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
Maximum potential intensity (MPI) Upper bound to TC intensity Important quantity to know when predicting TC intensity Approaches Empirical Theory Modeling (not discussed here)
Atlantic (DeMaria and Kaplan 1994) Clim SST/Vmax sample from Fit upper bound of Vmax (least squares, exponential) A = 28.2, B = 55.8, C = ,T 0 = 30.0 °C T in °C V given in ms-1 E. Pacific (Whitney and Hobgood 1997) Clim SST/Vmax sample from Fit upper bound of Vmax (least squares, linear) C 0 = ms -1, C 1 = 5.36 ms -1 SST in °C EPMPI given in ms -1
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
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…
Sometimes, Vmax > theoretical MPI Example: Hurricane Celia 2010
Re-examine empirical formulas using 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
10653 cases total Vmax > MPI for 30 cases (0.3%) Good upper bound for sample cases total Vmax > MPI for 8 cases (0.1%) Good upper bound for sample
First, try a few simple improvements... Vmax > MPI 12% of cases Vmax > MPI 16% of cases *Note: Atmospheric sounding comes from GFS model.
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
Vmax > MPI 10% of cases Vmax > MPI 14% of cases
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
Vmax > MPI 9% of cases Vmax > MPI 11% of cases
Observed Vmax Exceeds Theoretical MPI. Why? When?
“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 VMAX MPI EMPI RSST TGRD TADV T SHDC RHCN EPOS V V V20C TSPD TSPY LAT
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
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