Geoscience Reference
In-Depth Information
Table 5.2.
Relative transmission of cloud types for solar zenith angles (SZA) of 78° and
60° based on data from June through early August collected at St. Patrick Bay ice cap
(82°N, 64°W)
Cloud Type
a) SZA=78°
b) SZA=60°
c) b-a
Cirrus/Cirrostratus
0.84
0.89
0.05
Altocumulus
0.58
0.78
0.20
Stratus
0.50
0.54
0.04
Fog
0.46
0.67
0.21
Altostratus
0.46
0.65
0.19
Stratocumulus
0.35
0.66
0.31
Snow
0.33
0.59
0.26
Average
0.50
0.68
0.18
(All cloud types)
Note
: Values represent the ratio between hourly totals of the surface shortwave flux for cloud
cover of the given type >70% and all observations with cloud cover <30%.
Source
: From Serreze and Bradley (
1987
).
conditions on and around a small ice cap on the Hazen Plateau of northern Ellesmere
Island (N.W.T., Canada, approximately 82°N, 64°W). The data were collected dur-
ing the summers of 1982 and 1983.
Table 5.2
shows ratios of the solar radiation
received at the surface during cloudy skies (greater than 70 percent cloud fraction)
and nearly clear skies (less than 30 percent cloud fraction) for different cloud types
and two solar zenith angles.
As expected, relative transmission is greatest for the thinner, higher cloud types
(cirrus/cirrostratus) and least for the lower, thicker clouds and for conditions of
snowfall. Note also the dependence on the solar zenith angle, pointing to the higher
cloud albedos at large zenith angles. Polar clouds tend to have a higher relative
transmission than clouds in middle latitudes. This is attributed to polar clouds being
thinner and having lower water content than clouds in middle latitudes owing to
more limited convection and lower specific humidities.
5.3.3
Distribution of Global Radiation
Satellite retrievals offer systematic assessments of global radiation and other sur-
face radiative fluxes with complete Arctic coverage. Here we make use of two data
sets, APP-x, based on AVHRR data, and ISCCP-D, which is based on a number of
different satellite platforms. As discussed in
Chapter 2
, satellite-derived products
such as APP-x and ISCCD-D suffer from a number of shortcomings, but the fields
are certainly of high enough quality to illustrate basic spatial and temporal patterns
in the Arctic.
Maps of the mean-monthly global radiation flux for March through October based
on ISCCP-D are provided in
Figure 5.1
. March, September, and October are charac-
terized by a primarily zonal patterns (i.e., the isolines are roughly parallel to latitude
