Potential Vorticity Streamers and Tropical Cyclogenesis During the 2007 North Atlantic Hurricane Season T. J. Galarneau 1, L. F. Bosart 1, and R. McTaggart-Cowan 2 1 Department of Earth and Atmospheric Sciences University at Albany, State University of New York 2 Numerical Weather Prediction Research Section Meteorological Service of Canada 14th Cyclone Workshop Sainte-Adèle, Quebec, Canada 24 September 2008 Support provided by NSF grants ATM and ATM

Motivation I TC genesis events associated with PV streamers often occur in close proximity to US coastal regions For example,TC Diana (1984) formed via tropical transition (TT) on 7–8 Sept. Bosart and Bartlo (1991) Davis and Bosart (2001, 2002)

Motivation II Frequent PV streamers over the western Atlantic during much of the 2007 North Atlantic hurricane season For example: 18 UTC 8 Aug 2007 DT pressure (hPa) DT–850 hPa shear (kt) 925–850 hPa  (10  5 s  1 ) DT = dynamic tropopause = 2.0 PVU surface a b c d a = remnant PV streamer b = PV streamer c,d = future PV streamers

Goals Determine distribution of 2007 TC genesis events among the different pathways –Comparison with dynamically based TC genesis climatology (McTaggart-Cowan et al. 2008) Hess et al. (1995); Elser et al. (1996) Examine selected cases associated with PV streamers that impact US coastline –Role of PV streamers in TC genesis

Data and Methods 0.5  NCEP–GFS analyses NHC Best Track Dynamic Tropopause (DT) = 2.0 PVU surface NCDC GOES-12 satellite archive Nondivergent wind Q-vectors Time series plots are box average of: –DT–850 hPa wind shear (1000  1000 km) –400–200 hPa Q nd -vector divergence (500  500 km)

Outline Review TC genesis pathways Overview of 2007 season Wind shear reduction mechanisms Examine individual cases (synoptic-scale) –TS Gabrielle (8–11 Sept 2007) –Hurricane Humberto (12–14 Sept 2007) Concluding remarks

TC genesis pathways as defined in McTaggart-Cowan et al. (2008)

TC Genesis Pathways Fig. 9 from McTaggart-Cowan et al. (2008) Non-Baroclinic –Easterly waves in main development region Low-Level Baroclinic –Easterly waves near midlevel easterly jet (Cape Verde) Transient Trough Interaction –PV streamer interaction Trough Induced –PV streamer Weak TT –Baroclinic cyclone Strong TT –Baroclinic cyclone 40% 16% 13% 3% 13% 16%

Comparison with 2007 season

2007 North Atlantic Tropical Cyclones NHC Best Track Data Non-Baroclinic (4) Low-Level Baroclinic (1) Trans Trough Interact (5) Trough Induced (2) Weak TT (2) Strong TT (1)

Fig. 10 from McTaggart-Cowan et al. (2008) Spatial Distribution of TC genesis Events (1948–2004) 40% 3% 13% 15% 16% n=496 Dean, Felix, Ingrid, Karen Barry, Erin, Lorenzo, Noel, Olga Melissa Chantal, Humberto Gabrielle, Jerry Andrea D F I K M B E L N O H C J G A 2007 Genesis Positions

1 July–30 September –200 hPa Thickness (dam), 400 hPa Temperature Anomaly (K) and Normalized Height Deviation (%) W mid-ocean trough Warm anomaly Enhanced westerly flow 2.5  NCEP–NCAR Reanalysis

Shear Reduction Mechanisms (“nontraditional pathways”)

Shear Reduction Mechanisms Trough category –trough translates equatorward on southern flank of negative PV anomaly in the ridge Base category –Barotropic PV interaction reduces flow in base of the anticyclonic wave break (foldover ridge) anticyclonic wave break PV on  = 345 K McTaggart-Cowan et al. (2008 AMS Conf. on Hurricanes and Tropical Meteorology)

Shear Reduction Mechanisms Trough category –trough translates equatorward on southern flank of negative PV anomaly in the ridge Base category –Barotropic PV interaction reduces flow in base of the anticyclonic wave break (foldover ridge) anticyclonic wave break PV on  = 345 K McTaggart-Cowan et al. (2008 AMS Conf. on Hurricanes and Tropical Meteorology) Barotropic Shear Reduction S/SW trough translation

