4DVar Assimilation (physics) in ROMS ESPreSSO*

John Wilkin, Julia Levin, Javier Zavala-Garay

• 2006 reanalysis (SW06)• Operational system for OOI CI OSSE (ongoing)

Assimilating: altimeter SLA; satellite IR SST; CODAR surface currents; climatology; glider T,S; T,S from XBT/CTD, Argo, NDBC (via GTS)

• Use methodology developed for spring 2006 LaTTE reanalysis Zhang et al., Ocean Modelling, submitted 2009

• Skill assessed in forecast window– several days for T,S– 1-2 days for velocity

*Experimental System for Predicting Shelf and Slope Optics

ROMS ESPreSSO configuration• 5 km horizontal resolution Cape Cod to Cape Hatteras• 36 levels in traditional ROMS s-coordinate (stretching=1)• 4th order Akima T,S advection; 3rd order upwind u,v advection • Bathymetry, land-sea mask from NGDC Coastal Relief Model• Open boundary data HyCOM+NCODA (no bias correction)

– stiff boundary nudging in forward simulations • Meteorology forcing 3-hourly 12-km NCEP NAM-WRF

– 72-hour forecast window– sea level atmospheric conditions + bulk formulae = fluxes– [use NCEP NARR in 2006 reanalysis]

• River daily average discharge USGS gauges– adjusted for ungauged fraction of watershed

• Tides TPXO0.7 tides (5 harmonics)

We are working in a data rich location for 4DVar assimilation…

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Mid-Atlantic Regional Coastal Ocean Observing System (MARCOOS) CODAR, gliders, moorings, tide gauges, drifters, satellites …

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IS4DVAR*

• Given a first guess (the forward trajectory)…• and given the available data…and given the available data…

( )oR x

ox

*Incremental Strong Constraint 4-Dimensional Variational data assimilation

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IS4DVAR

• Given a first guess (the forward trajectory)…• and given the available data…• what change (or increment) to the initial what change (or increment) to the initial

conditions (conditions (ICIC) produces a new forward trajectory ) produces a new forward trajectory that better fits the observations?that better fits the observations?

ox

( )oR x

( )o oR x x

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The “best fit” becomes the analysis

assimilation window

ttii = = analysis analysis initial time initial time

ttff = analysis = analysis final time final time

The strong constraint requires the trajectory satisfies the physics in ROMS. The Adjoint enforces the consistency among state variables.

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The final analysis state becomes the IC for the forecast window

assimilation window forecast

ttff = analysis = analysis final time final time

ttff + + = forecast = forecast horizon horizon

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Forecast verification is with respect to data Forecast verification is with respect to data not yet assimilatednot yet assimilated

assimilation window forecast

verification

ttff + + = forecast = forecast horizon horizon

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1111

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LaTTE domain and observation locations.

Bathymetry of the New York Bight is in grayscale and dashed contours.

yellow star is location of Ambrose Tower

green squares are CODAR HF Radar sites

LaTTE 2006 reanalysis (60 days)

LaTTE 2006 reanalysis (60 days)

Comparison of observed and modeled sea surface temperature and current at 0700 UTC 20 April 2006.

2-D histograms comparing observed and modeled temperature, salinity, and u-component of velocity model before (control simulation) and after (analysis) data assimilation.

Color indicates the log10 of the number of observations.

LaTTE 2006 reanalysis (60 days)

Ensemble average of the DA skill for analysis and forecast periods for different data withheld from system

Vertical bars are 95% confidence.

Vertical dashed line is boundary between analysis and forecast.

LaTTE 2006 reanalysis (60 days)

Removing any data from the analysis system has more or less predictable negative impact on the forecast.

More data is always better.

Ensemble average of the DA skill for analysis and forecast periods for different data withheld from system,

evaluated with respect to:

(a)glider-measured temperature and (b)satellite-measured SST

Vertical bars are 95% confidence.

Vertical dashed line is boundary between analysis and forecast.

LaTTE 2006 reanalysis (60 days)

Satellite SST is crucial to forecast skill for this skill metric; namely, comparison to new observations in the forecast window.

