Uncertainty Estimation of SAR Current Retrievals using an Atmospheric
NWP Ensemble
Abstract
Retrieval of ocean surface current using single-antenna synthetic
aperture radar (SAR) relies on the removal of the wind and wave signal
(wind-wave bias) from the observed SAR Doppler shift. This is often
performed using deterministic atmospheric and wave models input into a
geophysical model function, though atmospheric models are chaotic, and
their predictability is flow-dependent. 251 Sentinel-1 SAR scenes,
obtained in 2023 over Skagerrak and Kattegat, are analyzed to assess the
uncertainty in the radial velocity estimation caused by uncertainties in
the wind speed and direction provided by MEPS, an operational regional
atmospheric ensemble system. Using ensemble surface wind direction and
speed from MEPS, we investigate the conditions under which the radial
velocity retrieval uncertainties exceed community-guided thresholds. The
maximum wind speed and direction values that meet the specified
thresholds are also estimated using two approaches. Our findings show
that retrievals have a higher degree of uncertainty when the wind speed
uncertainty exceeds 20% of the mean field, with larger uncertainties
associated with low-wind speed conditions. A strong dependency between
satellite antenna-look and maximum uncertainty values is reported.
Employing ensemble models on radial velocity uncertainty quantification
has promising potential and may be extended to operational global
products.