Large Scale conditions associated with tropical convection'

1
Large Scale conditions associated with tropical
convection.

• Dr John McBride
• Bureau of Meteorology Research Centre

2
Outline

• 1. Large scale structure of the tropics.
• 2. Some mathematics of the tropical waves
• equatorial trapping, symmetry properties, the
  Kelvin wave, the Rossby waves.
• 3. The Gill model of Large scale tropical
  structure
• a) The beauty of Gill theory
• b) the waves
• c) Does it work
• 4. Some examples of waves from day-to-day
  charts.

3
1. - Large Scale Structure of the Tropics
Transparencies of Monsoon versus Trade wind
ITCZ
4
(No Transcript)
5
Mean January 925 hPA (Generated interactively at
www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl)
Monsoon Tr Trade Wind ITCZ STR
6
Mean January 200 hPA
STR
Tropical Upper Tropospheric Trough (TUTT)
7
Mean July 925 hPA
Monsoon Tr Trade Wind ITCZ STR
8
Mean July 200 hPA
STR
9
2. Some mathematics of the tropical waves
equatorial trapping, symmetry properties, the
Kelvin wave, the Rossby waves.
10
(No Transcript)
11
(No Transcript)
12
3. The Gill model of Large scale tropical
structure a) The beauty of Gill theory b) the
waves c) Does it work
Go to transparencies
13
(No Transcript)
14
(No Transcript)
15
(No Transcript)
16
(No Transcript)
17
(No Transcript)
18
4. Some examples of waves from day-to-day
charts.
n 1 Rossby Wave McBride web-page 28
February 2002 7 May 2002 9 June 2002 6 Oct
2002 11 Oct 2002
19
(No Transcript)
20
4 Tropical Waves
21
Tropical Waves - 1

• This section deals with equatorially trapped
  tropical waves
• Simply the work by using linear terms of the
  shallow water equation
• Assume an equatorial b plane such that Coriolis
  force is given
• f b y

22
Tropical Waves - 2
Solutions of the form
23
Tropical Waves -3
Substitute gt dispersion equation
24
Tropical Waves - 4

• 2 closures of solution
• Equatorially trapped gravity waves
• Equatorially trapped planetary waves

25
Tropical Waves - 5

• Also have n 0 solution mixed Rossby / gravity
  wave
• And kelvin wave solution
• W k c (obtained by putting v 0 in set of
  equations)
• Non dispersive group velocity same as phase
  speed so wave packet does not disperse

26
Tropical Waves - 6

• Planetary waves phase speed westward
• So long (small k) waves are non dispersive

27
Tropical Waves - 7

• Gill 1980
• Physical model of tropical heat source (Quarterly
  Journal of the Royal Meteorological Society)

28
Tropical Waves - 8

• Turn on heating
• gt kelvin waves carry information eastwards
• Creates easterly surface trade winds and upper
  level westerlies

29
Tropical Waves - 9

• Add dissipation can reach steady state with trade
  easterlies (kelvin wave) dying out with distance
  from the heating source.
• To the west the information is carried by long
  planetary waves
• Fastest planetary wave speed travels at one third
  the kelvin wave speed gt effect penetrates only
  one third the distance

30
Tropical Waves - 10

• Gill model of large scale structure of tropics
  (QJ 1980)
• Heating in mid tropospheric associated with
  convection excites first tropospheric internal
  mode

31
Tropical Waves - 11

• Heating centred on Equator
• Q varies as D(y) parabolic cylinder formation
• Y is non dimensional length
• Excites 2 waves
• Kelvin wave
• Not long Rossby wave (both symetric U,
  anti-symetric v)

32
Tropical Waves - 12

• Heating on one side of equator, cooling on the
  other
• Q varies D(y) ye ..
• Excites 2 waves
• N 0 long mixed R-G wave
• N 2 long Rossby wave
• Monsoon Heating on equator and to one side
• Add solutions