Introduction to tropical meteorology and deep convection

Introduction to tropical meteorology and deep convection Roger K. Smith University of Munich 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

INDO IR 01010510 GMS IR 01022621

GOES IR 00112909 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 o N Maritime continent Tropic of Capricorn 23.5 o s 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 Next topic

300 energy gain + energy loss F (W m 2 ) 200 100 Radiative heat balance Absorbed solar flux Emitted infrared flux 0 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 mm 1000 Mean annual precipitation 500 0 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)

mean meridional circulation Hadley circulation The mean meridional circulation and main surface wind regimes. after Defant, 1958 ITCZ

ITCZ 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 Next topic

Field Experiments in the Tropics 1969-1979 Bermuda Oceanographic and Meteorological EXperiment (BOMEX) 1969 Holland & Rasmusson, (1973), MWR, 101, 44-55. Global Atmospheric Research Programme (GARP) Atlantic Tropical Experiment (GATE) July 1974 GARP: Fleming et al., (1979), BAMS, 60, 649-659. GATE: Kuettner et al. (1974) BAMS, 55, 711-744. The MONsoon EXperiment (WMONEX December 1978, SMONEX May August, 1979) WMONEX: Greenfield & Krishnamurti (1979), BAMS, 60, 649-659.SMONEX: Fein & Kuettner (1980) BAMS, 61, 461-474. Field Experiments in the Tropics 1987 - Australian Monsoon EXperiment & Equatorial Mesoscale EXperiment (AMEX, EMEX) January February 1987 AMEX Holland et al. (1986), BAMS, 67, 1466-1486. EMEX Webster & Houze (1991), BAMS, 72, 1481-1505. Tropical Oceans Global Atmosphere Couple Ocean Atmosphere Response Experiment (TOGA COARE) November 1992 February 1993 Webster & Lucas (1992), BAMS, 73, 1377-1416.

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 Next topic 200 mb JJA 850 mb

200 mb DJF 850 mb Velocity potential We can separate the three-dimensional velocity field into a rotational part and a divergent part (see e.g. Holton, 1972, Appendix C.) V = k ψ + χ rotational nondivergent ( k ψ) = k 2 ψ ( k ψ ) = 0 irrotational divergent χ=0 ( ) 2 ( χ= χ )

Upper tropospheric (200 mb) mean seasonal velocity potential indicating the divergent part of the mean seasonal wind which is proportional to χ. (Adapted from Krishnamurti et al., 1973). month OLR < 225 W m 2 From Lau & Chan, 1983

Walker circulation

January SE Asian Monsoon

Australasian Monsoon South America: January climatology 850 mb wind 200 mb wind

An example of monsoon variability West coast of India 1963 1971 (From Webster, 1983) 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 Next topic

August July June Time-longitude section of vis imagery - latitude band 10-15 N 01/06 0107 0108 time From Wallace, 1970 Winter MONEX GMS IR 25 Dec 1978 C B Fig. 1.23 A MSLP 25 Dec 1978 From Webster, 1983

Winter MONEX 250 mb 25 Dec 1978 A C Fig. 1.24 B shaded horizontal wind divergence 850 mb 25 Dec 1978 A C B 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)

The tropical troposphere z km 15-121 o C -73 o C observed -150 o C +150 o C 29 o C 77 o C θ observed Conclusion: Deep tropical circulations cannot be dry adiabatic The role of diabatic processes z 15 km Latent heat release opposes adiabatic cooling Fast ascent Radiative cooling opposes adiabatic warming Slow descent ITCZ subtropical high Conclusion: Deep convection occupies a small fractional area

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).

Radar reflectivity contours (dbz) of a tropical squall line observed during COPT81 (Ivory Coast, West Africa). Vertical cross-section along the axis of propagation. Types of convection

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 Winter Cell cell z Summer Cell cell Y - Eq. Y 0 Y 1 Y + Solar heating maximum Streamline dividing the winter and summer cells Eq. θ Μ0 u = U M Solution for θ M0 and Y θ Μ, θ Ε (y) unknowns u = 0 2 Ωθ0 4 θ Y 2 E0 Y 2 3a 2 2 Ωθ 2 Y θ Y 2 0 4 2 E0 θm0 =θ 10a gh θm0 =θ 2a gh a Y θ Μ (Y) = θ Ε (Y) y Pole 1/2 5 θgh Y = 2 3Ωθ0 2 5 θ gh M0 E0 2 2 18a Ωθ0 θ =θ Take θ 0 = 255 K, θ = 40 K and H = 12 km Y 2400 km and θ Μ0 0.9 K cooler than θ Ε (0). in agreement with obs. Scale and put y = L E Y Now dv ν µ u = ν Yv µ dy 2 2 ( ) dv ν µ η = µ Yv ν dy 2 2 ( ) vy ( ) = v = exp ( 1 2 2 Y) H ( Y) n n Kelvin wave Mixed R-G wave Parcel motions and dv Yv ( Y ) dh n = n + exp 1 2 2 dy dy Properties of the Hermite polynomials dhn = 2nHn 1( Y) dy H ( Y) = 2YH ( Y) 2nH ( Y) n n n + 1 1 Horizontal structure of the Kelvin wave and of a westward propagating Mixed Rossby-gravity wave. A few outstanding problems Interaction between moist convection and the large-scale flow. What are the controls on deep convection. Models 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 Thank you for your interest Talk available at: http://www.meteo.physik.uni-muenchen.de\~roger\030514-dlr.pdf