THE NEAR-SURFACE LAYER OF THE OCEAN
The rain-induced roughness increases strongly with rainrates up to about
2 mm h
-1 , while at higher rainrates 0r
z increases only slightly (Figure
2-22b). Only for air friction velocity less than approximately 0.15 m s
-1 , can
the rain-induced waves appreciably contribute to the surface roughness.
Charnock’s (1995) formula employed for the calculation of z 0C in Figure
2-22b, however, does not work under low wind speed conditions. In fact,
due to viscous effects, the surface roughness under low wind stresses
increases with decreasing wind.
Figure 2-22 exhibits considerable differences in the rain-induced
roughness lengths compared to those from laboratory studies by Houk and
Green (1976). This is because of a more realistic raindrop spectrum
employed by
et al. (1997), which also covers many small, submillimeter drops while in the laboratory the effect of large drops (several
mm in diameter) had been mainly investigated. The large drops in fact cover
a very small portion of the natural spectrum of raindrops (Pruppacher and
Klett, 1987).
The rain-induced stress leads, together with the wind stress, to increased
surface wind-drift currents. This effect coexists with the attenuation of short
gravity waves by enhanced turbulence in the upper ocean during rainfall
(Tsimplis and Thorpe, 1988). The rain-induced wind-drift currents also
reduce the amplitude threshold at which short gravity waves break (Philips
and Banner, 1974).
From investigations in a wind-wave tank, Poon et al. (1992) found that
gravity waves in the frequency range between 2 and 5 Hz decay during
rainfall. At the same time, the spectral density of wave slopes in the
frequency domain between 10 and 100 Hz drastically increases. The latter
effect is due to rain-induced waves; however, it is pronounced only under
low wind speed conditions. Results of experiments in a wind-wave tank by
Yang et al. (1997) are consistent with the Poon et al. (1992) findings but
provided some more details to the rain effects on fine structure of wind
waves.
Due to damping of short gravity waves, a substantial part of the
momentum transferred to the ocean by form drag under non-precipitating
situations is instead transferred by skin friction during rainfall. This results
in decrease of the surface renewal time, which is concurrent with the
damping of short gravity waves.
2.5.6 Flux of kinetic energy carried by rain
According to Tsimplis (1992), the flux of kinetic energy carried by rain
with a uniform drop size distribution is:
134
Schl ssel
ü
The rain-induced roughness increases strongly with rainrates up to about
2 mm h
-1 , while at higher rainrates 0r
z increases only slightly (Figure
2-22b). Only for air friction velocity less than approximately 0.15 m s
-1 , can
the rain-induced waves appreciably contribute to the surface roughness.
Charnock’s (1995) formula employed for the calculation of z 0C in Figure
2-22b, however, does not work under low wind speed conditions. In fact,
due to viscous effects, the surface roughness under low wind stresses
increases with decreasing wind.
Figure 2-22 exhibits considerable differences in the rain-induced
roughness lengths compared to those from laboratory studies by Houk and
Green (1976). This is because of a more realistic raindrop spectrum
employed by
et al. (1997), which also covers many small, submillimeter drops while in the laboratory the effect of large drops (several
mm in diameter) had been mainly investigated. The large drops in fact cover
a very small portion of the natural spectrum of raindrops (Pruppacher and
Klett, 1987).
The rain-induced stress leads, together with the wind stress, to increased
surface wind-drift currents. This effect coexists with the attenuation of short
gravity waves by enhanced turbulence in the upper ocean during rainfall
(Tsimplis and Thorpe, 1988). The rain-induced wind-drift currents also
reduce the amplitude threshold at which short gravity waves break (Philips
and Banner, 1974).
From investigations in a wind-wave tank, Poon et al. (1992) found that
gravity waves in the frequency range between 2 and 5 Hz decay during
rainfall. At the same time, the spectral density of wave slopes in the
frequency domain between 10 and 100 Hz drastically increases. The latter
effect is due to rain-induced waves; however, it is pronounced only under
low wind speed conditions. Results of experiments in a wind-wave tank by
Yang et al. (1997) are consistent with the Poon et al. (1992) findings but
provided some more details to the rain effects on fine structure of wind
waves.
Due to damping of short gravity waves, a substantial part of the
momentum transferred to the ocean by form drag under non-precipitating
situations is instead transferred by skin friction during rainfall. This results
in decrease of the surface renewal time, which is concurrent with the
damping of short gravity waves.
2.5.6 Flux of kinetic energy carried by rain
According to Tsimplis (1992), the flux of kinetic energy carried by rain
with a uniform drop size distribution is:
134
Schl ssel
ü