Abstract
When rain falls over the ocean, it produces a vertical salinity profile
that is fresher at the surface. This fresh water will be mixed downward
by turbulent diffusion through gravity waves and the wind stress, which
dissipates over a few hours until the upper layer (1-5 m depth) becomes
well mixed. Therefore, there will be a transient bias between the bulk
salinity, measured by in-situ instruments, and the satellite-measured
SSS (representative of the first cm of the ocean depth). Based on
observations of Aquarius (AQ) SSS under rain conditions, a rain impact
model (RIM) was developed to estimate the change in SSS due to the
accumulation of precipitation previous to the time of the satellite
observation. RIM uses ocean surface salinities, from the HYCOM (Hybrid
Coordinate Ocean Model) and the NOAA global rainfall product CMORPH, to
model transient changes in the near-surface salinity profile. Also, the
RIM analysis has been applied to SMOS (Soil Moisture and Ocean Salinity)
and SMAP (Soil Moisture Active Passive), with similar results observed.
The original version of RIM assumes a constant vertical diffusivity and
neglects the effects of wind and wave mixing. However, it has been shown
that the persistence of rain-induced salinity gradients depends on wind
speed, with freshening due to rain during weak winds (less than 2 m/s)
persisting for 8 hours or more. Moreover, the mechanical mixing of the
ocean caused by wind and waves rapidly reduces the salinity
stratification caused by rain. Also, previous results using RIM, in the
presence of moderate/high wind speeds, show that the model overestimates
the effect of rain on the SSS, which suggests that for RIM to accurately
model the near-surface salinity stratification, the effect of wind needs
to be included in the model. To address this issue, this paper will
focus on an improved RIM-2 that parameterizes the effects of wind on the
vertical diffusivity (Kz). Results will be presented that compare RIM
and RIM-2 calculations at different depths for several Kz
parametrizations. Also, comparisons, between RIM-2 at depths of several
meters with measurements from in-situ salinity instruments, will be
presented.