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Another Look at the Use of Maximum Potential Intensity of Tropical Cyclones. Andrea Schumacher, CIRA/CSU, Fort Collins, CO Mark DeMaria and John Knaff, NOAA/NESDIS/ StAR , Fort Collins, CO NCAR/NOAA/CSU Tropical Cyclone Workshop 16 November 2011 CIRA, Fort Collins, CO. Introduction.

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another look at the use of maximum potential intensity of tropical cyclones

Another Look at the Use of Maximum Potential Intensity of Tropical Cyclones

Andrea Schumacher, CIRA/CSU, Fort Collins, CO

Mark DeMaria and John Knaff, NOAA/NESDIS/StAR, Fort Collins, CO

NCAR/NOAA/CSU Tropical Cyclone Workshop

16 November 2011

CIRA, Fort Collins, CO

introduction
Introduction
  • Maximum potential intensity (MPI)
    • Upper bound to TC intensity
    • Important quantity to know when predicting TC intensity
  • Approaches
    • Empirical
    • Theory
    • Modeling (not discussed here)
empirical mpi
Empirical MPI
  • Atlantic (DeMaria and Kaplan 1994)
    • Clim SST/Vmax sample from 1962-1992
    • Fit upper bound of Vmax (least squares, exponential)
    • A = 28.2, B = 55.8, C = 0.1813,T0 = 30.0 °C
    • T in °C
    • V given in ms-1
  • E. Pacific (Whitney and Hobgood 1997)
    • Clim SST/Vmax sample from 1963-1993
    • Fit upper bound of Vmax (least squares, linear)
    • C0 = -79.17 ms-1, C1 = 5.36 ms-1
    • SST in °C
    • EPMPI given in ms-1
mpi theory
MPI Theory
  • Two main thermodynamic theories
    • Carnot heat engine (Emanuel 1986; 1991)
      • Acquire enthalpy from the ocean
      • Convert to wind energy
      • Function of sea surface and storm outflow temperature
      • Calculates MPI as a maximum surface wind speed
    • Eye subsidence (Miller 1958 and Holland 1996)
      • Requires SST, atmospheric sounding and surface pressure
      • Calculates MPI as a minimum central pressure
motivation predicting intensity
Motivation – Predicting Intensity
  • Current version of Statistical Hurricane Intensity Prediction Scheme (SHIPS) uses empirical MPI as a predictor
    • Pros:
      • Availability (function of SST only)
    • Cons:
      • Small sample set – 31 years
      • Doesn’t include atmospheric variables
      • Only uses SST – no ocean structure (e.g., OHC)
      • Doesn’t always serve as true “upper bound” to Vmax…
motivation cont
Motivation (Cont…)
  • Sometimes, Vmax > theoretical MPI
  • Example: Hurricane Celia 2010
project goals
Project Goals
  • Re-examine empirical formulas using 1982-2010 SHIPS sample set – valid?
  • Examine Emanuel’s theoretical MPI (1995) over the SHIPS dataset
    • Identify when valid / not valid
  • Create a combined theoretical / empirical formula for MPI in SHIPS
ships empirical mpi validation 1982 2010
SHIPS Empirical MPI Validation1982-2010

10653 cases total

Vmax > MPI for 30 cases (0.3%)

 Good upper bound for sample

13003 cases total

Vmax > MPI for 8 cases (0.1%)

 Good upper bound for sample

emanuel mpi in ships 1982 2010
Emanuel MPI* in SHIPS1982-2010

Vmax > MPI

12% of cases

Vmax > MPI

16% of cases

First, try a few simple improvements...

*Note: Atmospheric sounding comes from GFS model.

asymmetry adjustment
Asymmetry Adjustment
  • Vmax includes asymmetric adjustment, MPI does not
  • For proper comparison, remove asymmetric component from storm motion (Schewerdt):

Vmax(adjusted) = Vmax – 1.5*(TC speed)0.63

TC Motion

Symmetric

Vortex

Asymmetric

Vortex

emanuel mpi asymmetry adjustment
Emanuel MPI+ Asymmetry Adjustment

Vmax > MPI

10% of cases

Vmax > MPI

14% of cases

improved reduction factor
Improved Reduction Factor
  • Emanuel MPI assumes 700-mb winds reduced by factor of 0.8 at surface (friction)
  • Franklin et al. (2002) used dropwinsonde data to show reduction factor is closer to 0.9
  • Multiplied Emanuel MPI by 0.9/0.8
emanuel mpi asymmetry adjustment improved reduction factor
Emanuel MPI + Asymmetry Adjustment + Improved Reduction Factor

Vmax > MPI

9% of cases

Vmax > MPI

11% of cases

environmental differences atlantic
Environmental Differences - Atlantic

“Violators” have…

  • Higher intensity, weakening
  • Lower MPI, decreasing
  • Lower SST, decreasing
  • Larger low-level temperature gradient
  • Lower OHC
  • Higher latitude
summary
Summary
  • Empirical MPI provides good upper bound for intensity
  • SHIPS parameter based on Emanuel theoretical MPI improved by
    • Adding asymmetry adjustment due to storm motion
    • Using more appropriate reduction factor when converting 700-mb winds to surface
future work
Future Work
  • Objectively identifying conditions where Emanuel MPI should be adjusted or replaced with empirical
    • Fast-moving TCs moving over cooler SST/OHC, lack of equilibrium
    • Extratropical cases (Atlantic)
    • Annular cases (E Pacific)
  • Creating combination empirical/theoretical MPI for testing in SHIPS
  • Parameterizing SST change under inner core
    • Decrease MPI