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