Geoscience Reference
In-Depth Information
to the surface. Expanding on
Chapter 3
, the net surface flux can be partitioned into
three major terms, the magnitude and existence of which depend on the season and
nature of the column underlying the atmosphere, which could variously represent
ocean with or without at floating sea ice cover (and an overlying snow cover), bare
soil, soil with vegetation on the top (which may in turn be covered by snow), an ice
sheet or a glacier:
F
sfc
= - (R
sfc
+ Q
H
+ Q
E
) = - (M + C + B)
(5.2)
which can be rearranged as
(R
sfc
+ Q
H
+ Q
E
) - (M + C + B) = 0
(5.3)
M is melt, C is conduction of heat into or out of the underlying volume, and B is the
“bulk” heat gain (B), representing the absorbtion of solar radiation that penetrates
into the column. As previously discussed, this is a very important mechanism for
increasing the sensible heat content of the Arctic Ocean in spring and summer in
open water areas. There is also a significant penetration and absorbtion of solar
radiation within a snow pack and within sea ice and glacier ice. The bulk heating
term is negligible within a bare soil column.
5.2
Further Understanding the Budget Terms
R
sfc
is defined as the sum of the downward (sometimes termed “downwelling”) and
upward (“upwelling”) shortwave (solar) and longwave radiation components at the
surface, or
R
sfc
= R
ss
(1- α) + R
ls
- εσT
s
4
(5.4)
R
ss
is the flux of incoming solar radiation at the surface and α is the surface albedo
(the ratio between upward and downward solar radiation). Recall that solar radi-
ation is defined as radiation with wavelengths of 0.15 to 4.0 micrometers (µm).
R
ss
(1- α) hence represents the net solar radiation at the surface. During polar dark-
ness, and ignoring moonlight and starlight, the term is zero. Recall that longwave
radiation represents wavelengths of about 4-300 µm. The incoming longwave flux is
given by R
ls
, while εσT
s
4
is the outgoing longwave flux. The outgoing longwave flux
is dependent on the (dimensionless) surface emissivity ε, the Stefan-Boltzman con-
stant σ and T
s
, the skin temperature in Kelvin (not to be confused with SAT, which
is generally considered to be at about 1.5 m above the surface). Emissivity is defined
as the ratio of radiant flux from a body to that of a perfect emitter (a blackbody)
at the same kinetic temperature. A perfect emitter (ε = 1) is also a perfect absorber
of radiation. True blackbodies do not exist. However, most surfaces approximate
blackbodies in the longwave spectrum (ε > 0.9). Snow is an especially good emitter
(and consequently absorber), with a longwave emissivity of typically 0.98 to 0.99.
By comparison, the atmosphere is a selective or partial emitter, with an emissivity
much less than 1. R
ls
depends on atmospheric temperature, the water vapor content
