Sensitivity of airsea exchange coefficients Cd and Ch on hurricane size and intensity of HWRF

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Sensitivity of air-sea exchange coefficients (Cd
and Ch) on hurricane size and intensity of HWRF
Young Kwon and Robert Tuleya EMC/NCEP/NWS/NOAA HFI
P Regional model physics team
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• Motivations
• Intensity skill of HWRF is not as good as track
  forecast skill (sometimes worse than statistical
  models).
• Part of the poor intensity forecast skill might
  result from incorrect wind-pressure relationship
  of HWRF.
• Subjective verification indicates that HWRF has
  the tendency of producing a larger storm with
  time, and this tendency seems to cause the wrong
  wind-pressure relationship. (wind speed is
  proportional to dp not p).
• The goal of this work is to improve the intensity
  forecast skill of HWRF by correcting storm size
  and pressure-wind relationship with tuning Cd and
  Ch.

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Hurricane Katrina (2005082600) Simulation Result
(HWRF and GFDL)
P 939.7hPa
P 943.5hPa
W 58.4 m/s
W 74.1 m/s
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• Motivations
• Intensity skill of HWRF is not as good as track
  forecast skill (sometimes worse than statistical
  models).
• Part of the poor intensity forecast skill might
  result from incorrect wind-pressure relationship
  of HWRF.
• Subjective verification indicates that HWRF has
  the tendency of producing a larger storm with
  time, and this tendency seems to cause the wrong
  wind-pressure relationship. (wind speed is
  proportional to dp not p).
• The goal of this work is to improve the intensity
  forecast skill of HWRF by correcting storm size
  and pressure-wind relationship with tuning Cd and
  Ch.

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Background
Hurricane intensity is proportional to
Emanuel (1995)
Besides intensity, the size of storm might also
depends on surface exchange coefficients
(espeially Cd)
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Upper level outflow
H
Energy gain from sea surface (sensible and
latent heat) Ch
Low level inflow
Energy loss by surface friction Cd
OCEAN
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Method and Case

• 1. Change HWRF surface physics code in order to
  use prescribed Cd and Ch separately over the
  ocean.
• 2. Conduct experiments using fixed Ch/various Cd
  in order to examine the sensitivity of Cd on
  storm size and intensity forecast skill.
• Conduct experiment as in 3. except fixing Cd but
  varying Ch.
• Case for this study
• Hurricane Hanna(2009.08.30.12 UTC)
• Stays in the Ocean most of time, positive
  intensity bias

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Preliminary Results
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24hr Forecast (MSLP and 850mb wind speed)
Lin cd
H209
980mb 38.2m/s
980mb 37.8m/s
Inc cd
Dec cd
980mb 34.6m/s
981mb 40.7m/s
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72hr Forecast (MSLP and 850mb wind speed)
Lin cd
H209
942mb 65.6m/s
938mb 59.4m/s
Inc cd
Dec cd
956mb 54.7m/s
942mb 65.6m/s
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Future plan

• Conduct experiments using more Cd profiles with
  cycled simulation
• After investigating the sensitivity of Cd to
  hurricane, examine the Ch sensitivity to
  hurricane forecast skill with Cd value fixed
• Find the optimum combination of Cd/Ch values in
  order to produce most accurate HWRFs intensity
  and track forecast
• Tune the surface exchange coefficients with
  inclusion of sea spray parameterization

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Spray effects on Cd and Ch
Ch
Exchange Coeff.
Cd
Wind Speed