Satellite Derived Ocean Surface Vector Winds Joe Sienkiewicz, NOAA/NWS Ocean Prediction Center Zorana Jelenak, UCAR/NOAA NESDIS

Direct Measurements Buoy, Ships –Pros 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

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

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

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 (   0 ) Capillary waves infer wind speed –Multiple samples from different azimuth angles wind direction solutions

7 QuikSCAT Measurement Geometry WVC – Wind Vector Cell 25km → 76 WVC across swath

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

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

16 December 2009 Rain Effects The radar signal is attenuated by the rain as it travels to and from the Earth’s surface  σ 0 Retrieved wind speed The radar signal is scattered by the raindrops. Some of this scattered energy returns to the instrument  σ 0 Retrieved wind speed The roughness of the sea surface is increased because of the splashing due to raindrops  σ 0 Retrieved wind speed Directional information can be lost 11

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Rain Impact on OSCAT Retrievals Case 1

SUPER STORM Sandy – ASCAT 1419 UTC29 Oct 2012

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