satellite altimetry in the gulf of mexico: techniques ... · satellite altimetry in the gulf of...

Colorado Center for Astrodynamics Research University of Colorado at Boulder Remote Sensing Seminar • 14 April ’02

Satellite Altimetry in the Gulf of Mexico: Techniques, Tools and Theory


Today’s SSH/SST Overlay


Yesterday’s


Outline of Talk

Introduction to Gulf of Mexico OceanographyTechniques

Overview of CCAR near real-time altimeter data system.Recent update and validation of system.Sample near real-time products and applications.

ToolsOn-line forms for plotting GOM data.Online SSH/SST Overlay form.Loop Current Toolbox.

TheoryLoop Current Eddy Shedding and Upstream Influences?


Oceanographic Overview

The Gulf of Mexico (GOM) is an oceanographic region where the general circulation is dominated by mesoscale circulation.

Loop Current Loop Current eddiesanticyclones (highs)cyclones (lows)eddy pairs


JHUAPL 3-day composite SST 2/22/2003


Schematic of Altimeter System


Schematic of Corrections


CCAR Mesoscale SSH Map 2/22/2003


SST/SSH Overlay October 22, 2003


Altimetry Blended SSH


GOM SSH Map 10/22/2003


GOM SSH Anomaly 10/22/2003


Original CCAR Near Real-time Altimeter System

Quick-look processing designed to retain and map mesoscale variability.Went online in early 1996.Used NOAA Laboratory for Satellite Altimetry ERS and Geosat Follow-on near real-time altimeter data products.

Enhanced NOAA corrections -Delft Institute for Earth-Oriented Space Research orbits.

Used near real-time TOPEX RGDRsProcessed by the Naval Oceanographic Office Altimeter Data Fusion Center.JPL Earth Orbiter Systems Group GPS orbit.

CCAR system was designed so that quick-look products are processed and posted by 12:00 p.m. MST.


CCAR Quick-look mesoscale analysis

All altimeter data are referenced to an independent griddedmean sea surface.

The data are treated as non-repeating ground tracks and are referenced directly to the mean sea surface (non-collinear processing).Saves a significant amount of computational effort during near real-time processing.

Along track loess filtering is used to remove residual orbit andenvironmental correction errors.

The loess filtering removes a running least squares fit of a tilt and bias (line) within a sliding window from the along-track data.The window width is approximately 15 degrees (or 200 seconds) along track.

The detrended data are objectively mapped to a 1/4° grid.In the Gulf, a model mean or mean dynamic height is added to the anomaly to estimate the total sea surface height.


Current CCAR Near Real-time Altimeter System

Updated “collinear” processing and new reference mean sea surface.Went online in summer 2003.Uses NOAA Laboratory for Satellite Altimetry ERS and Geosat Follow-on near real-time altimeter data products.

Only North Atlantic and GOM ERS-2 data since tape recorder failure.NRT ERS-2 processing will be suspended soon.

Uses near TOPEX and Jason-1 RGDRsStack file processing (thanks to Eric Leuliette and Gerard Kruizinga)JPL Earth Orbiter Systems Group GPS orbits and real-time data.

Envisat data will be added this summer.


Updating of the Mean Reference Surface

The original system used the Ohio State University MSS 1995 (OSUMSS95 or “Rapp Mean”) as the mean reference surface.

1/16° spatial resolution.Based on one-year means from T/P (10-day repeat), ERS-1(35-day repeat) and Geosat (17-day repeat) and one 168-day repeat cycle from the ERS-1 Geodetic Mission Phase (Yi, 1995).

The updated system uses the Goddard Spaceflight Center MSS 00.1 (GSFC00.1_MSS or the “Goddard Mean”).

1/30 ° spatial resolutionBased on Geosat Geodetic & ERM data, multi-years of ERS -1&2 35-day repeat data, both cycles of ERS-1 168-day repeat data and 6 years of T/P data (Wang, 2001 in JGR Oceans).


