Thermodynamic Structure of Tropical Cyclones From Aircraft Reconnaissance

1. Thermodynamic Structure of Tropical Cyclones From Aircraft Reconnaissance Kay Shelton MS Thesis Presentation

2. Thesis Outline • Composite • Created composite radial and vertical profiles of qe categorised by storm intensity • Bret (1999) • Case study of the qe evolution in a TD–H4 storm • Claudette (2003) • Detailed case study of storm under strong shear • System very short-lived as a hurricane

3. Thesis Outline • Composite • Created composite radial and vertical profiles of qe categorised by storm intensity • Bret (1999) • Case study of the qe evolution in a TD–H4 storm • Claudette (2003) • Detailed case study of storm under strong shear • System very short-lived as a hurricane • How did Claudette intensify to a hurricane under strong shear? • Why was the hurricane phase so short-lived?

4. TC Genesis and Intensification • Mid-level moistening • Emanuel (1995), Bister and Emanuel (1997) • Moistening in the mid-troposphere helps to eliminate cold downdrafts reaching the surface • Vertical Wind Shear • Vertical shear is a negative factor • Downshear vortex reformation, eg. Hurricane Danny (1997) and Hurricane Gabrielle (2001) [Molinari et al 2004,2005]

5. WISHE Hypothesis • Proposed by Emanuel (1986) • Assumes a pre-existing axisymmetric (surface concentrated) disturbance in symmetric neutrality • Feedback mechanism between maximum surface winds and surface fluxes • Smith (2003) – modelled the boundary layer for an axisymmetric hurricane • Cold downdrafts not included • Calculated radial profiles of qe and latent and sensible heat fluxes • Results support WISHE hypothesis • Neither of the assumptions for WISHE are usually met in early stages of TC development

6. Pre-WISHE Hypothesis • Proposed by Molinari et al (2004) • Can have wind shear acting to create strong asymmetries, strong buoyant convection and cold downdrafts occurring • But, via • i) vortex merger processes, vorticity can be axisymmetrised • ii) convective mixing, profile becomes more nearly neutral and cold downdrafts are eliminated • System can approach a state where WISHE is valid and a hurricane can form

7. Background - Shear • Shear tilts vortex • Enhanced convection and maximum upward motion expected downshear and downshear-left • Dynamically induced downdrafts upshear cover a large area • Frank and Ritchie (2001) • Corbosiero and Molinari (2002) • Jones (1995) & Reasor et al (2004) From Eastin et al (2005) 

8. Claudette (2003) – Case Study • USAF reconnaissance data • Flight level observations • Dropsondes • IR, visible and microwave satellite imagery • Gridded ECMWF (1.125ox1.125o) analyses • NHC Best Track (modified using reconnaissance centre fixes)

9.  Track SSTs in central/western Caribbean: 26-28oC Case Study period H1 (12 UTC/10th July)  Pressure and Windspeed  Very rapid pressure change at H1 time

10. 850-200hPa Shear Storm reaches hurricane intensity when shear is close to its maximum and still increasing. A large portion of the storm’s life occurs with shear greater than 12.5ms-1, which is considered a cut-off value, above which storm’s can no longer sustain themselves (Zehr, 1992) H1  

11. IR Animation

12. Flight 6 - Composite Observations 400km • 850hPa • ~ 3 hours of observations • Centred on 0715 UTC on 10th July • 5 hours prior to H1 time Closed circulation is very small scale and located under deep convection. Larger-scale wave structure evident.

