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T799 October Joint OSSE Nature Run: very preliminary assessment of tropical cyclone activity

A very preliminary assessment of tropical cyclone activity in the Nature Run reveals concerning weaknesses in the African Easterly Jet (AEJ), suspicious tracks in the Atlantic, excessive weak TC activity in the Eastern Pacific, and unsatisfactory structure of intense systems. Further investigation is necessary before adopting this Nature Run for hurricane-focused applications.

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T799 October Joint OSSE Nature Run: very preliminary assessment of tropical cyclone activity

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  1. T799 October Joint OSSE Nature Run: verypreliminary assessment of tropical cyclone activity Oreste Reale Evaluation based on interpolated pressure level fields

  2. African Easterly Jet The AEJ intensity is 40% lower than 2005 NCEP analyses and climatology. In the analyses, the AEJ intensity is strongly controlled and constrained by boundary forcings, and only small interannual variability (1-2m/s) occurs even when northward or southward displacements occur. The weakness of the T799 AEJ is therefore a concern.

  3. African Easterly Jet (cont.)

  4. African Easterly Waves Memory of initialization persists for about 10 days. Then wave speed shows very good correspondence to 2005 analysis (and climatology). However, wave frequency is lower.

  5. Atlantic tropical systems Extremely suspicious eastern Atlantic TC tracks

  6. Strongest `Hurricane’ in the Atlantic Perplexingly large scale, relatively modest intensity. T511 contains equally strong systems, and better confined.

  7. Poor vertical structure Eye-like feature extremely unrealistic, scale resembles diluted vortices typical of much lower resolution models

  8. Eastern Pacific Tropical Systems One relatively strong system; evidence of excessive weak TC activity south of 15N; one nice feature is the orography-induced Hawaian wake

  9. Proliferation of nondeveloping (or weakly developing) tropical depressions over the Eastern TPAC Very large amounts of small unrealistic vortices at a10-15N. They tend to disappear only when a stronger system is present in the basin

  10. Intensity is very modest for the strongest EPAC Tropical Cyclone Wind speed barely reaches 30 m/s

  11. Western-central Pacific Tropical Systems Perplexing lack of strong activity on the western Pacific: however, the intensity of the strongest system is quite good (max of 60 m/s at 900hPa)

  12. Strongest TC in the T799 NR The system is strong, however there is a perplexing mid-tropospheric wind max. The scale is good: the system appears very compact as to be expected at such resolution.

  13. Summary • AEJ is 40% weaker than climatology • Atlantic TC activity contains some highly suspicious tracks • Eastern Pacific seems to present excessive proliferation of weak TCs • The intensity of the strongest ATL systems is not superior to T511 • Different behavior in different basins • Structure of some intense system not very satisfactory in terms of scale and size of eye-like feature

  14. Preliminary conclusions • The increased resolution does not necessarily provide stronger confidence in a much better Nature Run for the tropics • Representation of TC activity does not immediately appear superior to the T511 • Caution should perhaps be used in adopting this NR for applications centered on future instruments targeting hurricanes • Further investigation is needed

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