1. Atlantic Tropical Cyclogenesis
Reporter: Lin Ching
Ventrice, M. J., C. D. Thorncroft, and M. A. Janiga, 2012: Atlantic tropical cyclogenesis: A three-way interaction between an African easterly wave, diurnally varying convection, and a convectively coupled atmospheric Kelvin wave. Mon. Wea. Rev., 140, 1108–1124.
Ventrice, M. J., C. D. Thorncroft, and C. J. Schreck III, 2012: Impacts of convectively coupled Kelvin waves on environmental conditions for Atlantic tropical cyclogenesis. Mon. Wea. Rev., 140, 2198–2214. 

2. Atlantic tropical cyclones over the main development region (MDR; 5o–25oN, 15o–65oW) are commonly associated with African easterly waves (AEWs; e.g., Carlson 1969a; Zipser and Gautier 1978; Avila and Pasch 1992; Berry and Thorncroft 2005).
MDR
AEWs

3. Tropical Storm Debby (2006)
The pre-Debby AEW was quite weak over Africa just prior to tropical cyclogenesis.
The key processes, contributed to the rapid nature of the tropical cyclogenesis, included an interaction between the AEW and diurnally varying convection at the West African coast and a strong convectively coupled equatorial atmospheric Kelvin wave (CCKW) over the eastern tropical Atlantic.
2-km winds and PV (Sippel et al. 2011)
3-way interaction
diurnal cycle of convection over the Guinea Highlands (GHs)
幾內亞高地
Tropical
Cyclogenesis
convectively coupled atmospheric equatorial Kelvin wave (CCKW)
African easterly wave (AEW)

4. Saharan Air Layer (SAL)
African Easterly Wave (AEW)
The dominant synoptic weather systems observed over Africa and the tropical Atlantic during Northern Hemisphere boreal summer
westward propagating tropical waves that grow along the African easterly jet (AEJ)
Saharan Air Layer (SAL)

5. Aug. 1998-2009 averaged TRMM 3B42 rainfall rate
Guinea Highlands (GHs)
Aug averaged TRMM 3B42 rainfall rate
Chiao and Jenkins (2010) concluded that the GHs region played an important role by modulating the low-level westerly flow that deflected northward along the GHs. This deflection enhanced the low-level cyclonic circulation there and transported moist air toward the north, supporting the development of mesoscale convective systems (MCSs).
Convection is generally triggered close to the topography during the late afternoon in the vicinity of the GHs (Hodges and Thorncroft 1997; Yang and Slingo 2001; Laing et al. 2008).

6. Convectively Coupled Atmospheric Equatorial Kelvin Wave (CCKW)
Mounier et al. (2007) analyzed CCKWs over the Atlantic and Africa and found that the passage of the waves are preceded by low level easterly wind anomalies and followed by low-level westerly wind anomalies, partly in phase with negative and positive geopotential height anomalies, respectively.
Mounier et al. (2007) also demonstrate that MCSs traveling through the convective envelope of the CCKW over Africa often have higher amplitudes and tend to last longer.
CCKWs have been shown to modify the background state and to intensify westward moving features traveling through its convectively active phase (Mekonnen et al. 2008).
→ modulate tropical cyclone activity by enhancing westward propagating AEWs?
Ventrice et al. 2012a : the development of Tropical Storm Debby as a case study highlighting a CCKW–AEW interaction.
Ventrice et al. 2012a : the impact of strong CCKWs on the large-scale environmental conditions over the tropical Atlantic that is associated with tropical cyclogenesis.

7. Data and method
European Centre for Medium-Range Weather Forecasts (ECMWF) Interim Re-Analysis (ERA-Interim) dataset:
1989 to present
horizontal grid size of 1.5o
AEW tracking method (Berry et al., 2007):
objectively isolate trough axes of individual AEWs and locate the mean position of the AEJ.
Climate Prediction Center (CPC) merged IR dataset:
4-km spatial resolution every 30 min.
diurnal cycle of convection over the GHs region
development of Tropical Storm Debby
TRMM Multisatellite Precipitation Analysis:
1998 to present
original 3-hourly 0.25o latitude–longitude grids
averaged to 6-hourly 1o latitude–longitude grids

8. Data and method
National Oceanic and Atmospheric Administration’s (NOAA) daily averaged interpolated OLR dataset:
Convection associated with CCKWs
horizontal gridded resolution of 2.5o
Wavenumber-frequency filtering (Wheeler and Kiladis, 1999):
period range of 2.5–20 days
eastward wavenumbers 1–14
equivalent depths of 8–90 m.
A total of 142 CCKWs were objectively identified during the 1989–2009 June–September (JJAS) seasons
In lag analysis, ‘‘day 0’’ is when the minimum Kelvin filtered OLR anomaly moves over the selected base point (10oN, 15oW)
National Climatic Data Center’s (NCDC’s) International Best Track Archive for Climate Stewardship (IBTrACS) v3 dataset
analysis of tropical cyclogenesis events during 1979–2009

9. Evolution of the pre-Debby AEW

10. The exceedance frequency of pixels less than 233 K for Aug. 1998-2009
an approximate estimation of the coherent diurnal cycle of rainfall (Duvel 1989; Mounier et al. 2007; Nguyen and Duvel 2008).
solar heating increased
warming and destabilizing the BL
oceanic dominated convection
continental dominated convection

11. CPC IR brightness temperature
AEW forcing
merge → cyclogenesis
the synoptic forcing by the intensifying AEW influenced the development of both strong MCSs.