13. Fl. 6 Pass 1 – Centre Cross-section qe Windspeed T, Td 50m D-value     SW S E NE SW S E NE

14. Fl. 7 Pass 1 – Centre Cross-section qe Windspeed T, Td 145m 9oC D-value     WSW SW SSW S-N E SE WSW SSW SW E SE S-N

15. Surface Observation • At time of H1 • No eye seen in IR image • Dropsonde (cyan) • 1203 UTC • 850hPa winds (white) • 1131-1208 UTC • 700hPa winds (black) • 1209-1300 UTC • No vortex tilt from surface to 850hPa

16. Flight 7 - Composite Observations 400km • 700hPa • ~ 4.5 hours of observations • Centred on 1415 UTC on 10th July • 2 hours after Claudette named H1 Small-scale vortex embedded in larger-scale wave Embedded circulation under deep convection

17. Fl. 7 Pass 2 – Centre Cross-section qe Windspeed T, Td 100m 7oC D-value SW NE NE SW

18. 1330UTC/10th – SSM/I 85GHz HW Good for weak TCs Sensitive to warm precip. 85GHz PCT Isolates convection Storm centre

19. 1456UTC/10th – TRMM 85GHz – Colour Composite Cloud free/dry – grey Low level clouds – blue/green Deep convection - red 85GHz H Sensitive to ice phase Shows upper levels (5-9km)

20. Intensification Summary • Repeated convective pulses over centre • Initially a very small vortex forms embedded within a larger-scale wave • Vortex remains aligned from surface to 850hPa • Vortex deepens and eyewall structure begins to develop • High qe and winds clearly define eyewall structure • Dry air seen persistently upshear at 700hPa and moist air remains downshear • Hurricane strength reached at time of near-maximum and increasing shear • In which case, why did the convective pulses occur over the centre?

21. Fl. 7 Pass 3 – Centre Cross-section qe Windspeed T, Td 7oC 100m 7oC D-value NW SE SE NW

22. Fl. 7 Pass 4 – Centre Cross-section qe Windspeed T, Td 12oC D-value   SW S SSE N NNE N SW SSE NNE N N S

23. Weakening Summary • Dry air upshear and also to the left and right of the shear vector persistent at 700hPa • Strong small-scale circulation exists until 1530UTC • No evidence of this strong circulation or a D-value depression in Pass 4 • The dry air in three quadrants of the storm could possibly have allowed cold downdrafts to occur, acting to weaken the storm • Also the advection of the mid-level dry air around the storm by its own circulation could have brought about the weakening

24. Shear Shear Circulation spins up DRY MOIST DRY MOIST Dry air not advected by weak circulation Strong circulation advects dry air downshear Comparison with Theories 1. Mid-level moistening or rather, the lack of: • Why wasn’t this a factor in the spin-up phase of Claudette and yet, so instrumental in the weakening 2. Vertical wind shear: • This creates the dry/moist anomalies and drives the downshear convection • But, it is seemingly not a negative factor for this case • Why did the storm form if shear was so high?

25. Comparison with Theories 3. WISHE and Pre-WISHE: • Vortex interactions and convection create a symmetric, neutral system • But, multiple vorticies are not found in Claudette • The qe profile is not WISHE-like, and yet a hurricane develops 4. How did this hurricane form when all the conventional theories suggest it should not have?

26. Z Shear 700hPa A B C Surface Hypothesis • A: 700hPa circulation located upshear of surface circulation. Shear advects 700hPa circulation over surface circulation • B: Circulations are vertically aligned, convection breaks out  rapid intensification • C: Shear advects 700hPa circulation further downshear, circulations decouple  system weakens

27. Conclusions • Although Claudette did manage to become a hurricane, it was never going survive for long • How many more of these embedded, mini-hurricanes occur? • The flight level and dropsonde data available from the USAF from their missions into developing TCs has never been so abundant. If this type of system is fairly common, NHC forecasters will have to address the issue of being able to predict such small, short-lived hurricanes.

28. Acknowledgements • John Molinari • Dave Vollaro • Chris Thorncroft and the rest of the Faculty • Kevin Tyle and Dave Knight • Celeste, Diana, Sharon, Lynn and Sally • Kristen Corbosiero, Anantha Aiyyer and Eyad Atallah • Grad students past and present • Gareth Berry