1. A Taste of the Tropics Easterly waves Tropical Cyclones
Many Images in this lecture are courtesy of Kerry Emanuel

3. The Coupled Monsoon System
Key features of the WAM Climate System during Boreal summer
Cold Tongue
SAL
ITCZ
Heat Low
AEJ

4. Schematic of African Easterly Jet
AEJ
90oC
50oC θ θe θe θ
60oC
20oC

5. The Mean State over West Africa
Thorncroft and Blackburn 1999

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

7. Zonal Variations in the Mean State
925hPa q
315K PV
PV ‘strip’ present on the cyclonic shear side of AEJ.
Strong baroclinic zone 10o-20oN
925hPa qe
High qe strip exists near 15oN

8. 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

9. 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.

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

11. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day 0
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

12. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day-4
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

13. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day-3
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

14. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day-2
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

15. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day-1
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

16. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day 0
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

17. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day+1
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

18. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day+2
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

19. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day+3
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

20. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day+4
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

21. OLR and 850 hPa Flow Regressed against TD-filtered OLR (scaled -20 W m2) at 10N, 10W for June-September
Day+5
Streamfunction (contours 1 X 105 m2 s-1)
Wind (vectors, largest around 2 m s-1)
OLR (shading starts at +/- 6 W s-2), negative blue

22. Diagnostics for highlighting multi-scale aspects of AEWs
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.
Berry et al 2006

23. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

24. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

25. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

26. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

27. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

28. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

29. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

30. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

31. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

32. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

33. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

34. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

35. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

36. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

37. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

38. 315K Potential Vorticity (Coloured contours every 0
315K Potential Vorticity (Coloured contours every 0.1PVU greater than 0.1 PVU) with 700hPa trough lines and easterly jet axes from the GFS analysis (1 degree resolution), overlaid on METEOSAT-7 IR imagery.

39. Conceptual framework (ii) Baroclinic growth.
q’ Max
700hPa Trough

40. Conceptual framework (ii) Baroclinic growth.
q’ Max
700hPa Trough

41. Conceptual framework (ii) Baroclinic growth.
q’ Max
700hPa Trough

42. Synoptic-Mesoscale Interactions
PV-theta analysis of AEWs – Scale Interactions
Synoptic-Mesoscale Interactions

43. Synoptic-Mesoscale Interactions
PV-theta analysis of AEWs – Scale Interactions
Synoptic-Mesoscale Interactions

44. Synoptic-Mesoscale Interactions
PV-theta analysis of AEWs – Scale Interactions
Synoptic-Mesoscale Interactions
From a PV-theta perspective, the heating rate profiles are crucial to know and understand.

45. Synoptic-Mesoscale Interactions
PV-theta analysis of AEWs – Scale Interactions
Synoptic-Mesoscale Interactions
From a PV-theta perspective, the heating rate profiles are crucial to know and understand.
Mesoscale-Microscale Interactions
Ultimately these profiles are influenced by the nature of the microphysics!

46. Hurricanes
The terms "hurricane" and "typhoon" are regionally specific names for a strong "tropical cyclone". A tropical cyclone is the generic term for a non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation (Holland 1993).

47. Hurricanes: Definitions
Tropical cyclones with maximum sustained surface winds of less than 17 m/s (34 kt, 39 mph) are called “tropical depressions”. Once the tropical cyclone reaches winds of at least 17 m/s (34 kt, 39 mph) they are typically called a “tropical storm” and assigned a name. If winds reach 33 m/s (64 kt, 74 mph)), then they are called:
"hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east160E)
"typhoon" (the Northwest Pacific Ocean west of the dateline)
"severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E)
"severe cyclonic storm" (the North Indian Ocean)
"tropical cyclone" (the Southwest Indian Ocean)
(Neumann 1993)

