EFFECTS OF OCEAN MIXING ON HURRICANE HEAT CONTENT ESTIMATES: A NUMERICAL STUDY S. DANIEL JACOB and LYNN K. SHAY Meteorology and Physical Oceanography Rosenstiel School of Marine and Atmospheric Science University of Miami, Miami, FL Drs. Peter Black, Rainer Bleck and Arthur Mariano USWRP - NSF Sponsored project: ATM

OBJECTIVES USING A NUMERICAL MODEL INVESTIGATE:  Entrainment Mixing Schemes Gaspar (1988) Kraus and Turner (1967) Pollard, Rhines and Thompson (1973), Price (1981) Deardorff (1983) Compare model results to Gilbert (1988) observations  Role of Oceanic Variability Mixed Layer Temperature/ Depth evolution for realistic, climatological and quiescent initial conditions Momentum Response Compare model results to observations

Miami Isopycnic Coordinate Ocean Model Advantages –Explicit Mixed Layer Physics –Natural Discontinuity for different water masses Configuration –Domain: Gulf of Mexico –Resolution: 0.07 –15 Layers –Closed Boundaries Initial Conditions –Realistic Conditions from Yearday 200 data (Case E) –Climatological Conditions from Levitus (1982) (Case C) –Quiescent Conditions from average prestorm AXBT (Case Q) (Bleck and Smith 1992)

WIND FIELD STRUCTURE

ENTRAINMENT Based on Observational Analysis (Jacob et al., JPO 2000) –Entrainment is the dominant mechanism in the mixed layer. –Mixed layer heat and mass budgets strongly depend upon the entrainment scheme used. Numerical Modeling: –MICOM: Gaspar (1988) scheme that uses u * and Q 0 to prescribe entrainment rate. Observations suggest the presence of strong near-inertial shears at the mixed layer base. –Mixed layer response using four entrainment parameterizations is investigated.

ENTRAINMENT SCHEMES Turbulent Kinetic Energy Sources (Niiler and Kraus 1977): –wind stress (u * 3 ) –free convection (Q 0 ) –current shear (V 2 ) Kraus and Turner (1967) and Gaspar (1988) –u * 3 and Q 0 Pollard, Rhines and Thompson (1973); Price(1981) –V 2 Deardorff (1983) –All three source mechanisms Numerical simulations are performed for the four schemes and results are compared to observations.

NUMERICAL EXPERIMENTS

MOVIE 1 Gaspar KT PRT DDF

2R max Blue - Gaspar Green - KT Red - PRT Cyan - DDF

MOVIE 2 PRT

QUANTITATIVE COMPARISON: LOCATIONS AND SECTIONS

POINT 3 Blue - Gaspar Green - KT Red - PRT Cyan - DDF

MODEL-DATA COMPARISON

SUMMARY Areal extent of the mixed layer response differs for the four entrainment schemes used. PRT and Deardorff schemes predict similar response near the track. Large entrainment rates away from the track are predicted by the Deardorff scheme. Results in cooler and deeper mixed layers. Advective tendencies are clearly affected by the choice of entrainment scheme. Effects are minimal. PRT scheme fits the data better than other schemes.

TEMPERATURE-SALINITY DIAGRAM Case E Case E GCW Case C Case Q

PRE-STORM MLT AND MLD Case E Case Q Case C E-Q

STORM MLT AND MLD Case Q Case E Case C E-Q

WAKE 2 MLT AND MLD Case E Case Q Case C E-Q

POINT 3 EVOLUTION POINT 3 Blue - Case E Green - Case C Red - Case Q

ENTRAINMENT AND SURFACE FLUXES POINT 3 Blue -Case E Green -Case C Red -Case Q

MODEL-DATA COMPARISON: TEMPERATURE

MODEL-DATA COMPARISON: VELOCITIES

SUMMARY Results indicate a clear modulation of MLTs and MLDs by the currents associated with the eddy. Three dimensionality is important in the mixed layer. Model-Data comparison improves by using realistic initial conditions. The near-inertial pass band is shifted below f. Compares well to the theoretical estimates. Simulated MLTs are within observational limits. MLDs and currents in the mixed layer are higher than those observed. Time-averaged surface fluxes contributed up to 35% of the mixed layer heat budget.

CONCLUSIONS MLTs simulated using Pollard, Rhines and Thompson (1973) entrainment scheme compare better with observations. Oceanic background conditions are essential for realistic simulations in coupled models. Surface fluxes contribute up to 35%, thus cannot be neglected. Implications for storm intensity.

THINK 3D!

AXCTD PROFILES (SUMMER 1999)

AIR-SEA PARAMETERS DURING GILBERT

POINT 1 EVOLUTION POINT 1 U  h U  T Blue - Case E Green - Case C Red - Case Q

VARIATION OF ADVECTIVE TENDENCIES Case E Case Q Case E Case Q