Beyond L-band: 0.4-2 GHz wideband radiometry for observing surface salinity in cold seawater

Sea surface salinity (SSS) is a key variable for understanding ocean circulation, freshwater fluxes, and climate variability. While L-band radiometry (1.4 GHz) has enabled global monitoring of SSS from space, the errors of retrieved SSS are two to three times larger in cold waters than in warm waters due to reduced sensitivity of the radiometric signal to SSS. To address this limitation, this paper investigates the potential of wideband passive microwave radiometry (0.4–2 GHz). Using a multi frequency Bayesian retrieval scheme, and two extreme instrumental designs, we show that wideband radiometry has the potential to reduce SSS uncertainties by a factor of 2 to 3 in cold and salty waters, and by more than a factor 4 for SSS around 20 pss. Wideband SSS retrievals would also be more resilient to radiometric calibration issues than L-band. Consequently, wideband radiometry could enable the detection of geophysical SSS anomalies in many polar regions where this is not possible with current Earth observation satellites. The lowest frequencies are the ones contributing the most to the uncertainty decrease. Our study highlights the need to pursue research into technical and methodological development for optimizing radiometric measurements in the low frequency range, and minimize the impact of Radio Frequency Interference, of the galactic signal and, during the day, of the sun. Wideband radiometry could represent a breakthrough with respect to L-Band radiometry for monitoring SSS variations in the polar regions where large climate variability is ongoing, under the interaction between the ocean, atmosphere and cryosphere.