Gulf of Mexico - Goddard Mean Sea Surface (GSFC00.1_MSS - Wang, 2001 JGR Oceans)


“Goddard Mean” minus “Rapp Mean”

T/P and ERS 35-day repeat along-track differencesDifference of Gridded Fields


1993-1999 Mean Anomalies

“Rapp” Mean Anomaly “Goddard” Mean Anomaly

Centimeters


RMS Difference 1993-1999 Mean Anomaly


Online Data Access Tools

Gulf of Mexico Interactive Online Data Forms

Gulf of Mexico Near Real-Time Data ViewerGulf of Mexico Near Real-Time Geostrophic Velocity Vector ViewerGulf of Mexico Along-Track Data HostGulf of Mexico Along-Track Visualization (One Day) Gulf of Mexico Along-Track Visualization (Multiple Day)

http://www-ccar.colorado.edu/~realtime/gom-real-time_ssh

http://www-ccar.colorado.edu/~realtime/gom-real-time_ssh

http://www-ccar.colorado.edu/~realtime/gom-real-time_vel

http://www-ccar.colorado.edu/~realtime/gom_data_ssh/ssh.html

http://www-ccar.colorado.edu/~realtime/gom_data_ssh/sshpic.html

http://www-ccar.colorado.edu/~realtime/gom_data_ssh/sshpic2.html


Sample SSH data viewer output


Sample geostrophic velocity viewer output


Sample along track data viewer output


Near Real-time Applications

In this presentation, I will highlight example applications anddata syntheses by end users of these data bases including:

Sport sailingOffshore operational support in the Gulf of MexicoOceanographic survey designMarine mammal habitat monitoring


Sport Sailing


Loop Current and Drilling Platform


Sample Offshore Industry “Eddy Watch” Map from Horizon Marine


Offshore Operational Support: Gulf of Mexico

Data User: Capt. Karl Greig, captain of a large anchor handling tug boat owned by Edison Chouest Offshore.Application: Route finding for towing semi-submersible drilling rigs used in deepwater oil and gas exploration. Operation: Moving a rig from Mississippi Canyon block 68 to Mustang Island block 68, a total of 425 nautical miles. Typical towing speeds are 3 to 4 knots so avoiding and/or using eddy currents significantly reduces transit times, in this case by over 50 hours. Altimeter Product Used: Overlays of geostrophic velocity vectors on colored magnitudes values accessed on CCAR website by satellite phone.Estimated Savings: $650,000 in rig downtime and towing costs.


Near real-time currents: August 15, 2003

xx Mustang Island

lease block

Mississippi Canyonlease block


New Tools for Deriving Altimeter Products

Feedback from users of our altimeter data system has given us insight into a variety of oceanographic products of interest to scientific and operational users.Based on this feedback we have developed a number of tools for monitoring the Gulf of Mexico general circulation in deep water. These include:

Composite SST imagery overlaid with altimetry (with special thanks to JHU/APL Ocean Remote Sensing Group for online imagery).

Circulation monitoringSurvey design

Objective Loop Current and Eddy TrackingLoop Current Metrics

Site evaluationModel skill assessmentScientific studies of Loop Current dynamics and it’s effect.


Sample Survey Design for Whale Survey


Survey Design August 1997

GulfCetGulfCet cruise in August 97 cruise in August 97 documented spatially documented spatially shifted eddy geometry shifted eddy geometry detected in NRT product detected in NRT product used to design survey

-90 -88 -86 -84

Longitude

24

26

28

30

Lat

itude

100 m200 m

1000 m

3000 m

3000 m

2000 m

1000 m

2000 m

4020

60

0

0

used to design survey


Sperm Whale Contacts - August 1997


Online SSH/SST Overlay (IDL Version)


11/26/1999 SST image overlaid with contoured SSH field


Loop Current Metrics

Loop Current (LC) metrics are computed using the 17-cm sea surface height contour as a proxy for the high velocity core of the LC in the eastern Gulf of Mexico. This proxy allows objective computation of LC metrics such as:

Maximum northward and westward extent of LC penetration.LengthAreaCirculationVolume

The LC database can be used for site specific analyses.


Procedure for Computing Loop Current Metrics

The procedure for computing LC metrics from the SSH fields has been automated by a MATLAB program that accesses the GOM altimeter archive and computes the values. Daily values for each metric are computed as follows

Load the 1/4° gridded SSH filed and generate the coordinates of the 17-cm contours within the Gulf.Identify the LC core, which is defined as the continuous 17-cm contour that enters the Gulf through the Yucatan Channel and exits through the Florida Straits.Find the maximum west longitude and north latitude coordinates to determine the extent of westward and northward penetration.Compute the length of the LC by summing the distances between the coordinates on the 17-cm contour.Compute the area, circulation, and volume by evaluating the appropriate integral quantities.


Automated Loop Current Tracking

Tem

pera

ture

°C

January 12, 1998


Sample Loop Current Statistics Time Series


Distance to Loop Current Front

The Loop Current frontal database is useful for evaluating specific sites within the Gulf of Mexico for scientific and operational activities. The time series above was used to interpret measurements at a mooring on the West Florida shelf.


