Principles pertinent to the utilization of 1.55 cm wavelength radiation emanating from the surface of the earth for studying the changing characteristics of polar sea ice are briefly reviewed. Recent data obtained at that wavelength with an imaging radiometer on-board the Nimbus 5 satellite are used to illustrate how the seasonal changes in extent of sea ice in both polar regions may be monitored free of atmospheric interference. Within a season, changes in the compactness of the sea ice are also observed from the satellite. Some substantial areas of the Arctic sea ice canopy identified as first-year ice in the past winter were observed not to melt this summer, a graphic illustration of the eventual formation of multiyear ice in the Arctic. Finally, the microwave emissivity of some of the multiyear ice areas near the North Pole was found to increase significantly in the summer, probably due to liquid water content in the firm layer

Campbell, W. J.

Chang, T. C.

Gloersen, P.

Wilheit, T. T.

English

NASA Technical Reports Server

SX-652-73-341-
PRAINT .
- NASA TM X, - & f
POLAR'SEA ICE OBSERVATIONS BY MEANS
OF MICROWAVE RADIOMETRY
P. GLOERSEN
T. C. CHANG
T. T. WILHEIT
- --W. J. CAMPBELL
(NBS-TM-X7052 9) POLR
OBSERVATIONS BY ?!BPOLAR SEA ICER TIOS y E(NA NS OF ICROWAVE N74-1 3 090RDIOMETRY (ASA) 12 P HC $3.00
CSCL 08L1
Unclas
G3/13 24499
NOVEMBER 1973
GODDARD SPACE.FLIGHT CENTER
GREENBELT, MARYLAND
.L ".
https://ntrs.nasa.gov/search.jsp?R=19740004977 2020-03-23T11:51:52+00:00Z
X-652-73-341
POLAR SEA ICE OBSERVATIONS BY MEANS
OF MICROWAVE RADIOMETRY
P. Gloersen, T. C. Chang and T. T. Wilheit
NASA Goddard Space Flight Center
Greenbelt, Maryland
W. J. Campbell
U.S. Geological Survey
Tacoma, Washington
"Presented at the Interdisciplinary Symposium on Advanced Concepts and Tech-
niques in the Study of Snow and Ice Resources"
November 1973
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland
I
POLAR SEA ICE OBSERVATIONS
BY MEANS OF MICROWAVE RADIOMETRY
P. Gloersen
T. C. Chang
T. T. Wilheit
NASA Goddard Space Flight Center
W. J. Campbell
U.S. Geological Survey, Tacoma
ABSTRACT
Principles pertinent to the utilization of 1. 55 cm wavelength radiation
emanating from the surface of the Earth for studying the changing character-
istics of polar sea ice are briefly reviewed. Recent data obtained at that
wavelength with an imaging radiometer on-board the Nimbus 5 satellite are
used to illustrate how the seasonal changes in extent of sea ice in both polar
regions may be monitored free of atmospheric interference. Within a sea-
son, changes in the compactness of the sea ice are also observed from the
satellite. Some substantial areas of the Arctic sea ice canopy identified as
first-year ice in the past winter were observed not to melt this summer, a
graphic illustration of the eventual formation of multiyear ice in the Arctic.
Finally, the microwave emissivity of some of the multiyear ice areas near
the North Pole was found to increase significantly in the summer, probably
due to liquid water content in the firn layer.
BACKGROUND
The imaging microwave radiometer utilized in these studies has been
described elsewhere (Wilheit 1972, Gloersen et al. 1973). Briefly, The
Electronically-Scanned Microwave Radiometer (ESMR) operates at a wave-
length of 1.55 cm. In-this wavelength region, the Raleigh-Jeans approxima-
tion applies to all targets of interest, and the microwave power received from
a radiating target depends linearly on the brightness temperature of that tar-
get. This simplifies calibrating the output of the radiometer directly in terms
of the brightness temperature of the target. In general, the observed bright-
ness temperature depends on the physical temperature and emissivity of the
viewed surface, and both direct and reflected components from the inter-
vening atmosphere. The atmospheric components at a wavelength of 1.55 cm
result principally from the liquid water and water vapor present. Since these
quantities are small in the Arctic and Antarctic atmospheres, they have been
neglected in determinations of surface brightness temperatures reported
here. The stratus clouds usually obscuring the polar regions consist mostly
of ice crystals, which are transparent to 1.55 cm radiation.
The observed variation of the brightness temperatures result from the
combination of seasonal, diurnal, and climatic changes in the physical sur-
face temperatures amounting to differences of about 400K, and from
