Wind Stress Data Products for Model Comparison

2012 ECCO MeetingCalifornia Institute of Technology

David Moroni10/31/12

Introduction

• Wind stress is the most direct forcing mechanism for air-sea energy transfer over the global oceans.

• Aside from momentum exchange, wind stress is a significant component in air-sea fluxes of heat, fresh water, CO2, O2, and Nitrogen.

• GCMs, such as the MITgcm, need accurate estimates of wind stress to ensure the remaining state estimates of circulation and energy fluxes are physically consistent with the real world.

Which to Choose

• Many ocean wind and stress products are out there, but almost none are without their limitations.

• The most common limitations include:– sparse sampling– most are wind products

• issues converting from wind to stress (e.g., which Cd to use?)

– Scatterometer winds are not the same as reanalysis winds• significant differences in areas of strong currents and transient eddies

– local time-of-day sampling could present significant biases related to the diurnal cycle

• SST gradients are known to cause anomalous perturbations in the wind stress (O’Neill et al. 2003, 2010)

Latent Heat Flux: Global Probability Density

Momentum Flux: Global Probability Density

411/10/2009 ECCO2 Meeting – CalTech, Pasadena, CA

Comparisons to In Situ Stress• Recent developments in sea-state dependent stress

parameterizations have been adjusted to provide a better fit to in situ stress observations (Bourassa 2004, 2006):

• We now have a‘prototype’ QuikSCAT Level-2 wind stress retrieval product based on the Bourassa (2006) parameterization.

Large and Pond (1981) Bourassa (2006)

511/10/2009 ECCO2 Meeting – CalTech, Pasadena, CA

Daily Averaged Stress Magnitude 2004-2005: Mean Differences (Top: Iter00-QS, Bottom: Iter30-QS)

Daily Averaged Stress Magnitude 2004-2005: RMS Differences (Top: Iter00-QS, Bottom: Iter30-QS)

6-Hourly Comparisons:6 Hour Co-location

6-Hourly (6hr) Stress Magnitude 2004-2005: Mean Differences (Top: Iter00-QS, Bottom: Iter30-QS)

Daily Averaged Stress Magnitude 2004-2005: Mean Differences (Top: Iter00-QS, Bottom: Iter30-QS)

6-Hourly (6hr) Stress 2004-2005: RMS Differences (Top: Iter00-QS, Bottom: Iter30-QS)

Daily Averaged Stress Magnitude 2004-2005: RMS Differences (Top: Iter00-QS, Bottom: Iter30-QS)

Daily Scatter Plots (left: iter00-QS, right: iter30-QS): x-axis = QS, y-axis=ECCO

Stress Magnitude

Zonal Stress

Meridional Stress

r = .724

r = .801

r = .750

r = .721

r = .798

r = .747

6-Hourly Scatter Plots (left: iter00-QS, right: iter30-QS): x-axis = QS, y-axis=ECCO

Stress Magnitude

Zonal Stress

Meridional Stress

r = .761

r = .813

r = .784

r = .758

r = .810

r = .782

Consideration of Intercalibrated Level 4 Products

• Advantages:– Long time series (24+ years)– Gap-free, near-global coverage at sub-daily intervals (e.g., 6-hourly)– Sensor intercalibration to reduce sensor bias– Unlike standard ECMWF or NCEP analysis products, these products

are tuned to represent equivalent neutral scatterometer winds• Disadvantages:

– Time series inconsistency with number of assimilated satellite observations (only 1 sensor: 1987-1990)

– Most sensors are passive (i.e., no measurable wind direction) and must rely on NWP-based directional data

– Reduced spatial resolution noticeable in derivative fields

Monthly Averaged Curl (Top: June 1988, Bottom: June 2003)

Conclusions• Level 2 wind/stress products have generally been the more

preferred option for ECCO model comparison.• 6-Hourly co-location seems to remove some bias, but adds

additional “noise” to the signal.– the scatterometer is clearly seeing a different signal than the model,

primarily in areas with strong currents and eddies

• Level 4 products do offer some considerable advantages and compare well with Level 2 products, although it remains to be seen how the uncertainty differences in the time series would play into ECCO model comparison.

• Next steps to include ECCO model comparisons with:– updated re-processed L2 winds to generate a new wind stress product– intercalibrated L4 wind stress products

Questions?

Bonus Slides

6-Hourly vs. Daily Averaged Stress Magnitude 2004-2005: Difference of Mean Differences [Iter30-QS]-[Iter00-QS] (top: Daily, bottom:6hr)

6-Hourly vs. Daily Averaged Zonal Stress 2004-2005: Difference of Mean Differences [Iter30-QS]-[Iter00-QS] (top: Daily, bottom:6hr)

6-Hourly vs. Daily Averaged Meridional Stress 2004-2005: Difference of Mean Differences [Iter30-QS]-[Iter00-QS] (top: Daily, bottom:6hr)

6-Hourly vs. Daily Averaged Stress Magnitude 2004-2005: RMS Difference of Mean Differences (top: Daily, bottom:6hr)

6-Hourly vs. Daily Averaged Zonal Stress 2004-2005: RMS Difference of Mean Differences (top: Daily, bottom:6hr)

6-Hourly vs. Daily Averaged Meridional Stress 2004-2005: RMS Difference of Mean Differences (top: Daily, bottom:6hr)