High resolution vector wind retrieval from SeaWinds scatterometer data

A wind scatterometer is a radar designed to measure the normalized radar backscatter of the ocean's surface from multiple azimuth angles. These high precision backscatter measurements are used to estimate the near-surface vector wind (speed and direction) over the ocean with the aid of a geophysical model function relating the vector wind and the surface backscatter. The SeaWinds series of Ku-band scatterometer instruments use a dual pencil-beam, conically-scanning antenna to measure dual-polarization radar backscatter at a nominal spatial resolution of 25 km. Measurements are made at two incidence angles, 46 deg (H-pol) and 54 deg (V-pol). These measurements enable wind measurement at 25 km resolution with daily global coverage. The first SeaWinds instrument, known as QuikSCAT, has been operating since mid 1999 and has collected an extensive global data set.

The measurement geometry and sampling characteristics of SeaWinds are particularly well-suited for applying reconstruction/resolution enhancement algorithms to generate higher resolution backscatter estimates. By exploiting oversampling and the spatial overlap of the measurements, such techniques can be applied to create high-resolution backscatter estimates on a 2.5 km resolution spatial grid. The high resolution backscatter measurements support observation of sea ice extent, freeze/thaw, and iceberg tracking. The enhanced resolution measurements can also support high wind retrieval of wind speed and direction on a 2.5 km resolution grid. While the enhanced resolution QuikSCAT backscatter measurements are low-resolution compared to synthetic aperture radar (SAR) data, they are obtained at a diversity of azimuth angles that enable direct retrieval of the wind direction along with wind speed. By separately processing the backscatter measurements from each azimuth/incidence look combination, four high resolution backscatter observations for each grid element are generated. These are used with a conventional scatterometer wind retrieval algorithm to estimate the vector wind for each 2.5 km resolution grid element.

The resulting wind estimates reveal significant mesoscale features in both speed and direction, including convective events. Further the high resolution scatterometer winds can be used closer to shore than conventional 25 km wind estimates. As can be expected, the high resolution wind estimates are noisier than conventionally retrieved 25 km winds and tend to have larger biases. Nevertheless, simulation and actual data demonstrate the utility of the high resolution wind estimates. While the temporal sampling may not justify global wind retrieval at 2.5 km, near-coastal and severe weather systems can benefit from observations at this resolution. Experimental high resolution wind products are now being made available to both research and operational users. Data has proven particularly useful in hurricane tracking. To support operational tracking of extreme weather events, regional products centered on NOAA “invest” regions are produced at 2.5 km/pixel resolution and distributed in near real-time.

Here, we consider the application of QuikSCAT high resolution winds in mesoscale and severe weather studies. Selected examples and simulation are presented. QuikSCAT high resolution winds are compared to co-located Radarsat SAR-derived wind fields. A high degree of correlation and consistency in the estimates are found. While the SAR-derived winds exhibit finer resolution, the scatterometer data has broader coverage, though suffers from ambiguity selection problems at times. The effect of land contamination of near-coastal scatterometer measurements is studied and an improved land masking procedure is developed. High resolution scatterometer winds retrieved using different model functions are compared. The primary differences occur at high wind speeds, which are often associated with rain.