# Presentation: 498332

## Tropical Transition: The Genesis of Tropical Cyclones from Extratropical Disturbances

- Christopher A. Davis
*National Center for Atmospheric Research                      Boulder, Colorado  USA*

*Collaborator:*
- Michael Montgomery (Colorado State Univ., USA)

- Ron McTaggert-Cowan
*McGill University                                                                     Montreal, Quebec   Canada*

- Lance F. Bosart
*The University at Albany, SUNY                                                     Albany, New York   USA*

## Atlantic Genesis (1975-2005)

- Black = 1975-1993    Red = 1994-2005

## Shear exists in composite of developing depressions over Western Atlantic.

- Bracken and Bosart (2000) *Monthly Weather Review*

*Shear exists in composite of developing depressions over Western Atlantic.*

## Tropical Transition

- Subtropical or Extra-tropical precursors
- Cold-core upper tropospheric lows
- Fronts
- Mesoscale Convective Vortices
- Initially cold-core (vorticity maximizes at 700 hPa or above)
- May involve easterly waves
- Relies on finite vertical wind shear
- Diabatic-baroclinic development (strong precursor)
- Organization of convection (weak precursor)

## Atlantic Tropical Transitions by Season (defined as having non-tropical origin)

| Year | TTs | Other |
| --- | --- | --- |
| 2000 | 4 | 9 |
| 2001 | 8 | 7 |
| 2002 | 9 | 3 |
| 2003 | 6 | 9 |
| 2004 | 5 | 10 |
| 2005 | 8 | 19 |
| total | 40 | 57 |

## MM5 Simulation of Michael (2000)

- Potential Vorticity (PVU) and Wind on 340 K  Sfc.
- Sea-level Pressure
- Start: 00 UTC 15 Oct. 2000
- End: 00 UTC 17 Oct. 2000

## MM5 Simulation of Hurricane Michael (2000)

- Nested 12.3-km grid
- Cold start from global analysis (NCEP)
- 48-h integration
- Kain-Fritsch CP
- Blackadar PBL
- Schultz Cloud Physics

*Davis and Bosart, 2003: Monthly Weather Review*

## PV and Wind on 340 K Isentropic Surface

- 12 UTC 9 Sept

- 00 UTC 17 Sept

- 12 UTC 14 Sept

- 00 UTC 14 Sept

## Pre-Humberto Vortex

- Vortex centroid location on 340 K sfc.

- 1000 km

- Images obtained from *http://weather.unisys.com/archive/sat_ir/*

- IR Satellite Images 12Z 16 Sept. to 00Z 19 Sept. 2001

- 340 K PV at 00 UTC 19 Sept.

## Modification of Wind Profile

- y

- x

- z

- “Weak” PV

-  sfc

- p1

- p2

- p3

## Slower Development Without Shear

- CTRL: heavy   Weak PV: thin
- U: solid          V: dashed

- Wind Profiles prior to convection

*CTRL simulation is initialized with more shear on flank of upper-tropospheric cold low*

- Vorticity time-series

## What About 2005?

## Franklin

## Harvey

## Maria

## Nate

## Vince

## TT ?

- Katrina

- Rita

## I T C Z ?

- Emily

- Lee

- Irene

- Beta

## Cindy

- Stan

- Jose

- Wave Accumulation?
- dU/dx < 0

## Western Atlantic Genesis Issues

- Large-scale influences (waves, troughs)
- Mesoscale (MCSs, MCVs, fronts)
- Convective scale (convective bursts, VHTs)
- Numerical prediction capability
- Critical observations?

## WATTAGE                                                Western Atlantic Tropical Transition and Genesis Experiment (2008)

- 100-200 km

- 300-400 km

- (a)

- (c)

- (b)

- (d)

**NCAR G-V**
**NOAA G-IV**
**NASA DC-8**

**NCAR C-130**
**NRL P-3**

**NOAA P-3**
**UAVs**

## Conclusions

- Key Points about Tropical Transition

- Vertical shear organizes convection => vorticity production
- Convection decreases shear => warm core formation
- TT can occur with easterly waves: issue is shear => upper trough, lower tropospheric mesoscale ascent

- General Points

- Broad spectrum of genesis mechanisms, strong hints apparent in analyses
*Wave genesis*
*TT*
*ITCZ *
- Multi-scale observations crucial for sorting out genesis mechanisms and improving prediction