Numerical Simulations of the Extratropical Transition of Floyd 1999: Structural Evolution and Respon

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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
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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 Floyds evolution along the
  East Coast and the mechanisms for the heavy
  rainfall over southern New England, where 2040
  cm fell in 1218 h across northern New Jersey,
  southeastern New York, and central Connecticut.

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1 environmental equatorward flow of cooler,
drier air2 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 circulation4 ascent of warm,
moist inflow over tilted isentropic surfaces
associated with baroclinic zone (dashed line) in
middle and lower panels5 ascent (undercut by
dry-adiabatic descent) that produces cloudbands
wrapping westward and equatorward around the
storm center6 cirrus shield with a sharp cloud
edge if confluent with polar jet.
Klein et al. (2000)
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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
1999/09/16_0000 UTC
Surface
6
1999/09/17_0000 UTC
500 hPa
NJ New Jersey
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1999/09/17_0000 UTC
Surface
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1999/09/17_0000 UTCcentral pressure of 979 hPa
1999/09/16_0000 UTCcentral pressure of 951 hPa
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NJ New Jersey
1999/09/16_2100 UTC
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1999/09/16_1930 UTC
b. Mesoscale analysis
1999/09/16_2130 UTC
1999/09/16_2330 UTC
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1999/09/16_0600 1999/09/17_0600 UTC
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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 s 33 layers(full s)
Initial dataNCEP 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)
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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.
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Sea level presure
36 km domain
1999/09/17_0000 UTC
500 hPa hieght
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1999/09/16_0300 UTC
1999/09/16_1500 UTC
1999/09/17_0600 UTC
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c. Simulated mesoscale evolution of Floyd
12 km domain
1999/09/16_1500 UTC
1999/09/16_2100 UTC
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1999/09/16_1930 UTC
Observation of Radar
1999/09/16_2130 UTC
1999/09/16_2330 UTC
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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).
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(No Transcript)
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d. Precipitation verification
The model precipitation was interpolated to
observation stations using the Cressman (1959)
method
Pn is the model precipitation at the four model
grid points surrounding the observation.
The weight Wn 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.
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1999/09/16_0600 09/17_0600 UTC
shaded light 200250 mm medium 250350 mm
dark gt 350 mmlt 100 gt 130
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e. Frontogenesis calculations
1999/09/16_0000 UTC
Surface
500 hPa
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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.
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1999/09/16_1200 UTC for 500 hPa from 36 km domain
PA PennsylvaniaVA Virginia
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1999/09/16_1800 UTC
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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.
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Precipitation between 1999/09/16_0600 17_0600
UTC from 1.33 km domain
CTL
NOTER
shaded light 200250 mm medium 250350 mm
dark gt 350 mm
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b. No latent heating/cooling experiments
NOLH_sfc.
CTL_sfc.
-25 hPa
1999/09/17_0000 UTC
NOLH_500 hPa
CTL_500 hPa
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CTL
NOEVAP
1999/09/16_2100 UTC from 12 km domain
-5 hPa
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c. No surface heat flux experiment
CTL
NOHFLX
1999/09/16_2100 UTC from 12 km domain
-4 hPa
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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 Floyds pressure and temperature evolution and
  only a 1030 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 Floyds
  weakening after landfall and enhancing the
  frontogenetical circulations near the coast.