The Tropical Transition of Cyclones: Science Issues and Critical Observations or TC Genesis: A Global Problem Chris Davis (NCAR) Collaborators: Lance Bosart Ron McTaggart-Cowan Andy Heymsfield Michael Montgomery Jason Dunion

What is Tropical Transition? TC formation induced by disturbances of extratropical origin (cold core) Strong:  Baroclinic frontal cyclone Weak:  Cold upper-tropospheric troughs or weak baroclinic waves  Mesoscale Convective Vortices Bracken and Bosart (2000, MWR): Modest shear may assist TC development

Strong Baroclinic Precursors Images courtesy of NRL:

Synoptic Climatology of Atlantic TC Genesis Define storm-centred objective indicators of TT: –Upper level Q-vector convergence –Lower level thermal asymmetry Datasets ( ) –NCEP/NCAR Reanalysis –NHC Best Track Compute linear back-trajectories for storm centre locations from T-0h (NHC tracking) to T-36h

Results – Genesis Locations Strong TTWeak TTTr Induced TropicalWave InducedPerturbed

Development of Maria 2005 , Wind on DT,  at 950 hPa 00 Z 30 Aug00 Z 01 Sep 00 Z 02 Sep00 Z 31 Aug Potential Temperature on PV=1.5 PVU Red Contours = 900 hPa Relative Vorticity (1, 2, 4, and 8x10 -5 s -1 )

Genesis Mechanisms: Theories in need of Observations Synoptic-scale: –Disturbances from higher latitudes –Tropical waves –ITCZ and cross-equatorial flow Mesoscale –MCVs organizing convection –Vortex merger –Mid-tropospheric moistening and downdraft reduction Convective-scale vortices (VHTs) Observations required on multiple spatial scales

Genesis Hypotheses Governed by the synoptic-scale (global model success) Merger and/or downward migration of mid-tropospheric mesoscale vortices. Upscale influence of intense convective-scale vortices. Lower-mid-tropospheric relative humidity governs downdrafts and surface divergence Cloud physics critically affects downdrafts – strongly influenced by aerosols (dust) Prediction more limited by synoptic-scale errors than mesoscale errors. Note: The above are not mutually exclusive, but facilitate defining observing objectives

Key Observations Radar: convection structure, vertical circulations, vortices on multiple scales (vortical hot towers, MCVs) Aircraft (in situ) and dropsondes: Boundary-layer: water vapor, surface fluxes Aircraft (in situ) and dropsondes: Mesoscale structure of RH in the lower-middle troposphere. Long-range, high altitude aircraft: upper-tropospheric sub-synoptic-scale features (wind and temperature, deduce PV) Aircraft (  phys probes), aerosol lidar: cloud physics and dust concentration

Observing Challenges Multiple altitudes (12 km, 6-8 km; 3-4 km; BL) Long duration (genesis is an abrupt process with relatively long quiescent periods) Close coordination with satellites: major questions about next generation US platforms => international effort required Many precursors, not many cyclones Genesis often far at sea – aircraft ferry considerations

WATTAGE Western Atlantic Tropical Transition and Genesis Experiment L km km (a)(c) (b) (d) NCAR G-IV NOAA G-IV NASA DC-8 NCAR C-130 NRL P-3 NOAA P-3 UAVs P-3/C-130 flight tracks HIAPER flight tracks

Closing Remarks TC formation has global similarities obscured by regional taxonomy. Need to uncover the similarities. Observations of TC formation require new paradigm –Long duration, multiple scales, episodic –Synergy of aircraft, satellite and numerical models for deployment