WRF Model Forecast Track Sensitivities of Tropical Cyclone Ernesto (2006) to Various Parameterizations and Grid Spacing

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WRF Model Forecast Track Sensitivities of
Tropical Cyclone Ernesto (2006) to Various
Parameterizations and Grid Spacing
Nick P. Bassill Michael C. Morgan 2006
This work was supported by the National Science
Foundation Grant ATM-0125169
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National Hurricane Center Discussion for Ernesto
from 15 UTC 26 August 2006

• THE FORECAST TRACK IS MORE PROBLEMATIC AFTER 72
  HR. THE LARGE-SCALE MODELS AGREE THAT THE
  MID-LEVEL RIDGE OVER THE GULF OF MEXICO WILL
  WEAKEN AS A SHORTWAVE TROUGH DIGS SOUTHEASTWARD
  THROUGH THE MISSISSIPPI VALLEY. HOWEVER ... THERE
  ARE DIFFERENCES IN HOW MUCH WEAKENING WILL OCCUR.
  THE ECMWF AND NOGAPS CALL FOR ERNESTO TO RECURVE
  OVER THE EASTERN GULF OF MEXICO ... WHILE THE
  UKMET SHOWS ENOUGH RIDGE TO KEEP ERNESTO MOVING
  WEST-NORTHWESTWARD

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WRF Model Configuration

• ARW core (January 2006 release)
• YSU boundary layer scheme (bl_pbl_physics1)
• 2nd order diffusion (diff_opt1, km_opt4)
• Rapid radiative transfer model (RRTM) longwave
  radiation scheme (ra_lw1)
• Dudhia shortwave radiation scheme (ra_sw1)
• Five-layer thermal diffusion land-surface model
  (sf_surface_physics1)
• Monin-Obukhov similarity theory based surface
  layer (sf_sfclay_physics1)

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Changeable WRF Variables

• Grid Spacing
• Horizontal 30km, 45km, or 60km
  
• Vertical 31 or 54 levels
• Cumulus Parameters
• (1) Kain-Fritsch (cu_physics1)
• (2) Betts-Miller-Janjic (2)
• (3) Grell-Devenyi ensemble (3)
• Microphysics Parameters
• (1) Kessler (warm rain) (mp_physics1)
• (2) Eta-Ferrier (time efficient) (5)
• (3) WSM6 (most complex of these) (6)

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Continued

• 180 hour forecasts are created with every
  possible combination (48 forecasts)
• Forecasts are initialized with GFS 0.5 x 0.5
  forecast valid 12 UTC 26 August 2006
• The goal is to attempt to determine the relative
  importance of modifying grid spacings and
  parameterizations
• Any combinations including 54 vertical levels
  and the Betts-Miller-Janjic scheme fail due to
  unknown boundary condition problems

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There are two primary tracks
North across Cuba and Florida
West across the Yucatan Peninsula
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• Both are 30km x 30km x 31 levels and use
  Kain-Fritsch CP
• Blue run uses WSM6 MP
• Red run uses Kessler MP
• Future plots show blue minus red (i.e. west -
  north)

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150
120
90
150
60
120
180
90
60
30
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300-700 hPa Thickness Differences (Blue - Red)
TC John
Fli Version
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West minus North 90 hour total precipitation in
inches
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Difference in 300-700 hPa steering flow (blue -
red)
Average magnitude is 5 knots
Fli Version
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• Is this simply the result of forcing by the
  boundary?
• 9 additional runs were done using
• This larger domain
• 60 km grid spacing
• 31 vertical levels

- All possible combinations of previously used
microphysics and cumulus parameterizations
Answer No, the results show tracks which are
essentially identical to their respective smaller
domain versions
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Observations (1)

• Choice of MPs and CPs are most important in
  determining forecast track
• Choice of horizontal and vertical grid spacing
  modifies intensity, but not track philosophy
• Larger domain does not alter forecast track
  philosophy
• At these grid spacings, the Kessler MP scheme
  performs best
• Westward moving TCs prevent realistic formation
  of Hurricane John
• Eta/Ferrier microphysics scheme is generally
  unable to intensify Ernesto
• Example Average minimum central pressure of
  storms entering the Bay of Campeche

Eta/ Ferrier WSM6 ?SLP
30 km 989 952.5 36.5mb
45 km 1001 981.3 19.7mb
60 km 1002.7 991.3 11.4mb
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• Will a smaller grid spacing allow
• the Eta/Ferrier scheme to intensify the cyclone
  similarly to the other schemes?
• any models the potential to more accurately
  forecast the future path of the cyclone?

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• 4 km grid spacing over this domain
• 31 vertical levels
• No cumulus parameterization

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Observations (2)

• Smaller grid spacing allows WSM6 MP scheme to
  perform best
• Smaller grid spacing does not allow Eta/Ferrier
  scheme to comparably deepen the cyclone

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Future Work

• Use different sets of initial conditions to
  determine if these conclusions remain valid (ex.
  Eta ICs, 1 x 1 GFS ICs, different
  initialization times, etc.)
• Test additional microphysics schemes
• Test these results with other TCs