Environmental Engineering Reference
In-Depth Information
11.1
Introduction
The Earth's climate is shaped by a myriad of interacting processes between
components of the Earth system - the atmosphere, hydrosphere, lithosphere, biosphere,
and anthrosphere. Among the interacting processes are the energy cycle, water cycle,
and other biogeochemical cycles. Covering 70% of the Earth's surface and containing
96% of the global freshwater, the ocean is the major storage of water substances. The
radiant energy falling on the sea surface is mostly offset by the flux of longwave
radiation emanating from the sea surface and the fluxes of sensible heat and latent heat.
Oceanic evaporation is the dominant mass flux in the global water cycle. The conden-
sational heat release during the process of cloud formation is a major driving force for
atmospheric motions. With its huge storage and heat capacity, the ocean has a signifi-
cant role in shaping the climate of the Earth system and its variability.
Estimates of these air-sea fluxes have been based on marine surface
observations (Bunker 1976 ; Hastenrath 1980 ; Weare et al. 1981 ; Esbensen and
Reynolds 1981 ; Isemer and Hasse 1985 , 1987 ; Hsiung 1985 ; Oberhuber 1988 ;
Cayan 1992 ; da Silva et al. 1994 ; Josey et al. 1998 ). A major limitation of these
estimates is the inadequate sampling. Historically, measurements are collected by
Ships of Opportunity. The advent of satellite observations provided global ocean
coverage, reducing the uncertainty in these air-sea flux measurements.
In Sect. 11.2 , we discuss the transfer mechanisms occurring at the air-sea interface
and present the bulk formulae for estimating the flux transfers. Bulk parameters from
satellite observations are introduced in Sect. 11.3 .Section 11.4 describes various
research or operational products that integrate the bulk parameters to produce surface
flux products. An assessment of these products is presented in Sect. 11.5 . Finally,
Sect. 11.6 contains a summary and outlook for the use of these products in climate
research and operational applications.
11.2 Transfer at the Air-Sea Interface
The sea surface is the interface between the atmosphere and the ocean through
which the exchange of radiant energy, momentum, heat, and moisture occurs.
Atmospheric motion keeps the sea surface in motion, making the flux exchange
through a nonsteady material surface. The fluxes are defined relative to a predefined
fixed level above the average sea surface. The atmospheric boundary layer is
typically 1 km to a few km in height. The atmospheric boundary layer can be
divided into a constant flux sub-layer and a transition layer to the free atmosphere.
The constant flux sub-layer is typically 10-100 m thick or 10-20% of the boundary
layer height, in which the flux values do not deviate more than ~10% from their
mean. Right at the surface is the viscous sub-layer, typically < 1 cm thick, in which
molecular processes dominate. Through the boundary layer, the surface
communicates with the free atmosphere by the transfer of buoyancy and momen-
tum through the constant flux sub-layer and by entrainment from above the
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