THE NEAR-SURFACE LAYER OF THE OCEAN
growth (especially capillaries), and enhances wave energy dissipation.
Laboratory and field measurements show that the surface tension can be
reduced by up to 60% due to surface films (Baier, 1972; Huhnerfuss, 1977).
Removal of surface films by convection, rain, near-surface shear, and
breaking waves affects hydrodynamic processes at the air-sea interface,
especially the capillary wave field, which substantially determines the
surface roughness and thus air-sea exchanges. This process has a dual effect
on the gravity-capillary wave field: It damps waves due to increased
turbulence, and it enhances waves due to fragmentation and dissipation of
surface films.
The lack of in situ measurements of the viscoelastic properties of films
under various ocean regimes and particularly under different forcing
conditions, limits the direct estimates of the global surface film effects on
air-sea exchange. There are, however, indications that this uncertainty can be
largely reduced if the mean square wave slope due to capillary-gravity waves
is used rather than wind speed (Frew, 1997). Glazman and Greysuku (1993)
demonstrated the correlation between the surface wind stress and the sea
surface roughness associated with capillary-gravity waves detected by
backscattering from altimeters. This means that surface films may have less
effect on the air-sea exchange parameterizations that are derived in terms of
the mean square wave slope or friction velocity rather than in terms of the
wind speed.
2.3 Modeling Molecular Sublayers during Nighttime
Conditions
Conceptual models of the aqueous molecular sublayers can be divided
into two classes: surface renewal models and boundary layer models. In
renewal models, the properties of molecular sublayers depend on the surface
renewal time. The renewal time is then related to the environmental
parameters causing hydrodynamic instabilities that control the properties of
molecular sublayers. Boundary-layer models are based on the quasistationary representation of boundary-layer processes. Below we describe
these modeling approaches in detail.
2.3.1 Dimensional analysis
Dimensional considerations can provide initial insight into the dynamics
of aqueous molecular sublayers. Here, we ignore the bubble and droplet
production in whitecaps and hence account for only interfacial (direct) heat,
mass, and momentum transport. In the case of stationary meteorological and
wave conditions, the following set of functional dependences can be
formulated:
88
growth (especially capillaries), and enhances wave energy dissipation.
Laboratory and field measurements show that the surface tension can be
reduced by up to 60% due to surface films (Baier, 1972; Huhnerfuss, 1977).
Removal of surface films by convection, rain, near-surface shear, and
breaking waves affects hydrodynamic processes at the air-sea interface,
especially the capillary wave field, which substantially determines the
surface roughness and thus air-sea exchanges. This process has a dual effect
on the gravity-capillary wave field: It damps waves due to increased
turbulence, and it enhances waves due to fragmentation and dissipation of
surface films.
The lack of in situ measurements of the viscoelastic properties of films
under various ocean regimes and particularly under different forcing
conditions, limits the direct estimates of the global surface film effects on
air-sea exchange. There are, however, indications that this uncertainty can be
largely reduced if the mean square wave slope due to capillary-gravity waves
is used rather than wind speed (Frew, 1997). Glazman and Greysuku (1993)
demonstrated the correlation between the surface wind stress and the sea
surface roughness associated with capillary-gravity waves detected by
backscattering from altimeters. This means that surface films may have less
effect on the air-sea exchange parameterizations that are derived in terms of
the mean square wave slope or friction velocity rather than in terms of the
wind speed.
2.3 Modeling Molecular Sublayers during Nighttime
Conditions
Conceptual models of the aqueous molecular sublayers can be divided
into two classes: surface renewal models and boundary layer models. In
renewal models, the properties of molecular sublayers depend on the surface
renewal time. The renewal time is then related to the environmental
parameters causing hydrodynamic instabilities that control the properties of
molecular sublayers. Boundary-layer models are based on the quasistationary representation of boundary-layer processes. Below we describe
these modeling approaches in detail.
2.3.1 Dimensional analysis
Dimensional considerations can provide initial insight into the dynamics
of aqueous molecular sublayers. Here, we ignore the bubble and droplet
production in whitecaps and hence account for only interfacial (direct) heat,
mass, and momentum transport. In the case of stationary meteorological and
wave conditions, the following set of functional dependences can be
formulated:
88