1. 551 Tropical
Advanced Topics

2. Trade Wind Inversion

10. Trade Wind or Subtropical Inversion
The height of the base of this inversion varies from about 500 m at the eastern extremities of the subtropical highs to about 2000 m at the western and equatorial extremities.
In the equatorial trough zone and over the western portions of the trade-wind belt, the inversion does not exist as a mean condition, although it appears in certain weather patterns.
The inversion is generally strongest when the height of its base is lowest, and vice versa. The thickness of the inversion layer varies from tens of m to more than 1000 m.
On the average its thickness is about 400 m.

11. Tropical Analyses
See both midlatitude style, isobaric analysis, and streamline/isotach analyses.
Why the latter is most popular?

14. Why streamline/isotachs better than isobars/heights in the tropics?
Flow not geostrophic within roughly 5 degrees of the equator (so connection between pressure and winds are not necessarily straightforward)
Lot of pibals (just winds) in the tropics compared to radiosondes.

16. Easterly Waves: The Major Synoptic Disturbances in the Tropics
Westward moving synoptic waves characterize the whole tropics
They are tropospheric waves that modulate clouds/rainfall and move at about 5-10 m/s and have wavelengths of km.

20. Composite African Wave Structures

21. The Mean State over West Africa” The African Easterly Jet (AEJ)
Burpee, R.W The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29, 77-90
Why is the AEJ there?

22. Strong baroclinic zone 10o-20oN
Hint
Strong baroclinic zone 10o-20oN

23. 4.2 The Mean State over West Africa
Reed, R.J., Norquist, D.C. and Recker, E.E., The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105, (1977).

24. Zonal Variations in the Mean State
Mean 700hPa U wind, 16th July – 15th August 2000
Berry and Thorncroft 2005

25. 4.3 Observations of African Easterly Waves
Carlson 1969ab
Carried out case studies of several AEWs
Peak amplitudes at mb and at surface
Eastward tilt with height from the surface to the level of the AEJ
Two cyclonic centers at low-levels
Synoptic variations in cloud cover
Peak of cloudiness close to AEW trough

26. 4.3 Observations of African Easterly Waves
Burpee (1970)
Eastward tilt beneath the AEJ – Westward tilt above the AEJ
Northerlies dry and warm
Southerlies wet and cold

27. 4.3 Observations of African Easterly Waves
Reed et al, 1977
Composite AEW structures from phase III of GATE (after Reed et al, 1977). (a) and (b) are relative vorticity at the surface and 700hPa respectively with a contour interval of 10-5s-1. (c) and (d) show percentage cover by convective cloud and average precipitation rate (mm day-1) respectively. Category 4 is location of 700hPa trough and the “0” latitude is 11oN over land and 12oN over ocean.

28. 4.3 Observations of African Easterly Waves
Thorncroft and Hodges (2001)

29. 4.3 Observations of African Easterly Waves
Carlson, T.N., 1969a: Synoptic histories of three African disturbances that developed into Atlantic hurricanes. Mon. Wea. Rev., 97,
Carlson, T.N., 1969b: Some remarks on African disturbances and their progress over the tropical Atlantic. Mon. Wea. Rev., 97,
Burpee, R.W., 1970: The origin and structure of easterly waves in the lower troposphere of North Africa, J. Atmos. Sci. 29,
Reed, R.J., Norquist, D.C. and Recker, E.E., 1977: The structure and properties of African wave disturbances as observed during Phase III of GATE, Mon. Wea. Rev. 105,
Thorncroft, C.D. and Hodges: 2001 K.I., African easterly wave variability and its relationship to tropical cyclone activity, J. Clim. 14, (2001).
Kiladis, G., C. Thorncroft, and N. Hall, 2006: Three-Dimensional Structure and Dynamics of African easterly waves: part I: Observations, J. Atmos. Sci., 63,
Mekonnen, A., C. Thorncroft, and A. Aiyyer, 2006: On the significance of African easterly waves on convection, J. Climate, 19,
Berry, G., Thorncroft, C.D. and Hewson, T African easterly waves in 2004 – Analysis using objective techniques Mon. Wea. Rev., 133,

30. The End