Soil Moisture and Sea Surface Salinity Derived from Satellite-Borne Sensors

Abstract

The monitoring of soil moisture and sea surface salinity over the Earth has been profoundly enhanced during the last thirteen years due to a new generation of satellite sensors. L-band radiometry is currently the only technology providing direct measurements of soil moisture, insensitive to surface roughness and distribution of elements in the soil, and the only technology the only technology for measuring that allows us to measure sea surface salinity from space. The Soil Moisture and Ocean Salinity (SMOS) and Soil Moisture Active Passive (SMAP) satellite missions resolve global and local variability with a spatial resolution of approximately 43 km, a swath width close to 1000 km, and a sampling time, for each mission, of at least twice every 3 days. These resolutions and samplings can be increased by either merging data from the two sensors, and with complementary information gathered from other passive or active sensors, or with in situ information at higher spatial resolution. Numerous scientific studies based on the use of this new type of measurement have led to a better understanding and constraint of the processes governing the variability of the water cycle, ocean circulation and the Earth's climate. The continuity of measurements, and the increased spatial and radiometric resolution is critical for fulfilling scientific needs. Future L-band radiometry missions currently being planned in Europe (the Copernicus Imaging Microwave Radiometer), and in China (the Ocean Salinity mission) should provide better constraints on auxiliary parameters by combining multiple frequencies, but they will not have improved spatial resolution beyond SMOS and SMAP. The temporal continuity with SMOS and SMAP will likely not be ensured. In parallel, new concepts are being developed to increase spatial resolution of both land and ocean parameters.

Salinity Fronts in the South Atlantic

“…We used SSS data from SMOS (Kerr et al, 2010;Reul et al, 2014;Boutin et al, 2018;Reul et al, 2020;Boutin et al, 2021;Boutin et al, 2023). We used high-resolution SMOS SSS data provided by the Barcelona Expert Center (BEC; smos-bec@icm.csic.es), where nine years (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019) of SMOS data were reprocessed and daily global SSS maps generated as documented by Olmedo et al (2021).…”

“…Foreword: Remote sensing of salinity fronts on a global scale began in this century owing to three satellite missions -SMOS, Aquarius, and SMAP -that carried radiometers whose data are used to estimate sea surface salinity (SSS; Reul et al, 2014;Boutin et al, 2018;Vinogradova et al, 2019;Reul et al, 2020;Boutin et al, 2021;Boutin et al, 2023). The SSS data from all three missions have been used, albeit sporadically, to study space-time variability of salinity fronts, mostly in the tropics (Qu et al, 2014;Kao and Lagerloef, 2015;Yu, 2015;Melnichenko et al, 2016;Nyadjro and Subrahmanyam, 2016;Asto et al, 2019;Reul et al, 2020;Bao et al, 2021;Boutin et al, 2023Boutin et al, , pp. 1469Boutin et al, -1471Vazquez-Cuervo et al, 2023).…”

Salinity Fronts in the South Atlantic

“…In the tropics, copious rains and freshwater discharges from great rivers produce surface water masses of low density; therefore, salinity fronts in the tropics often feature strong density gradients. In both cases (tropics and subtropics), salinity fronts shape ocean circulation by either enhancing (in the tropics) or reducing (in the subtropics) horizontal density gradients created by temperature gradients (Boutin et al, 2023) [1]. Salinity fronts mark water mass boundaries in the tropics, where surface water masses may have similar temperatures, yet different salinities.…”

“…Remote sensing of salinity fronts on a global scale began in this century, owing to three satellite missions-SMOS, Aquarius, and SMAP-that carried radiometers whose data were used to estimate sea surface salinity (SSS; Reul et al, 2014 [2]; Boutin et al, 2018 [3]; Vinogradova et al, 2019 [4]; Reul et al, 2020 [5]; Boutin et al, 2021 [6]; Boutin et al, 2023 [1]). The SSS data from all three missions have been used, albeit sporadically, to study space-time variability of salinity fronts, mostly in the tropics (Qu et al, 2014 [7]; Kao and Lagerloef, 2015 [8]; Yu, 2015 [9]; Melnichenko et al, 2016 [10]; Nyadjro and Subrahmanyam, 2016 [11]; Asto et al, 2019 [12]; Reul et al, 2020 [5]; Bao et al, 2021 [13]; Boutin et al, 2023 [1]; Vazquez-Cuervo et al, 2023 [14]). Yet, despite the proven potential of satellite SSS data, the global climatology of salinity fronts is lacking.…”

Guest Editorial: Special Issue on Oceanography, Hydrology and Glaciology from Space

“…Space-based monitoring of soil moisture and sea surface salinity is reviewed by Boutin et al (2023). The paper discusses the various remote sensing techniques used for this application.…”