Improving the Initialization of Coupled Hurricane-Ocean Models by Feature-Based Assimilation of Mesoscale Oceanic Features Richard M. Yablonsky and Isaac Ginis URI Graduate School of Oceanography URI Graduate School of Oceanography June 2007 Third Northeast Tropical Workshop MIT Endicott House, Dedham, MA

Numerical models allow us to predict the future state of the atmosphere and ocean using mathematical equations based on physical laws… The Role of Numerical Models Image courtesy of Isaac Ginis Forecasters at NHC use output from the GFDL/URI coupled hurricane-ocean model and the coupled Hurricane WRF model (as well as other models) to help them make accurate 3-5 day hurricane forecasts

Hurricane prediction in the last 17 years… Good news and Bad news…

Track errors cut in half in 17 years! Image courtesy of James Franklin, NHC The Good News!

No progress with intensity in 17 years! Image courtesy of James Franklin, NHC The Bad News!

Hurricane Physics & Thermodynamics Image courtesy of Hurricanes develop and are maintained by transportation of water vapor from the ocean surface up a natural chimney to the upper atmosphere. Inward component of surface wind Inward component of surface wind Evaporation from ocean INFLOW (Isothermal expansion) OUTFLOW (Isothermal compression) Radiation to space UPDRAFT (Adiabatic expansion & cooling) DOWNDRAFT (Adiabatic compression & warming) DOWNDRAFT (Adiabatic compression & warming)

Is the ocean part simple? Heat energy for the hurricane depends on evaporation from the sea surface Evaporation rate depends on SST So to initialize a hurricane model, all you need to know about the ocean is the SST ahead of the storm, right? WRONG!!!WRONG!!! Let us now understand why…

Ocean’s Role in Hurricane Intensity Satellite Infrared Image of Sea Surface Temperature Hurricane Gert (1999)

Typical of Gulf of Mexico in September Typical of Caribbean in September Image courtesy of Isaac Ginis

Why is the cold wake “not as cold” here? Answer: We must look under the ocean surface!

Importance of Hurricane-Ocean Interaction: Katrina & Rita -- Nearly “Perfect Storms” Subsurface (75-m) ocean temperature in advance of Rita on 15 September 2005 Warm “Loop Current” water and a “Warm Core Ring” extend far into the Gulf of Mexico from the Caribbean… Directly under Rita’s and Katrina’s track!!!

Capturing the Loop Current and Rings You need real data to properly initialize the 3-D ocean temperature field… You can start with a monthly ocean climatology and assimilate the daily SST… But is a monthly climatology good enough to capture major oceanic fronts like the Gulf Stream, the Loop Current, & eddies (rings)? Let’s find out!!!

Question: How do we know our initialization is better than GDEM? Answer: We use satellite altimetry and real-time AXBT instruments for assimilation and/or validation! The depth of the 26°C isotherm on 15 Sept according to GDEM climatology The depth of the 26°C isotherm on 15 Sept after we assimilate mesoscale oceanic features (i.e. the Loop Current and Rings)

Sea surface height assimilation Satellite altimetry is used daily by the National Hurricane Center to calculate the depth of the 26°C isotherm Using maps of the 26°C isotherm depth, we obtain a “first-guess” for the Loop Current shape and position We also obtain a “first-guess” for the location of any warm & cold core rings in the Gulf of Mexico We then modify the GDEM climatology accordingly If there are real-time AXBTs, we assimilate them too…

Airborne eXpendable BathyThermograph (AXBT) (1)AXBT drops from a plane and floats on the ocean surface (2) Temperature probe is released and takes measurements as it sinks (3) Surface transmitter relays the ocean temperature profile to the airplane Images courtesy of US Coast Guard International Ice Patrol

Assimilating AXBT temperature profiles When available, we can adjust the shape and position of the Loop Current and rings from the form suggested by altimetry using AXBT profiles This is helpful because altimetry is not perfect… Also, AXBT profiles can be used to define the temperature profile in the core of the Loop Current & rings, rather than relying on the GDEM profile from the source region (i.e. the Caribbean) We call modified initialization “data-assimilated” But how exactly do we “modify” the GDEM climatology to obtain our “data-assimilated” initialization?...

GDEM Data-assimilated How we modify GDEM Climatology: Feature-based modeling! Look at altimetry/axbts Define LC & ring positions Use Caribbean water along LC axis & in warm core ring center Make center of cold core ring colder than surroundings Blend features w/ surroundings Altimetry x x x x x xx xxx x x x x x Start with GDEM Or… define using real data: e.g. AXBT 6 for LC and AXBT 13 (14) for WCR (CCR) Finally, assimilate SST and integrate ocean model for 48 hrs for geostrophic adjustment

Our feature-based model Continuous data assimilation

Our feature-based model Continuous data assimilation

Our feature-based model Continuous data assimilation

Our feature-based model Continuous data assimilation

Our feature-based model Continuous data assimilation

Tropical Cyclone Heat Potential: integrated oceanic heat content from the surface down to the 26°C isotherm Our data-assimilated tropical cyclone heat potential: 9/15/05 RSMAS HYCOM’s tropical cyclone heat potential: 9/15/05

GDEM climatology Data- assimilated RSMAS HYCOM Tropical Cyclone Heat Potential: RMS Error for all 18 AXBT locations

Operational Implementation Our feature-based ocean initialization with data assimilation was incorporated into the 2006 operational version of the GFDL model (along with other improvements from 2005) This initialization is now being used operationally in both the GFDL model and the Hurricane WRF model for the 2007 Atlantic hurricane season

Intensity Errors in Recent Seasons We are improving this model at URI… Note the reduced GFDL intensity error in 2006!!!

Future Work Analyze more historical ocean temperature profiles for validation and perhaps further improvements to the initialization procedure Consider using Pressure/Inverted Echo Sounder (PIES) instruments and/or drifters to supplement altimetry and AXBTs for defining the location of fronts (e.g. Loop Current & rings) in real time Extend the feature-based modeling procedure to the western North Pacific Ocean for typhoons and the western Indian Ocean for cyclones

Acknowledgements Biju Thomas, Yalin Fan, Zhitao Yu (GSO) Aleksandr Falkovich (NCEP) Eric Uhlhorn (HRD) Michelle Mainelli and Stephen Baig (NHC) HYCOM Consortium (RSMAS)

THANK YOU FOR YOUR TIME AND ATTENTION! QUESTIONS??? For more information, please go to: Yablonsky, R. M., and I. Ginis, 2007: Improving the initialization of coupled hurricane-ocean models by feature-based assimilation of mesoscale oceanic features. Mon. Wea. Rev., 135, submitted.