-01Tropical Weather Course (Keesler)Tropical Weather Course (Keesler)Tropical TIPSTropical TIPS

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OverviewOverviewOverviewOverview

Primary Physical Controls of the TropicsPrimary Physical Controls of the TropicsPlanetary-Scale Circulation in the TropicsPlanetary-Scale Circulation in the TropicsNon-Severe Weather Systems and Tertiary Non-Severe Weather Systems and Tertiary Circulation'sCirculation'sBasic Analysis and Circulation ModelsBasic Analysis and Circulation ModelsMonsoonsMonsoonsClimate AnomaliesClimate AnomaliesTropical CyclonesTropical CyclonesSevere Weather in the TropicsSevere Weather in the TropicsTropical ForecastingTropical Forecasting

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Primary Physical Controls of the tropicsPrimary Physical Controls of the tropicsPrimary Physical Controls of the tropicsPrimary Physical Controls of the tropics

Earth’s Energy BudgetEarth’s Energy Budget Effects of Land-SeaEffects of Land-Sea DistributionDistributionTerrain EffectsTerrain EffectsDiurnal EffectsDiurnal Effects

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Primary Physical Controls of the tropicsPrimary Physical Controls of the tropicsPrimary Physical Controls of the tropicsPrimary Physical Controls of the tropics

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AWS/TR-95/001AWS/TR-95/001Page 11Page 11Figure 2-1Figure 2-1

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Earth’s Energy BudgetEarth’s Energy BudgetEarth’s Energy BudgetEarth’s Energy Budget

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Vertical heat transfer mechanismsVertical heat transfer mechanisms– Radiation - Radiation - not very significantnot very significant

– Sensible heat transferSensible heat transfer

– Latent heat transfer - Latent heat transfer - Most important vertical mechanism in Most important vertical mechanism in

tropicstropics

» Conduction - molecular boundary layer» Convection - air warmed by conduction

» Condensation (latent heat becomes Condensation (latent heat becomes sensible heat)sensible heat)

» Evaporation (Sensible heat becomes Evaporation (Sensible heat becomes latent heat)latent heat)

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Horizontal Heat Transfer Horizontal Heat Transfer – Horizontal heat transfer mechanismsHorizontal heat transfer mechanisms

Sensible heat transfer through warm- or cold-Sensible heat transfer through warm- or cold-air advection air advection

About 40% of earth's total horizontal heat About 40% of earth's total horizontal heat exchange occurs through ocean currents.exchange occurs through ocean currents.

Latent heat transfer.Latent heat transfer.– Moisture advection.Moisture advection.– Latent heat release aloft, carried poleward by the Latent heat release aloft, carried poleward by the

Hadley cell.Hadley cell.

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Effects of Land-Sea DistributionEffects of Land-Sea DistributionEffects of Land-Sea DistributionEffects of Land-Sea Distribution

Determines climate type (monsoon, Determines climate type (monsoon, maritime, Continental.).maritime, Continental.).

Max seasonal variations occurs over land.Max seasonal variations occurs over land. Most convection and latent heat-sensible Most convection and latent heat-sensible

heat conversion takes place over land.heat conversion takes place over land. Land-sea breeze circulation's are a direct Land-sea breeze circulation's are a direct

result.result.

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Terrain EffectsTerrain EffectsTerrain EffectsTerrain Effects

High altitude more typical of mid-High altitude more typical of mid-latitude weather.latitude weather.

Rainfall:Rainfall:– Leeside or windward location usually most Leeside or windward location usually most

important factor.important factor.– In trade winds, heaviest rain usually on In trade winds, heaviest rain usually on

slopes just below trade-wind inversion.slopes just below trade-wind inversion.

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Terrain EffectsTerrain EffectsTerrain EffectsTerrain Effects

Whether a location is on the leeside or Whether a location is on the leeside or windward side is a very important factor windward side is a very important factor RainfallRainfall

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Diurnal EffectsDiurnal EffectsDiurnal EffectsDiurnal Effects

Temperature RangeTemperature RangeCloudsCloudsRainfallRainfall

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Temperature RangeTemperature Range– On small islands and coastlines 3-5°C On small islands and coastlines 3-5°C

with prevailing onshore flow.with prevailing onshore flow.– On inland locations or coasts 5-10°C On inland locations or coasts 5-10°C

with prevailing land breeze.with prevailing land breeze.– In the interior in the dry season > 10°C.In the interior in the dry season > 10°C.

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CloudsClouds–Oceans Oceans

Maximum (0400-0700L), Maximum (0400-0700L), minimum (1400L-1900L)minimum (1400L-1900L)

–Land Land Daytime maximum, Daytime maximum,

nocturnal minimumnocturnal minimum

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RainfallRainfall

– Nocturnal max over oceans, and Nocturnal max over oceans, and small islands.small islands.

– Shower maximum over land in Shower maximum over land in afternoon.afternoon.

– Monsoon areas and areas in Monsoon areas and areas in disturbances have night to early disturbances have night to early morning maximum.morning maximum.

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Planetary-Scale Circulation in Planetary-Scale Circulation in the Tropicsthe Tropics

Planetary-Scale Circulation in Planetary-Scale Circulation in the Tropicsthe Tropics

Definitions and General CharacteristicsDefinitions and General Characteristics Primary Weather ZonePrimary Weather ZoneTropical Wind ProfilesTropical Wind ProfilesUpper Tropospheric FeaturesUpper Tropospheric Features

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Definitions and General Definitions and General CharacteristicsCharacteristics


Classical Definitions. Classical Definitions. – Ancient Greeks - Ancient Greeks - Tropics of Cancer to Tropics of Cancer to

Capricorn (23.5N to 23.5S)Capricorn (23.5N to 23.5S)

– Alexander Supan - Average Annual Temp > Alexander Supan - Average Annual Temp > 68 F (20°C)68 F (20°C)

– Early 1900s - W.P. Koppen Early 1900s - W.P. Koppen World is divided into climatic Zones A to F.World is divided into climatic Zones A to F. A= tropical rainy, Af = rain forest, Aw = A= tropical rainy, Af = rain forest, Aw =

savanna, Am = monsoon.savanna, Am = monsoon.

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Dynamic Definitions - allow for seasonal, longitudinal variations.Dynamic Definitions - allow for seasonal, longitudinal variations.



H H H

H H H

– Dividing line Dividing line between mid-between mid-Tropospheric Tropospheric easterlies and easterlies and westerlies (axes westerlies (axes of STRs).of STRs).

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Tropical weather characteristicsTropical weather characteristics– Cumulus clouds predominate.Cumulus clouds predominate.

– Conditionally unstable.Conditionally unstable.

– Air temperatures comparable to Air temperatures comparable to ocean surface temperatures.ocean surface temperatures.

– Predominately easterly low-level Predominately easterly low-level flow.flow.



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Climatology is more important in Climatology is more important in the tropics than it is in the mid-the tropics than it is in the mid-latitudeslatitudes– Weather dominated by semi-Weather dominated by semi-

permanent, slowly migrating systemspermanent, slowly migrating systems Subtropical Ridge (STR)Subtropical Ridge (STR) Equatorial Trough (ET)Equatorial Trough (ET)



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Subtropical Ridge (STR)Subtropical Ridge (STR)– SurfaceSurface

Centered on five oceanic anticyclones.Centered on five oceanic anticyclones. Over the oceans: Over the oceans:

– shrinks and retreats equatorward in winter, shrinks and retreats equatorward in winter, grows and builds poleward in summer. grows and builds poleward in summer. (inducing TUTT formation)(inducing TUTT formation)

Primary Weather Zones and Primary Weather Zones and SystemsSystems


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Subtropical Ridge (STR)Subtropical Ridge (STR)– SurfaceSurface

Axis tilts ENE-WSW in Northern Axis tilts ENE-WSW in Northern Hemisphere, ESE-WNW in Southern Hemisphere, ESE-WNW in Southern Hemisphere.Hemisphere.

