Tropical Meteorology Flashcards
State the approximate latitude limits of the tropics.
- Between the Tropic of Capricorn 23.5 degrees S (southern hemisphere)
- Tropic of Cancer 23.5 degrees N (northern hemisphere)
Describe the Hadley cell.
- A large-scale atmospheric convection cell in which air rises at the equator and sinks at medium latitudes
- Sinks at around 30 degrees N/S
- Responsible for the trade winds in the Tropics and control low-latitude weather patterns
Describe what is meant by horse latitudes.
- Located between 30-40 degrees in both hemispheres
- Light winds, little rain fall due to anticyclones that form here
Describe what is meant by doldrums.
- Exists with the intertropical convergence zone
- Between 5 degrees N/S
- Trade winds converge resulting in light winds
- Frequent heavy thunderstorms due to convection cause by sun and moist air
Describe the equatorial trough and the inter-tropical convergence zone (ITCZ)
- The trough is centered on the thermal equator
- Near continuous belt of low pressure around the low latitudes
- Very high dew point therefore large amounts of moisture in the air
- Contains the doldrums, ITCZ and part of the trade winds converge resulting
State the seasonal location of the equatorial trough and ITCZ.
- Both move with the sun into warmer hemisphere
- Causes the trade winds to change from SE to SW when crossing over the equator in NS summer.
- NE trade winds become NW in SH summer
- Smaller movements over the ocean than land
State the typical low and mid-level weather in an active and inactive ITCZ.
- If trade winds meet head on there is strong convergence
State the typical low and mid-level weather in an active and inactive ITCZ.
- Active = extensive Cu, TCu, Cb and thunderstorms with heavy showers, icing and strong up/downdrafts
- Inactive = Low level fair weather cumulus, light winds converge resulting
Explain the origin and common location of the South Pacific Convergence Zone (SPCZ).
- Most active in SH summer
- Cause by semi-stationary high eastern pacific and the anti-cyclones originating/traveling east from Australia/New Zealand
- NE wind meets SE wind creating zone of intense cloudiness between Papa New Guinea and French Polynesia
Describe the weather associated with the SPCZ.
- Summer = convective thunderstorms and tropical disturbances
- Winter = weaker convective thunderstorms or fair weather Cu.
With regard to the Trade Winds, describe the mechanisms that drive the Trade Winds.
- Pressure differential between the anti-cyclones at 30 degrees and depressions at the equator
- As it move from high to low, Coriolis force deflects the wind left in SH and right in NH.
- Ultimately cause by the sun and aid in heat distribution.
With regard to the Trade Winds, describe the approximate latitudinal and vertical limits.
- Between 30 degrees N and S.
- Generally restricted to 8,000ft AMSL
With regard to the Trade Winds, describe the seasonal location and direction.
- Follows the sun
- Moves north in NZ winter and south in summer
- SH = SE in NZ summer, SW in winter
- NH= NW in NZ summer, NE in winter
With regard to the Trade Winds, describe the commonly associated weather.
- Weather associated with high pressure systems closer to 30 degrees latitude
- The closer to the equator the trade winds get, the more unstable the weather and clouds become
- Wind strength = 10-20kts occasionally 30kts
- Trade winds strong in winter than summer
With regard to the Trade Winds, describe the winds and weather usually experienced above the Trade Winds.
- Above trade wind inversion at closer to 30 degrees = good visibility dry air and clear skies
- Closer to the equator moisture content and instability increase creating TCu and Cb aloud at altitude
- Above the trade winds strong = light westerlies
With regard to the Trade Winds, describe the topographical influences on the Trade Winds.
- Land heats up and cools down faster than the ocean
- Increases instability in summer on coasts exposed to SE trade wind
- Increases cloud development
- Steeper terrain will also cause orographic
- In winter stability is increased reversing the effects
Define monsoon.
- Prevailing winds bringing wet or dry seasons.
- Caused by continental highs and lows creating giant land breezes or sea breezes
describe the mechanisms involved with regard to wet monsoons.
