Geology Reference
In-Depth Information
300
T
b
-19V
T
b
-37V
T
b
-85V
T
b
-19H
T
b
-37H
T
b
-85H
Rain on OW
280
260
T
b
-85V
240
220
T
b
-37V
200
T
b
-19V
180
160
T
b
-85H
140
T
b
-37H
120
T
b
-19H
100
Figure 8.22
Evolution of
T
b
during an event of rainfall on simulated seawater in an outdoor tank. The rain started
on 1 Dec. at 8:46 a.m. (335:08:46 denotes ddd:hh:mm). As soon as rain ended at 336:08:58
T
b
resumed its
typical value from OW [
Shokr and Kaleschke
, 2012].
estimate the rate of rainfall over ocean [
Lin and Rossow
,
1997]. Using surface‐based radiometer measurements
from simulated seawater
Shokr and Kaleschke
, [2012]
found that the increase of
T
b
during rainfall events is
particularly pronounced at higher frequencies, and the
increase from the horizontally polarized emission is sig-
nificantly larger than that from the vertically polarized.
This is shown in Figure 8.22, which depicts the evolution
of
T
b
during rainfall event. It peaked when the rainfall
peaked. The maximum values of
T
b
‐19h,
T
b
‐37h, and
T
b
‐85h, observed during the peak of the rain were 122.0,
161.1, and 239.9 K, respectively, with the corresponding
PR values of 0.190, 0.155, and 0.043. These values
are significantly higher than those obtained from water
surface with no rain (0.27, 0.25, and 0.19, respectively). It
is worth mentioning that rainfall over land reverses the
trend of
T
b
, i.e., the emitted radiation from the radiomet-
rically warmer land surface is scattered by the hydromete-
ors, resulting in less observed
T
b
during rainfall.
a crucial component in the radiative schemes of climate
and sea ice models. The conversion of spectral to broad-
band albedo is accomplishedvia measured band data and
simulation of unavailable spectral bands. Multispectral
sensors are important for this purpose.
Albedo contributes to the self‐perpetuating nature of
sea ice as it triggers a positive feedback that enhances the
growth of sea ice. Sea ice albedo varies over a wide range
from 0.07 to 0.7, depending on the spectral band, with
possible higher values around 0.9 when covered by snow
[
Allison et al
., 1993]. This is almost an order of magni-
tude higher than the typical albedo from the seawater sur-
face (≅0.07). Therefore, when albedo increases as sea ice
grows, the absorbed radiation decreases markedly. This
results in less surface temperature and therefore allows
growth of more ice. The opposite is true when sea ice melts.
Flooded ice surface has low albedo, which means more
absorption of solar energy and therefore an enhanced melt
situation. This is how the ice‐albedo feedback loop acts
positively to enhance the ice growth and decay once they
start. For that reason, sea ice albedo has been identified
as an important climatic parameter, not only because it
modifies the surface radiation balance but also due to its
possible role in perpetuating global temperature change.
The interest of the community of climate studies in the
parameterization of sea ice albedo has recently been rein-
forced in order to strengthen the sea ice components in
climate models. The purpose is to develop methods for
estimating the global radiative force caused by sea ice‐
albedo feedback in the polar regions [
Hudson
, 2011].
Inaccurate estimate of this forcing leads to inaccurate
8.3. visiBle and neaR‐infRaRed
Reflectance and alBedo
As defined in section 7.4, reflectance is the fraction
of electromagnetic power reflected from a specific inter-
face with respect to the power of the incident radiation.
Albedo is the hemispheric integration of reflectance in
the optical spectrum (wavelength 0.30-3.0
μ
m). When
albedo is determined for successive narrow bands, it is
called spectral albedo. When spectrally integrated over all
solar‐reflective bands, it is called broadband albedo. It is