Ridge axis normally 23-35 degrees Ridge axis normally 23-35 degrees latitude North and Southlatitude North and South



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Subtropical Ridge (STR)Subtropical Ridge (STR)– AloftAloft

Primary Weather Primary Weather Zones and SystemsZones and SystemsPrimary Weather Primary Weather

Zones and SystemsZones and Systems

H

H

H

H

H

HL

»Slopes to equator Slopes to equator with height.with height.

»Position of axis at Position of axis at 200 mb is 15-20° 200 mb is 15-20° latitude.latitude.»More elongated east-west More elongated east-west

than the surface ridgethan the surface ridge.

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Trade-wind regionTrade-wind region– Region of low-level prevailing easterlies Region of low-level prevailing easterlies

between STR and ET.between STR and ET.

– NE trades in Northern Hemisphere, SE NE trades in Northern Hemisphere, SE trades in Southern Hemisphere.trades in Southern Hemisphere.

– General subsidence with capping trade-General subsidence with capping trade-wind inversion.wind inversion.

Primary Weather Primary Weather Zones and SystemsZones and SystemsPrimary Weather Primary Weather

Zones and SystemsZones and Systems

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Other termsOther terms– Near equatorial trough (NET).Near equatorial trough (NET).

– ITCZ or ITC.ITCZ or ITC.

– Doldrums.Doldrums.

– Meteorological or thermal equator.Meteorological or thermal equator.

– Near-equatorial Tradewind Convergence Near-equatorial Tradewind Convergence (NETWC).(NETWC).

Equatorial Trough (ET)Equatorial Trough (ET)Equatorial Trough (ET)Equatorial Trough (ET)

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Location, strength, circulations varyLocation, strength, circulations vary– Generally found where trade winds from Generally found where trade winds from

each hemisphere convergeeach hemisphere converge

– Low pressure from heating and upward Low pressure from heating and upward motionmotion

– May be axis of convergence or series of May be axis of convergence or series of cyclonescyclones


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Several streamline configurations:Several streamline configurations:– Near Equatorial Convergence Zone (or Near Equatorial Convergence Zone (or

trade-wind trough).trade-wind trough).

– Monsoon troughMonsoon trough

– Confluent westerlies.Confluent westerlies.

– Equatorial Buffer ZoneEquatorial Buffer Zone


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Near Equatorial Convergence Near Equatorial Convergence Zone (or trade-wind trough).Zone (or trade-wind trough).– Confluent zone between trade Confluent zone between trade

winds of each hemisphere.winds of each hemisphere.

– Usually over oceans, but can Usually over oceans, but can extend over adjacent land areas.extend over adjacent land areas.

(ET) Streamline (ET) Streamline ConfigurationsConfigurations

(ET) Streamline (ET) Streamline ConfigurationsConfigurations

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Trade Wind TroughTrade Wind Trough

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Monsoon troughMonsoon trough–Often a series of cyclonic centers:Often a series of cyclonic centers:

Heat lows (over land primarilyHeat lows (over land primarily).). Thunderstorm clusters, non Thunderstorm clusters, non

developing.developing. Monsoon depressions. Monsoon depressions. Tropical disturbances.Tropical disturbances.

(ET) Streamline Configurations(ET) Streamline Configurations

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Monsoon troughMonsoon trough–LocationsLocations

Africa - Sub-Sahara in July and Africa - Sub-Sahara in July and Kalahari-Madagascar in January.Kalahari-Madagascar in January.

Central America - East Pacific: Central America - East Pacific: south of Central America all year south of Central America all year and occasionally crosses into and occasionally crosses into Caribbean.Caribbean.

(ET) Streamline Configurations(ET) Streamline Configurations(ET) Streamline Configurations(ET) Streamline Configurations

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Africa - Sub-Sahara in JulyAfrica - Sub-Sahara in July

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Monsoon troughMonsoon trough–LocationsLocations

South Asia - West Pacific: Iraq to South Asia - West Pacific: Iraq to northern India through SE Asia to northern India through SE Asia to Guam.Guam.

Australia - along the northern coast Australia - along the northern coast from December through February.from December through February.

(ET) Streamline Configurations(ET) Streamline Configurations(ET) Streamline Configurations(ET) Streamline Configurations

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• Confluent westerlies.Confluent westerlies.Locations:Locations:

West of Central America in fall.West of Central America in fall.NW of Australia in Southern NW of Australia in Southern Hemisphere summer.Hemisphere summer.

Winds generally light, speed Winds generally light, speed convergence increasing convergence increasing downstream.downstream.


C

H H

C

HH

H

TradewindTradewindTroughTroughTradewindTradewindTroughTrough

MonsoonMonsoonTroughTroughMonsoonMonsoonTroughTrough

ConfluentConfluentWesterliesWesterliesConfluentConfluentWesterliesWesterlies

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Equatorial Buffer ZoneEquatorial Buffer Zone– July:July:

Southern Hemisphere SE winds become Southern Hemisphere SE winds become SW entering monsoon trough.SW entering monsoon trough.


Equatorial Buffer ZoneEquatorial Buffer Zone– January:January:

Northern Hemisphere NE winds become NW Northern Hemisphere NE winds become NW entering Southern Hemisphere monsoon trough.entering Southern Hemisphere monsoon trough.

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Equatorial Buffer ZoneEquatorial Buffer Zone– Buffer CellsBuffer Cells

Weak, closed circulations near Weak, closed circulations near equator where curving winds "cut equator where curving winds "cut off".off".

No net inflow or outflow.No net inflow or outflow. Labeled with a "B" on streamline Labeled with a "B" on streamline

analysis.analysis.


B

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Tropical Vertical Wind Tropical Vertical Wind Profiles: Deep EasterliesProfiles: Deep EasterliesTropical Vertical Wind Tropical Vertical Wind Profiles: Deep EasterliesProfiles: Deep Easterlies

Low-level east winds remain easterly with Low-level east winds remain easterly with heightheight

Found within 15° of equatorFound within 15° of equator May extend to 30° in summer hemisphereMay extend to 30° in summer hemisphere Not in all seasons or regionsNot in all seasons or regions Narrowest north-south extent at 200 mbNarrowest north-south extent at 200 mb

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Shallow EasterliesShallow Easterlies– Low-level easterlies become westerly with Low-level easterlies become westerly with

heightheight

– Average 15-30° latitude, closer to equator Average 15-30° latitude, closer to equator in winter hemispherein winter hemisphere

– Example - Hawaiian islands (20°N) in Example - Hawaiian islands (20°N) in shallow easterlies all yearshallow easterlies all year

Tropical Vertical Wind Profiles: Shallow Easterlies

Tropical Vertical Wind Profiles: Shallow Easterlies

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Low-level westerlies become easterly Low-level westerlies become easterly with height.with height.

Found in monsoon regions in Found in monsoon regions in summer.summer.

Examples: western Pacific, Indian Examples: western Pacific, Indian Ocean, western Africa (shallower in Ocean, western Africa (shallower in Africa).Africa).

Tropical Vertical Wind Tropical Vertical Wind Profiles: Shallow WesterliesProfiles: Shallow Westerlies

Tropical Vertical Wind Tropical Vertical Wind Profiles: Shallow WesterliesProfiles: Shallow Westerlies

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Subtropical Jet (STJ)Subtropical Jet (STJ)– Found 20-35°N and 25-32°S Found 20-35°N and 25-32°S

(average).(average).

