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Numerical Simulations of Extratropical Transition of Floyd: Structural Evolution and Responsible Mechanisms for Heavy Ra

This study explores the evolution of Hurricane Floyd during its extratropical transition and the mechanisms behind the heavy rainfall over southern New England. Numerical simulations using the MM5 model were conducted to investigate the storm's structure and precipitation distribution. The results were compared with observed data and verified through frontogenesis calculations. The study also includes additional experiments, such as a no-terrain simulation, to further understand the storm's behavior.

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Numerical Simulations of Extratropical Transition of Floyd: Structural Evolution and Responsible Mechanisms for Heavy Ra

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  1. Numerical Simulations of the Extratropical Transition of Floyd (1999): Structural Evolution and Responsible Mechanisms for the Heavy Rainfall over the Northeast United States Brain A. Colle, 2003: Mon. Wea. Rev.,131, 2905-2925. 2004/07/13

  2. Introduction • Tropical cyclones undergoing an extratropical transition (ET) can develop into powerful midlatitude cyclones that cause significant damage from wind and waves in coastal areas. • Although there is no strict definition of an ET, typically such transitions are associated with the development of storm asymmetries in the precipitation, temperature, and wind fields as the cyclone moves toward higher latitudes. • Klein et al. (2000) provide a conceptual model for ET events over the western Pacific that illustrates the development of cloud and precipitation asymmetries as a tropical cyclone interacts with an approaching midlatitude trough. • This paper discusses Floyd’s evolution along the East Coast and the mechanisms for the heavy rainfall over southern New England, where 20–40 cm fell in 12–18 h across northern New Jersey, southeastern New York, and central Connecticut.

  3. 1: environmental equatorward flow of cooler, drier air;2: decreased tropical cyclone convection in the western quadrant (with corresponding dry slot);3: environmental poleward flow of warm, moist air is ingested into tropical cyclone circulation;4: ascent of warm, moist inflow over tilted isentropic surfaces associated with baroclinic zone (dashed line) in middle and lower panels;5: ascent (undercut by dry-adiabatic descent) that produces cloudbands wrapping westward and equatorward around the storm center;6: cirrus shield with a sharp cloud edge if confluent with polar jet. Klein et al. (2000)

  4. SC GA NC AL FL Observation analysis a. Synoptic-scale evolution Hurricane Floyd at 1999/09/16_0000 UTC 500 hPa FL: FloridaAL: AlabamaGA: GeorgiaSC: South CarolinaNC: North Carolina

  5. NC FL 1999/09/16_0000 UTC Surface

  6. NJ NC 1999/09/17_0000 UTC 500 hPa NJ: New Jersey

  7. NJ NC 1999/09/17_0000 UTC Surface

  8. 1999/09/17_0000 UTCcentral pressure of 979 hPa 1999/09/16_0000 UTCcentral pressure of 951 hPa

  9. NJ NJ: New Jersey 1999/09/16_2100 UTC

  10. 1999/09/16_1930 UTC b. Mesoscale analysis 1999/09/16_2130 UTC 1999/09/16_2330 UTC

  11. 1999/09/16_0600 ~ 1999/09/17_0600 UTC

  12. Model simulation of the Floyd transition a. Model description • The MM5(version 2.12) was used. 1999/09/16_0000 UTC D1: 36 km D2: 12 km D3: 4 km D4: 1.33 km σ= 33 layers(full σ) Initial data:NCEP Eta Model 221 grids (32 km grid spacing)SST data: used Navy OIST (~ 30 km grid spacing)Microphysics scheme: Reisner et all. (1998)Cumulus parameterization: Grell et al. (1994)PBL parameterization: MRF scheme (Hong and Pan 1996)

  13. b. Large-scale verification and evolution of Floyd 980hPa 976hPa 951hPa The goal of this study was to document the larger-scale changes in storm structure and the developing mesoscale precipitation and temperature distributions across southern New England.

  14. Sea level presure 36 km domain 1999/09/17_0000 UTC 500 hPa hieght

  15. 1999/09/16_0300 UTC 1999/09/16_1500 UTC 1999/09/17_0600 UTC

  16. c. Simulated mesoscale evolution of Floyd 12 km domain 1999/09/16_1500 UTC 1999/09/16_2100 UTC

  17. 1999/09/16_1930 UTC Observation of Radar 1999/09/16_2130 UTC 1999/09/16_2330 UTC

  18. 1.33 km domain 1999/09/16_1930 UTC 1999/09/16_2130 UTC 1999/09/16_2330 UTC At 1 km above sea level (ASL).

  19. d. Precipitation verification The model precipitation was interpolated to observation stations using the Cressman (1959) method Pnis the model precipitation at the four model grid points surrounding the observation. The weight Wngiven to the surrounding gridpoint values is given by R is the model horizontal grid spacing,D is the horizontal distance from the model grid point to the observation station.

  20. 1999/09/16_0600 ~ 09/17_0600 UTC shaded light: 200~250 mm medium: 250~350 mm dark: > 350 mm< 100 % > 130 %

  21. e. Frontogenesis calculations 1999/09/16_0000 UTC Surface 500 hPa

  22. The Miller (1948) frontogenesis equation was calculated on pressure levels. The scalar frontogenesis, which is defined as the Lagrangian rate of change of the horizontal temperature gradient, can be written A: includes the deformation and divergence effects, which together are important in driving an ageostrophic direct circulation, B: the vertical circulation can change the horizontal temperature gradient through the tilting process,C: the differential diabatic processes, includes only heating/cooling from precipitation output from the model.

  23. PA VA 1999/09/16_1200 UTC for 500 hPa from 36 km domain PA: PennsylvaniaVA: Virginia

  24. 1999/09/16_1800 UTC

  25. Discussion and additional experiments a. No terrain experiment 1999/09/16_2100 UTC from 12 km domain CTL NOTER The Appalachians and coast hills over the eastern United States were replaced by flat land at sea level.

  26. Precipitation between 1999/09/16_0600 ~ 17_0600 UTC from 1.33 km domain CTL NOTER shaded light: 200~250 mm medium: 250~350 mm dark: > 350 mm

  27. b. No latent heating/cooling experiments NOLH_sfc. CTL_sfc. -25 hPa 1999/09/17_0000 UTC CTL_500 hPa NOLH_500 hPa

  28. CTL NOEVAP 1999/09/16_2100 UTC from 12 km domain -5 hPa

  29. c. No surface heat flux experiment CTL NOHFLX 1999/09/16_2100 UTC from 12 km domain -4 hPa

  30. Summary • The operational NCEP models performed poorly for this event; therefore, an understanding of the mechanisms for the heavy rainfall and physical sensitivities within the models is important for forecasting. • The MM5 at 4- and 1.33-km grid spacing was able to realistically reproduce the narrow and intense band of precipitation that developed just inland of the coast over southern New England. • A separate simulation without the Appalachians and the coastal terrain resulted in little change in Floyd’s pressure and temperature evolution and only a 10%–30% reduction in precipitation over some upslope areas; therefore, terrain played a secondary role in the devastating flooding for this particular event. • The experiments with no latent heating, evaporation, and surface fluxes illustrate the importance of diabatic effects in slowing Floyd’s weakening after landfall and enhancing the frontogenetical circulations near the coast.

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