### Abstract

Tropical cyclones (TCs) are strongly influenced by fluxes of momentum and moist enthalpy across the air-sea interface. These fluxes cannot be resolved explicitly by current-generation numerical weather prediction models, and therefore must be accounted for via empirical parameterizations of surface exchange coefficients (C_{D} for momentum and C_{k} for moist enthalpy). The resultant model uncertainty is examined through hundreds of convection-permitting Weather Research and Forecasting Model (WRF) simulations of Hurricane Katrina (2005) by varying four key parameters found in commonly used parameterizations of the exchange coefficient formulas. Two of these parameters effectively act as multiplicative factors for the exchange coefficients over all wind speeds (one each for C_{D} and C_{k}); the other two parameters control the behavior of C_{D} at very high wind speeds (i.e., above 33 m s^{-1}). It is found that both the intensity and the structure of TCs are highly dependent upon the two multiplicative parameters. The multiplicative parameter for C_{D} has a considerably larger impact than the one for C_{k} on the relationship between maximum 10-m wind speed and minimum sea level pressure: C_{D} alters TC structure, with higher values shifting the radius of maximum winds inward and strengthening the low-level inflow; C_{k} only affects structure by uniformly strengthening/weakening the primary and secondary circulations. The TC exhibits the greatest sensitivities to the two multiplicative parameters after a few hours of model integration, suggesting that these parameters could be estimated by assimilating near-surface observations. The other two parameters are likely more difficult to estimate because the TC is only marginally sensitive to them in small areas of high wind speed.

Original language | English (US) |
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Pages (from-to) | 2290-2308 |

Number of pages | 19 |

Journal | Monthly Weather Review |

Volume | 142 |

Issue number | 6 |

DOIs | |

State | Published - Jan 1 2014 |

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### All Science Journal Classification (ASJC) codes

- Atmospheric Science

### Cite this

*Monthly Weather Review*,

*142*(6), 2290-2308. https://doi.org/10.1175/MWR-D-13-00208.1

}

*Monthly Weather Review*, vol. 142, no. 6, pp. 2290-2308. https://doi.org/10.1175/MWR-D-13-00208.1

**Sensitivity of tropical cyclone simulations to parametric uncertainties in Air-Sea fluxes and implications for parameter estimation.** / Green, Benjamin W.; Zhang, Fuqing.

Research output: Contribution to journal › Article

TY - JOUR

T1 - Sensitivity of tropical cyclone simulations to parametric uncertainties in Air-Sea fluxes and implications for parameter estimation

AU - Green, Benjamin W.

AU - Zhang, Fuqing

PY - 2014/1/1

Y1 - 2014/1/1

N2 - Tropical cyclones (TCs) are strongly influenced by fluxes of momentum and moist enthalpy across the air-sea interface. These fluxes cannot be resolved explicitly by current-generation numerical weather prediction models, and therefore must be accounted for via empirical parameterizations of surface exchange coefficients (CD for momentum and Ck for moist enthalpy). The resultant model uncertainty is examined through hundreds of convection-permitting Weather Research and Forecasting Model (WRF) simulations of Hurricane Katrina (2005) by varying four key parameters found in commonly used parameterizations of the exchange coefficient formulas. Two of these parameters effectively act as multiplicative factors for the exchange coefficients over all wind speeds (one each for CD and Ck); the other two parameters control the behavior of CD at very high wind speeds (i.e., above 33 m s-1). It is found that both the intensity and the structure of TCs are highly dependent upon the two multiplicative parameters. The multiplicative parameter for CD has a considerably larger impact than the one for Ck on the relationship between maximum 10-m wind speed and minimum sea level pressure: CD alters TC structure, with higher values shifting the radius of maximum winds inward and strengthening the low-level inflow; Ck only affects structure by uniformly strengthening/weakening the primary and secondary circulations. The TC exhibits the greatest sensitivities to the two multiplicative parameters after a few hours of model integration, suggesting that these parameters could be estimated by assimilating near-surface observations. The other two parameters are likely more difficult to estimate because the TC is only marginally sensitive to them in small areas of high wind speed.

AB - Tropical cyclones (TCs) are strongly influenced by fluxes of momentum and moist enthalpy across the air-sea interface. These fluxes cannot be resolved explicitly by current-generation numerical weather prediction models, and therefore must be accounted for via empirical parameterizations of surface exchange coefficients (CD for momentum and Ck for moist enthalpy). The resultant model uncertainty is examined through hundreds of convection-permitting Weather Research and Forecasting Model (WRF) simulations of Hurricane Katrina (2005) by varying four key parameters found in commonly used parameterizations of the exchange coefficient formulas. Two of these parameters effectively act as multiplicative factors for the exchange coefficients over all wind speeds (one each for CD and Ck); the other two parameters control the behavior of CD at very high wind speeds (i.e., above 33 m s-1). It is found that both the intensity and the structure of TCs are highly dependent upon the two multiplicative parameters. The multiplicative parameter for CD has a considerably larger impact than the one for Ck on the relationship between maximum 10-m wind speed and minimum sea level pressure: CD alters TC structure, with higher values shifting the radius of maximum winds inward and strengthening the low-level inflow; Ck only affects structure by uniformly strengthening/weakening the primary and secondary circulations. The TC exhibits the greatest sensitivities to the two multiplicative parameters after a few hours of model integration, suggesting that these parameters could be estimated by assimilating near-surface observations. The other two parameters are likely more difficult to estimate because the TC is only marginally sensitive to them in small areas of high wind speed.

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U2 - 10.1175/MWR-D-13-00208.1

DO - 10.1175/MWR-D-13-00208.1

M3 - Article

VL - 142

SP - 2290

EP - 2308

JO - Monthly Weather Review

JF - Monthly Weather Review

SN - 0027-0644

IS - 6

ER -