48. Hurricanes: Atlantic Names
2007
2008
2009
2010
2011
2012
Andrea
Arthur
Ana
Alex
Arlene
Alberto
Barry
Bertha
Bill
Bonnie
Bret
Beryl
Chantal
Cristobal
Claudette
Colin
Cindy
Chris
Dean
Dolly
Danny
Danielle
Don
Debby
Erin
Edouard
Erika
Earl
Emily
Ernesto
Felix
Fay
Fred
Fiona
Franklin
Florence
Gabrielle
Gustav
Grace
Gaston
Gert
Gordon
Humberto
Hanna
Henri
Hermine
Harvey
Helene
Ingrid
Ike
Ida
Igor
Irene
Isaac
Jerry
Josephine
Joaquin
Julia
Jose
Joyce
Karen
Kyle
Kate
Karl
Katia
Kirk
Lorenzo
Laura
Larry
Lisa
Lee
Leslie
Melissa
Marco
Mindy
Matthew
Maria
Michael
Noel
Nana
Nicholas
Nicole
Nate
Nadine
Olga
Omar
Odette
Otto
Ophelia
Oscar
Pablo
Paloma
Peter
Paula
Philippe
Patty
Rebekah
Rene
Rose
Richard
Rina
Rafael
Sebastien
Sally
Sam
Shary
Sean
Sandy
Tanya
Teddy
Teresa
Tomas
Tammy
Tony
Van
Vicky
Victor
Virginie
Vince
Valerie
Wendy
Wilfred
Wanda
Walter
Whitney
William

49. Hurricane: Categories
The USA utilizes the Saffir-Simpson hurricane intensity scale (Simpson and Riehl 1981) for the Atlantic and Northeast Pacific basins to give an estimate of the potential flooding and damage to property given a hurricane's estimated intensity:
Saffir-Simpson Scale
Saffir-Simpson Category
Maximum sustained wind speed
Minimum central pressure
Storm surge
mph
m/s
kts mb ft m 1
74-95
33-42
64-82
> 980
3-5 2
96-110
43-49
83-95
6-8 3
50-58
96-113
9-12 4
59-69
13-18 5
156+
70+
136+
< 920
19+
5.7+
Note : Classification by central pressure was ended in the 1990s, and wind speed alone is now used. These estimates of the central pressure that accompany each category are for reference only.

50. Tropical Cyclones are Important!
"The death toll in the infamous Bangladesh Cyclone of 1970 has had several estimates, some wildly speculative, but it seems certain that at least 300,000 people died from the associated storm tide [surge] in the low-lying deltas." (Holland 1993)
The largest damage caused by a tropical cyclone as estimated by monetary amounts has been Hurricane Katrina (2005) as it struck the Bahamas, Florida and Louisiana: US $40.6 Billion in insured losses, and an estimated $81 billion in total losses. However, if one normalizes hurricane damage by inflation, wealth changes and coastal county population increases, then Katrina is only the third worst, after the 1926 Great Miami Hurricane and the lethal 1900 Galveston Hurricane. If the 1926 storm hit in 2005, it is estimated that it would cause over $140 billion in damages, and the 1900 storm about $92 billion (Pielke, Gratz, Landsea, Collins, Saunders, Musulin 2006).

51. Climatology and Structure: Regions of Formation
low latitudes, 5-20o (not at the equator): f important
none in SE Pacific or S. Atlantic: SSTs important
more appear in the Pacific than in the Atlantic: SSTs important

52. Climatology and Structure: Seasonality
Most Tropical Cyclones occur in Summer and early Fall

53. Climatology and Structure: Tracks
Tropical cyclones generally recurve polewards from their easterly track

54. Climatology and Structure: Conditions for occurrence
Gray (1979)
Warm ocean waters (of at least 26.5°C [80°F], some say 26oC) throughout a sufficient depth (unknown how deep, but at least on the order of 50 m [150 ft]), and area.
Why is this important?
Warm waters are necessary to fuel the heat engine of the tropical cyclone. Warm SSTs allows cumulus convection to reach the tropopause and give maximum warming in the troposphere.

