Comparison of Aquarius Measurements over Oceans with Radiative Transfer Models at L-band

Spaceborne radiometric measurements at L-band from the Aquarius instrument are compared to numerical model simulation. The empirical calibration of the data, performed over oceans only, is checked for consistency with measurements over the celestial sky. The calibration for the horizontal polarization is found to extend properly to the cold sky temperature, but the vertical polarization exhibit large biases, questioning the calibration accuracy over the whole dynamic range of measurements. The dependence of measurements on wind speed and wind direction is compared to model predictions. The accuracy of the model for surface roughness impact is estimated to be of the order of 0.25K over a large range of wind speeds, but is less accurate at the low and high end of the wind speed range, particularly for wind speeds much larger than 15 m/s. The impact of wind direction, while measureable at large wind speeds, is more uncertain to quantify because its amplitude is close to that of the radiometric noise.