1. Applications of Satellite Derived
Ocean Surface Vector Winds in Operational Marine Weather Forecasting
Joe Sienkiewicz, NOAA/NWS Ocean Prediction Center
Zorana Jelenak, UCAR/NOAA NESDIS

2. How is Wind Measured Over the Ocean?

3. Direct Measurements Buoy, Ships Pros Cons
Provide frequent updates at a single point, meeting or exceeding 1-hr refresh
Provide reference points for real-time diagnostics in the generation of the forecast/warning package
Complements synoptic scale satellite information by providing a frequently updated reference measurement
Cons
Limited coverage – only provide limited point measurements

4. Indirect Measurements
ASCAT Descending Passes
OSCAT Descending Passes
Wind speed and direction information needs to be derived from satellite measurements

5. Wind Vector Measurements Through History
Ku-band
QuikSCAT
June 1999-Nov 2009
SeaSAT 1978
110 days
OceanSat-2
Sep 2009
NSCAT
August 1996
Stick
Scatterometer
Pencil-beam
Scatterometer
Pencil-beam
Scatterometer
SeaWinds+AMSR
Dec 2002
ERS 1 & 2
Aug 1991, April 1995
ASCAT
Oct 2006
Stick
Scatterometer
Stick
Scatterometer
ASCAT-B
Sep 2012
C-band

6. What are Scatterometers?
Microwave Radars on Polar orbiters
C-band ~5GHz (~5cm), Ku-band ~14GHz (~2cm)
C-band (less impacted by rain than Ku)
Ku-band (rain impact)
designed to measure ocean surface backscatter (s 0 )
Capillary waves
infer wind speed
Multiple samples from different azimuth angles
wind direction solutions

7. Pr Pt
Surface roughness relates to s 0 and depends on friction velocity

9. QuikSCAT Measurement Geometry
The conical scanner onboard the QuikSCAT satellite provides a distinctive alternate method (compared to either the ERS or the NSCAT scatterometer) to receiving a multi-view observation of the backscattered microwave energy. In addition, the SeaWinds instrument takes advantage of the slightly different incident (look) angles (46 and 52 deg for the h- and v- pol emitters, respectively) and emittance properties of the two polarizations in order to obtain two distinct measurements for most azimuthal position. (Note: only one solution exists along the swath edge where the h- cone does not see out as far as the v-cone.) The required differences in viewing angle in order to solve the wind retrieval solution is then obtained by catching the same footprint on the ocean surface while in front of the satellite, then again, while behind the satellite. The best geometries are when the same area is caught at 90 deg differences (off to the side of the subtrack) while the worse geometry occurs along the subtrack because 0 deg (in front) and 180 deg (in back) differences do not provide a unique difference in viewing angle. As previously discussed, since the microwave emission off the ocean varies with the orientation of the Bragg waves , a series of possible speed/wave angle solutions are possible for each geometry.
WVC – Wind Vector Cell
25km → 76 WVC across swath 9 9

10. Measurement Geometry Far Swath Mid Swath Nadir Mid Swath Far Swath
H-pol
V-pol
2 observations
4 observations good azimuth diversity
4 observations poor azimuth diversity

11. 4-Look Solution
Most likely solution U=10m/s, χ=150°

13. Rain Effects
The radar signal is attenuated by the rain as it travels to and from the Earth’s surface  σ0
Retrieved wind speed
The radar signal is scattered by the raindrops. Some of this scattered energy returns to the instrument  σ0
The roughness of the sea surface is increased because of the splashing due to raindrops  σ0
Directional information can be lost
One of most significant limitations of QuikSCAT winds for use in areas of high precipitations is the known rain contamination of the measurement signal. Three main effects of rain on scatterometer measurements are:
1) Double attenuation of the radar signal while passing through the atmosphere
2) Enhancement of the radar return due to volume scattering from rain drops
3) Enhancement of the radar return due to rain modified ocean surface roughness 13
16 December 2009 13

15. A S C T R K

16. O S C A T R K

19. Rain Impact on OSCAT Retrievals Case 1

20. Rain Impact on OSCAT Retrievals Case 1

21. Examples

22. ASCAT 2100 UTC 02 NOV 2012

23. GDAS 2100 UTC 02 NOV 2012

30. NOAA Ocean Prediction Center – scatterometer impacts on operations
OSCAT and ASCAT
Forecasters use every pass to:
make warning and short-term
forecast decisions
and to estimate
- cyclone location, intensity,
extent of wind field
- strength and extent of
orographically enhanced jets
- wind field near strong SST
gradients
Results in improved warning and
forecast services over otherwise
data sparse oceans

31. SUPER STORM Sandy – ASCAT 1419 UTC29 Oct 2012