12. large-scale upper-level wind divergence and consistent
Color: unfiltered OLR anomalies
Contour: KW filtered OLR anomalies
Vector: 200-hPa wind anomalies
‘‘suppressed–active–suppressed’’ convective pattern associated with the CCKW travels eastward with an average phase speed of roughly 15 m s-1.
Off-equatorial convection is collocated with the convectively active phase of the CCKW between day -2 and day +1.
large-scale upper-level wind divergence and consistent
with the deep convection.

13. This excaptional CCKW : 2σ negative KW filtered OLR anomaly
Color: unfiltered OLR anomalies
Contour: negative KW filtered OLR anomalies N
This excaptional CCKW :
2σ negative KW filtered OLR anomaly
the strongest 5% of all JJAS ( ) CCKWs
beginning over the east Pacific (120oW) on 14 August
ending over the central Pacific on 6 September (180o)

14. Shading is unfiltered OLR anomalies.
Black dashed contours are negative Kelvin filtered OLR anomalies.
South–north-oriented black lines highlight individual AEW trough axes.
red dashed line identifies the mean location of the AEJ.
Vectors represent 200-hPa wind anomalies.

15. Tropical cyclogenesis becomes most frequent just after the passage of the convectively active phase of the CCKW.
CCKW may be impacting the large-scale environmental conditions associated with tropical cyclogenesis.
Time–longitude plot of composited OLR anomalies averaged along 10oN during June–September 1979–2009.

16. Conclusions
Pre-Debby AEW interacted with the coherent diurnal cycle of convection generated over the GHs region.
CCKWs might modulate tropical cyclogenesis over the MDR by directly amplifying westward propagating AEWs.
Tropical cyclogenesis becomes significantly more frequent two days after convection is initially excited by the CCKW.

17. subtropical westerly jet
5oS–10oN averaged, when minimum Kelvin filtered OLR anomaly is located at 15oW
JJAS climatological 925–200-hPa vertical wind shear vectors
westerly shear
subtropical westerly jet
easterly shear
tropical easterly jet

18. Before day 0, westerly vertical wind shear phase of the CCKW opposes the climatological easterly shear and reduces the background vertical wind shear.
After day +1, vertical wind shear increases over equatorial Africa as the easterly shear phase of the CCKW increases the climatological easterly.
westerly shear
easterly shear
easterly shear
westerly shear

19. Negative Kelvin-filtered OLR anomalies are dashed.
westerly shear
easterly shear
Time–longitude plot of 925–200-hPa vertical wind shear magnitude anomalies averaged within the 0o–25oN band:
Red: large shear
Blue: small shear
Negative Kelvin-filtered OLR anomalies are dashed.
shear increase
shear decrease

20. Total column water vapor anomaly
Saharan Air Layer (SAL)
TCWV increases over the tropical Atlantic during and after the passage of the convectively active phase of the CCKW.

21. 925-hPa relative vorticity anomaly

22. anomalies of Kelvin-filtered OLR (shaded)
925-hPa streamfunction (contoured)
The minimum negative/positive stream-function anomaly is collocated with the convectively active/suppressed phase of KW.
large couplet of negative and positive stream-function anomalies is located slightly more westward.
negative streamfunction anomalies are once again collocated with the convectively active phase of the CCKW.
anomalous streamfunction couplet over the tropical Atlantic is located farther west
a new area of anomalous negative stream-function develops east of the couplet over 40oW, confirming the train of westward propagating disturbances

23. vertical cross section of meridional wind anomalies through 10oN on day +2

24. time–longitude composite of unfiltered 200-hPa velocity potential anomalies.
Kelvin-filtered OLR is represented by the black contours.
MJO-filtered OLR is represented by the orange contours.
strongest CCKWs observed over the tropical Atlantic region are commonly associated with the decay of the active convective signal of the MJO over the Pacific and eastward progression over the Western Hemisphere. (Straub and Kiladis, 2003 and Mekonnen et al., 2008)
CCKWs might be an important feature in the genesis of the MJO. (Straub et al., 2006).

25. Conclusions
Over the eastern Atlantic, genesis is less frequent just prior to the passage of the minimum Kelvin-filtered OLR anomaly as Kelvin wave induced westerly shear reduces the climatological easterly shear.
More frequent genesis events are observed as Kelvin wave–induced easterly shear over the eastern MDR increases the climatological easterly shear.
Kurihara (1981) performed a modeling study showing that mean easterly shear is more favorable for tropical cyclogenesis than westerly shear with respect to a westward-moving wave in the Northern Hemisphere.
The anomalous easterly shear phase of the CCKW is associated with a region of enhanced AEW activity (Mekonnen et al. 2008; Leroux et al. 2010).
This increased AEW activity may be more important for tropical cyclogenesis frequency there than the shear.
The increased low-level cyclonic flow and increased easterly wave activity over the tropical Atlantic during and after the passage of the CCKW occurs during a time when genesis is most frequent.

26. Conclusions
The overall low tropical cyclogenesis activity ahead of the convectively active phase of the CCKW arises from unfavorable large-scale environmental conditions (including enhanced westerly vertical wind shear and reduced easterly vertical wind shear, reduced atmospheric moisture, and anomalous anticyclonic low-level relative vorticity), along with the influence of the westward-moving SAL or mid-latitude dry air outbreak (Dunion and Velden 2004).
The increasing trend of tropical cyclogenesis events occurs from the combination of the direct enhancement of convection associated with the convectively active phase of a CCKW, as well as more favorable large-scale environmental conditions (reduced westerly vertical wind shear and enhanced easterly vertical wind shear, increased atmospheric moisture, and anomalous cyclonic low-level relative vorticity) for tropical cyclogenesis.

28. African easterly wave (AEW)