Rain Impact Model V2.0 for Sea Surface Salinity: A Flag for Salinity Stratification

This research addresses the effects of rainfall on Aquarius (AQ) and Soil Moisture Ocean Salinity (SMOS) satellite sea surface salinity (SSS) retrievals using a macro-scale Rain Impact Model (RIM) that predicts transient SSS stratification based upon the rain accumulation and ocean surface wind speed over the previous 24 hours. The RIM V2.0 is an extension of V1.0, which only includes the rain accumulation effect but ignores the effect of wind speed on the vertical mixing. This new version introduces a term in the model that takes into account the effect of wind speed in the mechanical mixing, which translates into a dynamic vertical diffusivity. Thus, RIM can provide a valuable quality flag for the interpretation of AQ & SMOS SSS retrievals, and comparisons among 2-D rain patterns between SMOS, AQ and RIM will be presented as validation of the RIM performance.

A common reference for satellite SSS measurements is the Hybrid Coordinate Ocean Model (HYCOM), but there is a significant mismatch between the remote sensing sampling depth of approximately 0.01 m and the typical range of 5-10 m of in situ instruments. Under normal ocean conditions the upper layer of the ocean is well mixed and there is approximately uniform salinity; therefore satellite SSS retrievals are good estimates of the bulk salinity. Conversely, under rainy conditions, there is a dilution of the near-surface salinity that mixes downward by diffusion and mechanical mixing of gravity waves/wind speed forcing. This transient salinity stratification, significantly modifies the salinity gradient in the upper 1-2 m of the ocean; and therefore invalidates the usual assumption of well-mixed salinity. Typically, these salinity stratifications dissipate in a few hours and the upper layer becomes well mixed at a slightly fresher salinity.