Improving the Improving the Initialization of Coupled Initialization of Coupled Hurricane-Ocean Models Hurricane-Ocean Models

by Feature-Based by Feature-Based Assimilation of Mesoscale Assimilation of Mesoscale

Oceanic FeaturesOceanic Features

Richard M. Yablonsky and Isaac GinisRichard M. Yablonsky and Isaac Ginis URI Graduate School of OceanographyURI Graduate School of Oceanography

18-20 June 200718-20 June 2007Third Northeast Tropical WorkshopThird Northeast Tropical WorkshopMIT Endicott House, Dedham, MAMIT 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!Track errors cut in half in 17 years!

Image courtesy of James Franklin, NHC

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0

TheGoodNews!

No progress with intensity in 17 years!No progress with intensity in 17 years!

Image courtesy of James Franklin, NHC

TheBad

News!

Hurricane Physics & ThermodynamicsImage courtesy of http://hurricanes.noaa.gov

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 Evaporation from ocean

INFLOW (Isothermal expansion) INFLOW (Isothermal expansion)

OUTFLOW (Isothermal compression) OUTFLOW (Isothermal compression)Radiation to space Radiation to space

UPDRAFT(Adiabaticexpansion& cooling)

DOWNDRAFT(Adiabaticcompression& warming)

DOWNDRAFT(Adiabaticcompression& 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 IntensitySatellite Infrared Image of Sea Surface Temperature Hurricane Gert (1999)

Typical of Gulf of Mexicoin September

Typical of Caribbeanin 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 inadvance of Rita on15 September 2005

Warm “Loop Current”water and a “Warm Core Ring” extend far into the Gulf of Mexicofrom the Caribbean…

Directly under Rita’sand 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 ourinitialization is better than GDEM?

Answer: We use satellite altimetry and real-time AXBT instrumentsfor assimilation and/or validation!

The depth of the 26°C isotherm on 15 Sept. 2005 according to GDEM climatology

The depth of the 26°C isotherm on 15 Sept. 2005 after we assimilate mesoscale oceanic features (i.e. theLoop 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 dropsfrom a planeand floats on theocean surface(2) Temperature probe is releasedand takesmeasurementsas it sinks (3) Surface transmitter relays the ocean temperature profile to the airplane

Images courtesy of US Coast Guard International Ice Patrol

http://www.uscg.mil/lantarea/iip/General/ocean2.shtml

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?...

GDEMData-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 xx

xx

xxxx

xxxx xx

xx

xx

xx xx

xxxx

xx

xx

• Start with GDEM

Or… define using realdata: e.g. AXBT 6 for LC and AXBT 13 (14) for WCR (CCR)

Finally, assimilate SST and integrate ocean model for 48 hrs forgeostrophic adjustment

Our feature-based model

Continuous dataassimilation

Our feature-based model

Continuous dataassimilation

Our feature-based model

Continuous dataassimilation

Our feature-based model

Continuous dataassimilation

Our feature-based model

Continuous dataassimilation

Tropical Cyclone Heat Potential:integrated oceanic heat content fromthe surface down to the 26°C isotherm

Our data-assimilated tropicalcyclone heat potential: 9/15/05

RSMAS HYCOM’s tropicalcyclone 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 areimprovingthis modelat URI…

Note thereducedGFDLintensityerror in2006!!!

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: http://www.gso.uri.edu/~richard/publications.html

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.