– CausesCauses Upward branch of Hadley cell Upward branch of Hadley cell

circulation.circulation. Conservation of angular momentum Conservation of angular momentum

accelerates air to east as it moves north.accelerates air to east as it moves north.

Upper-Tropospheric FeaturesUpper-Tropospheric Features

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Sub Tropical JetSub Tropical Jet

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Tropical Easterly Jet (TEJ)Tropical Easterly Jet (TEJ)– Feature of northern hemisphere (NH) summer Feature of northern hemisphere (NH) summer

monsoon in Asiamonsoon in Asia– Very persistent in NH summer monthsVery persistent in NH summer months– Location: central Africa to SE Asia (from 5 Location: central Africa to SE Asia (from 5

degrees to 20 degrees North latitude)degrees to 20 degrees North latitude)– Strongest winds (80-100 kts) over Arabian Sea Strongest winds (80-100 kts) over Arabian Sea

between 40,000-55,000 ft (200-100 mb). between 40,000-55,000 ft (200-100 mb). Highest observed winds: 152 ktsHighest observed winds: 152 kts

Upper-Tropospheric FeaturesUpper-Tropospheric FeaturesUpper-Tropospheric FeaturesUpper-Tropospheric Features

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Tropical Easterly Jet (TEJ)Tropical Easterly Jet (TEJ)– CausesCauses

Uniquely high temperatures and heights of the Uniquely high temperatures and heights of the isobaric surfaces of the Tibetan Plateau during isobaric surfaces of the Tibetan Plateau during summer (See Southwest Monsoon later)summer (See Southwest Monsoon later)

The above causes an upper-level high to develop The above causes an upper-level high to develop over Tibetan Plateau.over Tibetan Plateau.

Flow from high conserves angular momentum as it Flow from high conserves angular momentum as it moves south, accelerates toward the west.moves south, accelerates toward the west.


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Tropical Easterly Jet (TEJ)Tropical Easterly Jet (TEJ)– CausesCauses

Creates upper level divergence over India, and Creates upper level divergence over India, and speed convergence over eastern Africaspeed convergence over eastern Africa

This circulation contributes to the widespread This circulation contributes to the widespread lifting and convection north of the jet over southern lifting and convection north of the jet over southern Asia, and strong subsidence and aridity (dryness) Asia, and strong subsidence and aridity (dryness) over North Africa and Middle East. over North Africa and Middle East.


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Tropical Upper-Tropospheric Trough Tropical Upper-Tropospheric Trough (TUTT)(TUTT)

Forms in summer hemisphere over mid-oceans.Forms in summer hemisphere over mid-oceans. Subtropical ridge - part that moves Subtropical ridge - part that moves

poleward over land masses (North America, poleward over land masses (North America, eastern Asia).eastern Asia).

Subequatorial ridge - part that stays near Subequatorial ridge - part that stays near equator over the ET.equator over the ET.


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Tropical Upper-Tropospheric Trough Tropical Upper-Tropospheric Trough (TUTT) - Continued -(TUTT) - Continued -

Level: Most intense - 200 mb.Level: Most intense - 200 mb.

Orientation: ENE-WSWOrientation: ENE-WSW

Season: Late April - Mid November (most Season: Late April - Mid November (most intense in Augustintense in August


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Position: South of Surface Subtropical Position: South of Surface Subtropical Ridge over trade winds (convergent weather Ridge over trade winds (convergent weather associated with TUTT occurs in tradewinds) associated with TUTT occurs in tradewinds)

Major Convergent Weather: Few degrees Major Convergent Weather: Few degrees southeast of upper level centersoutheast of upper level center

Weather in axis or Cyclonic Cell Center: Weather in axis or Cyclonic Cell Center: Few clouds, general sinking motion Few clouds, general sinking motion


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Temperature: Cold trough (3-5 degrees Temperature: Cold trough (3-5 degrees colder than environment, Isold TCU and colder than environment, Isold TCU and TSTMs in TUTT, and center of cellsTSTMs in TUTT, and center of cells

Axis Windshift: Often an abrupt 180 Axis Windshift: Often an abrupt 180 degree turn at trough axisdegree turn at trough axis

Cirrus Tracers: Indicates Cirrus Tracers: Indicates >> 50 kts either 50 kts either side of TUTT or embedded cell.side of TUTT or embedded cell.


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Non-severe Weather Systems Non-severe Weather Systems and Tertiary Circulationsand Tertiary Circulations

Non-severe Weather Systems Non-severe Weather Systems and Tertiary Circulationsand Tertiary Circulations

•LinesLines•Tropical WavesTropical Waves•VorticesVortices•Land and Sea BreezesLand and Sea Breezes•Valley and Mountain BreezesValley and Mountain Breezes

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Non-severe LinesNon-severe Lines

Lines - Lines - synoptic scale convergence with synoptic scale convergence with length much greater than width.length much greater than width.

– Squall lines Squall lines

– Cold frontsCold fronts

– Shear linesShear lines

– Surge LinesSurge Lines

– Near Equatorial Convergence ZonesNear Equatorial Convergence Zones

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Shear LineShear Line

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Non-Severe Tropical WavesNon-Severe Tropical Waves

National Hurricane Center (NHC) National Hurricane Center (NHC) definition - a trough or cyclonic curvature definition - a trough or cyclonic curvature maximum in the trade-wind easterlies.maximum in the trade-wind easterlies.

Sometimes called “Tropical Easterly Sometimes called “Tropical Easterly Waves” in the Atlantic.Waves” in the Atlantic.

Originally, this term was based on older Originally, this term was based on older research on the tropics that was based on research on the tropics that was based on sparse surface and upper air data. sparse surface and upper air data.

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Non-Severe Tropical WavesNon-Severe Tropical Waves When NHC started using METSAT imagery When NHC started using METSAT imagery

in 1967, they saw new aspects of the so-called in 1967, they saw new aspects of the so-called “wave” that didn’t fit the theoretical model.“wave” that didn’t fit the theoretical model.

Not as common as previously thought; many Not as common as previously thought; many systems called waves were actually vortices systems called waves were actually vortices

This model is falling out of favor with many This model is falling out of favor with many in the tropical community.in the tropical community.

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Non-Severe Tropical WavesNon-Severe Tropical Waves AFWA/XOGM policy is to AFWA/XOGM policy is to notnot use this term use this term

(“(“wavewave”) to describe tropical phenomena of ”) to describe tropical phenomena of any kind, any kind, despitedespite the fact that NHC and the fact that NHC and other DoD units use it - we believe many so-other DoD units use it - we believe many so-called “waves” are actually vortices, and called “waves” are actually vortices, and that the term is that the term is overusedoverused. Further, it has . Further, it has been shown that these “waves” very seldom been shown that these “waves” very seldom develop into cyclones. develop into cyclones.

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Non-Severe Tropical WavesNon-Severe Tropical Waves If you call it anything, call it a “tropical If you call it anything, call it a “tropical

disturbance”.disturbance”.

The next slide shows examples of The next slide shows examples of phenomena that used to be called “waves”phenomena that used to be called “waves”

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The late, great The late, great ““Tropical Easterly Wave”Tropical Easterly Wave”

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Non-Severe Tropical WavesNon-Severe Tropical WavesNon-Severe Tropical WavesNon-Severe Tropical Waves

CharacteristicsCharacteristics– Many stem from upper-level cyclones (cold Many stem from upper-level cyclones (cold

lows).lows).– Characteristics of Atlantic (easterly) waves.Characteristics of Atlantic (easterly) waves.