- Occurs when land is heated up in spring/summer
- Creates intense instability causing air to rise
- Replacement air from the oceans travels inland and creates large thunderstorms, heavy precipitation and squalls
- Mountain barriers increases the development of
State the seasons during which the Australian monsoons occur.
- Occurs between November and April, peaks in February
- Summer
- Occurs when moist NW trade wind blows onto northern Australia
- In winter the dry monsoon blows from the SE trade wind bringing dry weather
Describe the requirements for the formation and development of tropical cyclones.
- Sea temperature must be 26.5 degrees Celsius or higher
- Associated with low pressure area known as equatorial trough
- Between 5 and 30 degrees latitude due to Coriolis force being too weak above 5 degrees.
- Must develop over ocean due to moisture requirement
Describe the requirements for the formation and development of tropical cyclones.
- Must be high-level divergence just below the tropopause
- Requires warm core created by release of latent heat
- Warm core creates the upper level high pressure zone
- A cyclone eye to reduce pressure below 1000hPa
Describe the weather conditions associated with tropical cyclones.
- Air pressures as low as 950hPa
- Hurricane forces winds of 64kts or greater are sustained
- Torrential rain, thunderstorms and violent squalls
- Can last up too two weeks
Explain the factors causing El Niño events.
- Caused by the collapse of the Walker Cell
- Occurs when pressure increases in the west near Darwin and decreases int he east near Tahiti
- Trade winds can reverse in direction
- Enhances westerlies in mid-latitudes
Explain the factors causing La Niña events.
- Caused by the Walker Cell and Humboldt current
- Occurs when pressure decreases in the west near Darwin and increases in the east near Tahiti
- Trade winds enhanced
- Westerlies in mid-latitudes reduced or even reversed into easterlies
Describe how El Niño and La Niña events influence the weather in New Zealand.
- El Niño causes more rain on the West Coast and dryer conditions on the east coast
- La Niña causes more rain on the east coast and dryer conditions on the West Coast
- Higher chance of ex-tropical cyclones in summer months with La Niña
Explain what is meant by equatorial trough.
- Meteorological equator is the annual mean latitude of the ITZC
- Meteorological equator at 5 degrees N
- The thermal equator is a different line jointing all points of highest mean annoyance surface temperature
Explain what is meant by ITCZ.
- Marks the places on earth where the trade winds from northern and southern hemispheres converge
- Centred on the thermal equator
Explain what is meant by South Pacific convergence zone (SPCZ).
- Formed where the outflow from the Walker cell clashes with the semi-permanent highs at 30 degrees south
- About 200 to 400km wide
- Is the birth place for all South Pacific tropical cyclones
Relative to the equatorial trough, state the region where maximum convergence occurs.
- 20 degrees N 15 degrees S
Relative to the equatorial trough, state the region where the maximum convection occurs.
- 20 degrees N 15 degrees S
Relative to the equatorial trough, state the region where the maximum cloud development occurs.
- 20 degrees N 15 degrees S
Describe the weather features commonly associated with an ‘active’ or an ‘inactive’ ITCZ.
- Active = thunderstorms and heavy showers of rain up to 600km (300nm) wide
- Most active in the afternoon during summer
- Inactive = some showers, light winds, 500km wide
- Least active over ocean in the winter.
Describe the icing commonly associated with an ‘active’ or an ‘inactive’ ITCZ.
- Active = Moderate to severe icing above the FL, especially within the first 10,000 to 15,000ft above it.
- Inactive = light to none
Describe the turbulence commonly associated with an ‘active’ or an ‘inactive’ ITCZ.
- Active = moderate to severe due to Cb clouds
- Can’t fly under turbulence because cloud bases are 1000ft
- Can’t fly over turbulence as clouds are too high
- Inactive = light to none
Describe the cloud related factors commonly associated with and ‘active’ or an ‘inactive’ ITCZ.
- Active = Wall of Cu, Cb, TCu clouds going all the way up to into the stratosphere
- Large deviations required of wanting to go around an active ITCZ
- Inactive = various okras of cumulus
Describe the preferred location and characteristics of the South Pacific Convergence Zone.
