High Resolution Simulations of Floyd (1999): Structural Evolution and Responsible Mechanisms for the Heavy Rainfall over the Northeast U.S.        Dr. Brian A. Colle Institute for Terrestrial and Planetary Atmospheres Marine Sciences Research Center Stony Brook University / SUNY

989 mb Eta Eta track

0000 UTC 16 Sept 1999 Surface 500 mb

(a) Observed (in cm) OBS Total 32-km Eta Eta Total (init 09/16/00Z)

Motivational Questions How well can the Penn State/NCAR Mesoscale Modeling System (MM5) simulate the Floyd (1999) event? What processes led to the narrow and intense band of precipitation just inland of the coast? What was the role of the inland hills and Appalachians on the mesoscale precipitation distribution and amounts? What role did the diabatic effects (surface heat fluxes and latent heating) have on the Floyd evolution? What are the uncertainties in the model physical parameterizations associated with the Floyd event?

MM5 Initialized: 0000 UTC 9/16/1999 36 km domain MM5 physics: MRF PBL, Grell CP, Reisner2 microphysics, Dudhia radiation

500-m MM5 Frontogenesis (oC 100 km-1 h-1) at 1800 UTC

Surface: 21Z 9/16/1999

1.33 km MM5 (21 h) WSR-88D: 21 UTC 9/16/99 A B

B A

at 500 m

Conclusions The MM5 initialized at 0000 UTC 16 Sept 1999 was able to realistically simulate the extratropical transition of Floyd. The 1.3-km MM5 captured the heavy precipitation, but overpredicted the amounts by 30-40% on average. A deep frontogenetical circulation combined with moist symmetric instability at low-level levels and slantwise neutrality aloft resulted in an intense band of precipitation just inland of the coast. The terrain played a secondary role in the heavy precipitation. Surface fluxes and especially latent heating were important in maintaining Floyd’s strength as it transitioned to extratropical. The cyclone strength was important in enhancing the deformation frontogenesis just inland of the coast. There were large sensitivities with model moist and PBL physics as well as SST. In particular, the BMJ scheme resulted in a much weaker cyclone and an underprediction of precipitation.

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