Tropical Cyclone Formation and Extratropical Transition IWTC – V Recommendations

1. Tropical Cyclone Formation and Extratropical TransitionIWTC – V Recommendations • There is a strong need for a consistent definition of tropical cyclone formation such that operational priorities may be satisfied, and high-quality data sets will be defined based on physically relevant formation characteristics • region-independent • benefit the research community for studies that address synoptic-scale and mesoscale environmental influences on tropical cyclone formation plus allow testing of theories of tropical cyclone formation in observation and modeling frameworks • Tropical cyclone formation is dependent on interactions among a variety of space and time scales. There is a strong need to define factors that would provide diagnostic evaluation of potential tropical cyclone formation. • examination of tropical cyclone formation in numerical prediction models • measure of uncertainty in the ability for tropical cyclone formation to be related to large-scale factors in numerical model simulation

2. Tropical Cyclone Formation and Extratropical TransitionIWTC – VI Summary • Two-stage process (Zehr 1993, Gray 1998, Karyampudi and Pierce 2000) • (I) Preconditioning via sub-synoptic (synoptic-) scale organization of an environment favorable for genesis [EXTERNAL] • (II) Concentration of environment vorticity by the mesoscale [INTERNAL] • Kinematic – establish low-level cyclonic vorticity • Thermodynamic – establish a moist-neutral environment, downdraft-free convection

3. Synoptic-scale [External] InfluencesStage I • Tropical waves that propagate zonally provide local flow perturbations that contribute to a favorable environment for tropical cyclone formation via enhancement of • Vertical motion • Low-level vorticity • Changing vertical wind shear • Most tropical cyclones form under the influence of wave circulations. • If the waves could be forecast (with demonstrable skill), genesis could be forecast. 1200 UTC 07 Oct 2002

4. Synoptic-scale [External] InfluencesStage I • Such predictions are very difficult in the tropics using current deterministic models. • Tropical waves are predictable using statistical techniques. Other statistical genesis prediction techniques are also showing promise. • Therefore, combined statistical/deterministic forecasts offer promise for forecasts of genesis. Possible lead times of up to the time-scale of the MJO. 1200 UTC 07 Oct 2002

5. Mesoscale Organization [Internal] InfluencesStage II Bottom Up Top Down

6. TD BU Top-Down vs. Bottom-Up

7. TD BU Top-Down vs. Bottom-Up

8. TD BU Top-Down vs. Bottom-Up

9. TD BU Top-Down vs. Bottom-Up

10. TD BU Top-Down vs. Bottom-Up

11. Mesoscale Organization [Internal] InfluencesStage II • Issues associated with the relative roles of stratiform and convective-type vortex enhancement • Establishment of downdraft-free convection • Vortex hot tower contribution to the establishment of a moist-neutral environment • Necessity of these conditions? • Factors that impact generation of sufficient vortex hot tower activity • Scales: pre-genesis clusters are observed on the scale of 100 km. • Are they comprised of downdraft-free convection? • What mechanisms are responsible for these conditions? • Why do most fail to warm core, surface-concentrated vortices?

12. Stages I and II • Genesis process is driven by large-scale dynamics • Given proper initialization and representation of the large-scale environment (Gray’s necessary genesis conditions) models should be able to simulate the genesis process. • Contains the dynamical forcing that initiates convection

13. Forecast Issues • Diagnosis of tropical cyclone formation based on the 850 to 500 hPa wind profiles. • Cases of operational global model success • Favorable large-scale environment • Tropical waves • MJO • Cases of operational global model failures • Smaller scale, external influences • Trough intrusions • Low shear • Differential steering • Major factor is to identify role of large scale versus mesoscale processes

14. Formation Alert: 0600 UTC 4 July 2006 First Forecast: 0800 UTC 8 July 2006 Images from: http://www.nrlmry.navy.mil/sat_products.html

15. All 2005 Atlantic Tropical Cyclones through Alpha (excluding Vince) Too strong Too weak Pasch et al. 2006, AMS Conference on Hurricanes and Tropical Meteorology

16. UKM: slightly better detection relative to vorticity threshold GFS: equal detection relative to dynamic and thermodynamic parameters NOGAPS: rarely exceeds vorticity threshold, but does exceed thermodynamic threshold Pasch et al. 2006 AMS

17. Extratropical TransitionIWTC-V Summary • IWTC-V recognises that a consistent definition of extratropical transition (ET) of a tropical cyclone does not exist and recommends that WMO support the development of an operationally and physically consistent definition for ET for use by the operational and research communities, and that this ET definition be presented at IWTC-VI. This should include conceptual models of wind, precipitation and ocean surface wave distributions. • To advance the understanding of the process of extratropical transition, it is recommended that a comprehensive research program be developed including the use of existing data sets, and field experiments in co-operation with existing programs such as THORPEX • Precipitation • Expansion of wind field • Ocean wave fields • Role of the ocean in ET

18. Extratropical TransitionIWTC-V SummaryObservations and Forecasts • Timing of extratropical transition: • Phase with the midlatitude trough • Structural changes • Physical processes • Important observations • Impacts on forecasts of downstream weather • Re-intensification? • Dissipation?

19. Timeline of events during Extratropical Transition over the western North Pacific Transformation Re-Intensification 993 mb +/- 7 mb Little or no re-intensification (SLP > 1000 mb) Moderate re-intensification (980 mb < SLP < 999 mb 976 mb +/- 18 mb SLP (mb) Deep re-intensification, including rapid deepening (SLP < 980 mb) 957 mb +/- 25 mb Step 1 Step 2 Step 3 End ET 0 30 46 81 Time (h) Adapted from Klein et al. (2000) Weather and Forecasting

20. Hurricane Floyd 0000 UTC 16 Sep 1999 Deep Re-intensification 10-5 s-1 10-10 K m-1 s-1

21. 10-10 K m-1 s-1 Hurricane Fran 0000 UTC 06 Sep 1996 Dissipation 10-5 s-1 10-10 K m-1 s-1

22. Hurricane Bonnie 1200 UTC 26 Aug 1998 Moderate Re-intensification 10-5 s-1 10-10 K m-1 s-1

23. Extratropical TransitionPhysical Processes and Downstream Impacts • Scale of physical processes involved in the ET process ranges from microscale to planetary scale – most are associated with phase changes and are difficult to observe/model/diagnose • There are variety of physical processes that contribute to the sensitivity of the downstream response to the TC and the upstream state during ET?

24. GFS ensemble members +00 0000 UTC 16 Sep 2003 500 hPa height (m) at a 240 m interval Hurricane Isabel GFS 500 hPa ensemble +108 h VT 1200 UTC 20 Sep 03

25. Extratropical Transition as Tropical/Extratropical Interaction • Role of the tropical cyclone structure • prior to recurvature • at recurvature • following recurvature • What must be observed (space, time, parameter) to increase predictability of • important weather parameters? • downstream impacts? • Role in seasonal variability (Hart 2006 AMS Conference)

26. Summary • Progress • Observational capabilities • Operational Model capabilities • Recognition of the importance of scale interactions • Requirements • Explaining variability • Continued analysis of scale interactions • Definitions