The Tropical Transition of Cyclones: Science Issues and Critical Observations

orTC Genesis: A Global Problem

Chris Davis (NCAR)

Collaborators:Lance Bosart

Ron McTaggart-CowanAndy Heymsfield

Michael MontgomeryJason 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: http://www.nrlmry.navy.mil/tc_pages/tc_home.html

Synoptic Climatology of Atlantic TC Genesis

• Define storm-centred objective indicators of TT:– Upper level Q-vector convergence– Lower level thermal asymmetry

• Datasets (1948-2004)– 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 TT Weak TT Tr Induced

TropicalWave InducedPerturbed

Development of Maria 2005, Wind on DT, at 950 hPa

00 Z 30 Aug 00 Z 01 Sep

00 Z 02 Sep00 Z 31 Aug

Potential Temperature on PV=1.5 PVURed 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

45,000 ft.

30,000 ft.

15,000 ft.

10,000 ft.

2,000 ft.

500 ft.

Stage 1: C-130 cloud physics module

Stage 2: C-130 PBL module0ºC

45,000 ft.

30,000 ft.

15,000 ft.

10,000 ft.

2,000 ft.

500 ft.

Stage 1: C-130 cloud physics module

Stage 2: C-130 PBL module0ºC

L

100-200 km

300-400 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