55. Climatology and Structure: Conditions for occurrence
2. A minimum distance of at least 500 km [300 mi] from the equator.
Why is this important?
Must have background rotation (f > 0) large enough to convert inflow to tangential flow (cf conservation of angular momentum).
Consider vorticity equation.

56. Climatology and Structure: Conditions for occurrence
Gray (1979)
3. Low values (less than about 10 m/s [20 kts 23 mph]) of vertical wind shear between the surface and the upper troposphere.
Why is this important?
From thickness considerations the minimum surface pressure is related to the mean temperature of the column above. If shear tilts the column of warm air over then the surface pressure will rise.
Other possible reasons include “ventilation” effects or more simply understood “shearing out”.
Some shear may actually be beneficial!

57. Climatology and Structure: Conditions for occurrence
Gray (1979)
4. A pre-existing near-surface disturbance with sufficient vorticity.
Why is this important?
Tropical cyclones cannot be generated spontaneously. They must be triggered.
What can serve as a seedling?
Easterly wave, MCSs, Upper-level midlatitude troughs.
Problem: these are often cold core!

58. Climatology and Structure: Conditions for occurrence
Gray (1979)
4. A pre-existing near-surface disturbance with sufficient vorticity.
Why is this important?
Tropical cyclones cannot be generated spontaneously. They must be triggered.
What can serve as a seedling?
Easterly wave, MCSs, Upper-level midlatitude troughs.
Problem: these are often cold core!

59. 6.2 Climatology and Structure: Conditions for occurrence
In Summary:
Environment: Need significant f, SSTs and low shear.
Seedling: Need finite amplitude disturbance.

60. Climatology and Structure: Observed Structure

61. Climatology and Structure: Observed Structure

62. Climatology and Structure: Observed Structure

63. Climatology and Structure: Observed Structure

64. Climatology and Structure: Observed Structure

65. Climatology and Structure: Observed Structure

66. Climatology and Structure: Observed Structure

67. Climatology and Structure: Observed Structure
The Thetae Structure
Key is the strong surface radial gradient of thetae
Mature hurricanes are nearly moist adiabatic in the eye wall.
Thus the strong surface radial gradient in thetae leads to the large gradient in T in the troposphere.
Hydrostatically this leads to the large surface pressure gradient and associated large surface winds.

68. AEWs and Tropical Cyclones
Key weather systems in the West African and Tropical Atlantic regions
An ideal region to study scale interactions
SAL
AEWs TC
MCSs

69. AEWs and Tropical Cyclogenesis
AEWs often get a “boost” before they leave Africa; associated with mergers of PV from upstream and in situ generation.
The Guinea Highlands region is one of the wettest regions of tropical North Africa.
GPCP rainfall (mm/day) for Aug-Sep, )

70. Importance of Guinea Highlands Region
Coherent cyclonic centers are tracked within the ITCZ at 700hPa and in the low-level baroclinic zone at 850hPa
Average vorticity tracking statistics for June-July-August at 700hPa and 850hPa based on ERA40 using methodology of Thorncroft and Hodges (2001).

71. Importance of Guinea Highlands Region
Composites of East Atlantic Developing and Non-Developing AEWs ( )
Developing Non-Developing
Hopsch , Thorncroft and Tyle 2010

72. Importance of Guinea Highlands Region
Composites of East Atlantic Developing and Non-Developing AEWs ( )
Developing (33) Non-Developing (512)
Hopsch , Thorncroft and Tyle 2010

73. Importance of Guinea Highlands Region
Composites of East Atlantic Developing and Non-Developing AEWs ( )
Developing (33) Non-Developing (33 most intense)
Hopsch , Thorncroft and Tyle 2010

74. Most Intense Non-Developing AEWs
Importance of Guinea Highlands Region
Most Intense Non-Developing AEWs
DAY 0 DAY DAY 2
Hopsch , Thorncroft and Tyle 2010