5.11-1
significant variations in the emissivity of the surface, as illustrated in Figure
1, which translate into brightness temperature contrasts of 20K and 120K for
multiyear sea ice and open water vs. first-year sea ice, respectively. The
emissivity differences shown for first-year sea ice, multiyear sea ice, and
open water result in microwave image patterns that are readily discernable
in spite of variations in the physical temperature of the surface, since the
latter vary sufficiently slowly in space and time compared to the emissivity
differences. It should be noted that the emissivity of the central Arctic ice
canopy, where most of the multiyear ice is located, has been observed to
increase markedly over the values shown in Figure 1 in the summer months,
probably due to a liquid water content in the firn layer. (Edgerton et al.
1971, Meier 1972) When the instantaneous field-of-view of the ESMR contains
a mixture of the three polar sea components, intermediate brightness tem-
peratures are observed. This may be used to infer compactness of sea ice,
as illustrated in Figure 2.
The curve in Figure 2 is simply a linear interpolation between the cases
of completely open water (TB = Tw = 1310K) and completely consolidated first-
year sea ice with a surface temperature, Ts , of 2600K, typical of areas near
the edge of the ice pack, especially in Spring and Fall. The shading indicates
the climatic variation of Ts expected, in addition to the uncertainty of the
brightness temperature measurement. This leads to an average uncertainty
in the determination of the fraction of open water of about 4% from the ob-
served brightness temperature. It has been observed that measurable open
water usually occurs only in areas where little or no multiyear ice is pre-
sent. Therefore, the curve fits most cases of interest; some downward ad-
justments to T s would be required for sea ice well-removed from its outer
boundary in Antarctica during Austral Winter.
SEA ICE COMPACTNESS
The curve of Figure 2 has been used to analyze the sea ice cover around
Greenland and Antarctica, as shown in Figures 3 and 4, respectively. The
distribution of compactness shown on these particular days has been observed
to change significantly from one polar projection to another over time inter-
vals as short as 10 days, the minimum time interval utilized so far in the
data processing. It is evident that such determinations, combined with
records of wind stress, would provide useful data for the study of ice dyna-
mics. Such studies are currently in progress. In addition to the areas dis-
cussed so far here, other areas of considerable ice dynamic activity, as
evidenced by the data on hand, are the Hudson Bay and the Bering, Chuckchi,
East Siberian, Laptev, and Beaufort Seas.
SEA ICE BOUNDARIES
Nimbus 5 ESMR data are now available for determining the near minimum
and maximum sea ice boundaries for both polar regions in 1973. Such data
are illustrated in Figures 5 and 6, illustrating quantitatively what is already
5.11-2
known in a qualitative way, i. e. that the amount of ice cover changes by about
a factor two in both polar. regions during a given year.
In Antarctica (Figure 6), the observations might seem to indicate the
formation of multiyear ice in some regions, since not all of the ice formed
in 1972 melted during Austral Summer. On the other hand, these same areas
had a distinct first-year signature in Austral Spring 1972 and Austral Winter
1973 indicating, at least, that it was not ice formed in previous years. This,
coupled with the knowledge obtained by observers in these areas indicating
high. ice dynamic activity and the absence of multiyear sea ice leads to the
conclusion that the sequential images are representative of a steady-state
phenomenon, i. e. the areas of persistent sea ice cover are accumulation
regions in a system of steady depletion and replenishment.
In the Arctic region (Figure 5), there are a number of noteworthy points
to be made. First, the near-minimum boundary of the sea ice illustrated
will be the basis for following the changing distribution of multiyear ice in
the winter of 1973-4, giving valuable insight into the macroscale ice dynamics
in the north polar region. It was also observed that not all of the sea ice iden-
tified as first-year in Winter 1972-3, particularly in the Kara, northern
Chukchi, and Beaufort Seas, melted in Summer 1973. Thus, observing the
