Nonspinning QuikSCAT Winds

Retrieving Ocean Surface Wind Speeds from the Nonspinning QuikSCAT ScatterometerBryan W. Stiles, R. Scott Dunbar, and Alexandra H. ChauSimon Yueh (presenting for the authors) Jet Propulsion Laboratory, California Institute of Technology

Jet Propulsion Laboratory California Institute of TechnologyQuikScatOverviewCurrent Status of QuikSCATQuikSCAT stopped spinning in November 2009.Current data is all single azimuth dataLarge number of looks; reduced noiseNo directional discriminationNarrow swath, global coverage once per month.Single look wind retrieval methodDetermine wind speed from backscatter by assuming ECMWF wind direction is correct.Spectral and Noise characteristics of single look windsComparison of spinning and non-spinning QuikSCAT wind spectraInvestigation of residual differences from ECMWF windsMap of Differences from ECMWFRain effects Overall westward bias in scatterometer windsOcean current effectsConclusions2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 2Jet Propulsion Laboratory California Institute of TechnologyQuikScatCurrent Status of QuikSCATQuikSCAT stopped spinning on November 23, 2009Since then we have obtained single azimuth data from a variety of incidence angles and polarizations.Data will be used to Develop geophysical model functions for OceanSAT-2Calibrate cryosphere products for OceanSAT-2Retrieve accurate wind speed profiles on a narrow (30 km) swath with global coverage once per month.

Start DateEnd DatePolarizationIncidence Angle(deg)16 March 201014 April 2010Vertical co-pol5415 July 20101 Sept 2010Vertical co-pol591 Sept 20104 March 2011Horizontal co-pol504 March 20109 June 2011Vertical co-pol599 June 201112 July 2011Vertical co-pol5412 July 2011presentHorizontal co-pol46This is what we are discussing today.2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 3Jet Propulsion Laboratory California Institute of TechnologyQuikScatCurrent Status of QuikSCATThe two primary purposes of this talk are toCompare the spectra of the nonspinning wind data with that of the spinning wind data to quantify the resolution and noise characteristics of each data set.Compare the relatively noise-free non-spinning winds to ECWMF winds, SSM/I rain rate, and OSCAR current information in order to better quantify the differences between scatterometer and NWP winds and to demonstrate the utility of the data set.We have concentrated on the June-July 2011 data, because it has the same incidence angle and polarization as the nominal QuikSCAT outer beam and thus we can use the same GMF to retrieve winds and more readily compare the spinning and nonspinning data sets.2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 4Jet Propulsion Laboratory California Institute of TechnologyQuikScatSingle Look Wind Retrieval MethodStep 1: Average 50 consecutive footprint (egg) measurements to produce a ~ 30 km by 30 km backscatter measurement.Slice processing is not done because it would require extensive recalibration and accurate attitude knowledge.Footprints move 3.8 km on ground during averaging.Step 2: Determine co-located ECMWF direction and thus relative azimuth.Step 3: Assuming ECMWF direction is correct, invert geophysical model function (GMF) to obtain retrieved speed.Use Remote Sensing Systems Ku2011 GMF, (Ricciardulli and Wentz)Step 4: Co-locate SSM/I rain rate measurements for use in rain flagging. 26% of wind vector observations within 30 minutes of a SSM/I co-location.58% with 90 minutes2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 5Jet Propulsion Laboratory California Institute of TechnologyQuikScatComparison of Spinning and Non-Spinning Wind Profiles

Here we compare a 1000-km long non-spinning wind speed profile (bottom) with a similar profile (top) obtained when QuikSCAT was spinning.

Both profiles are compared with co-located ECWMF and SSM/I wind speeds.Rainy data is omitted.2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 6Jet Propulsion Laboratory California Institute of TechnologyQuikScatSpectra ComparisonNon-spinning QuikSCAT data is compared withECMWF (much lower energy at meso-scales)Spinning QuiKSCAT data obtained from slices binned in 12.5 km by 12.5 km cells.Factor of two better resolution than non-spinning data which uses whole footprints rather than slices.2 orders of magnitude more instrument noise than non-spinning data.An analytical spectrum is produced assuming a k-2 slope with added white noise and a low pass filter representing the typical resolution of the backscatter measurements used in the retrievals.The analytical spectra for the spinning and nonspinning QuikSCAT wind speeds are consistent with the observed spectra.Nonspinning Spectra were computed using data from the month of June 2011.Spinning QuikSCAT spectra were computed using a full year (2008) of wind data.Wind speed spectra are computed for four different spatial regions.

2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 7Jet Propulsion Laboratory California Institute of TechnologyQuikScatEquations for Analytical Spectra

Half power contour FWHMGaussian filter models antenna spatial responseA = constant scale factor used to match the observed magnitude of the spectraAnalytical Spectrum S(k)2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 8Jet Propulsion Laboratory California Institute of TechnologyQuikScatSpectra by region

Freilich and Chelton, Journal of Physical Oceanography, 19862011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 9Jet Propulsion Laboratory California Institute of TechnologyQuikScatWavenumber Spectra of Pacific Winds Measured by the Seasat Scatterometer (article)AuthorM.H. Freilich and D.B. CheltonJournalJournal of Physical OceanographyYear1986Volume16Pages741-757KeywordsspectraAbstractVector winds measured by the Seasat-A Satellite Scatterometer (SASS) are analyzed to determine the spatial structure of oceanic surface winds over wavelengths from 200 to 2200 km. The analysis is performed in four latitudinal bands in the Pacific Ocean. Sampling characteristics of SASS preclude the possibility of determining full two-dimensional spectra; the analysis is therefore limited to one-dimensional (along the satellite ground track) spectra of vector wind components and kinetic energy.