Form over Ethiopian Highlands June to October.Form over Ethiopian Highlands June to October. Move across baroclinic zone south of Sahara - Move across baroclinic zone south of Sahara -

often form squall lines.often form squall lines. Dampen under STR axis in eastern Atlantic and Dampen under STR axis in eastern Atlantic and

strengthen near Lesser Antilles.strengthen near Lesser Antilles.

70Inverted “Vee” or Screaming Eagle - really a circulationInverted “Vee” or Screaming Eagle - really a circulation

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Screaming EagleScreaming Eagle

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Non-Severe VorticesNon-Severe VorticesNon-Severe VorticesNon-Severe Vortices

Fair weather vortices - Fair weather vortices - (Equatorial (Equatorial anticyclone, heat low, and TUTT)…anticyclone, heat low, and TUTT)…

Bad weather vortices - Bad weather vortices - (Tropical cyclones, (Tropical cyclones, monsoon depression, west African cyclones, monsoon depression, west African cyclones, and mid Tropospheric cyclones).and mid Tropospheric cyclones).

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Fair Weather VorticesFair Weather VorticesFair Weather VorticesFair Weather Vortices

Equatorial anticyclonesEquatorial anticyclones– Found where monsoon trough is more than Found where monsoon trough is more than

10° from equator10° from equator– Surge from opposite hemisphere crosses Surge from opposite hemisphere crosses

equator and turns anticyclonicallyequator and turns anticyclonically– Curved band of cloud seen at leading edge Curved band of cloud seen at leading edge

of anticycloneof anticyclone

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Fair Weather VorticesFair Weather VorticesFair Weather VorticesFair Weather Vortices

Heat lows/ heat troughsHeat lows/ heat troughs– Low-level air rises over hot land with Low-level air rises over hot land with

subsidence aloft.subsidence aloft.– Lowest pressure coincides with highest Lowest pressure coincides with highest

temperature.temperature.– ExamplesExamples

Sahara and SW Asia in summer (may be part of Sahara and SW Asia in summer (may be part of monsoon trough).monsoon trough).

Southern South America, southern and eastern Southern South America, southern and eastern Africa all year.Africa all year.

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Bad Weather VorticesBad Weather Vortices

Mid-tropospheric cyclones.Mid-tropospheric cyclones.– Subtropical cyclones.Subtropical cyclones.

Cut-off portion of deep mid-latitude trough in Cut-off portion of deep mid-latitude trough in the tropics in winter.the tropics in winter.

Example: "Kona" storms in Hawaii, cause SW Example: "Kona" storms in Hawaii, cause SW winds and heavy rains.winds and heavy rains.

– Arabian Sea cyclonesArabian Sea cyclones Major rainmaker on west coast of India in SW Major rainmaker on west coast of India in SW

monsoon. monsoon. Cloud pattern resembles a typhoon.Cloud pattern resembles a typhoon.

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Tertiary CirculationsTertiary Circulations

Land and sea breezesLand and sea breezes– Sea-breeze characteristics.Sea-breeze characteristics.

Sets up a few hours after sunrise.Sets up a few hours after sunrise. Moves inland until late afternoon to evening.Moves inland until late afternoon to evening. Strong breezes may extend 30 - 50 miles inland.Strong breezes may extend 30 - 50 miles inland.

– Land-breeze characteristics.Land-breeze characteristics. Generally shallower with weaker winds than sea breeze.Generally shallower with weaker winds than sea breeze. Normally does not penetrate as far offshore as sea breeze Normally does not penetrate as far offshore as sea breeze

penetrates onshore.penetrates onshore. Land breeze front often triggers convection, especially Land breeze front often triggers convection, especially

converging land breezesconverging land breezes

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Prevailing flow onshore:Prevailing flow onshore:

Rainfall - Special CasesRainfall - Special Cases

Day

Night

Daytime max on inlandDaytime max on inlandslopesslopes

Nighttime max on coastlineNighttime max on coastline

Prevailing flow offshorePrevailing flow offshoreAfternoon max onAfternoon max oncoastlinecoastline

Sea BreezeSea Breeze

Land BreezeLand Breeze

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Sea Breeze

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Sea BreezeSea Breeze

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Land BreezeLand Breeze

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Tertiary CirculationsTertiary CirculationsTertiary CirculationsTertiary Circulations

Orographically-induced winds.Orographically-induced winds.– Valley breeze.Valley breeze.

Slopes warm during day.Slopes warm during day. Upslope wind.Upslope wind. Clouds and convection over peaks during daytime.Clouds and convection over peaks during daytime.

– Mountain breeze.Mountain breeze. Slopes cool at night.Slopes cool at night. Downslope wind.Downslope wind. Clouds and convection in valleys at night.Clouds and convection in valleys at night.

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Tertiary CirculationsTertiary CirculationsTertiary CirculationsTertiary Circulations

Orographically-induced winds.Orographically-induced winds.– Mountain gap winds.Mountain gap winds.

Funneling through passes when surface gradient Funneling through passes when surface gradient across mountains is strong.across mountains is strong.

Example: Tehuantepecer winds off Mexico.Example: Tehuantepecer winds off Mexico.

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TehuantepecersTehuantepecers

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Tehuantepecers

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Basic Analysis and Circulation Basic Analysis and Circulation ModelsModels

Basic Analysis and Circulation Basic Analysis and Circulation ModelsModels

•DefinitionsDefinitions•General TechniquesGeneral Techniques•Draw AsymptotesDraw Asymptotes•Streamlining to satellite ImagesStreamlining to satellite Images•Recommended ProceduresRecommended Procedures•Auxiliary Analysis TechniquesAuxiliary Analysis Techniques

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DefinitionsDefinitions

Streamline - Streamline - solid lines which are tangent to the solid lines which are tangent to the

instantaneous wind directioninstantaneous wind direction Isotachs - Isotachs - lines which connect points of equal wind lines which connect points of equal wind

speed. Not necessarily parallel to the flowspeed. Not necessarily parallel to the flow Isogons - Isogons - lines which connect points of equal wind lines which connect points of equal wind

directiondirection Asymptotes - Asymptotes - special streamlines onto which other special streamlines onto which other

streamlines intersect (confluent) or emanate from streamlines intersect (confluent) or emanate from (diffluent)(diffluent)

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DefinitionsDefinitions

Singular point - a point into which or Singular point - a point into which or from which more than one streamline from which more than one streamline can be drawn. Examples are:can be drawn. Examples are:– VorticesVortices– Neutral points (Cols)Neutral points (Cols)– Cusp Cusp – Buffer Cell Buffer Cell

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Streamline spacing should not necessarily Streamline spacing should not necessarily correspond to wind speed.correspond to wind speed.

Try to find neutral points and vortices first.Try to find neutral points and vortices first. Consider all wind reports carefully before Consider all wind reports carefully before

ruling them out. ruling them out. Draw arrowheads and tails at edges of Draw arrowheads and tails at edges of

charts and at singularities.charts and at singularities.

General TechniquesGeneral TechniquesGeneral TechniquesGeneral Techniques

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In generalIn general–ConvergersConvergers begin at neutral begin at neutral

points and flow into cyclonespoints and flow into cyclones

–DivergersDivergers begin at anticyclones begin at anticyclones and end at neutral points and end at neutral points (Cols)(Cols)

Drawing AsymptotesDrawing AsymptotesDrawing AsymptotesDrawing Asymptotes

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Rules for Rules for confluentconfluent asymptotes. asymptotes. Flow should converge smoothly Flow should converge smoothly

from either side.from either side. They should line up with They should line up with

convective cloud bands on low-convective cloud bands on low-level charts and clear areas on level charts and clear areas on upper-level charts.upper-level charts.

