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, /07/13

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.

Klein et al. (2000) 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.

Observation analysis a. Synoptic-scale evolution Hurricane Floyd at 1999/09/16_0000 UTC FL: Florida AL: Alabama GA: Georgia SC: South Carolina NC: North Carolina GA SC AL FL NC 500 hPa

FL 1999/09/16_0000 UTC NC Surface

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

1999/09/17_0000 UTC NC NJ Surface

1999/09/16_0000 UTC central pressure of 951 hPa 1999/09/17_0000 UTC central pressure of 979 hPa

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

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

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

Model simulation of the Floyd transition The MM5(version 2.12) was used. a. Model description 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) 1999/09/16_0000 UTC

b. Large-scale verification and evolution of Floyd 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. 951 hPa 976 hPa 980 hPa

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

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

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

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

1.33 km domain 1999/09/16_1930 UTC1999/09/16_2130 UTC1999/09/16_2330 UTC At 1 km above sea level (ASL).

d. Precipitation verification The model precipitation was interpolated to observation stations using the Cressman (1959) method P n is the model precipitation at the four model grid points surrounding the observation. The weight W n given 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.

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

e. Frontogenesis calculations Surface500 hPa 1999/09/16_0000 UTC

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.

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

1999/09/16_1800 UTC

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

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

b. No latent heating/cooling experiments NOLH_sfc. CTL_sfc. 1999/09/17_0000 UTC -25 hPa NOLH_500 hPaCTL_500 hPa

CTLNOEVAP 1999/09/16_2100 UTC from 12 km domain -5 hPa

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

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.