The salient features of the results are summarized as follows. (i) For each of the four geographic regions, the spectra of meridional and zonal wind components and of kinetic energy are consistent with a power-law dependence on wavenumber for midlatitude regions in both the Northern and Southern hemispheres the wave-number dependence of kinetic energy is k2.2, while for tropical regions in both hemisphere it is k1.9. (ii) For each individual region, the spectral dependence on wavenumber is nearly the same for both velocity components and for kinetic energy. (iii) Comparisons of zonal and meridional component spectra indicate that midlatitude winds may be isotropic, while tropical winds may be significantly anisotropic. (iv) The coherence between wind components is small everywhere.

92011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 10

Jet Propulsion Laboratory California Institute of TechnologyQuikScatSpectra of regions 3 and 4

2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 11Jet Propulsion Laboratory California Institute of TechnologyQuikScatResults from Spectra StudyNoise floor for non-spinning data is ~ 2 orders of magnitude lower than for spinning data.The non-spinning wind speed spectrum (red ) is consistent with a spectrum (black solid curve) with 30-km measurement resolution and k-2 slope.The noise in the new 12.5 km JPL reprocessing (cyan) of the nominal (spinning) QuikSCAT data is less than that of the current (blue) JPL data set.The spinning wind speed spectrum (cyan ) is consistent with a spectrum (black dashed curve) with 15-km measurement resolution and k-2 slope.The spectra of the non-spinning wind speeds (red) are similar to the spectra of the backscatter (green). Interesting Observation: There is excess energy in all the observed spectra at 80-km scale for regions 2 and 3 as compared to regions 1 and 4.2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 12Jet Propulsion Laboratory California Institute of TechnologyQuikScatMap of Difference of Non-Spinning Wind Speeds from ECWMFSpeed Bias (Non-spinning QuikSCAT ECMWF m/s)Standard Deviation of Difference (m/s)

The speed bias (top) of the non-spinning QuikSCAT speeds w.r.t. ECMWF winds shows prominent discontinuities around + or 40 deg latitude and features of rain.

The standard deviation (bottom) of the difference also shows the effect of rain.

All data was included in these plots. No rain flagging was applied.2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 13Jet Propulsion Laboratory California Institute of TechnologyQuikScatRain Effects on Retrieved Winds

0.0 1.5 3.0 mm/hr

The top panel shows the average SSM/I rain rate from http://www.ssmi.com for the month of June 2010.

The standard deviation of the difference between QuikSCAT non-spinning wind speeds and ECMWF wind speeds is highly correlated with rain.Standard Deviation of Wind Speed Difference form ECMWF, June 2011 SSM/I Rain Rate, June 20102011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 14Jet Propulsion Laboratory California Institute of TechnologyQuikScat

Overall Westward Bias

The prominent latitudinal discontinuity in the wind speed bias (top) is due to a 0.3 m/s westward bias in the zonal component of the scatterometer winds w.r.t the ECMWF winds that has been observed for multiple scatterometers (Hristova and Rodriguez).

The middle panel shows the residual bias when 0.3 m/s is added to the zonal component of the QuikSCAT winds.

The bottom panel is the average zonal winds. Note the sign change between the tropics and the high latitudes. This sign change is why a constant offset in zonal winds yields a speed reduction in high latitudes and a speed increase in the tropics.Speed Bias (QuikSCAT ECMWF m/s)Zonal Component of QuikSCAT non-spinning winds (m/s)2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 15Residual Speed Bias with 0.3 m/s Westward Bias Removed Jet Propulsion Laboratory California Institute of TechnologyQuikScatOverall Westward Bias

Speed Bias (QuikSCAT ECMWF m/s) with 0.3 m/s Eastward Bias removed

0.0 1.5 3.0 mm/hrSSM/I Rain Rate, June 2010After the 0.3 m/s westward bias is removed from the QuikSCAT winds, the remaining bias is highly correlated with the distribution of rain.

Arguably, the westward scatterometer bias is actually an eastward ECMWF bias (Hristova and Rodriguez).2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 16Jet Propulsion Laboratory California Institute of TechnologyQuikScatOcean Current Effects

OSCAR monthly average current June 2011 (m/s)After omitting wind vectors that are co-located with nonzero SSM/I rain rates (within 30 minutes), we compared the residual differences from ECMWF with OSCAR ocean currents.

The residual differences when binned by OSCAR ocean current speed are consistent with the root sum square of a nominal ECMWF error (1 m/s) and the OSCAR current.2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 17Jet Propulsion Laboratory California Institute of TechnologyQuikScatConclusionsBecause QuikSCAT stopped spinning, nominal operation ceased in November 2009.The current non-spinning state of QuikSCAT limits global coverage to once a month; disallows wind direction determination due to single azimuth looks;makes rain detection difficult due to single polarization availability.Nonetheless, the currently acquired data has a unique feature.Due to the large number of looks, non-spinning QuikSCAT winds have negligibly small errors due to instrument noise.Nonspinning QuikSCAT winds can be useful for analyzing the differences between scatterometer winds and numerical wind products.detecting small effects on sigma-0 that are harder to observe under higher noise conditions.monitoring long term trends in 6:00 AM / 6:00 PM local time wind speeds.

2011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 18Jet Propulsion Laboratory California Institute of TechnologyQuikScatReferencesRicciardulli and Wentz, (Ku2011 Ku-band Geophysical Model Function) manuscript in preparation, technical report on http://www.ssmi.comHristova-Veleva, S. M., and E. Rodriguez, 2010: SST-Induced Surface Wind Response: Comparison of QuikSCAT and ASCAT depiction of the phenomenon, OVWST meeting, Barcelona, Spain, May 20102011/07/28 IGARSS 2011QuikSCAT Nonspinning Winds 19Jet Propulsion Laboratory California Institute of TechnologyQuikScat