1. Convective Oscillations in a Strongly Sheared Tropical Storm Jaclyn Frank and John Molinari The University at Albany, SUNY

2. Introduction Tropical cyclones usually do not form or intensify in vertical shear greater than ~12.5 ms -1. Tropical cyclones usually do not form or intensify in vertical shear greater than ~12.5 ms -1. Tropical Storm Edouard formed off the coast of Florida in 2002 and reached a peak intensity of 55 kt despite vertical shear in excess of 13 ms -1. Tropical Storm Edouard formed off the coast of Florida in 2002 and reached a peak intensity of 55 kt despite vertical shear in excess of 13 ms -1. Although shear increased with time to well over 20 ms -1, Edouard maintained tropical storm intensity until landfall. Although shear increased with time to well over 20 ms -1, Edouard maintained tropical storm intensity until landfall.

3. Convective Oscillations Forecasters have long been aware of the pulsing nature of convection in tropical cyclones. Forecasters have long been aware of the pulsing nature of convection in tropical cyclones. “Always try to analyze more than one image leading up to the analysis time…This is particularly applicable to shear patters that often go through a cyclic regime of convection blow-up near the low-level center followed by increasing separation of the overcast from the low-level center. This can lead to rapidly varying DT numbers over several hours.” ― A. Burton, Australian Bureau of Meteorology “Notes on the Application of the Dvorak Technique” “Notes on the Application of the Dvorak Technique”

4. Convective Oscillations in Edouard Edouard underwent a series of pulses in which deep convection formed near the storm center, then shifted more than 100 km downshear within a few hours. Edouard underwent a series of pulses in which deep convection formed near the storm center, then shifted more than 100 km downshear within a few hours. Bursts of convection near the storm center continued in spite of strong vertical shear, probably helping the storm maintain intensity. Bursts of convection near the storm center continued in spite of strong vertical shear, probably helping the storm maintain intensity. One such event will be examined here. One such event will be examined here.

5. 0715 UTC 3 Sept. IR image 0715 UTC 3 Sept. IR image 1500 m recon winds (white barbs, 0613 UTC center pass) 1500 m recon winds (white barbs, 0613 UTC center pass) Positive ( ) and negative ( ) lightning flashes (30 min. centered on 0615 UTC) Positive ( ) and negative ( ) lightning flashes (30 min. centered on 0615 UTC) Shear direction (yellow arrow; 850-200 hPa, 13-15 ms -1 ) Shear direction (yellow arrow; 850-200 hPa, 13-15 ms -1 )

6. 1115 UTC 3 Sept. IR image 1115 UTC 3 Sept. IR image 250 m recon winds (1154 UTC center pass) 250 m recon winds (1154 UTC center pass) 30 min. lightning centered on 1115 UTC 30 min. lightning centered on 1115 UTC 850-200 hPa shear 13-15 ms -1 850-200 hPa shear 13-15 ms -1

7. 1515 UTC 3 Sept. IR image 1515 UTC 3 Sept. IR image 325 m recon winds (1546 UTC center pass) 325 m recon winds (1546 UTC center pass) 30 min. lightning centered on 1115 UTC 30 min. lightning centered on 1115 UTC 850-200 hPa shear 13-15 ms -1 850-200 hPa shear 13-15 ms -1

8. 1915 UTC 3 Sept. IR image 1915 UTC 3 Sept. IR image 350 m recon winds (1942 UTC center pass) 350 m recon winds (1942 UTC center pass) 30 min. lightning centered on 1115 UTC 30 min. lightning centered on 1115 UTC 850-200 hPa shear 13-15 ms -1 850-200 hPa shear 13-15 ms -1

9. A plot of flash count versus distance from storm center reveals that lightning seemed to move inward with time and increase in frequency from 05-12 UTC 3 September.

10. A similar plot from 13- 17 UTC 3 September shows the inner core convective maximum waning and a new burst of lightning reforming more than 100 km from the storm center.

