1. TC Development from a Train of Interacting Easterly Waves
60% of Atlantic TCs from AEWs
Figure obtained from COMET
James P Fowler
Thomas Galarneau Jr.
Department of Hydrology and Atmospheric Sciences
University of Arizona
Hi I’m James, advisee of Dr. Thomas Galarneau, and today I will be talking with you guys about how Tropical cyclones develop in a rapid train of African Easterly Waves (which are synoptic systems of organized convection coming off the Sahal region in Africa and is the major source of TCs in the Atlantic.)
97th AMS Annual Meeting
26 January :15 PM

2. Motivation
Four tropical cyclones (TCs) formed over the main development region (MDR) of the North Atlantic during the last two weeks of August 2010
Observed TC Tracks from HURDAT
The close proximity of these TCs in both time and space provided an opportunity to study the multiscale interactions between these TCs and their synoptic environment
<= 33 kts (Tropical Depression)
34–64 kts (Tropical Storm)
65–83 kts (Cat 1 Hurricane)
84–95 kts (Cat 2 Hurricane)
96–113 kts (Cat 3 Hurricane)
114–134 kts (Cat 4 Hurricane)
>= 135 kts (Cat 5 Hurricane)
I will be focusing on an active period of AEWs during end of August into early September 2010 where 4 AEWs entered the Atlantic in a short period of time.
This gave us an unique opportunity to study how these systems interacted with one another as well as with the environment and see what influence these interaction have in their development into TCs using global reanalysis data and ensemble forecasts.
This plot here of best track data shows the 6 hrly position of the 4 TCs we will be focusing on and the dots are colored based on intensity.
Fiona
Gaston
Earl
Danielle

3. Goals and Data
Examine how storm-storm and storm-environment interactions influence the development of these TCs
Utilize the 0.5° Climate Forecast System Reanalysis (CFSR) in our analysis and diagnosis

4. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface
Flow zonal initially
This next series of maps shows data from CFSR and each system will be circled by its strength shown by the key in the lower left. The maps shows low level cyclonic relative vorticity (solid contours), with potential temp (colors) and winds (barbs) on the dynamic tropopause (which is where there is a surface of 2 Potential vorticity units (its just showing the height of the tropopause)). Where the pot temp is higher, the tropopause is high in the atmosphere and is associated with anticyclonic motion and when potential temperature is low, the tropopause is low in the atmosphere associated with cyclonic motion.
Here on the 23rd of August, there is a lot of zonal flow across the Atlantic and Danielle has formed into a TS around 35W.
Danielle develops
into TS K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

5. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

6. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

7. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

8. East coast cyclogenesis Flow becomes meridional
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface
East coast cyclogenesis
Flow becomes meridional
Danielle is a
mature hurricane
Moving 4 days later, Danielle has matured into a hurricane while Earl has now formed into a TS near where Danielle developed. Due to a low pressure system developing over New England, the flow across the mid-latitudes has become meridional which was further enhanced by the outflow from Danielle. Also notice the AEW that has just moved into the Atlantic that will eventually become Fiona.
Earl develops
into TS K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

9. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

10. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface
Danielle is a
mature hurricane
Couple of days later, Danielle is nearing peak intensity with a large upper level anticyclone. To the south, Earl is taking a much longer time to develop into a hurricane because of the interference from Danielle’s outflow along with the upperlevel trough to the NE. Fiona is still trying to get its act together and wont be anytime soon because of that trough upstream being enhanced by Danielles outflow. This map is a great example of how the storms are influencing the development of other systems.
Earl’s development into mature hurricane is delayed by Danielle’s outflow K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

11. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

12. Now Fiona must contend with trough
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface
Without the interference from Danielle, Earl entered a period of rapid intensification off of Puerto Rico. Fiona has finally reached TS status, but much further west than the previous storms since it is still dealing with the upper-level trough.
Earl develops into mature hurricane as it escapes trough enhanced by Danielle’s outflow
Now Fiona must contend with trough K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

13. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

14. Danielle Earl Fiona Gaston
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

15. Earl’s outflow destroys Fiona
DT 𝜽 (shaded in K), wind (barbs in knots), and 925–850 hPa 𝜻 (contours every 𝟎.𝟓× 𝟏𝟎 −𝟒 𝒔 −𝟏 )
Dynamic tropopause (DT)=2.0 potential vorticity unit surface
Danielle has be fully absorbed by the mid-latitude trough. Earl is just to the SE of the Outerbanks, NC with its own large upper level anticyclone. Fiona, still a TS, is being shredded apart by Earl’s outflow. As Earl recurves to the NE, the meridional winds will increase in its wake and the downstream trough gets enhanced. Gaston, which briefly made TS status before weakening, will have to face that strong trough that will stop it from ever reaching TS status again.
Earl’s outflow destroys Fiona
Recurvature of Earl into midlatitudes contributes to reinforcement of trough over central Atlantic, disrupting Gaston K
tropical depression/low
Danielle Earl Fiona Gaston
CFSR: 0.5o NCEP Climate Forecast System Reanalysis (Saha et al. 2010)
tropical storm
hurricane