Loop Current Monitoring for Scientific Studies

“A Loop Current Jet Induced Along the West Florida Shelf.”Hetland et al. (GRL 1999)

Young Phase Mature Phase


Loop Current Ring Separation

Eddy Shedding FrequencyPrimary peaks are at 6 and 11 months.A smaller peak occurs at 9 months.Little or no power at annual frequency.

Eddy shedding is not a discrete process, so we are hopeful that continuous monitoring of the Loop Current with altimetry will allow more detailed spectral analysis in the coming decades.

For the past thirty years we have had nearly “continuous” observations of Loop Current ring separations. This is a relatively short time series considering that only 34 observed consecutive ring separations have been observed. Based on this limited record attempts have been made to identify significant peaks in the eddy shedding spectrum (Sturges and Leben, 2000).


Loop Current Eddy Shedding Theory

“The Momentum Imbalance Paradox” Pichevin and Nof (Tellas, 1997)

The theory is based on momentum balance within a control volume applied to a northward flowing jet through gap in a wall.The theory when applied to the Loop Current predicts:

Periodic shedding at 300 days.Eddy diameter lower bound of 480 km.Upper bound for eddy generation period of 297 days (~10 months)West propagation of eddies at 12km/dayThis is the only analytical theory that I know of which predicts the regular shedding of Loop Current eddies.


Control Volume Paradox


Momentum Balance


Does this theory agree with the observed distribution of LC penetration?

Northward Penetration of Loop Current Front

CUPOM Forward Model RunHistograms

Altimetry


Does the idealized configuration match the Gulf inflow/outflow geometry?

The Campeche Shelf may act as a barrier, which hasn’t been accounted for in the theory.

Campeche Shelf


Comparison of quasi-periodic shedding and altimeter observations

Histograms of Northward Penetration of Loop Current

Forward CUPOM Model Altimetry 1993-1999


Locations of Peaks in HistogramThe latitudes of the histogram peaks associated with the retreated Loop Currentfront after eddy shedding are shown by the white horizontal lines below.


Geometry with Yucatan Shelf


What is a fluidic switch?

Is the Loop Current be acting like a fluidic switch?• Two states: quasi-periodic eddy shedding and a retreated locked or static phase.

• The locked phase is characterized by extended periods of non-intrusion, possibly acted on by an outside agent.

•Dynamics of locked phase need to be investigated further and may be influenced by a “wall effect” or even a “Coanda effect”.

The Coanda effect is the tendency of a jet near a wall or boundary to attach to the boundary.

Example fluidic switch


Upstream Influences on the Loop Current


Yucatan Vorticity Influx Effects

Candela et al., 2002“Periods of negative (anticyclonic) cumulative vorticity influx are

related to the Loop Current extending into the Gulf of Mexico, whereas periods of positive (cyclonic) cumulative vorticity influx relate to a Loop Current retraction, sometimes coincident with the shedding of an anti-cyclonic eddy.”Oey et al. (2004) Modeling Study

“Anticyclonic vorticity at Yucatan deters northward extrusion of the Loop Current , which explains the prolonged period in the later.” (Referring to a model experiment where Caribbean eddies caused longer (14-16 month) eddy shedding periods.)


Yucatan Channel Mooring


Vorticity Flux due to Horizontal Shear through the Yucatan Channel


Maps of the Loop Current at the beginning and endsof periods of negative/anticyclonic vorticity flux


Maps of the Loop Current at the beginning and ends of periods of positive/cyclonic vorticity flux


Vorticity Flux from Altimetry

The northward vorticity anomaly flux is computed from altimetry by:

Vorticity Flux = vgeostrophic ⋅gf∇2h

The northward geostrophic velocity is computed by adding the northward geostrophic velocity anomaly from altimetry to the model mean northward geostrophic velocity. No advection of mean vorticity is calculated.


Altimeter vs. Mooring Vorticity Flux


Mooring Time Period Altimeter Derived Metrics


9-year Time Series of Metrics


Summary

After over twenty years, we still do not have a comprehensive theory for the behavior of the Loop Current. This says something about the complexity of the problem.

Predictability proceeds from knowledge. Any theory is better than no theory, even a wrong one!

This seminar was intended to motivate you to think about the altimeter observations and what they might tell us about this interesting and complex fluid system.


Parting Comments

CCAR near real-time altimetry web pages and forms are accessible from: http://ccar.colorado.edu/~lebenThe site is being updated to the new mean reference surface, web page design and additional altimeter data streams as they become available.Digital data is available by ftp (contact [email protected]).Online Gulf of Mexico near real-time and historical Ocean Color overlaid images and Loop Current/ Loop Current eddy analyses and metrics in preparation.

satellite altimetry in the gulf of mexico: techniques ... · satellite altimetry in the gulf of...

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