meteorology and deep convection

Introduction to tropical

meteorology and deep convection

Roger K. Smith

University of Munich University of Munich

Thai Meteorological Department Lecture 1

Topics

 A satpix tour of the tropics

 Th l i l ti (H dl i l ti ) I t

 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 burst

INDO IR 01010510

GMS IR 01022621

GOES IR 00112909

MovieMETSAT MEUM

on simulati m erical s A n u m

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

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

From Nieuwolt (1977)

How does one define the tropics?

Tropic of Cancer 23.5

N

M iti

Maritime continent

Tropic of Capricorn 23.5

s

Next topic

 A satpix tour of the tropics

 Th l i l ti (H dl i l ti ) I t

 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

energy gain

)

300 energy loss

+

Radiative heat balance

(W m

)

200 

F (

100

Absorbed solar flux Emitted infrared flux

 

0

90 50 30 10 0 10 30 50 90

Zonally averaged components of the absorbed solar flux and 90 50 30 10 0 10 30 50 90

South Latitude North

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)

2000

1500

Mean annual precipitation 1000

mm

500 0

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

Mean annual precipitation as a function of latitude.

(after Sellers, 1965)

idi l i l ti mean meridional circulation





The mean meridional circulation and main surface wind regimes The mean meridional circulation and main surface wind regimes.

after Defant, 1958

ITCZ

ITCZ

Next topic

 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

 Moist convection and convective systems

 The Madden-Julian Oscillation, westerly wind bursts

Field Experiments in the Tropics p p

 Global Atmospheric Research Programme (GARP) Atlantic

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

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

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

 T ropical Oceans Global Atmosphere Couple Ocean

Atmosphere Response Experiment (TOGA COARE)

November 1992 – February 1993

Next topic

 A satpix tour of the tropics

 Th l i l ti (H dl i l ti ) I t

 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

200 mb

850 mb

JJA

200 mb

850 mb

DJF

Velocity potential

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

V      k  

t ti l i t ti l

2

rotational nondivergent

irrotational divergent

( )

  0

  ( k    )   k

     ( ) 0

( ) 0

 k         ( )

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

is proportional to .

(Adapted from Krishnamurti et al., 1973).

OLR 225 W

month

F L & Ch 1983

OLR < 225 W m

From Lau & Chan, 1983

Walker circulation

Next topic

 A satpix tour of the tropics

 Th l i l ti (H dl i l ti ) I t

 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

Monsoons

July

SE Asian Monsoon

January

Australasian Monsoon

South America: January climatology

200 mb wind

850 mb wind

An example of monsoon variability

West coast 1963 of India

1971

(From Webster, 1983)

Next topic

 A satpix tour of the tropics

 Th l i l ti (H dl i l ti ) I t

 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

Time-longitude section of vis imagery - latitude band 10 -15 N 01/06

01/06

0107

0108

e

From Wallace, 1970

tim

Winter MONEX

GMS IR

A GMS – IR B

25 Dec 1978

C Fig. 1.23

C

MSLP MSLP 25 Dec 1978

From Webster, 1983

Winter MONEX

250 mb

A 25 Dec 1978

C B

Fig. 1.24

C

shaded

horizontal wind divergence

850 mb A

850 mb 25 Dec 1978

B

C B

C

From Webster, 1983

The variation of surface pressure at Darwin for the period

23 - 28 Dec. 1978.

Easterly waves over Africa

WV Imagery 17 June 1997 00Z

Tropical cyclones

Super typhoon Wynnie 11 Aug 1997

TC tracks (1979-1988)

Next topic

 A satpix tour of the tropics

 Th l i l ti (H dl i l ti ) I t

 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

Moist Convection

Trade Cumuli

The tropical troposphere

z km

-73

C 15

-121

C

b d

observed -150

C +150

C

29

C 77

C



29

C

observed 77

C

Conclusion: Deep tropical circulations cannot be dry adiabatic

The role of diabatic processes

z km 15

Latent heat release opposes

Radiative cooling opposes adiabatic

warming Fast

ascent opposes

adiabatic cooling

ascent g

Slow descent

ITCZ subtropical high

Conclusion: Deep convection occupies a small fractional area

S i l ti f d Radar reflectivity contours (dBZ) of Successive locations of radar

reflectivity contours in a tropical squall line.

From Chang et al (1987)

y ( )

a tropical squall line observed during

(Ivory Coast, West Africa).

Horizontal cross-section at low levels From Chang et al. (1987)

(0.5 – 1.5 km).

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

Vertical cross-section along the axis of propagation.

Types of convection

“Hector”, Darwin, Australia

Next topic

 A satpix tour of the tropics

 The zonal mean circulation (Hadley circulation) Inter

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

 Fi ld i t

 Field experiments

 Macroscale circulations, The Walker circulation, Monsoons

 Synoptic scale weather systems, tropical waves

 M i t ti d ti t

 Moist convection and convective systems

 The Madden-Julian Oscillation, westerly wind bursts

Madden-Julian Oscillation Madden Julian Oscillation

From JAS, Sept. 1972

Madden-Julian Oscillation

MJO

From Webster 1987

Indian Ocean Feb/Mar 2001

06/02 07/02 13/02

17/02 24/02 06/03

West Pacific Ocean – Feb 2001

06/02 09/02 13/02

21/02 24/02 29/02

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

Westerly wind bursts

GOES-9 12 UTC 2 Dec 1997 10.7 micron image

The extended Held-Hou Model z

Solution for _M0andY u = U_M

_Y_Y

Summer Cell Winter

Cell Winter

cell

Summer cell

Eq. Pole

__y

u = 0 y

_

2

4 2

0

M 0   Y E0  Y

    

5 gH 1/ 2

Y   

  

unknowns

Y

Y_- Eq. Y₀ Y₁ Y₊

M 0 2 E0 2

2

4 2

0

M 0 2 E0 2

Y Y

10a gH 3a

Y Y

2a gH a

 

  

    

2 0

2

M 0 E0 2 2

0

Y 3

5 gH 18a

    

    

 

Take₀= 255 K,= 40 KandH = 12 kmY2400 kmand Solar heating maximum

Streamline dividing the winter and summer cells

Take ₀ 255 K,  40 Kand H 12 kmY 2400 kmand

_0.9 Kcooler than _. in agreement with obs.

Kelvin wave

 ^{2 2} ^{  dv}

Parcel Mixed R-G wave

² ²u Y v dY

^{2   2}^{  Y v dv}

dY Scale and put y = L_EY

ce motions ed G w ve

 

( )  exp ( )

v Y v_n  ¹₂Y² H_n Y

 

dv

dY Y v Y dH

n dY

 n

 exp

   ¹₂ ² Now

and

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

dH

dYⁿ 2n H_n1( )Y H_n1( )Y 2YH_n( )Y 2n H_n1( )Y Properties of the

Hermite polynomials

A few outstanding problems

 Interaction between moist convection and the large-scale flow

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!

The End