Rules for Drawing AsymptotesRules for Drawing Asymptotes

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Rules for Rules for diffluent diffluent asymptotes.asymptotes. Flow should diverge smoothly from a Flow should diverge smoothly from a

single axissingle axis In low-levels, they should be in clear areas In low-levels, they should be in clear areas

(on satellite pictures) or in areas of closed-(on satellite pictures) or in areas of closed-cell stratocumulus.cell stratocumulus.

In upper-levels, they should overlay areas In upper-levels, they should overlay areas of upper-level cloudsof upper-level clouds

Rules for Drawing AsymptotesRules for Drawing AsymptotesRules for Drawing AsymptotesRules for Drawing Asymptotes

93

Rules for linked vortices.Rules for linked vortices.– Two like-vortices must have a neutral Two like-vortices must have a neutral

points in between.points in between. A series of cyclones is bounded by A series of cyclones is bounded by

diffluent asymptotes. A series of diffluent asymptotes. A series of anticyclones is bounded by confluent anticyclones is bounded by confluent asymptotes.asymptotes.

Rules for Drawing AsymptotesRules for Drawing AsymptotesRules for Drawing AsymptotesRules for Drawing Asymptotes

96

Recommended proceduresRecommended proceduresRecommended proceduresRecommended procedures

Overlay current chart on 24-hour Overlay current chart on 24-hour continuity and mark important continuity and mark important features in yellow. Keep previous features in yellow. Keep previous chart for reference.chart for reference.

Overlay current chart on satellite Overlay current chart on satellite image, or draw the significant image, or draw the significant features from the image on your features from the image on your chart.chart.

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Recommended proceduresRecommended proceduresRecommended proceduresRecommended procedures

Tentatively mark major features (large Tentatively mark major features (large cyclones and anticyclones, hurricanes, etc.).cyclones and anticyclones, hurricanes, etc.).

Start streamlining near dominant features like Start streamlining near dominant features like the STR and undisturbed trade-wind areas. the STR and undisturbed trade-wind areas. Work in toward complicated areas. Work in toward complicated areas.

Light winds occur with cols, high centers, Light winds occur with cols, high centers, buffer cells, and areas of sharp curvature.buffer cells, and areas of sharp curvature.

Strong winds occur with cyclones and broad Strong winds occur with cyclones and broad areas of little curvature.areas of little curvature.

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Auxiliary Analysis TechniquesAuxiliary Analysis TechniquesAuxiliary Analysis TechniquesAuxiliary Analysis Techniques

Vertical Time Cross-sectionVertical Time Cross-section– An analysis of changes in the atmosphere with An analysis of changes in the atmosphere with

time over a stationtime over a station– Useful in the tropics because small changes in the Useful in the tropics because small changes in the

vertical over a station become readily apparentvertical over a station become readily apparent Vertical Space Cross-sectionVertical Space Cross-section

– An analysis of a cross section of the atmosphere An analysis of a cross section of the atmosphere along a line or route at a particular time.along a line or route at a particular time.

– Selected where reporting stations are available Selected where reporting stations are available reasonably close to the line of interest.reasonably close to the line of interest.

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Checkerboard DiagramCheckerboard Diagram Can show current weather over an areaCan show current weather over an area Diurnal weather pattern recognition and Diurnal weather pattern recognition and

monitoringmonitoring Is used to represent climatologyIs used to represent climatology Identifying small changes in weather that Identifying small changes in weather that

may be masked by normal diurnal patternsmay be masked by normal diurnal patterns

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Skew-T / Rawinsonde data.Skew-T / Rawinsonde data.– Determine thermodynamic variablesDetermine thermodynamic variables

Stability - determine indices particular to Stability - determine indices particular to station.station.

Moisture availability - LCL, CCL, LFC, Moisture availability - LCL, CCL, LFC, etc.etc.

Height rises/falls.Height rises/falls. Analysis same as in mid-latitudes.Analysis same as in mid-latitudes.

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MonsoonsMonsoonsMonsoonsMonsoons*** Monsoon rains flooding BangladeshMonsoon rains flooding Bangladesh

(AP report from 22 July 1999)(AP report from 22 July 1999) DHAKA, Bangladesh (AP) - Two days of torrential DHAKA, Bangladesh (AP) - Two days of torrential rains flooded another 10 villages in northern rains flooded another 10 villages in northern Bangladesh, marooning at least 20,000 people, Bangladesh, marooning at least 20,000 people, authorities said Tuesday. Rescue teams were authorities said Tuesday. Rescue teams were dispatched with food and medicine after heavy dispatched with food and medicine after heavy rains swelled the Padma and Jamuna rivers in an rains swelled the Padma and Jamuna rivers in an

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MonsoonsMonsoonsMonsoonsMonsoons*** Monsoon rains flooding Bangladesh (con’t)Monsoon rains flooding Bangladesh (con’t) area about 65 miles north of Dhaka, the area about 65 miles north of Dhaka, the Bangladeshi capital. Last week, 55,000 people Bangladeshi capital. Last week, 55,000 people were washed out of their homes when the Gumti were washed out of their homes when the Gumti River overflowed its banks and submerged 200 River overflowed its banks and submerged 200 villages in Comilla district, 55 miles east of villages in Comilla district, 55 miles east of Dhaka. Most of those people are now camped Dhaka. Most of those people are now camped on the muddy river embankment.on the muddy river embankment.

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MonsoonsMonsoonsMonsoonsMonsoons

DefinitionsDefinitions– Glossary of Meteorology, Huschke, 1959: "A Glossary of Meteorology, Huschke, 1959: "A

name for a seasonal wind. Derived from Arabic name for a seasonal wind. Derived from Arabic 'Mausim', a season": 'Mausim', a season":

– Monsoon is one of those overused words in Monsoon is one of those overused words in meteorology, often used to describe any meteorology, often used to describe any regularly occurring summertime rainy period. regularly occurring summertime rainy period.

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DefinitionsDefinitions– Colin Ramage, 1971, defined a monsoon in Colin Ramage, 1971, defined a monsoon in

terms of four criteria:terms of four criteria: Prevailing wind directions shifts by at least 120° Prevailing wind directions shifts by at least 120°

between January and July.between January and July. Average frequency of occurrence of the prevailing Average frequency of occurrence of the prevailing

wind in both January and July exceeds 40%.wind in both January and July exceeds 40%. Mean resultant winds in at least 1 of these 2 months Mean resultant winds in at least 1 of these 2 months

exceeds 3 m/s (6 kts).exceeds 3 m/s (6 kts). Definite lack of migratory systems.Definite lack of migratory systems.

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General characteristics of monsoonsGeneral characteristics of monsoons– SummerSummer

Surface pressure falls over continents due to heating.Surface pressure falls over continents due to heating. Increased pressure gradient between continent and Increased pressure gradient between continent and

nearby ocean causes onshore flow.nearby ocean causes onshore flow. Circulation forms.Circulation forms.

– Rising air over continent.Rising air over continent.

– Offshore return flow aloft.Offshore return flow aloft.

– Sinking air over oceans.Sinking air over oceans.

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General characteristics of monsoonsGeneral characteristics of monsoons– WinterWinter

Surface pressure rises over continents due to cooling.Surface pressure rises over continents due to cooling. Pressure gradient reverses from summer situation, Pressure gradient reverses from summer situation,

causing offshore flow.causing offshore flow. Circulation forms:Circulation forms:

– Rising air over ocean.Rising air over ocean.

– Onshore return flow aloft.Onshore return flow aloft.

– Sinking air over continent.Sinking air over continent.

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Asian MonsoonAsian Monsoon African MonsoonsAfrican Monsoons Australian MonsoonsAustralian Monsoons Central American MonsoonsCentral American Monsoons

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Monsoon Seasonal LocationsMonsoon Seasonal Locations

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The Asian MonsoonsThe Asian MonsoonsThe Asian MonsoonsThe Asian Monsoons

Division of Asia into 3 monsoonal regionsDivision of Asia into 3 monsoonal regions– East of TibetEast of Tibet

Damp cold winters, wet summers.Damp cold winters, wet summers.

