ACCESS NUMBER: ?????

Technical Summary

Study Title:Lagrangian Data Assimilation in Ocean Model Calculations

Report Title:Lagrangian Data Assimilation in Ocean Model Calculations

Contract Number: MMS 1435-01-04-CT-35279

Sponsoring OCS Region: ?????

Applicable Planning Area: Gulf of Mexico

Fiscal Year(s) of Project Funding: 2005, 2006, 2007.

Completion Date of Report: April, 2007.

Cost(s): FY 2005: $120,000; FY 2006: $110,000; FY 2007: $50,000; cumulative

Project Cost: $280,000

Project Manager: L. Oey

Affiliation: Princeton university

Address: AOS Program, Sayre Hall, PrincetonUniv., PrincetonNJ08544.

Principal Investigator: L. oey

Key Words: Gulf of Mexico; Drifters; Lagrangian assimilation; Loop Current; rings.

Background: Surface drifters (or in general floats, surface or subsurface) potentially can provide additional useful information about ocean currents, hence the movement of contaminants such as oil spills. These Lagrangian data can be used in conjunction with other more conventional data for assimilations into ocean models, so that a more accurate estimate of ocean currents may be derived for Oil Spill Risk Assessment Analysis. This project implements, tests and verifies such Lagrangian assimilation methodologies in the Gulf of Mexico.

Objective: To implement, test and verify Lagrangian assimilation methodologies in a time-dependent, three-dimensional circulation model of the Gulf of Mexico. Additionally, we also demonstrate and quantify the usefulness of Lagrangian data compared to fixed-point mooring data in producing more accurate surface fields.

Description: Various Lagrangian data assimilation schemes are tested in conjunction with satellite-data assimilations for the period Sep/1999-Sep/2000 in the Gulf of Mexico when good data - surface drifters, ship-board and moored ADCP’s, as well as satellite sea-surface height anomaly (SSHA) data were available. We have conducted detailed assessments of the model skills – in terms of how close the analysis currents are compared with observations. We also conducted extensive experiments comparing the accuracies of assimilating Lagrangian versus fixed-mooring data, and assessing the sensitivity of the model analyses to drifter distributions and densities in the Gulf of Mexico.

Significant Conclusions: We show that the SSHA+Drifter analysis consistently outperforms the analysis that assimilates only SSHA, especially at smaller scales. Drifter-assimilation alone also constrains the pressure field, such that the Loop and eddies compare quite favorably with altimetry SSH field, and also with moored and ship-board ADCP measurements.

Study Results: Because our Lagrangian assimilation methodologies adjust also the pressure fields, the benefits of assimilating surface drifters extend to subsurface 300~500 m. Assimilating drifters produce model currents that are also more accurate than fixed-point mooring data. The reason is that drifters can provide information over much wider “eddy’ areas. These resultsare important because they suggest that, if upper currents (surface ~ 500m) of the Gulf are of interest, assimilating drifters would produce improved analyses.

Study Product(s):

1.Oey, L.-Y., 2007: Lagrangian Data Assimilation in Ocean Model Calculations. Final Report, OCS Study MMS 2007-028, U.S. DOI Minerals Management Service, Herndon, Virginia. 80pp.

2.All model inputs and outputs delivered on 8 DvD’s to MMS;

3.Model code and runscripts;

4.All drifter data.

1