Assessment of Rain Freshening Effects and Salinity Stratification in the Tropics Based on the Aquarius Version 3 Salinity Product
[26-Nov-2014] Meissner, T., Wentz, F., Scott, J., and Hilburn, K.
Presented at the 2014 Ocean Salinity Science and Salinity Remote Sensing Workshop
"The Aquarius Version 3.0 salinity product has been released to the public in June 2014. The retrieval algorithm and the quality of the data constitute a major improvement from earlier versions. Our presentation starts with a brief overview of the major components of the Aquarius V3.0 salinity retrievals and upgrades that were implemented since prior releases.
A validation study of the Aquarius V3.0 salinity product comparing it with measurements from ARGO drifters, moored buoys and the HYCOM model shows a global RMS error of about 0.24 psu for Aquarius monthly 150 km salinity maps, which is close to the requirement of 0.2 psu. However, local biases up to 0.4 psu are observed when comparing Aquarius salinities with those other products. Aquarius is salty at high latitudes and fresh at low latitudes. These biases show strong correlation with sea surface temperature. In Aquarius V3.0 mitigation has been implemented for the biases that are allocated to the imperfection of the geophysical model function used in the salinity retrievals. In order to do this it is essential to separate those biases from real rain induced freshening effects which occur in the upper ocean layer. The Aquarius radiometer measures the salinity within a few centimeters of the ocean surface, whereas the ARGO and moored buoys measurements are taken at 5 meter and 1 meter depth, respectively. Other rain induced effects that obscure the freshening are surface splashing and atmospheric liquid water absorption. The CONAE K/Ka-band microwave radiometer MWR on board the SAC-D spacecraft provides observation of surface rain rate and atmospheric absorption collocated to Aquarius in space and time. Rain rate observations from other microwave satellites (SSMIS, WindSat, TMI) can be used to quantify rain effects that occur within a certain time before the Aquarius measurements. The surface splashing effect can be removed by analyzing combinations of the vertical and horizontal polarizations, which are measured by the Aquarius radiometer and which have little or no dependence on salinity. It is thus possible to quantitatively assess the size of the rain freshening at the ocean surface and the stratification within the upper 5 m of the surface layer. On average the rain freshening is about 0.2 psu between surface and 5 m depth and about 0.1 psu between surface and 1m depth. We discuss the dependence on rain accumulation before the Aquarius observation."