11. 15:17:30 – 15:59:0011:04:40 - 12:33:50 23:19:40 – 00:28:10 Boundary layer storm-relative tangential winds peak when convection is near the center, then weaken as it moves away once again. The wind profile becomes more flat and symmetric with time. SW-NE cross- section of 250- 350 m storm- relative recon tangential winds

12. Summary of Convective Oscillation In the convective burst shown here, lightning seemed to propagate inward toward the storm center. In the convective burst shown here, lightning seemed to propagate inward toward the storm center. The apparent inward propagation and subsequent downshear reforming made the convection seem to oscillate, with a period of less than one day. The apparent inward propagation and subsequent downshear reforming made the convection seem to oscillate, with a period of less than one day. Edouard intensified briefly when lightning was near the storm center, then weakened again rapidly. Edouard intensified briefly when lightning was near the storm center, then weakened again rapidly.

13. Possible Mechanisms for Oscillation of Convection Vertical shear effects Vertical shear effects Downdrafts induced by mid-level dry air Downdrafts induced by mid-level dry air Boundary layer recovery Boundary layer recovery

14. Vertical Shear Effects Asymmetric convection Asymmetric convection Vertical tilt Vertical tilt 0715 UTC 3 September IR, 1500 m recon winds (green barbs, 0613 UTC center pass), and dropsonde surface wind (orange barb, 0616 UTC). 1535 UTC 3 Sept. MODIS image

15. Vertical Shear Effects Dropsonde winds and visible satellite imagery clearly reveal the convective asymmetry and downshear tilt with height of Edouard. Dropsonde winds and visible satellite imagery clearly reveal the convective asymmetry and downshear tilt with height of Edouard. To maintain balance conditions in a tilted vortex, a vertical circulation develops (e.g. Jones 1995, DeMaria 1996). To maintain balance conditions in a tilted vortex, a vertical circulation develops (e.g. Jones 1995, DeMaria 1996). This circulation favors upward motion downshear and downward motion upshear of the storm center. This circulation favors upward motion downshear and downward motion upshear of the storm center. Convection only occurs in the downshear direction of the center of Edouard throughout the oscillation. Convection only occurs in the downshear direction of the center of Edouard throughout the oscillation. Vertical shear can account for the re-development of convection downshear, but not the inward propagation. Vertical shear can account for the re-development of convection downshear, but not the inward propagation.

16. Dry Air / Downdrafts Radiosondes over Florida and dropsondes over Edouard reveal very dry air above 500 hPa. Radiosondes over Florida and dropsondes over Edouard reveal very dry air above 500 hPa. Dry air in the middle troposphere can fuel convective downdrafts. Dry air in the middle troposphere can fuel convective downdrafts. Downdrafts can bring cool, dry air to the boundary layer, impeding development of convection near the storm center for a time. Downdrafts can bring cool, dry air to the boundary layer, impeding development of convection near the storm center for a time. Conversely, downdrafts can create a gust front, which can trigger new convection, and may have fueled the apparent inward propagation of lightning. Conversely, downdrafts can create a gust front, which can trigger new convection, and may have fueled the apparent inward propagation of lightning.

17. Boundary Layer Recovery Once convection forms near the storm center, the atmosphere stabilizes due to vertical mixing and the effects of cold downdrafts. Once convection forms near the storm center, the atmosphere stabilizes due to vertical mixing and the effects of cold downdrafts. Convection may be impeded near the center for a time, but still able to form downshear of the center. Convection may be impeded near the center for a time, but still able to form downshear of the center. Since the SSTs are warm, surface fluxes of heat and moisture allow the boundary layer to recover relatively quickly. Since the SSTs are warm, surface fluxes of heat and moisture allow the boundary layer to recover relatively quickly. As the atmosphere destabilizes, convection can again occur near the center. As the atmosphere destabilizes, convection can again occur near the center.

18. Hypothesis Ambient or shear-induced dry air fuels convective downdrafts. Ambient or shear-induced dry air fuels convective downdrafts. Downdraft cooling, followed by boundary layer recovery due to surface fluxes over warm water could cause the oscillation. Downdraft cooling, followed by boundary layer recovery due to surface fluxes over warm water could cause the oscillation. The cause of the apparent inward propagation of convection remains unclear, but may be forced by gust fronts. The cause of the apparent inward propagation of convection remains unclear, but may be forced by gust fronts. The reason convection recurs downshear after collapsing at the core may be related to the shear-induced circulation. The reason convection recurs downshear after collapsing at the core may be related to the shear-induced circulation. Periodic deep convection near the storm center probably helped Edouard resist the strong shear to some extent and maintain tropical storm intensity. Periodic deep convection near the storm center probably helped Edouard resist the strong shear to some extent and maintain tropical storm intensity.