- Papua New Guinea to French Polynesia
Use diagrams to explain the anticyclonic subsidence and associated meteorological conditions of the ‘Trade Winds’ in both hemispheres of the Pacific Ocean.
- Semi-permanent highs occur between 25 to 35 degrees latitude in both hemispheres and travel eastward
- Clear skies, light winds that are deflected by the Coriolis force
- Causes trade winds to be NE in NH and SE in SH
Used diagrams to explain the approximate latitudinal and vertical limits of the Trade Winds in both hemispheres of the Pacific Ocean.
- Between 30 degrees north and south.
- Vertical limits of 8000ft AMSL
Use diagrams to explain the seasonal changes in location and their effect on wind direction, of the Trade Winds in both hemispheres of the Pacific Ocean.
- Follows the sun
- Moves north in NZ and south in summer
- SH = SE in NZ summer, SW in winter
- Northern hemisphere = NW in NZ summer, NE in winter
Use diagrams to explain the typical wind strengths, including variation from summer to winter.
- Steady and persistent in strength
- Wind strength = 10-20kts occasionally 30kts
- Trade winds slightly stronger in winter than summer
- Winds above are generally light westerly
Use diagrams to explain the effect of trade winds on the weather experienced in island groups and northern Australis in both hemispheres in the Pacific Ocean.
- Increased rain in summer on SE coasts of islands and Australia’s, drier on the West Coast’s or inland
- In winter, trade wind strength is reduced due to land breeze
- Winter might still have some cloud on the SE coasts from the trade winds but overall much drier and stable
Outline how the tropical convection can occur as individual convective cells or as organised clusters.
- Individual cells caused by turbulence in the friction layer creating long sheets of cumulus aligning itself parallel to the trade winds
- Organised clusters are mesoscale convective areas. Caused by heating and instability through deep layers
Describe the factors involved in wet monsoons in terms of seasonal factors.
- The wet monsoons occurs in summer for wither hemisphere
- Enhanced more over land than over sea due to specific heat differences
- Also caused by the trade winds being deflected as they cross the equator and collide with the opposite trade winds at 180 degrees rather than 90 degrees
Describe the factors involved in wet monsoons in terms of effect of large land masses and orographic obstructions.
- The larger the land mass, the greater the difference in temperature between the land and the sea, increasing the wet monsoon in summer.
- Mountains and hills encourage rising of air so that the moist air reaches its dew point, increasing rainfall
Describe the factors involved in wet monsoons in terms of the location of the major monsoon regions.
- Concentrated along the South Asia continent
- Moist air from Indian Ocean causes monsoons in India, Pakistan, Myanmar etc
- NH monsoon from June to September
- Also the north Australian continent in summer
State the global basins where tropical cyclones form.
- South Pacific Ocean
- Indian Ocean
State the global basins where typhoons form.
- North West Pacific Ocean
State the global basins where tropical where hurricanes form.
- North Atlantic Ocean
With regard to the formation development and decay of tropical cyclones, describe the relationship with the equatorial trough and/or the SPCZ.
- A pre-existing disturbance at low levels provides a focal point from which rotation of large masses of thunderstorms
- The wave disturbance is caused by convergence zones or quasi-stationary fronts
- These are most likely to occur in the equatorial trough or South Pacific convergence zone
With regard to the formation, development and decay of tropical cyclones, describe the requirement for and supply of sensible and latent heat.
- Sensible heat is the increase in temperature of a body as more heat is added
- Latent heat is released as clouds form increasing instability and sensible heat
- This sensible hear is delivered to the centre of the storm through radial flow
With regard to the formation development and decay of tropical cyclones, describe the requirement for and supply of sensible and latent heat.
- developing tropical cyclones also extract sensible heat from the warm ocean below
With regard to the formation and development and decay of tropical cyclones, describe the effect of upper level divergence.