– West of TibetWest of Tibet Mostly Desert.Mostly Desert.

– South of TibetSouth of Tibet Dry mild winters, hot wet summers.Dry mild winters, hot wet summers.

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The Asian MonsoonsThe Asian Monsoons Summer (Southwest) Monsoon (Jul-Aug)Summer (Southwest) Monsoon (Jul-Aug)

– The Himalayan Mountain range prevents CAAThe Himalayan Mountain range prevents CAA– Monsoon trough extends from NE Africa to near Monsoon trough extends from NE Africa to near

GuamGuam– Tropical CyclonesTropical Cyclones

Rare in South Asia in summer.Rare in South Asia in summer.– Vertical shear associated with TEJ.Vertical shear associated with TEJ.– Subsidence over oceans.Subsidence over oceans.– Monsoon trough anchored over land.Monsoon trough anchored over land.

Typhoons increase over western Pacific in summer.Typhoons increase over western Pacific in summer.

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The Asian MonsoonsThe Asian Monsoons Summer (Southwest) Monsoon (Con’t)Summer (Southwest) Monsoon (Con’t)

– Onset: Main Driving Force - extreme differential heating between Onset: Main Driving Force - extreme differential heating between land and water areas, which results in:land and water areas, which results in:

– Strong Subtropical highs in the Southern Hemisphere (Strong Subtropical highs in the Southern Hemisphere (SHSH))

– Strong heat lows develop over Northern Africa, Saudi Arabia, Strong heat lows develop over Northern Africa, Saudi Arabia, Pakistan, Northern India, and SE Asia.Pakistan, Northern India, and SE Asia.

– Strong surface heating that causes the land mass to become Strong surface heating that causes the land mass to become warmer then the equatorial regionwarmer then the equatorial region

– Over land, heights are lower in the low levels and higher in the Over land, heights are lower in the low levels and higher in the upper levels (compared to equatorial region) creating a strong upper levels (compared to equatorial region) creating a strong pressure gradient forcepressure gradient force

– Low level flow is directed out of the Low level flow is directed out of the SHSH Subtropical Ridge, across Subtropical Ridge, across the equator and into monsoon trough.the equator and into monsoon trough.

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The Asian MonsoonsThe Asian MonsoonsThe Asian MonsoonsThe Asian Monsoons Summer (Southwest) Monsoon (Con’t)Summer (Southwest) Monsoon (Con’t)

– Moisture:Moisture: Air travels a great distance over water and becomes Air travels a great distance over water and becomes

high in mixing ratio (moisture content).high in mixing ratio (moisture content). Upward vertical motion within the trough produces Upward vertical motion within the trough produces

condensation and release of large amounts of latent condensation and release of large amounts of latent heat.heat.

Latent Heat warms the atmosphere which results in Latent Heat warms the atmosphere which results in stronger pressure gradient force (PGF).stronger pressure gradient force (PGF).

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– Coriolis:Coriolis: As southeasterly winds cross the equator, the As southeasterly winds cross the equator, the

coriolis force turns the winds to southwesterly, coriolis force turns the winds to southwesterly, resulting in the Indian Ocean/Westpac buffer zone.resulting in the Indian Ocean/Westpac buffer zone.

Southwesterlies tend to produce upward vertical Southwesterlies tend to produce upward vertical motion, resulting in another mechanism for the motion, resulting in another mechanism for the release of latent heat and stronger PGF.release of latent heat and stronger PGF.

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– Key Features:Key Features: TEJ - Tropical Easterly Jet (mentioned previously)TEJ - Tropical Easterly Jet (mentioned previously) Somalian JetSomalian Jet

– A low level jet that extends from the east coast of Africa A low level jet that extends from the east coast of Africa across the North Arabian Sea (NAS)across the North Arabian Sea (NAS)

– Core of Jet: between 3000 and 5000 ft (850 mb)Core of Jet: between 3000 and 5000 ft (850 mb)

– Max winds: 55 ktsMax winds: 55 kts

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The Asian MonsoonsThe Asian Monsoons Southwest Monsoon Key FeaturesSouthwest Monsoon Key Features (con’t) (con’t)

– Somalian Jet DynamicsSomalian Jet Dynamics: Winds Cross the equator near Africa and turn to southwesterliesWinds Cross the equator near Africa and turn to southwesterlies Terrain over Eastern Africa creates a barrier and wind speeds Terrain over Eastern Africa creates a barrier and wind speeds

increase due to a channeling effectincrease due to a channeling effect Winds parallel the coast of Somalia while heat lows inland and Winds parallel the coast of Somalia while heat lows inland and

friction create cross isobaric flow over land This results in low level friction create cross isobaric flow over land This results in low level diffluence and upwelling of cold water off the Somali coast.diffluence and upwelling of cold water off the Somali coast.

Cycle then feeds off itself - clear skies bring more heating, stronger Cycle then feeds off itself - clear skies bring more heating, stronger diffluence, cooler sea surface temps, and stronger wind speeds.diffluence, cooler sea surface temps, and stronger wind speeds.

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The Asian MonsoonsThe Asian Monsoons

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The Asian MonsoonsThe Asian MonsoonsThe Asian MonsoonsThe Asian Monsoons Southwest Monsoon Key FeaturesSouthwest Monsoon Key Features (con’t) (con’t)

– Monsoon DepressionsMonsoon Depressions Bad weather vortices that form in the monsoon trough.Bad weather vortices that form in the monsoon trough. Most common in northern Bay of Bengal, occasionally over land or the Arabian Most common in northern Bay of Bengal, occasionally over land or the Arabian

Sea.Sea. Resemble Tropical Cyclones on satellite images. Seldom become Tropical Resemble Tropical Cyclones on satellite images. Seldom become Tropical

Cyclones, but they can develop as suchCyclones, but they can develop as such Characteristics.Characteristics.

– Horizontal diameter: 100 kmHorizontal diameter: 100 km– Can persist a week or more, usually 3 - 4 days.Can persist a week or more, usually 3 - 4 days.– Locally very heavy rain (mainly in southwest quadrant ), winds commonly 40 kts Locally very heavy rain (mainly in southwest quadrant ), winds commonly 40 kts

sustained.sustained.– Can be cold core.Can be cold core.

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– Mid-Tropospheric DisturbancesMid-Tropospheric Disturbances North-South wind shear causes these systems to developNorth-South wind shear causes these systems to develop Most common in Northern Arabian Sea.Most common in Northern Arabian Sea. Occur between the low level westerlies and the upper level Occur between the low level westerlies and the upper level

easterlies (600 mb)easterlies (600 mb) Characteristics.Characteristics.

– Horizontal diameter: 500 kmHorizontal diameter: 500 km– Unlike monsoon depressions, they can persist for many days.Unlike monsoon depressions, they can persist for many days.– Also produce locally very heavy rainAlso produce locally very heavy rain

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– Onset Vortex:Onset Vortex: Cyclonic Circulation off the southern coast of India (in the NAS) prior Cyclonic Circulation off the southern coast of India (in the NAS) prior

to the onset of the SW Monsoonto the onset of the SW Monsoon Caused when the southwesterlies are deflected around the Ghats Caused when the southwesterlies are deflected around the Ghats

Moutains (Southern India)Moutains (Southern India) Characteristics.Characteristics.

