Geoscience Reference
In-Depth Information
of the atmosphere, other trace gases such as CO
2
and cloud cover. Whereas the
last three influence the emissivity of the atmosphere (emissivity increases with an
increase in all three), the temperature of the cloud base is also very important.
The net longwave budget (longwave in minus longwave out) is generally nega-
tive, but tends to become less negative (or sometimes even positive) when low,
warm cloud cover (Arctic stratus) is present. Although Equation
3.1
(the top-of-
atmosphere radiation budget) is analogous to Equation
5.4
, in the latter equation we
are dealing with the surface albedo (α) as opposed to the planetary albedo (A) and
the upward longwave flux depends on the temperature and emissivity of the surface
only (as opposed to the combined effects of the atmosphere and surface).
The sensible heat flux Q
H
is directed upward (negative on our convention) when
temperature decreases with height, and positive (downward) when temperature
increases with height. The sensible heat flux is driven by both conduction and tur-
bulent eddies. These turbulent eddies represent irregular fluctuations in the atmo-
spheric flow near the surface in the boundary layer. Conduction is important in
the first few millimeters above the surface, where large temperature gradients can
exist and where turbulence is small (wind speed declines logarithmically toward the
surface, and by definition drops to zero at the very surface). Above this very shal-
low near-surface layer, temperature gradients are much smaller. Conduction in the
atmosphere is inefficient, so the vertical sensible heat transfer is mainly by turbulent
eddies. The overall turbulent process can be thought of as a mixing of air molecules
always working to reduce the vertical temperature gradient in an attempt to restore
thermal equilibrium.
Similarly, when specific humidity decreases with height, Q
E
is directed upward,
again taken as negative on our convention. In the first few millimeters, the latent
heat flux is mainly by diffusion. Above this shallow near-surface layer, vapor gradi-
ents are much smaller, and diffusion is inefficient, so the latent heat transfer is hence
mainly by turbulent eddies. The latent heat flux works to destroy the humidity gra-
dient. The latent heat flux is typically thought of as referring to phase changes from
liquid to gas (evaporation) and gas to liquid (condensation), but may also include
transfers from solid to gas (sublimation) and gas to solid (deposition). This would
be the situation when it is very cold and no liquid water is present. Energetically
sublimation and deposition involve the sum of the latent heat of vaporization (2.5 ×
10
6
J kg
−1
at 0°C, note that the value depends on temperature) and the latent heat of
fusion (3.34 × 10
5
J kg
−1
).
As Q
H
and Q
L
are driven by turbulence, except very near the surface, they are
commonly referred to as the turbulent heat fluxes.
The traditional approach to estimating Q
H
and Q
L
is through “profile” meth-
ods, based on vertical gradients of the horizontal wind, temperature, and specific
humidity. The profile method contains various assumptions regarding the structure
of the turbulent eddies. The preferred approach, now widely used because appropri-
ate instruments are readily available, is termed the “eddy correlation method.” The
magnitude and direction of the sensible heat flux can be stated in terms of the time
average of the instantaneous covariance between anomalies of the vertical wind