- Subsiding air in the storm eye should increases pressure
- Upper-level divergence removes air from the column at the top of the troposphere fast than it is entering the system at the surface
- This reduces surface pressure increasing convergence
With regard to the formation development and decay of tropical cyclones, describe the characteristics of the cyclone ‘eye’.
- Light winds
- Subsiding air causes adiabatic warming which dissipates clouds
- Clear skies
- Where the lowest surface pressure exists
With regard to the formation development and decay of tropical cyclones, describe the requirement for a ‘warm core’.
- Instrumental as it encourages the development of an upper level high-pressure zone
- This creates more upper level divergence and therefore more low levels convergence
State the stages of development of tropical cyclones.
- Tropical disturbance
- Tropical depression
- Tropical storm
- Tropical cyclone
For each stage of development, describe the atmospheric pressure tendency.
- Tropical disturbance - pressure starts reducing
- Tropical depression (formative) - approaching 1000hPa
- Tropical storm (immature) - below 1000hPa
- Tropical cyclone (mature) - at its lowest, around 950hPa
For each stage of development, describe the typical wind strengths, including variations in wind velocity, either side of,the cyclone eye.
- (formative) - One quadrant increases to gale force (sustained 34kt wind)
- (immature) - Hurricane force (sustain 64kt wind)
- (mature) - Hurricane force or stronger
For each stage of development, describe the typical wind strengths, including variations in wind velocity in, the cyclone eye.
- Light winds and clear skies within the eye
- Around the eye is dense wall of cloud about 16km high known as the eye wall. Marks the belt of heaviest winds and strongest rainfall.
For each stage of development, describe the typical radii of the affected areas.
- (formative) - few closed isobars have a radius of 50km
- (immature) - defined eye forms with radius of 40km, hurricane force winds felt within 50km of the eye
- (mature) - hurricane force winds felt within 200km of the centre, gale force winds are experience out to 500km
- Life span of tropical cyclone = 10 days
For each stage of development, describe the associated weather, and the location of the most severe conditions.
- (formative) - clear rotation of cloud mass begins and pulls In surrounding Cb clouds intensifying storm
- (immature) - heavy frequent showers with no gaps, the heaviest rain around the eye wall
- (mature) - flooding caused by heavy rainfall, huge ocean swells and extreme winds
- Most severe weather located in the forward left quadrant of the storm in the SH and forward right in the NH
Describe the mechanisms for the decay of tropical cyclones.
- Movement over substantial land area removing water source
- Moving to higher latitude where water is too cold for required evaporation
- Ingestion of cold air or dry air from another source
- Separation from high level divergence
State the seasons during which tropical cyclones are generally experienced.
- SH is October to March (peaks in Feb)
- NH is June to November (peaks in Sep)
Describe the Walker Cell in the South Pacific Ocean.
- Entire Pacific Ocean south of the equator is influenced by the Walker circulation.
- Caused by two areas. One off the coast of Peru and the other, seas around the Indonesian islands/Northern Australis
- Westbound air traveling at low levels and easterly up top
Define the ENSO index.
- Method used to characterise the intensity of an El Niño Souther Oscillation (ENSO) event.
- It is an irregular variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean
- Can be La Niña, El Niño or neutral depending on surface pressures
Outline the characteristics of positive (La Nina) and negative (El Nino) phases of the ENSO index, including the effect on prevailing winds in tropical and mid-latitude regions.
- La Niña - pressure is 4hPa or more higher in the east pacific
- Increases strength of easterly trade winds, reduces westerlies in mid-latitudes
- Neutral Index is when pressure difference is less than 4hPa but still positive/higher in the Eastern Pacific Ocean south
- El Niño - pressure higher in the Western Pacific
- Easterly trade winds weaken or reverse direction and become westerlies
- Increases strength of mid-latitude westerlies
Outline the characteristics of positive (La Nina) and negative (El Nino) phases of the ENSO index, including the effect on meteorological conditions in Australasia.
- La Niña - Storms and bad weather on the east coasts of Pacific Islands, Australia and NZ
- El Niño - Bad weather on West Coasts, dry on East Coasts
