Assessment of Rain Impact on the Aquarius Salinity Retrievals
[13-Nov-2014] Meissner, T., Wentz, F., Scott, J., and Hilburn, K.
Presented at the 2014 Aquarius/SAC-D Science Team Meeting
Fresh biases up to 0.4 psu are observed when comparing Aquarius salinities with ARGO or HYCOM in the tropical and subtropical regions. These biases are partly due to inaccuracies in the geophysical model function that correlate with sea surface temperature and partly due to salinity stratification in the upper ocean layer. Aquarius measures salinity within a few centimeters of the surface, which is fresher during and after rain effects than the 5 m depth layer, to which ARGO and HYCOM refer to. In order to make improvements to the salinity retrieval algorithm and avoid overcorrecting it is essential to separate these two effects. Our presentation shows how to do that and addresses the following issues:
- We have collocated rain rate measurements from various sources with Aquarius: the CONAE MWR, SSMIS, WindSat, TMI, GMI, and the CMORPH data set. That allows us not only to filter out rain at the Aquarius observation but also quantify the salinity stratification in the upper ocean layer as function of rain accumulation. Salinity measurements from the PMEL moored buoy array at 1m depth help further to assess the stratification effect.
- We find that the rain freshening amounts to only a third of the observed fresh biases in the tropics and thus conclude that the bulk of the freshening is spurious and due to inaccuracies in the geophysical model function. We have derived a mitigation for these biases which is to be implemented in the upcoming V4.0 release
- We have studied the effect of rain splashing on the ocean surface. It is possible to find combinations between the Aquarius V-pol and H-pol channels that are insensitive to changes in the surface salinity but sensitive to surface roughness. Analyzing these brightness temperature differences shows little to no sensitivity to rain at the surface if scatterometer derived wind speed is used in the surface roughness correction. We conclude that the scatterometer wind speed itself is an appropriate proxy for surface roughness including rain splash effects and no splash correction should be done in addition.
- We have also studied the impact of rain and stratification on the sensor calibration. The calibration of Aquarius V3.0 is based on matching the Aquarius with the HYCOM reference globally. No rain filtering is done for that matching and the argument has been made that, as a consequence, the global Aquarius salinity average is too high. We have computed the impact of rain filtering on the calibration and found that filtering rain changes the global average by only about 0.03 psu and thus the rain freshening in the tropics has very little impact on the sensor calibration.