Introduction to tropical meteorology and deep convection

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Introduction to tropical

meteorology and deep convection

Roger K. Smith

University of Munich

TMD Lecture 1

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 A satpix tour of the tropics

 The zonal mean circulation (Hadley circulation), Inter- Tropical Convergence Zone (ITCZ)

 Field experiments

 Macroscale circulations, The Walker circulation, Monsoons

 Synoptic scale weather systems, tropical waves

 Moist convection and convective systems

 The Madden-Julian Oscillation, Westerly wind bursts

 Concluding remarks

Topics

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INDO IR 01010510

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GMS IR 01022621

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GOES IR 00112909

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EUMETSAT Movie

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Principal land and ocean areas between 40°S and 40°N.

The shaded areas show tropical highlands (over 1000 m).

From Nieuwolt (1977)

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How does one define the tropics?

Tropic of Capricorn 23.5^os Tropic of Cancer 23.5^oN

Maritime continent

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Next topic

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Radiative heat balance

Zonally averaged components of the absorbed solar flux and emitted thermal infrared flux at the top of the atmosphere.

(after Vonder Haar and Suomi, 1971, with modifications)

F (W m-2 )

100 0 200 300

90 50 30 10 0 10 30 50 90 South Latitude North

Absorbed solar flux Emitted infrared flux energy gain

energy loss

+

- -

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Mean annual precipitation

Mean annual precipitation as a function of latitude.

(after Sellers, 1965)

2000

1500 1000

500 mm

0

90 60 40 30 20 10 0 10 20 30 40 60 90 North Latitude South

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mean meridional circulation

The mean meridional circulation and main surface wind regimes.

after Defant, 1958



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ITCZ

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ITCZ

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Next topic

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Field Experiments in the Tropics

 Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) July 1974

 The MONsoon EXperiment (WMONEX December 1978, SMONEX May – August, 1979)

 Australian Monsoon EXperiment & Equatorial Mesoscale EXperiment (AMEX, EMEX) January – February 1987

 Tropical Oceans Global Atmosphere Couple Ocean Atmosphere Response Experiment (TOGA COARE) November 1992 – February 1993

 DYNAMO (Dynamics of the MJO)/CINDY(Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011)

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Next topic

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850 mb 200 mb

JJA

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200 mb

850 mb

DJF

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Velocity potential

V      k  

  ( k    )   k

²



We can separate the three-dimensional velocity field into a rotational part and a divergent part (see e.g. Holton, 1972, Appendix C.)

rotational nondivergent

irrotational divergent

( )

    0

( ) 0

 k         ( )

²

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Upper tropospheric (200 mb) mean seasonal velocity potential indicating the divergent part of the mean seasonal wind which

is proportional to .

(Adapted from Krishnamurti et al., 1973).

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OLR < 225 W m^-2

month

From Lau & Chan, 1983

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Walker circulation

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Next topic

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Monsoons

July

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SE Asian Monsoon

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January

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Australasian Monsoon

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South America: January climatology

200 mb wind 850 mb wind

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(From Webster, 1983)

An example of monsoon variability

1963

1971 West coast

of India

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Next topic

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Time-longitude section of vis imagery - latitude band 10 -15 N

From Wallace, 1970

0107 01/06

0108

time

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Fig. 1.23

GMS – IR 25 Dec 1978

MSLP 25 Dec 1978 Winter MONEX

A B

C

From Webster, 1983

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Fig. 1.24

250 mb 25 Dec 1978

850 mb 25 Dec 1978

Winter MONEX

A

C B

A

B C

shaded

horizontal wind divergence

From Webster, 1983

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The variation of surface pressure at Darwin for the period 23 - 28 Dec. 1978.

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Easterly waves over Africa

WV Imagery 17 June 1997 00Z

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Tropical cyclones

TC tracks (1979-1988)

Super typhoon Wynnie 11 Aug 1997

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Next topic

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Moist Convection

Trade Cumuli

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The tropical troposphere

q z km

29^oC

-73^oC 15

-121^oC

observed

observed -150^oC +150^oC

77^oC

Conclusion: Deep tropical circulations cannot be dry adiabatic

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The role of diabatic processes

z km 15

Conclusion: Deep convection occupies a small fractional area Latent

heat release opposes adiabatic

cooling

Radiative cooling opposes adiabatic

warming

ITCZ subtropical high Fast

ascent

Slow descent

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Successive locations of radar reflectivity contours in a tropical

squall line.

From Chang et al. (1987)

Radar reflectivity contours (dBZ) of a tropical squall line observed during

COPT81 (Ivory Coast, West Africa).

Horizontal cross-section at low levels (0.5 – 1.5 km).

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Radar reflectivity contours (dBZ) of a tropical squall line observed during COPT81 (Ivory Coast, West Africa).

Vertical cross-section along the axis of propagation.

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Types of convection

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Hector Movie

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Next topic

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From JAS, Sept. 1972

Madden-Julian Oscillation

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From Webster 1987

Madden-Julian Oscillation

MJO

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Indian Ocean Feb/Mar 2001

06/02 07/02 13/02

17/02 24/02 06/03

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West Pacific Ocean – Feb 2001

06/02 09/02 13/02

21/02 24/02 29/02

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http://www.bom.gov.au/bmrc/clfor/cfstaff/matw/maproom/OLR/hov.last6mths.anom.S.gif

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GOES-9 12 UTC 2 Dec 1997 10.7 micron image

Westerly wind bursts

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The extended Held-Hou Model

Y_- Eq. Y₀ Y₁ Y₊ z

Summer Cell Winter

Cell

Solar heating maximum

Streamline dividing the winter and summer cells

Winter cell

Summer cell

Solution for q_M0 andY

Eq. Pole

q_q_y

u = U_M

u = 0 y

q_

2

4 2

0

M 0 2 E0 2

2

4 2

0

M 0 2 E0 2

Y Y

10a gH 3a

Y Y

2a gH a

 q q

q -  q -

 q q

q -  q -

q_Yq_Y

1/ 2

2 0

2

M 0 E0 2 2

0

5 gH

Y 3

5 gH 18a

 q 

   q  q  q - q

 q Take q₀= 255 K, q= 40 Kand H = 12 km Y 2400 kmand q_0.9 Kcooler than q_. in agreement with obs.

unknowns

Y

Horizontal structure of the Kelvin waveand of a westward propagating Mixed Rossby-gravity wave.

Parcel motions Kelvin wave

Mixed R-G wave

^² ^-^²^u^ ^^^{Y v}^^-^ ^dv^

dY

^² ^-^{  }² ^ ^ ^{Y v}^ ^-^^dv^

dY

 

( )  exp ( )

v Y v_n  -¹₂Y² H_n Y

 

dv

dY Y v Y dH

n dY

 n

 exp

 -  -₂¹ ²

dH

dYⁿ 2n H_n_-₁( )Y H_n_₁( )Y 2YH_n( )Y -2n H_n_-₁( )Y Scale and put y = L_EY

Now

and

Properties of the Hermite polynomials

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 Interaction between moist convection and the large-scale flow.

 What are the controls on deep convection?

 Models usually don’t get the diurnal variation correct.

 No generally accepted theory of the Madden-Julian Oscillation.

 Weather forecasting in the tropics is still very difficult compared with that in the middle latitudes!

A few outstanding problems

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Introduction to tropical meteorology and deep convection