Chapter 5

Question 1

tropical waves

Answer 1

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

Question 2

Easterly waves

Answer 2

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

Question 3

Easterly waves are classified as (stability)

Answer 3

• 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.

Question 4

easterly waves can also be classified as

Answer 4

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

Question 5

Waves over the North Atlantic and Caribbean

Answer 5

• 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.

Question 6

African Easterly Waves

Answer 6

• 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.

Question 7

How is perturbation created in African easterly waves

Answer 7

• 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.

Question 8

explain how the axial tilt of the AEW varies:

Answer 8

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.

Question 9

Dynamics of African waves appear to be a combination of

Answer 9

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

Question 10

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

Answer 10

• 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.

Question 11

African waves increase in amplitude as they approach

Answer 11

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

Question 12

Divergence/convergence patterns in african waves:

Answer 12

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

Question 13

Waves in the Easterlies account for the majority of

Answer 13

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

Question 14

Barotropic Instability dependient on

Answer 14

horizontal shear of the mean flow.

Question 15

Barotropic disturbances derive their energy from

Answer 15

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

Question 16

Upper-level Cyclones

Answer 16

• 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.

Question 17

Tropical Upper Tropospheric Trough (TUTT)

Answer 17

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.

Question 18

formation of TUTT is usually caused by

Answer 18

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

Question 19

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

Answer 19

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

Question 20

TUTTs are important for

Answer 20

• 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

Question 21

Under specific circumstances, TUTTs can grow into

Answer 21

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

Question 22

Squall Lines

Answer 22

• 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.

Question 23

Shear Lines

Answer 23

• 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.

Question 24

Cold Surges

Answer 24

• 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.