– Vorticies are small and last 2-3 daysVorticies are small and last 2-3 days– Also produce locally very heavy rain (they dumped 10 cm of rain in 24 Also produce locally very heavy rain (they dumped 10 cm of rain in 24

hours!)hours!)– Existence is not always noticeable from year to yearExistence is not always noticeable from year to year– Rarely, can become tropical cyclones Rarely, can become tropical cyclones

126


– Breaks in the Monsoon:Breaks in the Monsoon:

A decrease in rainfall in central IndiaA decrease in rainfall in central India

Caused by an extreme northward shift in the Tropical Easterly Caused by an extreme northward shift in the Tropical Easterly Jet and Monsoon troughJet and Monsoon trough

Causes an increase in rainfall over the foothills of the Causes an increase in rainfall over the foothills of the Himalayas and extreme southerm tip of IndiaHimalayas and extreme southerm tip of India

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Winter (Northeast) Monsoon (Nov.-April)Winter (Northeast) Monsoon (Nov.-April)– Siberian High sets-up, CAA kills Indian Ocean Siberian High sets-up, CAA kills Indian Ocean

cyclonescyclones– Crachin" can form in coastal Southern China Crachin" can form in coastal Southern China

northern Indochina.northern Indochina. Persistent stratus regime.Persistent stratus regime. Rain, fog, drizzle, low visibilities.Rain, fog, drizzle, low visibilities. Sets in about January, may last till April.Sets in about January, may last till April.

128


Winter (Northeast - NE) Monsoon (Con’t). Winter (Northeast - NE) Monsoon (Con’t). – Mainly southern Asian regionMainly southern Asian region– During NE monsoons, strong winds flow from During NE monsoons, strong winds flow from

the extremely cold continental High centered the extremely cold continental High centered near Lake Baykal in southern Siberia (near near Lake Baykal in southern Siberia (near Mongolia) in a series of surges often greater Mongolia) in a series of surges often greater than gale force speedsthan gale force speeds

129


Winter (Northeast - NE) Monsoon (Con’t). Winter (Northeast - NE) Monsoon (Con’t). – Onset:Onset:

Sun position migrates southward (decrease in solar Sun position migrates southward (decrease in solar declination), thus decreasing solar insolation (heating) declination), thus decreasing solar insolation (heating) over Asia. This will, in turn, fill the heat lows south of over Asia. This will, in turn, fill the heat lows south of the Tibetan Plateauthe Tibetan Plateau

The Tibetan Plateau cools rapidly, resulting in a The Tibetan Plateau cools rapidly, resulting in a temperature/pressure reversal.temperature/pressure reversal.

Polar outbreaks begin to occur north of the Himalayas Polar outbreaks begin to occur north of the Himalayas as the Siberian High continues to intensify. as the Siberian High continues to intensify.

130

The Asian MonsoonsThe Asian Monsoons Winter (Northeast - NE) Monsoon (Con’t). Winter (Northeast - NE) Monsoon (Con’t).

– Onset (con’t):Onset (con’t): Surface winds over Asia become northeasterly.Surface winds over Asia become northeasterly. As this takes place, the upper level anticyclone which As this takes place, the upper level anticyclone which

feeds the Tropical Easterly Jet weakens. This allows the feeds the Tropical Easterly Jet weakens. This allows the more conventional subtropical westerly jet to become more conventional subtropical westerly jet to become predominant.predominant.

By the last week in November, the NE monsoon is By the last week in November, the NE monsoon is normally established. This seems to be a fairly firm time, normally established. This seems to be a fairly firm time, and corresponds to a dramatic decrease in tropical cyclone and corresponds to a dramatic decrease in tropical cyclone activity in the South China Sea.activity in the South China Sea.

131

The Asian MonsoonsThe Asian MonsoonsThe Asian MonsoonsThe Asian Monsoons

Fall/Spring Transition Fall/Spring Transition – SpringSpring..

Mei-yu (Baiu) front is most active from Japan to Mei-yu (Baiu) front is most active from Japan to Taiwan.Taiwan.

Peak formation time for tropical cyclonesPeak formation time for tropical cyclones

– FallFall.. Cold surges start in Southeast AsiaCold surges start in Southeast Asia Tropical cyclones increase.Tropical cyclones increase.

135

The African MonsoonsThe African MonsoonsThe African MonsoonsThe African Monsoons

West African Monsoons West African Monsoons – July: North African summer monsoon, South July: North African summer monsoon, South

African winter monsoon.African winter monsoon. Heat low forms over Sahara.Heat low forms over Sahara. High over southern Africa and South. Atlantic.High over southern Africa and South. Atlantic. Pressure gradient sets up strong southerly flow.Pressure gradient sets up strong southerly flow. Circulation shallower than Asian monsoon (onshore Circulation shallower than Asian monsoon (onshore

flow up to 850 mb).flow up to 850 mb).

138

1000 mb Winds1000 mb Winds

139


140


141


142


143


144


145


146


147


148

The African MonsoonsThe African MonsoonsThe African MonsoonsThe African Monsoons

West African Monsoons West African Monsoons – January: North African winter monsoon, South January: North African winter monsoon, South

African summer monsoon African summer monsoon Cooling produces high pressure over Sahara.Cooling produces high pressure over Sahara. Heat low forms over Kalahari, not as intense or dry as Heat low forms over Kalahari, not as intense or dry as

over Sahara.over Sahara. PrecipitationPrecipitation

– Drought over North Africa.Drought over North Africa.

– Scattered Thunderstorms over southern Africa.Scattered Thunderstorms over southern Africa.

– Precipitation less organized and persistent than Asian or North Precipitation less organized and persistent than Asian or North Africa monsoon.Africa monsoon.

149

The Australian MonsoonsThe Australian MonsoonsThe Australian MonsoonsThe Australian Monsoons

Summer (Northwest) monsoonSummer (Northwest) monsoon– Low pressure over Australia, high over Asia.Low pressure over Australia, high over Asia.– Monsoon trough Monsoon trough

Anchored from low near 13°S, 170°EAnchored from low near 13°S, 170°E Extends across northern Australian interior to Extends across northern Australian interior to

southern Africa.southern Africa. Monsoon strongest N - NW of Australia due to Monsoon strongest N - NW of Australia due to

land/sea temperature contrast and the NW flow from land/sea temperature contrast and the NW flow from buffer zone.buffer zone.

150

The Australian MonsoonsThe Australian MonsoonsThe Australian MonsoonsThe Australian Monsoons

Winter (Southeast) MonsoonWinter (Southeast) Monsoon– Easterlies over most of Southern Hemisphere Easterlies over most of Southern Hemisphere

tropics.tropics.– Continuous STR northern Australia.Continuous STR northern Australia.– STJ strongest (> 100 kts), near 30°S over STJ strongest (> 100 kts), near 30°S over

central Australia.central Australia.

151

The Central American MonsoonsThe Central American MonsoonsThe Central American MonsoonsThe Central American Monsoons

Regional Effects Regional Effects Region normally dominated by NE trade windsRegion normally dominated by NE trade winds..

– Channeling causes 3 quasi stationary cyclones Channeling causes 3 quasi stationary cyclones on Pacific side of Central America which on Pacific side of Central America which anchor monsoon trough.anchor monsoon trough.

– Northern Hemisphere easterlies to north and Northern Hemisphere easterlies to north and Southern Hemisphere westerlies to south Southern Hemisphere westerlies to south maintain cyclones.maintain cyclones.

152


Panama Bay cyclonePanama Bay cyclone Strongest anchoring cyclone of the three.Strongest anchoring cyclone of the three.

– Reinforced by warm ocean currents.Reinforced by warm ocean currents.

– Southwesterly flow channeled cyclonically by Southwesterly flow channeled cyclonically by Andes year-round.Andes year-round.

– Persistent feature May through January.Persistent feature May through January.

