Geoscience Reference
In-Depth Information
Solar
radiation
Precipitation
Terrestrial
radiation
Sensible and
latent heat fluxes
Refection
Absorption
Transmission
Heat flux to ice
Heat flux from lake bottomt
Fig. 4.1
Schematic picture of lake cross-section with the heat fluxes
p ð v ; T Þ ¼ ð 1 v Þq i ð L f þ c i ð T T 0 ÞÞ þ v q w c w ð T T 0 Þ
ð
4
:
4
Þ
fluxes into lakes is a major issue. They can be measured
directly, but for most situations they are parameterized from available weather and climate
data. In the following sections these parameterizations are discussed in detail.
The radiation balance is the governing factor in the Earth
To obtain the surface heat
fl
is heat budget. The basis of
the physics of radiation was introduced in Sect. 3.4 and serves here as the background for
the heat transfer by solar and terrestrial radiation. Planck
'
s law (Eq. 3.9 ) gives the radi-
ation intensity as a function of temperature and wavelength. Solar radiation and terrestrial
radiation are also called, respectively, as shortwave and long-wave radiation, because
solar radiation comes mostly from the wavelength band 0.2
'
3
ʼ
m, while terrestrial
-
radiation covers the band 5
m. The turbulent transfer of sensible and latent heat is
estimated using atmospheric boundary layer models of different degree of complexity.
Heat transfer from precipitation is a minor factor in general and contains sensible and
latent heat. E.g., if P = 10 mm day 1 liquid water and T
15
ʼ
-
T 0 =5
°
C, the sensible heat
fl
ux
2Wm 2 . This is not much. But for the same rate of snowfall at
is
5
°
C the heat
fl
flux is
-
30 W m 2 , which is comparable to the turbulent heat
fluxes. Consequently, rainfall on
snow or ice and snowfall on open water are important transient heat transfer mechanisms.
Also snow cover is permeable to liquid water and therefore the latent heat of raindrops can
be convected deeper. The time scale of precipitation events is of the order of 1 day. In the
boreal zone the monthly average precipitation is around 50 mm covering a few events, and
therefore in the climatological heat balance precipitation remains a small factor.
fl
-
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