changes in brightness temperature of these areas during Winter 1973-4 may
result in new knowledge of the emissivity of sea ice vs. age at some inter-
mediate points. In addition, such observations will give insight into the
sources of multiyear ice in the Arctic. Finally, there is clear evidence that
the sea ice northwest of Svalbard is strongly affected by ocean currents, since
the boundaries are observed to change well out-of-phase with expectations
based on seasonal variations alone.
APPLICATIONS
Since the amount of open water in the polar regions has a profound impact
on global circulation patterns of the atmosphere (Fletcher 1972), the detailed
information of extent and location of open water in the polar regions is a val-
uable input for computer models of the atmosphere and climate.
In terms of shipping, the Fleet Weather Facility of the U. S. Navy at
Suitland, Maryland is utilizing Nimbus 5 ESMR data obtained in near real-
time to produce weekly ice limit reports for military, commercial, and
international use. Plans for substituting an automated system for the present
hand-analysis from quick-look ESMR records are under consideration.
The nature of the persistent radiometric patterns that have been observed
in the continental ice sheets on Greenland and Antarctica have been described
elsewhere (Gloersen et al. 1973). These patterns may be related to snow
accumulation rates on these continents, but the correlation in Greenland is
not high, based on the available contours drawn through relatively few meas-
ured points on the basis of a climatic model (Mock 1967). If subsequent sur-
face measurements give more substantial support to a correlation with accu-
mulation rates, it may be possible to monitor long-term changes in such rates
with this instrument.
Although no such occurrances have been observed to date, it may be pos-
sible to monitor the calving of shelf ice of sufficient size from sources such
5.11-3
as the Ross Ice Shelf. Similar studies of such enormous glaciers as the
Lambert Glacier in Antarctica may also be possible, but the spatial resolu-
tion capability of the instrument would be stressed.
REFERENCES
Edgerton, A. T., A. Stogryn, and G. Poe, 1971. Microwave radiometric
investigations of snowpack. Final Report No. 1285 R-4 for U. S. G. S.
contract No. 14-08-001-11828, Aerojet-General Corp., Microwave
Division, El Monte, California.
Gloersen, P., T. T. Wilheit, T. C. Chang, W. Nordberg, and W. J. Camp-
bell, 1973. Microwave maps of the polar ice of the earth. NASA/GSFC
Report No. X-652-73-269.
Meier, M. F., 1972. Measurement of snow cover using passive microwave
radiation. Proc. Intl. Symposia on the Role of Snow and Ice in Hydro-
logy, Banff, Canada.
Mock, S. J., 1967. Accumulation patterns on the Greenland ice sheet. USA/
CRREL Report No. 233. Also, 1973. Priv. Comm.
Wilheit, T. T., 1972. The electronically scanning microwave radiometer
(ESMR). The Nimbus 5 User's Guide, pp. 59-104, U.S. Government
Printing Office 1972-735-963/259.
5.11-4
1.0
FIRST-YEAR
SEA ICE
0.9 -
0.8
0.7'-
I-
uj
0.6
0.5 - <,
0.4 -
0.3
0.5 1 1.55 5 10
WAVELENGTH-cm
Figure' 1. Microwave Emissivities of the Polar Seas
5.11-5
100
80 -
F = (TB - 0.95 Ts)/(Tw- 0.95 TS )
S60 -
0.
40-
L.
20 -
0 I I I260
140 160 180 200 220 240 260
TB - BRIGHTNESS TEMPERATURE - oK
Figure 2. Determination of Unresolved Open Water in Areas Containing Sea Ice from the
Microwave Brightness Temperature at = 1. 55 cm
% ICE COVER
90 - 100%
80 - 90%
S70 - 80%
S60 - 70%
50 - 60%
< 50%
Figure 3. Compactness of Sea Ice Around Greenland on February 26, 1973.
5.11-7
% ICE COVER
90 - 100% 90E
80 - 90%
-70 - 80%
~ 60 - 70%
l 50 - 60%
< 50%
Figure 4. Compactness of Sea Ice Around Antarctica on December 26, 1972.
5.11-8
NEAR-MINIMUM AND NEAR-MAXIMUM ARCTIC SEA ICE
BOUNDARIES FOR 1973 (FEBRUARY 10 AND SEPTEMBER 9)
90E
A ASKA
SCANADA
90W
S1972-3 WINTER ICE MELTED IN SUMMER 1973
SVARIATION OUT-OF-PHASE WITH SEASON
1972-3 FIRST-YEAR ICE SURVIVING THE SUMMER MELTING
SPERSISTENT MULTIYEAR SEA ICE
Figure 5. Seasonal Variation of Sea Ice in the Northern Hemisphere
5.11-9
NEAR-MINIMUM AND NEAR-MAXIMUM ANTARCTIC SEA ICE
BOUNDARIES FOR 1973 (FEBRUARY 10 AND JULY 16)
90W
180 00
80
70
60
50
90E
SSEA - ICE FORMED IN 1973
S1972 ICE SURVIVING THE SUMMER MELTING
Figure 6. Seasonal Variation of Sea Ice in the Southern Hemisphere
5.11-10


Polar sea ice observations by means of microwave radiometry

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