TS Gabrielle/Hurricane Humberto 8–14 Sept 2007 NHC Best Track Weak TT Trough Induced

DT Analysis and QG Forcing DT Pressure, DT–850 hPa shear, 925–850  400–200 hPa , Q-vectors,  Q nd G G 06 UTC 7 Sept T  36 h 09 UTC 7 Sept GOES-12 IR G Gabrielle G = Gabrielle

DT Analysis and QG Forcing DT Pressure, DT–850 hPa shear, 925–850  G G 06 UTC 8 Sept T  12 h 09 UTC 8 Sept GOES-12 IR G 400–200 hPa , Q-vectors,  Q nd Gabrielle G = Gabrielle

DT Analysis and QG Forcing DT Pressure, DT–850 hPa shear, 925–850  H H 12 UTC 9 Sept T  72 h 12 UTC 9 Sept GOES-12 IR H 400–200 hPa , Q-vectors,  Q nd Humberto X H = Humberto X = PV streamer

DT Analysis and QG Forcing DT Pressure, DT–850 hPa shear, 925–850  H H 12 UTC 11 Sept T  24 h 12 UTC 11 Sept GOES-12 IR H 400–200 hPa , Q-vectors,  Q nd Humberto X2X2 X1X1 H = Humberto X = PV streamer

DT–850 hPa wind shear Hours Prior to TS Wind Shear (m s  1 ) Wind shear weakened as QG forcing became small in response to eroding PV streamer Wind shear weakened by t-48 h likely in: –response to momentum and PV redistribution –association with barotropic PV interaction in Humberto case Environment Evolution Gabrielle Humberto 400–200 hPa Q nd divergence  Q nd (10  12 m kg  1 s  1 ) Hours Prior to TS 500  500 km avg 1000  1000 km avg

40% 3% 13% 15% 16% Dean, Felix, Ingrid, Karen Barry, Erin, Lorenzo, Noel, Olga Melissa Chantal, Humberto Gabrielle, Jerry Andrea D F I K M B E L N O H C J G A Concluding Remarks I Frequent PV streamers over western North Atlantic during 2007 may have limited “traditional easterly wave” development pathway relative to climatology

Concluding Remarks II Gabrielle formed via TT east of PV streamer in region of percolating convection that coincided with QG forcing for ascent During genesis, deep-layer shear is reduced by vertical momentum redistribution by convection G G +PV  PV +PV  PV +PV PV on  = 345 K N E Time = t Time = t  t QG forcing for ascent G = Gabrielle

Concluding Remarks II Gabrielle formed via TT east of PV streamer in region of percolating convection that coincided with QG forcing for ascent During genesis, deep-layer shear is reduced by vertical momentum redistribution by convection G G +PV  PV +PV  PV +PV PV on  = 345 K N E Time = t Time = t  t QG forcing for ascent G = Gabrielle McTaggart-Cowan et al. (2008) AMS Conf. on Hurricanes and Tropical Meteorology G shear reduction mechanism–trough category

Concluding Remarks III Incipient Humberto vorticity organized in region of deep convection coinciding with QG ascent forcing During genesis, deep-layer shear is reduced by vertical momentum redistribution and barotropic PV interaction H +PV  PV PV on  = 345 K N E Time = t Time = t  2  tTime = t  t H H  PV +PV Fracture QG forcing for ascent H = Humberto

Concluding Remarks III Incipient Humberto vorticity organized in region of deep convection coinciding with QG ascent forcing During genesis, deep-layer shear is reduced by vertical momentum redistribution and barotropic PV interaction H +PV  PV PV on  = 345 K N E Time = t Time = t  2  tTime = t  t H H  PV +PV Fracture QG forcing for ascent H = Humberto H shear reduction mechanism–“pseudo” base category McTaggart-Cowan et al. (2008) AMS Conf. on Hurricanes and Tropical Meteorology

Future Work Examine recent TC seasons to develop climatology and assess important physical processes among each pathway Develop TC genesis pathway classification scheme to be computed in real-time If large-scale flow patterns are predictable on seasonal timescales, then are predominant TC genesis pathways predictable on seasonal timescales?

15 UTC 24 Sept GOES-12 IR DT  (K), Wind (kt), 925–850 hPa  (10  4 s  1 ) 12-h Forecast v18Z/24 Image source: Image source: DEAS