153


Lake Nicaragua cycloneLake Nicaragua cyclone Anchored between Nicaragua and Costa Rica.Anchored between Nicaragua and Costa Rica. Weaker than Panama bay cyclone, persists May - Weaker than Panama bay cyclone, persists May -

November. November.

Gulf of Tehuantepec (Guatemalan) cyclone.Gulf of Tehuantepec (Guatemalan) cyclone. Weakest of the three.Weakest of the three. Maximum strength in OctoberMaximum strength in October Merges with NECZ to west, which is anchored over Merges with NECZ to west, which is anchored over

SST thermal maximum.SST thermal maximum.

154


American Monsoons don't meet Ramage's American Monsoons don't meet Ramage's Criteria becauseCriteria because

Semi-annually reversing North-South pressure gradients do Semi-annually reversing North-South pressure gradients do not form.not form.

– Instead, Central America has a "transitional" Instead, Central America has a "transitional" monsoon.monsoon.

Oscillates north - south semiannually.Oscillates north - south semiannually. Location determined by strengths of NE trades and Location determined by strengths of NE trades and

Southern Hemisphere southwesterlies.Southern Hemisphere southwesterlies.

155


Monsoon SurgesMonsoon Surges– Often caused by low-latitude (10 - 20°N) Often caused by low-latitude (10 - 20°N)

tropical cyclones.tropical cyclones. Acceleration of southwesterlies from south of Acceleration of southwesterlies from south of

monsoon trough.monsoon trough.

NE flow aloft back into Southern Hemisphere.NE flow aloft back into Southern Hemisphere.

156


Monsoon Surges (Con’t)Monsoon Surges (Con’t) Atemporalado IndexAtemporalado Index

– ““Atemporalado”: term used to describe a winter Atemporalado”: term used to describe a winter rain event in Central America due to a vogorous rain event in Central America due to a vogorous cold front or shear line that crosses far enough cold front or shear line that crosses far enough south.south.

– Often results in Tehuanapecers.Often results in Tehuanapecers.

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Monsoon Surges (Con’t)Monsoon Surges (Con’t) Rule of Thumb:Rule of Thumb:

Take SLP difference (Take SLP difference (P) between Merida P) between Merida Mexico and Houston TX. Mexico and Houston TX.

• If If P < 12 MB, no SURGEP < 12 MB, no SURGE

• If If P 12-14 MB, marginal SURGEP 12-14 MB, marginal SURGE

• If If P 15-19 MB, Nominal SURGEP 15-19 MB, Nominal SURGE

• If If P >20 MB, P >20 MB, STRONGSTRONG SURGE SURGE

158SurgeSurge

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Tropical AnomaliesTropical AnomaliesTropical AnomaliesTropical Anomalies El Nino / Southern Oscillation (ENSO)El Nino / Southern Oscillation (ENSO) - - Warming of equatorial Pacific watersWarming of equatorial Pacific waters

- - Occurs aOccurs att irregular intervals of 2 to 12 years irregular intervals of 2 to 12 years 30 - 50 Day Oscillation30 - 50 Day Oscillation - - Observed cycles in tropical storm frequencyObserved cycles in tropical storm frequency worldwideworldwide Quasi-biennial Oscillation (QBO)Quasi-biennial Oscillation (QBO) - - Periodic wind direction change in stratospherePeriodic wind direction change in stratosphere - 2 year cycle of increased occurrence of Atlantic- 2 year cycle of increased occurrence of Atlantic HurricanesHurricanes

~

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Tropical Cyclone WeatherTropical Cyclone WeatherTropical Cyclone WeatherTropical Cyclone Weather

PrecipitationPrecipitation– Reaches a maximum over oceans in the right rear Reaches a maximum over oceans in the right rear

quadrantquadrant– Reaches a maximum after landfall in the right front Reaches a maximum after landfall in the right front

quadrantquadrant

TornadoesTornadoes– Most likely near time of landfall due to strong shearingMost likely near time of landfall due to strong shearing

Storm surgeStorm surge– Storm surge is responsible for most of the deaths along Storm surge is responsible for most of the deaths along

the coast.the coast.

162

Severe Weather In The TropicsSevere Weather In The TropicsSevere Weather In The TropicsSevere Weather In The Tropics

•Severe ThunderstormsSevere Thunderstorms•Non-Convective WindsNon-Convective Winds

163

ThunderstormsThunderstormsThunderstormsThunderstorms

More common in tropics than in high More common in tropics than in high latitudeslatitudes

82 % of thunderstorms are over South 82 % of thunderstorms are over South America, Africa, and IndonesiaAmerica, Africa, and Indonesia

18% of thunderstorms are over water18% of thunderstorms are over water Most are not severe by mid-latitude Most are not severe by mid-latitude

(midwest) standards(midwest) standards

164

Severe ThunderstormsSevere ThunderstormsSevere ThunderstormsSevere Thunderstorms HailHail

– Rare in tropicsRare in tropics The typical WBZ in tropics above 12,000 ft The typical WBZ in tropics above 12,000 ft

and is usually over 15,000 ftand is usually over 15,000 ft Tornadoes and waterspoutsTornadoes and waterspouts

– Most common in the U.S. and AustraliaMost common in the U.S. and Australia.. LocationLocation

– Most severe storms occur when continental polar Most severe storms occur when continental polar air penetrates the tropics and squall lines or shear air penetrates the tropics and squall lines or shear lines developlines develop

165

Severe ThunderstormsSevere ThunderstormsSevere ThunderstormsSevere Thunderstorms

166

Severe WindsSevere Winds

Severe Winds (winds greater than 30 kts)Severe Winds (winds greater than 30 kts)

– Nonconvective - usually due to Nonconvective - usually due to orographic channelingorographic channeling Surges through Taiwan Strait (up to 50 kt Surges through Taiwan Strait (up to 50 kt

winds)winds) Shearlines moving through Central Shearlines moving through Central

America (Northers, Tehuantepecers)America (Northers, Tehuantepecers)

167


168

Tropical ForecastingTropical ForecastingTropical ForecastingTropical Forecasting

Short-Range Forecasting.Short-Range Forecasting. Medium-Range and Long-RangeMedium-Range and Long-Range

169

Schools of Thought in Tropical Schools of Thought in Tropical ForecastingForecasting

Schools of Thought in Tropical Schools of Thought in Tropical ForecastingForecasting

Climatological Method.Climatological Method.– Day-to-day weather varies little from Day-to-day weather varies little from

climatology.climatology.

– The best guide to forecasting is The best guide to forecasting is detailed knowledge of climatology.detailed knowledge of climatology.

170

Short-Range Forecasting Short-Range Forecasting TechniquesTechniques

Short-Range Forecasting Short-Range Forecasting TechniquesTechniques

Persistence and Extrapolation.Persistence and Extrapolation.– Works 80% - 85% of time (up to Works 80% - 85% of time (up to

95% in monsoon or wet/dry climate).95% in monsoon or wet/dry climate).

– Satellite data is great aid in Satellite data is great aid in extrapolation.extrapolation.

171

Medium-Range and Long Range Medium-Range and Long Range Forecasting TechniquesForecasting Techniques

Medium-Range and Long Range Medium-Range and Long Range Forecasting TechniquesForecasting Techniques

Climatology is best forecast beyond two to three daysClimatology is best forecast beyond two to three days Numerical Weather PredictionNumerical Weather Prediction

– Current models barely beat persistence over 24 hoursCurrent models barely beat persistence over 24 hours

– Models work best when weather is linked to mid-latitude systemsModels work best when weather is linked to mid-latitude systems

172

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1 afwa/xogm originally produced by: tsgt nast initial tropical training for satellite analysts...

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