16. 600 hPa 𝝍 (×106 m2 s-1), 500 hPa ascent (Pa s-1), and 700–500 hPa layer-mean RH (%)
Danielle
When Danielle and Earl are entering the Atlantic, there is an area of high pressure to the north, the flow is generally zonal and the Atlantic basin is relatively moist.
Earl

17. 600 hPa 𝝍 (×106 m2 s-1), 500 hPa ascent (Pa s-1), and 700–500 hPa layer-mean RH (%)
Earl H
Fiona
However, by the time Gaston enters the Atlantic, Danielle is recurving toward the North Atlantic causing the flow to become more meridional. The Atlantic basin is much drier compared to before and the area of high pressure has strengthened and moved south.
Gaston

18. Time-series of area-average 700–500 hPa relative humidity (%) and 600 hPa meridional flow index (m s-1)
Danielle at 40°W
Earl at 40°W
Fiona at 40°W
RH in subtropics
Gaston at 40°W
So a quick summary, the outflow from both Danielle and Earl enhanced the meridional winds over the Atlantic which would inhibit the development of subsequent storms. It turns out that the moisture content in the Tropics will decrease as the magnitude of the meridional winds increases. The graph here shows that Danielle and Earl had a relatively moist atmosphere in their formative stages, but as the meridional winds increased, the Atlantic became drier inhibiting the growth of Fiona and especially Gaston.
𝒗 in mid-latitudes
Enhanced meridional flow in mid-latitudes led to drier conditions over the subtropics, inhibiting the development of Fiona and Gaston
20-Aug Aug Aug Aug Aug Aug Sep Sep Sep Sep Sep

19. Summary of Synoptic-Scale Flow Evolution
Midlatitude flow amplified in response to cyclogenesis over eastern US and Danielle’s subsequent recurvature
Upper-level trough enhanced by Danielle’s outflow anticyclone delayed Earl’s development
Upper-level trough reinforced by Earl’s outflow anticyclone and abundant dry air in subtropics disrupted development of Fiona and Gaston

20. Storm-Relative Time Series and Time-Height Diagram Methodology
SW Quadrant
NW Quadrant
SE Quadrant
NE Quadrant
Tropical Cyclone 2˚
10˚
Environment 900–500 hPa vertical shear was computed by removing the TC vortex within 4˚ of the storm center using vorticity inversion (Davis et al. 2008)
Relative vorticity was computed using area-average within 2˚ of the storm center
Relative humidity for the TC “core” was computed within 2˚ of the storm center and for the quadrants within 10˚ latitude-longitude box centered on storm with “core” masked out
Longitude 4˚
Latitude

21. Time series of 600 hPa 𝜻 (×10-5 s-1)
Area-Average within 2° of TC Center (“inner core”)
Danielle
Earl
Fiona
Gaston
TC at 40°W
TC at 60°W
Differences in vorticity evolution occur after TCs reach 40°W

22. Time series of 700–500 hPa RH (%)
Area-Average within 2° of TC Center (“inner core”)
Danielle
Earl
Fiona
Gaston
Earl temporarily dries out during interaction with Danielle’s outflow
TC at 40°W
TC at 60°W
Gaston rapidly entrains dry air after 40°W
Fiona undergoes rapid drying after 60°W while being sheared apart by Earl’s outflow

23. Upshear Environment 700–500 hPa RH (%)
Upshear environment dries overall as TCs move westward across Atlantic
Gaston environment drier overall
Fiona and Gaston drier especially west of 60°W

24. Time-Height of Environment RH (%) Northeast of TC
Danielle
Gaston
40°W
60°W
40°W
60°W
pressure (hPa)
pressure (hPa)
Days since emerging from African coast
Days since emerging from African coast %

25. Conclusions
Close proximity of multiple TCs in time and space facilitates multiscale interactions between TCs and their synoptic environment
Upper-level outflow from TC can increase vertical wind shear over subsequent TC
TC recurvature can have downstream impacts, such as increased meridional flow and drying in the subtropics, that influence development of subsequent TCs
Predicting these complex multiscale interactions is key for medium-range prediction of TC genesis, track, and intensity
Thank you for your time and I am now open to any questions.
James Fowler:
Thomas Galarneau:

26. Future work: Use of Ensembles
120-h ECMWF 51-Member Ensemble Track Forecast for Gaston
Initialized at 1200 UTC 1 Sep 2010
Almost all ensemble members take Gaston on a more westward course compared to observed storm
HURDAT (thick black)
Ensemble members (thin black)
Tropical depression
Tropical storm or greater
120-h Ensemble Forecast Intensity
32/51 (63%) =TD
19/51 (37%) >=TS
37% of ensemble members developed Gaston into TS
Future: Compare members that developed Gaston into TS with members that did not
Goal: Understand what environment factors TC genesis is sensitive to in ensemble forecast
Did a quick look at how the ensemble members of the ECMWF handled Gaston’s development. Nearly all of the members took Gaston on a more direct westward motion and nearly a third had Gaston growing into TS status by the end of the 120-hr run. See histogram of storm intensity based on 10m wind at the end of the run: right of blue line are members that reached TS and left never developed into TS. In the future, I am aiming to study why most of the members had a poor understanding of Gaston’s motion and why did so many develop Gaston back into a TS. And eventually want to look at how the model ensembles handle the other systems in this train.