THE NEAR-SURFACE LAYER OF THE OCEAN
rainrates and downwelling short- and long-wave radiation.
et al.
(1997) averaged these data sets over hourly intervals and used them with the
surface renewal model to estimate the parameters of the freshwater skin of
the ocean.
Figure 2-25 shows time series of the rainrate, the salinity difference
across the freshwater skin and the salinity measured at the surface for a rainy
time period in January 1993. The direct comparison of the model and field
data is not feasible because the model calculation is for the freshwater skin
(the upper few tens of micrometers of the ocean only) while the measured
salinities include surface water from the upper few centimeters.
et al. (1997) assumed that the freshwater skin layer must
account for the greater part of the observed salinity changes because even
heavy rainfall ( 60
| mm h
-1 ) alters the salinity in the upper mixed layer by
less than 1 psu (Kantha and Clayson, 1994). Because the wind speeds during
this experiment were rather low, the salinity changes in the mixed layer can
be expected to be slightly higher. Calculations with the model of Price et al.
(1986) show maximum differences exceeding 2 psu over the top half meter
on 3 January that are of the same magnitude as the difference across the
freshwater skin. The observed salinity changes on this day are up to 4 psu
(some of this signal may, however, be associated with nonlocal rain).
For other rainfall events during this time series the observed salinity
differences in the mixed layer are generally smaller than those across the
skin. A full explanation of the salinity deficits measured can only be given
by considering the combined differences across the skin and mixed layers,
which has not yet been done.
2.5.9 Discussion
Of the various processes that can modify the molecular layers during
rainfall, the most important appear to be the rain-induced mixing that leads
to enhanced surface renewals and the additional surface and volume cooling
by raindrops with temperatures below the sea surface temperature.
Simulations with the renewal model have shown that the additional surface
cooling by rain could exceed 1
o C, if the rain did not affect the surface
renewal time. Nevertheless, the additional mixing and the implied increase
in renewal frequencies limit the effect to less than 0.1
o C when the rainwater
is 5 K cooler than the sea surface. The more impressive effect is the creation
of a haline molecular diffusion layer during rainfall—a freshwater skin of
the ocean—that exhibits salinity differences greater than 4 psu during strong
rains.
Since laboratory studies overemphasized large drops that are a small
fraction of the natural raindrop spectrum, the calculations of the rain-induced
surface roughness for a realistic drop size spectrum shows that this effect has
been overestimated in the past (Houk and Green, 1976). The wind-induced
140
Schl ssel
ü
Schl ssel
ü
rainrates and downwelling short- and long-wave radiation.
et al.
(1997) averaged these data sets over hourly intervals and used them with the
surface renewal model to estimate the parameters of the freshwater skin of
the ocean.
Figure 2-25 shows time series of the rainrate, the salinity difference
across the freshwater skin and the salinity measured at the surface for a rainy
time period in January 1993. The direct comparison of the model and field
data is not feasible because the model calculation is for the freshwater skin
(the upper few tens of micrometers of the ocean only) while the measured
salinities include surface water from the upper few centimeters.
et al. (1997) assumed that the freshwater skin layer must
account for the greater part of the observed salinity changes because even
heavy rainfall ( 60
| mm h
-1 ) alters the salinity in the upper mixed layer by
less than 1 psu (Kantha and Clayson, 1994). Because the wind speeds during
this experiment were rather low, the salinity changes in the mixed layer can
be expected to be slightly higher. Calculations with the model of Price et al.
(1986) show maximum differences exceeding 2 psu over the top half meter
on 3 January that are of the same magnitude as the difference across the
freshwater skin. The observed salinity changes on this day are up to 4 psu
(some of this signal may, however, be associated with nonlocal rain).
For other rainfall events during this time series the observed salinity
differences in the mixed layer are generally smaller than those across the
skin. A full explanation of the salinity deficits measured can only be given
by considering the combined differences across the skin and mixed layers,
which has not yet been done.
2.5.9 Discussion
Of the various processes that can modify the molecular layers during
rainfall, the most important appear to be the rain-induced mixing that leads
to enhanced surface renewals and the additional surface and volume cooling
by raindrops with temperatures below the sea surface temperature.
Simulations with the renewal model have shown that the additional surface
cooling by rain could exceed 1
o C, if the rain did not affect the surface
renewal time. Nevertheless, the additional mixing and the implied increase
in renewal frequencies limit the effect to less than 0.1
o C when the rainwater
is 5 K cooler than the sea surface. The more impressive effect is the creation
of a haline molecular diffusion layer during rainfall—a freshwater skin of
the ocean—that exhibits salinity differences greater than 4 psu during strong
rains.
Since laboratory studies overemphasized large drops that are a small
fraction of the natural raindrop spectrum, the calculations of the rain-induced
surface roughness for a realistic drop size spectrum shows that this effect has
been overestimated in the past (Houk and Green, 1976). The wind-induced
140
Schl ssel
ü
Schl ssel
ü