Mitigation of Large Scale Biases in the Aquarius Salinity Retrievals
[13-Nov-2014] Meissner, T., Wentz, F., and Scott, J.
Presented at the 2014 Aquarius/SAC-D Science Team Meeting
The Aquarius Version 3.0 salinity retrievals have salty biases at mid-high latitudes and fresh biases in the tropics and subtropics when compared to ARGO or HYCOM. In addition we observe biases between the three Aquarius beams. These biases are largely due to imperfection in the geophysical model function that us used in the salinity retrieval algorithm, mainly the dielectric constant, the oxygen absorption and the surface roughness model. Version 3.0 provides an empirical post-hoc adjusted salinity that mitigates these biases by stratifying them as function of SST and removing them from the retrieved value. For upcoming releases it is necessary to obtain a better understanding of the physical cause for these biases and also to perform the correction at the TB level rather the salinity level. We have analyzed measured minus expected TB as 2-dimensional function of SST and wind speed. The result suggests that the deficiencies on the geophysical model function are a combination of several effects:
- Small errors in the dielectric constant model.
- Small errors in the oxygen absorption model.
- Small uncertainties in the auxiliary SST fields.
- The assumed SST dependence in the wind induced emissivity.
We have derived and analyzed the magnitude and channel signature of each of these error sources. We find that the largest part of the observed biases is due to errors in the SST dependence in the wind induced emissivity. Based on our analysis, we derive an adjustment to the geophysical model function and discuss its impact on the performance of the salinity retrievals, in particular the large-scale biases and the residual interbeam biases.