THE NEAR-SURFACE LAYER OF THE OCEAN
Figure 2-24 suggests that the most pronounced effect is due to additional
mixing caused by the rain. This is observed at low rainrates where the rather
high temperature differences prevailing in the non-precipitating situations
quickly drop to low values when the rainrate increases to few mm h
-1 .
However, with further rainrate increase the strong decrease of the
temperature differences weakens. It is remarkable that the temperature
differences do not vanish even for precipitation rates of 50 mm h
-1 . The
additional skin cooling by the rain increases with rainrate though it does not
exceed -0.1 K. At low wind speeds and very low rainrates the combined
effect of solar warming and cooling by the turbulent and long wave fluxes
results in a warm skin. At higher wind speeds the combined effect shows a
minimum skin cooling at a distinct rainrate, which depends on the actual
friction velocity. Further increasing rainrates lead to a slightly enhanced
cooling due to the rain-induced heat flux (Figure 2-24b-d).
2.5.8 Comparison with data
The surface renewal model including the precipitation component
described above has been tested by
et al. (1997) with the COARE
data taken from the R/V Vickers in January-February 1993 near 156
o E, 2
o
S
and from the R/V Wecoma near 156
o E, between 2
o S and 5
o N. The frequent
convective rain in the western equatorial Pacific shows strong variability
with rainrates ranging from less than 0.1 to more than 100 mm h
-1 ; this tests
the parameterization under a great variety of situations.
The R/V Vickers surface fluxes in the
et al. (1997) comparison
were identical to those shown in Figure 2-19. The calculated temperature
differences across the cool skin that were caused by surface and volume
cooling due to the rain showed values generally below 0.05
o C; only one case
with heavy precipitation exceeding 100 mm h
-1 gave a cooling by more than
0.1
o C. There were just a few cases with strong daytime precipitation; during
these rare periods the solar warming of the skin was about halved, according
to the model. During nighttime the net effect of rain on the cool skin was
rather small. Apart from several cases with strong rain when the cool skin
was reduced by rain-induced mixing to values below 0.1
o C, the effects of
mixing and additional cooling partly cancelled each other during this
observation in the western equatorial Pacific Ocean.
Similar to the analysis shown in Figure 2-19, a quantitative comparison
of the parameterization with measurements of the skin versus bulk
temperature difference from the R/V Vickers required that the temperature
difference between the bulk sampling depth (i.e., the depth of the
thermosalinograph intake) and the surface microlayer was accounted for.
This temperature difference was calculated with the mixed-layer model of
Price et al. (1986) (PWP).
138
Sch
Schl ssel
ü
l ssel
ü
Figure 2-24 suggests that the most pronounced effect is due to additional
mixing caused by the rain. This is observed at low rainrates where the rather
high temperature differences prevailing in the non-precipitating situations
quickly drop to low values when the rainrate increases to few mm h
-1 .
However, with further rainrate increase the strong decrease of the
temperature differences weakens. It is remarkable that the temperature
differences do not vanish even for precipitation rates of 50 mm h
-1 . The
additional skin cooling by the rain increases with rainrate though it does not
exceed -0.1 K. At low wind speeds and very low rainrates the combined
effect of solar warming and cooling by the turbulent and long wave fluxes
results in a warm skin. At higher wind speeds the combined effect shows a
minimum skin cooling at a distinct rainrate, which depends on the actual
friction velocity. Further increasing rainrates lead to a slightly enhanced
cooling due to the rain-induced heat flux (Figure 2-24b-d).
2.5.8 Comparison with data
The surface renewal model including the precipitation component
described above has been tested by
et al. (1997) with the COARE
data taken from the R/V Vickers in January-February 1993 near 156
o E, 2
o
S
and from the R/V Wecoma near 156
o E, between 2
o S and 5
o N. The frequent
convective rain in the western equatorial Pacific shows strong variability
with rainrates ranging from less than 0.1 to more than 100 mm h
-1 ; this tests
the parameterization under a great variety of situations.
The R/V Vickers surface fluxes in the
et al. (1997) comparison
were identical to those shown in Figure 2-19. The calculated temperature
differences across the cool skin that were caused by surface and volume
cooling due to the rain showed values generally below 0.05
o C; only one case
with heavy precipitation exceeding 100 mm h
-1 gave a cooling by more than
0.1
o C. There were just a few cases with strong daytime precipitation; during
these rare periods the solar warming of the skin was about halved, according
to the model. During nighttime the net effect of rain on the cool skin was
rather small. Apart from several cases with strong rain when the cool skin
was reduced by rain-induced mixing to values below 0.1
o C, the effects of
mixing and additional cooling partly cancelled each other during this
observation in the western equatorial Pacific Ocean.
Similar to the analysis shown in Figure 2-19, a quantitative comparison
of the parameterization with measurements of the skin versus bulk
temperature difference from the R/V Vickers required that the temperature
difference between the bulk sampling depth (i.e., the depth of the
thermosalinograph intake) and the surface microlayer was accounted for.
This temperature difference was calculated with the mixed-layer model of
Price et al. (1986) (PWP).
138
Sch
Schl ssel
ü
l ssel
ü