Comparison of Synthetic Aperture Radar–Derived Wind Speeds with Buoy Wind Speeds along the Mountainous Alaskan Coast

Abstract

Satellite-borne synthetic aperture radar (SAR) offers the potential for remotely sensing surface wind speed both over the open sea and in close proximity to the coast. The resolution improvement of SAR over scatterometers is of particular advantage near coasts. Thus, there is a need to verify the performance of SAR wind speed retrieval in coastal environments adjacent to very complex terrain and subject to strong synoptic forcing. Mountainous coasts present a challenge because the wind direction values required for SAR wind speed retrieval algorithms cannot be obtained from global model analyses with as much accuracy there as over the open ocean or adjacent to gentle coasts where most previous SAR accuracy studies have been conducted. The performance of SAR wind speed retrieval in this challenging environment is tested using a 7-yr dataset from the mountainous coast of the Gulf of Alaska. SAR-derived wind speeds are compared with direct measurements from three U.S. Navy Oceanographic Meteorological Automatic Device (NOMAD) buoys. Both of the commonly used SAR wind speed retrieval models, CMOD4 and CMOD5, were tested, as was the impact of correcting the buoy-derived wind speed profile for surface-layer stability. Both SAR wind speed retrieval models performed well although there was some wind speeddependent bias. This may be either a SAR wind speed retrieval issue or a buoy issue because buoys can underestimate winds as wind speed and thus sea state increase. The full set of tests is performed twice, once using wind directions from the U.S. Navy Operational Global Atmospheric Prediction System (NOGAPS) model analyses and once using wind direction observations from the buoys themselves. It is concluded that useful wind speeds can be derived from SAR backscatter and global model wind directions even in proximity to mountainous coastlines.

Remote Sensing of the Surface Wind Field over the Coastal Ocean via Direct Calibration of HF Radar Backscatter Power

“…For example, a number of recent studies have compared remote QuikSCAT or similar satellite-based extractions for wind speed and direction to in situ, normally buoy-based, wind observations (Carvalho et al 2014) and various reanalysis products (see Carvalho et al 2012), finding rms errors for speed and direction of 1.7-2 m s 21 and 408-508, respectively. Synthetic aperture radar (SAR) satellite-based winds have been shown to have rms errors of 2 m s 21 and directional ambiguities of 6408 (Fisher et al 2008). While generally comparable to that shown here, it is important to note that only SAR observations have potential resolutions that approach HF radar in the coastal ocean and that none of these observational platforms has the temporal sampling abilities of HF radar.…”

Wind Class Sampling of Satellite SAR Imagery for Offshore Wind Resource Mapping

“…The root-mean-square error on SAR wind speeds, with respect to other data sources, has been reported by several authors to be 1-2 m s 21 (Furevik et al 2002;Hasager et al 2004;Monaldo et al 2001). Fisher et al (2008) have shown that the use of SAR in high-latitude, coastal regions with strong mesoscale forcing is as accurate as using it in the open ocean.…”

Wind-speed inversion from HF radar first-order backscatter signal

“…Schroeder et al 1985) and Synthetic Aperture Radar (SAR) (cf. Monaldo et al 2001;Fisher et al 2008) provide wind information over large areas but with a temporal resolution that is limited by their orbit repetition time that may extend to several days.…”