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Flashcards in Chapter 5 Deck (24)
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1
Q

tropical waves

A

a trough, or cyclonic curve maximum, in the trade wind easterlies, that moves from east to west.

2
Q

Easterly waves

A
  • Northern Hemisphere phenomena.
    • need deep, easterly current to form an propagate
  • lie within the boundaries (25°N - 20°S) of the tropical easterlies.
3
Q

Easterly waves are classified as (stability)

A
  • stable
    • slope east with height and
    • have divergence ahead, and
    • convergence to the rear of their troughs.
  • Neutral
    • vertical
    • have a short life (often less than 24 hours) and
    • often revert to stable, but sometimes become unstable waves.
  • Unstable
    • When a wave slopes westward with height
    • has the most violent weather.
      • showers and thunderstorms are found west, or ahead, of the trough line
    • associated with the development of tropical cyclones that reach hurricane intensity.
4
Q

easterly waves can also be classified as

A
  1. Waves over the North Atlantic and Caribbean and
  2. The African Waves
5
Q

Waves over the North Atlantic and Caribbean

A
  • persist for 1-2 weeks.
  • Vorticity maximum occurs near 700 mb.
  • Associated trough tilts upstream (towards east) with height.
    • due to colder air behind the wave.
  • Convection often amplifies as the waves approach the eastern Caribbean Sea.
    • due to the presence of the TUTT,
      • enhance the upper-level outflow from the convection.
6
Q

African Easterly Waves

A
  • form during the monsoon season.
  • They are well-defined wave perturbations with periods of roughly 3–5 days
  • spatial scale about 1000 km.
  • They occur as waves with maximum amplitude close to the level of the African Easterly Jet (AEJ).
  • move westward at speeds of 7–8 m s-1.
  • An easterly wave that forms over Africa every few days moves west across the North Atlantic into the Caribbean Sea.
7
Q

How is perturbation created in African easterly waves

A
  • Convection associated with the West African mid-level (easterly) jet,
    • acting as a forcing mechanism to create a perturbation in the flow
      • the disturbance can grow into a wave.
8
Q

explain how the axial tilt of the AEW varies:

A

varies with its location on the surface (continental or coastal) and in vertical (relative to the position of AEJ). When the trough axis is:

  • over the interior continent, AEWs tilt eastwards with height
  • near the coast, AEWs tilt eastwards with height below the AEJ level (700 mb) and have westward tilt above the AEJ.
9
Q

Dynamics of African waves appear to be a combination of

A

barotropic and baroclinic instability associated with the West African mid-level (easterly) jet.

10
Q

Dynamics of African waves appear to be a combination of both barotropic and baroclinic instability associated with the West African mid-level (easterly) jet.

A
  • Forcing mechanism (convection)
    • to create a perturbation in the flow from which
      • the disturbance can grow into a wave
  • Wave axis generally tilt from southwest to northeast, and is opposite to horizontal shear south of the jet.
  • When tilt and shear are opposite,
    • barotropic instability removes energy from the jet and puts it into the perturbation (wave).
  • growth due to barotropic instability will mainly be south of the jet core.
11
Q

African waves increase in amplitude as they approach

A

the west coast of Africa, and weaken as they head out to sea.

12
Q

Divergence/convergence patterns in african waves:

A

opposite of waves over the North Atlantic. Low- level convergence, clouds and rain ahead of wave axis, rather than behind.

13
Q

Waves in the Easterlies account for the majority of

A

North Atlantic tropical cyclones. An increased number of waves means an increased number of cyclones.

14
Q

Barotropic Instability dependient on

A

horizontal shear of the mean flow.

15
Q

Barotropic disturbances derive their energy from

A
  • the mean flow.
    • Energy considerations show that for a barotropic disturbance to grow it must tilt opposite t o 𝑑 𝑢 ⁄ 𝑑 𝑦 .
16
Q

Upper-level Cyclones

A
  • Most intense in the 200 – 300 mb level. Many only show up above 700 mb.
  • Associated with
    • upper-level convergence and
    • suppressed convection.
  • Occasionally convection fires in center of low since
    • cool upper-levels are associated with static instability.
  • Convection usually short-lived due to:
    • upper-level convergence
    • entrainment of dry air
  • Upper-level lows rarely transform directly into warm-core surface lows, though they can interact with surface features to form new surface lows to the east of the upper-level cyclone.
17
Q

Tropical Upper Tropospheric Trough (TUTT)

A

a trough in upper-level (at about 200 hPa) tropics. It is a persistent feature in the North Pacific and North Atlantic during the summer months.

18
Q

formation of TUTT is usually caused by

A

the expansion of the Trough of Westerlies Wind to the tropics.

19
Q

TUTTs are different than mid-latitude troughs in that they are

A

maintained by subsidence warming near the tropopause which balances radiational cooling.

20
Q

TUTTs are important for

A
  • tropical cyclone forecasting as they can
    • suppress or support tropical cyclone formation, depending on
      • its location in relationship to the surface circulation.
    • They can force large amounts of vertical wind shear over tropical disturbances and tropical cyclones which may inhibit their strengthening or
    • The enhanced upper-level divergence to the east of TUTT may assist tropical cyclone genesis and intensification by providing additional forced ascent
21
Q

Under specific circumstances, TUTTs can grow into

A
  • upper cold lows and
  • may enhance the development of low level disturbances.
22
Q

Squall Lines

A
  • Non-frontal lines of active thunderstorms. Can be hundreds of miles in length
  • Persists much longer than lifetime of individual cells that make up line.
    • Usually last for 3 – 15 hours
    • May last for several days in West Africa
  • Occur in northern Australia, northwestern India, Bangladesh, Malaysia, Indonesia, South America/Caribbean, and West Africa.
  • Most frequently occur in mid to late afternoon. Require:
    • moist low levels with relatively dry mid levels.
  • Tropical squall lines have trailing anvil, while mid-latitude squall lines have preceding anvil.
  • Leading edge of gust front sparks new development and propagates line.
23
Q

Shear Lines

A
  • Equatorward extension of a midlatitude cold front.
  • No large temperature or humidity shift. Zone of convergence.
  • Characterized by stratiform clouds with embedded convection.
  • Can stall out and cause low ceilings and rain for extended periods.
24
Q

Cold Surges

A
  • Surge of cold air from midlatitudes into Tropics.
  • Surge from winter hemisphere can enhance Equatorial westerlies in summer hemisphere, and increase cyclonic vorticity.
    • Can result in development of tropical cyclones in monsoon trough.