Studies of the effects of rain on the performance of the SMAP radiometer surface salinity estimates and applications to remote sensing of river plumes

In the coastal ocean, rivers provide a major source of biogeochemical, ecological, hydrographic and dynamical variability. Freshwater input by rivers produces buoyant low salinity plumes impacting ocean currents and stratification, which in turn influence air-sea heat exchange and oceanic vertical mixing. Rivers are also sources of nutrients and organic material, which can be transported far from the river mouth impacting marine ecosystems over large areas. As such, river plumes have long been an active area of study within the oceanographic scientific community. Despite their importance and vast study, there remains a striking lack of observations within regions influenced by river plumes that are suitable for characterizing the spatial and temporal scales and magnitudes of variability of their impacts on ocean salinity. Salinity measurements in the coastal region are difficult to make over long time periods due to the need for frequent servicing of instruments caused by biofouling and instrument drift. Because of this and the expense of moorings, time series of salinity measurements at fixed locations are not available at high enough density to capture the spatial variability in regions influenced by river plumes. The Aquarius mission helped pioneer the era of spaceborne sea surface salinity measurements [Lagerloef, et al; 2008]. Salinity estimates derived from its L-band radiometer provided an unprecedented view of the state of global ocean salinity and its dynamics, but its relatively low (100–150 km) resolution permitted only observations of very distant effects of the largest rivers (e.g., Mississippi and Amazon) on the ocean salinity [Gierach et al., 2013; Korosov et al, 2015]. The SMAP (Soil Moisture Active Passive) Mission was originally intended to observe land. Since Aquarius stopped working (June 10, 2015), the operational plan for SMAP now is to continue data collection over the oceans. The microwave sensors, the radar and radiometer, operate at the same frequency as those on Aquarius. SMAP instruments became the only NASA mission with the capability to monitor sea surface salinity. The combination of fully polarized, simultaneous active and passive L-band measurements, including their much higher spatial resolution than Aquarius, and with excellent radiometric precision, makes this satellite superior and unique. Due to a component failure, the radar instrument on SMAP became inoperable in early July 2015. The radiometer continues to collect valuable data for the application to sea surface salinity observation. This project is focused on improving methods of estimating salinity from SMAP data that will be applied to yield new information on salinity variability in regions impacted by river plumes. SMAP now allows salinity measurements at 40 km resolution, which allows observations of the salinity fields associated with smaller rivers (such as the Apalachicola River, with mean discharge rate approximately 5% that of the Mississippi River, but at times impacting the salinity field 200km from the coast; Morey et al., 2009).