GPS Doppler & Buoy 0 – 3 km 3 – 6 km 6 – 10 km Obs Cell Bunkers SFC Cell C CSCN Cell B BSBN 1040 UTC C E B F D A 1108 UTC Cell mergers Upwind flank development Weak Rotation NO tornadoes Cell Y Cell X YWYE XWXE Miniature Supercells Observed in Offshore Outer Rainbands of Hurricane Gustav (2008) Matthew D. Eastin and Jennifer Edwards Department of Geography and Earth Sciences, University of North Carolina at Charlotte Objectives The majority of tropical cyclone tornadoes are spawned by “miniature” supercells embedded in the outer convective rainbands ( km from the center), moving onshore in the right-front (or northeast) quadrant. The objectives of this study are to document the structure and evolution of numerous convective cells within offshore outer rainbands, as well as their local environment, in order to diagnose the essential physical processes involved with miniature supercell formation prior to any tornadogenesis. Summary of Significant Results Differences in the local environmental vertical wind shear profile appears to the most significant difference between the Gustav and Ivan supercells. The stronger 0-6 km shear and SRH in Ivan should promote stronger low-level updrafts (forced by dynamic pressure perturbations) and greater vertical vorticity (via tilting and stretching of ambient horizontal vorticity ) associated with the mesocyclone. Differences in the observed cell structures are consistent with such forcing, as Gustav’s (Ivan’s) cells are smaller (large) in diameter with weaker (stronger) low-level mesocyclones and updraft maxima located above (below) the freezing level. These results re- enforce the idea that careful monitoring of the offshore wind profile may benefit forecasters charged with issuing tornado watches and warnings. References and Additional Reading Baker, A.K., M.D. Parker, and M. D. Eastin, 2009: Environmental ingredients for supercells and tornadoes in Hurricane Ivan. Weather and Forecasting, 24, Eastin, M. D, and M. C. Link, 2009: Miniature supercells in an offshore outer rainband of Hurricane Ivan (2004). Monthly Weather Review, 137, Hurricane Gustav - 01 September UTC Rainband Environment: Modest CAPE (~ J/kg) Modest 0-3 km SRH (~100 m 2 /s 2 ) Modest 0-6 km shear (~12 m/s) Modest mid-level dry air (~50-70%) TC / Rainbands: Category – 3 Moving NW at 6.5 m/s Rainbands in NE quadrant Radii ~ km Miniature Supercells: Observed >100 km offshore New cells on upwind flank Merge into rainband segments No reported tornadoes TLH TBW GPS Soundings Dry air Rainbands Buoy GPS Modest dry air TBW Modest dry air Hurricane Ivan - 15 September UTC Rainband Environment: Modest CAPE (~ J/kg) Large 0-3 km SRH ( ~220 m 2 /s 2 ) Large 0-6 km shear (~28 m/s) Modest mid-level dry air (~50-70%) TC / Rainband: Category – 4 Moving N at 5.7 m/s Rainband in NE quadrant Radius ~350 km Miniature Supercells: Observed >100 km offshore Long-lived ( >2 hours) Remained distinct cells Multiple tornado reports TLH TBWGPS Soundings Apparent dry air intrusion Rainband Dual-Doppler Analysis Radar reflectivity Updrafts > 2 m/s Downdrafts < -2 m/s Vorticity 2 × s km ASL Cell X Cell Y Cell Z YW XE XW YE 1945 UTC Z Y Y Z X X 2044 UTC Radar-detected mesocyclone Mesocyclone and tornado Dual-Doppler Analysis Radar reflectivity Cell-relative winds Aircraft track 2.0 km ASL Cell X Cell Y Cell Z Buoy TLH GPS Z Y X Aircraft Doppler Analysis Region 1804 UTC Cell E ESEN Cell D DSDN Dual-Doppler Analysis Radar reflectivity Updrafts > 2 m/s Downdrafts < -2 m/s Vorticity 2 × s km ASL Cell D Cell E Cell F EN DS DN ES Dual-Doppler Analysis Radar reflectivity Cell-relative winds Aircraft track 2.0 km ASL Cell A Cell B Cell C Dual-Doppler Analysis Radar reflectivity Updrafts > 2 m/s Downdrafts < -2 m/s Vorticity 2 × s km ASL Cell A Cell B Cell C BN CS CN BS Dual-Doppler Analysis Radar reflectivity Cell-relative winds Aircraft track 2.0 km ASL Cell D Cell E Cell F GPS Doppler & Buoy TLH 0 – 3 km 3 – 6 km 6 – 10 km Obs Cell Bunkers SFC Doppler & Buoy & GPS GPS 0 – 3 km 3 – 6 km 6 – 10 km Obs Cell Bunkers SFC Very dry air Modest dry air TLH GPS C B A Doppler Analysis Regions and Cells 0857 UTC D Buoy E F