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External Influences on Cyclone Formation

External Influences on Cyclone Formation. Working Group 2.1 W. M. Frank, G. J. Holland, P. Klotzbach, J. L. McBride, P. E. Roundy Contributions: J. Molinari. Two Scales of Genesis. Individual Storm Formation Climatological Variations

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External Influences on Cyclone Formation

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  1. External Influences on Cyclone Formation Working Group 2.1 W. M.Frank, G. J. Holland, P. Klotzbach, J. L. McBride, P. E. Roundy Contributions: J. Molinari.

  2. Two Scales of Genesis Individual Storm Formation Climatological Variations e.g. – interannual variability, global change

  3. Climatological Factors • Warm ocean (SST of 26.5 C or greater) • Persistent deep convection and upward motion • Enhanced low-level cyclonic vorticity • Weak vertical shear directly over the center • Enhanced low-level moisture

  4. Formation of Individual Storms • Climatological conditions favorable for growth • Mean flow may or may not be unstable (e.g. – ITCZ breakdown) • Transient perturbations usually speed up the process

  5. Recent Research Areas • Individual Genesis Events Tropical waves Basin genesis parameters Synoptic-scale analyses • Climatological Variations in Storm Numbers Interannual variability Climate change

  6. Example Genesis Parameter – N. Atlantic Based on: vertical shear, convective instability, mid-level moisture DeMaria et al. (2001)

  7. Spectral Bands for Filtering 0.5 2.0 2.22 h=50m h=50m 2.50 0.4 3 days 2.86 0.3 3.33 4.0 TD-type Period (Days) h=12m Kelvin Frequency (CPD) 0.2 5.0 MRG 6 days 6.67 10.0 0.1 h=50m h=12m ER h=12m 20.0 30 days MJO-ISOe 0 -14 -10 -6 -2 0 2 6 10 14 Westward Zonal Wavenumber Eastward *Adapted from Wheeler and Kiladis, 1999

  8. Total Wind Fields, Westerly Phase of MJO, NWP (composite of 20 strong cases) Maloney and Hartman (2001)

  9. Genesis and ER Waves in the South Indian Ocean Bessafi and Wheeler (2006)

  10. Series of TC formations in the NW Pacific that result from a train of MRG waves interacting with the large-scale flow. Dickinson and Molinari (2002)

  11. Continued

  12. NW Pacific Composites Frank and Roundy (2006)

  13. Implications for Forecasts Consistent phase relationship suggests waves are modulating genesis Anomalies can be detected in advance of genesis by at least: ISOe: 13-20 days ER: 7-20 days MRG/TD: 3-6 days Kelvin: 3-7 days

  14. Statistical forecast of genesis probability, 15-day lead time Paul Roundy - http://www.atmos.albany.edu/facstaff/roundy/tcforecast/tcforecast.html

  15. Relationships Between Genesis and Basin-Scale Variations • Several studies show apparent long-term trends in cyclone activity and/or intensity • There are also long-term trends in some TC-related climatological variables (e.g. – SST) • Relationships between large-scale/long-term variables and the occurrence of local, necessary conditions for genesis need to be explored.

  16. Interannual Variability • There are persistent patterns of correlation between annual storm numbers in different basins. • These patterns are related to ENSO, and perhaps to other large-scale circulation modes. • There is no discernable tendency for inter-basin compensation in terms of storm numbers or storm days. • There is some suggestion that storm activity for Category 4-5 storms tends to be positively correlated between basins. Frank and Young (2006)

  17. Summary, Storm-Scale Predicting genesis requires accurate synoptic-scale (mesoscale?) forecasts of the wind and moisture fields at the time and place of storm formation. Longer range genesis predictions are very difficult in the tropics using current deterministic models. There is strong evidence that tropical waves play a major role in triggering genesis in all cyclone basins.

  18. Summary, cont. Tropical waves are predictable using statistical techniques. Other statistical genesis prediction techniques are also showing promise. Combined statistical/deterministic forecasts offer promise for forecasts of genesis with lead times of up to the time-scale of the MJO.

  19. Summary, Longer-Term Genesis Variability Long-term variability of tropical cyclone formation depends on the long-term variability of the occurrence of local genesis conditions. The missing link is to determine how long-term variations in large-scale fields (SST, winds, etc.) are related to the frequency of occurrence of local necessary conditions for genesis.

  20. Recommendations • Experiment with statistical techniques for longer-range genesis forecasts. • Improve understanding of the relationships between basin-scale variability and the incidence of genesis conditions.

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