1. 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

2.  Maximum potential intensity (MPI)  Upper bound to TC intensity  Important quantity to know when predicting TC intensity  Approaches  Empirical  Theory  Modeling (not discussed here)

3.  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,T 0 = 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)  C 0 = -79.17 ms -1, C 1 = 5.36 ms -1  SST in °C  EPMPI given in ms -1

4.  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

5.  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…

6.  Sometimes, Vmax > theoretical MPI  Example: Hurricane Celia 2010

7.  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

8. 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

9. First, try a few simple improvements... Vmax > MPI 12% of cases Vmax > MPI 16% of cases *Note: Atmospheric sounding comes from GFS model.

10.  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

11. Vmax > MPI 10% of cases Vmax > MPI 14% of cases

12.  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

13. Vmax > MPI 9% of cases Vmax > MPI 11% of cases

14. Observed Vmax Exceeds Theoretical MPI. Why? When?

15. “Violators” have…  Higher intensity, weakening  Lower MPI, decreasing  Lower SST, decreasing  Larger low-level temperature gradient  Lower OHC  Higher latitude Average (t=0)Avg 12-hr Change V > MPIMPI > VV > MPIMPI > V VMAX55.843.1-7.31 MPI36.5128.3-15.6-4.2 EMPI95.2134-7.4-2 RSST23.827.6-0.7-0.2 TGRD21.918.90.50.3 TADV-1.1-0.50.50 T150-65.3-66.20.20.1 SHDC12.1141.10.6 RHCN0.721.5-1.4-2.4 EPOS4.39.5-0.8-0.3 V5004.53.4-0.40 V3001.50.6-0.4-0.1 V20C7.22.30.50.7 TSPD9.58.9-0.40.1 TSPY3.82.8-0.50.1 LAT21.316.10.80.5

18.  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

19.  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