SURFACE FLUXES
235
1972). The albedo for the remaining direct solar radiation, α dr varies
with the solar zenith angle. The solar energy transferred to the ocean is
then given by
Q S = Q I [ 0.6 (1 − α dr ) + 0.4 (1 − α df )] = α Q I .
(6)
At the temperature of the ocean surface and of the atmosphere above,
blackbody radiation occurs at longer wavelengths (up to 50µ) and comprises the net longwave radiation,
Q L = Q A − σ (SST )
4 .
(7)
In (7), the downwelling (positive) radiation from the atmosphere, Q A ,
increases with cloud cover. The upwelling radiation from the ocean is
given by (−σ(SST ) 4 ), where σ = 5.67 × 10 −8 W/m 2 /K 4 is the StefanBoltzmann constant and taking the surface emissivity as 1, accounts
for reflected Q A (Lind and Katsaros 1986). This high emissivity and the
usually warmer SST compared to the radiating atmosphere and clouds,
makes Q L negative.
Since SST is a product of GODAE, the radiative flux problem can be
thus reduced to one of specifying Q I , α and Q A (Table 1). Fortunately,
these fields have been derived from satellite observations over the ocean
(Section 5), so that there is no need to use empirical formulae, such as
those developed by Smith and Dobson (1984) and examined by Fung et
al. (1984). Nonetheless, these expressions do quantify to first order the
decrease in Q I and the increase in Q A due to clouds. The compensating
effect on the daily surface heat flux can be nearly complete in some
situations. Therefore, it is important to use consistent data sets for
both radiation components, so that errors due to clouds are minimized.
The sea surface is usually warmer than the overlying air, leading to an
upward (negative) molecular diffusion of heat across the surface. At the
same time evaporation from the surface is assumed to keep the surface
air saturated, and hence usually more moist that the air above. These
gradients are maintained by the vertical turbulent transports of sensible
heat flux and moisture (latent heat), which in steady state must match
the surface sensible heat flux, Q H , and E (Q E ), respectively. Except in
the rare circumstance of very warm moist air over a cold sea, evaporation
takes water from the ocean and cools.
The major particle fluxes are precipitation, P, over the ocean due to
rain, P R and snow, P S :
P = P R + P S .
(8)
These fluxes are positive definite and account for about 90% of the evaporation (WGASF, 2000). Snowfall has an associated negative particle
235
1972). The albedo for the remaining direct solar radiation, α dr varies
with the solar zenith angle. The solar energy transferred to the ocean is
then given by
Q S = Q I [ 0.6 (1 − α dr ) + 0.4 (1 − α df )] = α Q I .
(6)
At the temperature of the ocean surface and of the atmosphere above,
blackbody radiation occurs at longer wavelengths (up to 50µ) and comprises the net longwave radiation,
Q L = Q A − σ (SST )
4 .
(7)
In (7), the downwelling (positive) radiation from the atmosphere, Q A ,
increases with cloud cover. The upwelling radiation from the ocean is
given by (−σ(SST ) 4 ), where σ = 5.67 × 10 −8 W/m 2 /K 4 is the StefanBoltzmann constant and taking the surface emissivity as 1, accounts
for reflected Q A (Lind and Katsaros 1986). This high emissivity and the
usually warmer SST compared to the radiating atmosphere and clouds,
makes Q L negative.
Since SST is a product of GODAE, the radiative flux problem can be
thus reduced to one of specifying Q I , α and Q A (Table 1). Fortunately,
these fields have been derived from satellite observations over the ocean
(Section 5), so that there is no need to use empirical formulae, such as
those developed by Smith and Dobson (1984) and examined by Fung et
al. (1984). Nonetheless, these expressions do quantify to first order the
decrease in Q I and the increase in Q A due to clouds. The compensating
effect on the daily surface heat flux can be nearly complete in some
situations. Therefore, it is important to use consistent data sets for
both radiation components, so that errors due to clouds are minimized.
The sea surface is usually warmer than the overlying air, leading to an
upward (negative) molecular diffusion of heat across the surface. At the
same time evaporation from the surface is assumed to keep the surface
air saturated, and hence usually more moist that the air above. These
gradients are maintained by the vertical turbulent transports of sensible
heat flux and moisture (latent heat), which in steady state must match
the surface sensible heat flux, Q H , and E (Q E ), respectively. Except in
the rare circumstance of very warm moist air over a cold sea, evaporation
takes water from the ocean and cools.
The major particle fluxes are precipitation, P, over the ocean due to
rain, P R and snow, P S :
P = P R + P S .
(8)
These fluxes are positive definite and account for about 90% of the evaporation (WGASF, 2000). Snowfall has an associated negative particle