Near-surface salinity stratification has been a topic of ongoing research, particularly for the calibration and validation of aquarius/ SAC-d and smap satellite data. A discrepancy between satellite…

This $442,503.36 grant award to Earth And Space Research (ESR), a nonprofit oceanographic research institute, was made by the National Aeronautics and Space Administration (NASA) to study near-surface salinity stratification. The research aims to investigate the processes controlling the generation and evolution of freshwater lenses in the Intertropical Convergence Zone, which create discrepancies between satellite-derived sea surface salinity data and in-situ measurements. ESR will combine ship-based observations, precipitation and wind data, and regional ocean modeling to characterize the frequency and impact of these salinity features on satellite salinity retrievals. This work is highly relevant to NASA's efforts to calibrate and validate salinity data from the Aquarius/SAC-D and Soil Moisture Active Passive (SMAP) satellite missions. The contract does not have a set-aside designation and is scheduled for completion by July 25, 2023.

Near-surface salinity stratification has been a topic of ongoing research, particularly for the calibration and validation of aquarius/ SAC-d and smap satellite data. A discrepancy between satellite retrievals and in-situ data, such as those from the argo system and shipboard thermosalinograph measurements, is present in regions of high precipitation, such as the intertropical convergence zone (itcz) due to the difference in measurement depth of approximately 1-2 CM and 5 m. An instrument capable of measuring the salinity and temperature at the l-band radiometric depth of 1-2 CM (the 'salinity snake') was deployed during the salinity processes in the upper ocean regional study 2 (spurs-2) cruise during august􀁇september 2016 and will be deployed during the second part of the spurs-2 field campaign in october of 2017. Additional measurements at 1 m, 2 m, and 5 m are provided by the ship, allowing the near-surface stratification to be estimated. Within the first deployment period of 35 days in the itcz, approximately 50 freshwater lenses with significant (>0.5 g/KG) salinity differences between the surface and 5M were found, ranging in size from 3 to 50 KM. A significant portion (>40%) of these lenses were encountered during calm conditions without any measured precipitation on the ship and some without any visible rain backscatter on a radar capable of observing precipitation within 40KM of the ship. While not conclusive, this suggests the potential persistence of such freshwater lenses for many hours or even days, and a strong dependence on wind-driven mixing, which is expected to act to destroy very-near surface stratification. The proposed study aims to investigate the processes controlling the generation of such freshwater lenses and to track their evolution. To perform a robust statistical analysis between in-situ observations and satellite retrievals, a correlation between the two types of data will be used to augment the existing observations. For this purpose, along-track data will be combined with the evolution of precipitation from ship-mounted radar, as well as large-scale satellite-derived precipitation and wind speed products. These are combined with statistical mean fields of surface forcings of rain events to produce realistic forcing fields that will be used to initialize and run a regional ocean modelling system (roms) to track the evolution of freshwater lenses. Our approach may be illustrated for the case of wind speed: a small convective cell in the atmosphere has a characteristic spatial extent (typically 3-6 KM) and an associated modification of the background wind field. By combining the point measurements of wind from the ship, along with radar information about precipitation, a two-dimensional wind-field around the ship is derived. Application of these techniques will allow US to realistically model the influence of wind speed, precipitation rates, advection, and mixing terms on near-surface salinity stratification. This analysis will be performed to classify both precipitation events and resulting freshwater lenses. IT will also characterize their frequency of occurrence and evolution, and hence ultimately estimate their effect on l-band satellite salinity retrievals. As such, the proposed study is highly responsive and relevant to items 2, 3, and 4 of the scope of the program.