Tropical Meteorology
Meteorology
Kerry Emanuel
Emanuel, Instructor
Allison Wing, TA
1
Course Outline
Outline
•
Radiative-Convective Eq
quilibrium
–
–
–
–
–
•
General principles of radiative transfer
Simple models without phase change
General princip
ples of moist convection
Simple models with phase change
Quantitative assessments of the equilibrium state comp
parisons to observations
The Zonally-Averaged Circulation
–
–
–
–
–
–
The observed climatology
Breakdown of the radiative
radiative-convective
convective equilibrium state
Dry theory
Moist theory
Regulation of intensity
2
•
Asymmetric Steady Circulations
– Monsoons
•
•
•
Development and onset of the Asian monsoon
Monsoon breaks
Nonlinear, asymmetric theory
– The Walker Circulation
•
•
•
Observations
Theory
Interannual Fluctuations of the Walker
Circulation – ENSO
– Observed behavior
– Theory and modeling of ENSO
3
•
Intraseasonal Oscillations
–
–
–
Observations
GCM simulations
Theory of equatorial waves
•
•
–
–
•
Dry
Moist
WISHE
Cloud-radiation
Cloud
radiation interactions and ISOs
Higher Frequency Disturbances
–
–
–
Monsoon depressions
Equatorial waves
Easterlyy waves
4
•
Tropical Cyclones
–
–
–
–
Structure and climatology
Steady state physics
Steady-state
Genesis
Ocean interaction
5
Brief Overv
verviiew o
off th
the
the Gl
Glo
Global
Atm
tmosp
osp
osph
here
6
Atmospheric Composition
Composition
Gas Name
Nitrogen
Chemical Formula
N2
Percent Volume
78.08%
Oxygen
O2
20.95%
*Water
H2O
0 to 4%
Argon
Ar
0.93%
CO2
0.0360%
Neon
Ne
0.0018%
Helium
He
0.0005%
*Methane
CH4
0.00017%
Hydrogen
H2
0 00005%
0.00005%
N2O
0.00003%
O3
0.000004%
*Carbon Dioxide
*Nitrous Oxide
*Ozone
* variable gases
7
8
100000
Thermosphere
90000
Mesopause
80000
Altitude (meters)
70000
60000
Mesosphere
Stratopause
50000
40000
30000
20000
10000
0
-100
Stratosphere
Tropopause
Troposphere
-80
-60
-40
-20
0
20
Temperature oC
Image by MIT OpenCourseWare.
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10
11
12
Seasonal variation of solar radiation
13
14
A One
One--Dimensional Description
Description of tth
he Tropi
ropical At
Atmosp
mosph
here
ere
15
100000
Thermosphere
90000
Mesopause
80000
Altitude (meters)
70000
60000
Mesosphere
Stratopause
50000
40000
30000
20000
10000
0
-100
Stratosphere
Tropopause
Troposphere
-80
-60
-40
-20
0
20
Temperature oC
Image by MIT OpenCourseWare.
16
Elements of Thermal Balance:
Solar Radiation
• Luminosity: 3.9 x 1026 J s-1 = 6.4 x 107 Wm-2
at top of photosphere
• Mean distance from earth: 1.5 x 1011 m
• Flux densityy at mean radius of earth
L
2
2
0
S 
 1370 Wm
W
0
2
4 d
17
Steffan-B
St
Bolt
ltzmann Equati
tion: F  T 4
8
2
4
 5.67
5.6710
10
Wm K
Sun:

7
4
2
2
 T 66.4410
10 W
Wm
T 6,,000 K
18
Disposition of Solar Radiation:


2
Total absorbed solar radiation  S 1 a   r
p p
0
a  planetary
l t albedo
lb d   30% 
p
T t l surface
Total
f
area  4 r 2
p
S
Absorption per unit area  0 1 a 
p
4 
Absorption by clouds, atmosphere, and surface
19
Terrestrial Radiation:
Effective emission temperature:
S
 T 4  0 1 a 
e
4 
p 
Earth: T  255K   18C
e
Observed average
g surface tempperature  288 K  15C 20
Highly Reduced Model
• Transparent to solar radiation
• Opaque to infrared radiation
• Blackbody emission from
surface and each lay
yer
21
Radiative Equilibrium:
Top of Atmosphere:
S


4
4
0
1 a    T
T 

A
p
e
4 

TA  Te
Surface:
S0
4
 Ts   TA  1 a p   2 Te
4
4
4
1
1
4
 Ts  2 Te  303 K
22
Surface temperature too large because:
because:
• Real atmosphere is not opaque
as
• Heat transported by convection as well as
by radiation
23
Energy Balance
Balance
24
Principal Atmospheric Absorbers
• H2O: Bent triatomic, with permanent dipole
dipole
moment and pure rotational bands as well
as rotation-vibration transitions
• O3: Like water, but also involved in
photodissociation
• CO2: No permanent dipole moment, so no
pure rotational transitions, but temp
poraryy
dipole during vibrational transitions
• Other gases: N2O, CH4
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26
Radiative Equilibrium
• Eq
quilibrium state of atmosp
phere and
surface in the absence of non-radiative
py fluxes
enthalpy
• Radiative heating drives actual state toward state of radiative equilibrium
equilibrium
27
Extended Layer Models
Models
TOA :  T2 4   Te 4  T2  Te
Middl Layer
Middle
L
: 2 T   T2   Ts   Te   Ts
4
1
4
Surface :  Ts 4   Te 4   T14
1
 Ts  3 4 Te
1
T1  2 4 Te
28
4
4
4
Effects of emissivity<1
Surface : 2 A TA 4   A T14   A T
s 4
 
1
4
5
5
 TA 
Te  321K  Ts
2
Stratosphere : 2 t Tt 4   A T2
4
 2
 Tt  1
1
4
Te  214K
29
 Te
Full
u ca
calculation
cu at o o
of radiative
ad at e e
equilibrium:
qu b u
30
Time scale of approach to equilib
equilib
ilibrri
um
um
31
32
Contributions of various absorbers
absorbers
Note: All
simulations
have
variable
clouds
interacting
with
radiation
33
34
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12.811 Tropical Meteorology
Spring 2011
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