4DVar Assimilation (physics) in ROMS ESPreSSO

• 2006 reanalysis (SW06)– Basis for experiments with ecosystem and bio-optical

modeling

• 2009- operational System for OOI CI OSSE (ongoing)– 72-hour forecast (NAM-WRF meteorology)– tides, rivers, OBC HyCOM NCODA etc.– assimilates:

• altimeter along-track SLA• satellite IR SST• CODAR surface currents• climatology• glider T,S• GTS: XBT/CTD, Argo, NDBC

Work flow for operational ESPreSSO/MARCOOS 4DVar

Analysis interval is 00:00 – 24:00 UTCInput data preparation commences 01:00 EST (05:00 UT)• RU CODAR is hourly - but with 4-hour delay• RU glider T,S where available (approx 1 hour delay) • USGS daily average flow available 11:00 EST

– persist in forecast

• AVHRR IR passes (approx 2 hour delay)• HyCOM NCODA forecast updated daily• Jason-2 along-track SLA via RADS (4 to 16 hour delay)• GTS XBT/CTD, Argo, NDBC from AOML (intermittent)• T,S climatology (MOCHA)

Work flow for operational ESPreSSO/MARCOOS 4DVar Input preprocessing• RU CODAR de-tided (harmonic analysis) and binned to 5km

– variance within bins and OI combiner expected u_err used for QC – ROMS tide solution added to de-tided CODAR – this approach reduces

tide phase error contribution to cost function

• RU glider T,S averaged to ~5 km horiz. and 5 m vertical bins– developed thermal lag salinity correction using constrained parametric

fit to minimize statically unstable profiles

• AVHRR IR individual passes 6-8 per day– use Matt Oliver’s cloud mask; bin to 5 km resolution– [2006 reanalysis uses REMSS daily SST OI combination of AVHRR,

GOES, AMSR]

• Jason-2 alongtrack 5 km bins (no coastal corrections)– MDT from 4DVAR on “mean model” (climatology 3D T,S, usurface, wind)

Comparison of HF Radar observed and modeled M2 tide in LaTTE.

Work flow for operational ESPreSSO/MARCOOS

• Input preprocessing completes approximately 05:00 EST

• 4DVAR analysis completes approx 08:00 EST• 24-hour analysis is followed by 72-hour forecast

using NCEP NAM 00Z cycle available from NOMADS OPeNDAP at 02:30 UT (10:30 pm EST)

• Forecast complete and transferred to OPeNDAP by 09:00 EST

• Effective forecast is ~ 60 hours

OPeNDAP http://tashtego.marine.rutgers.edu:8080/thredds/catalog.htmlncWMS http://tashtego.marine.rutgers.edu:8081/ncWMS/godiva2.html

Output OPeNDAP http://tashtego.marine.rutgers.edu:8080/thredds/catalog.html ncWMS http://tashtego.marine.rutgers.edu:8081/ncWMS/godiva2.html

ESPreSSO operational system

Lessons from operational IS4DVAR for ESPreSSO

• More and diverse data is better– use all available observations and platforms

• Quality control– outliers in CODAR– cloud clearing from IR– coastal altimetry

• High resolution regional climatology– removes bias from Open Boundary Condition– improves representation of dynamic modes and adjoint-based

increments • IR SST individual passes work best with 4DVAR

– time variability is explicitly resolved– implications for optics

• Useful skill for operational applications– glider reachability forecast

• Physics analysis affects ecosystem/optics model skill

RU Endurance Line glider transect May 18-24, 2006

Small scales may be important to large scale dynamics (and ecosystem)

ttff = analysis = analysis final time final time

assimilation window

The analysis final hours becomes the data for the high-res 4DVAR, then forecast

high-res assimilationwindow high-res forecast

ttff = analysis = analysis final time final time

ttff + + = forecast = forecast horizon horizon assimilation window

Issues/Tasks ahead for 4DVAR ESPreSSO• High frequencies

– Filter inertial oscillations/tides in increment when updating outer loop– High frequencies in coastal altimetry (keep or remove IB correction?)

• Background error covariance– Can we use multi-variate “balance” constraint in coastal ocean?– Wide shelf, steep slope, anisotropic variability– Ecosystem / bio-optics?

• Ecosystem/bio-optics assimilation in 4DVar– have Adjoint for NZPD model (ecosystem emphasis) – have Adjoint for simple bio-optical model (IOP emphasis)

– Climatology? Initialization? – need dense data set for assimilation development

– twin experiments?– optics in Community Sediment Transport Model?

• Couple ecosystem/optics with thermodynamics– interaction is significant on NJ inner shelf (“just do it”)

• Downscaling– 1 km resolution grid (better bathymetry and land/sea mask)– 5 km IS4DVAR analysis at end of interval treated as “data”– assimilate analysis into 1 km model with 1 4DVAR cycle