The analysis of the data observed over a snow field with a breadboard model of MSR (microwave scanning radiometer) to be installed in MOS-1 (Marine Observation Satellite-1) indicates that: (1) the influence of incident angle on brightness temperature is larger in horizontal polarization component than in vertical polarization component. The effect of incident angle depends upon the property of snow with larger value for dry snow; (2) the difference of snow surface configuration consisting of artifically made parallel ditches of 5 cm depth and 5 cm width with spacing of 10 and 30 cm respectively which are oriented normal to electrical axis do not affect brightness temperature significantly; and (3) there is high negative correlation between brightness temperature and snow depth up to the depth of 70 cm which suggests that the snow depth can be measured with a two channel microwave radiometer up to this depth

Takeda, K.

Tsuchiya, K.

English

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STUDIES ON PHYSICAL PROPERTIES OF SNOW BASED
ON MULTI CHANNEL MICROWAVE RADIOMETER*
Kiyoshi Tsuchiya
Institute of Color & Image Technology, Chiba University
1-33, Yayoi-cho, Chiba-city, Chiba-ken, Japan, 260
Kaname Takeda
National Institute of Resources, Science & Technology Agency
2-2-1, Kasumigaseki, Chiyoda-ku, Tokyo, Japan, 100
ABSTRACT
The analysis of the data observed over a snow field with a breadboard model of MSR
(Microwave Scanning Radiometer) to be installed in MOS-1 (Marine Observation Satellite-l) indi-
cates the following features.
(1) The influence of incident angle on brightness temperature is larger in horizontal polarization
component than in vertical polarization component. The effect of incident angle depends upon the
property of snow with larger value for dry snow. (2) The difference of snow surface configuration
consisting of artificially made parallel ditches of 5 cm depth and 5 cm width with spacing of 10 and
30 cm respectively which are oriented normal to electrical axis do not affect brightness temperature
significantly. (3) There is high negative correlation between brightness temperature and snow depth
up to the depth of 70 cm which suggests that the snow depth can be measured with a two channel
microwave radiometer up to this depth.
INTRODUCTION
As a part of Japan/US Cooperative Research Program on Snow Properties/Evapotranspiration,
the first field experiments on snow properties using a breadboard model of 2 channel MSR
(Microwave Scanning Radiometer) to be installed in Japanese first earth observation satellite, MOS-1
(Marine Observation Satellite-l) were made in January of 1982 the result of which was presented by
Tsuchiya (1982) at the first Japan/US Workshop held in March 1982 at Tokyo.
The result of the first experiment indicated that in spite of the fact that the general tendency of
the brightness temperature vs snow property relationship was similar to that obtained by other
investigators, i.e., England (1975), Hofer and Shanda (1978), Rango and et al (1979), Stiles and
Ulaby (1980), Tiuri and Schultz (1980) etc., there was a discrepancy in absolute values.
In an attempt to clarify the discrepancies as well as characteristics of snow for microwave radia-
tion, the second experiements were made in the winter of 1983 in the test site of Hokkaido Univer-
sity. The result of the experiments is reported in the following sections.
*Presented at the 2nd US/Japan Snow/Evapotranspiration Workshop, Nov. 15-19, 1983. East-West Center, Honolulu, Hawaii.
75
https://ntrs.nasa.gov/search.jsp?R=19850014920 2020-03-22T18:39:15+00:00Z
CHARACTERISTICS OF MSR USED IN THE EXPERIMENT
The characteristics of MSR (Microwave Scanning Radiometer) used in the experiments are
shown in Table 1 and Fig. 1 respectively.
Table 1
Characteristics of MSR (Microwave Scanning Radiometer)
to be Installed MOS-1, the Breadboard Model of Which was Used.
Frequency 23.8 GHz 31,4 GHz,
RF Bandwidth 400 MHz 500 MHz
Beamwidth 1.99 deg. 1.45 deg.
Integration I0 & 47 msec I0 & 47 msecTime
Radiometri c 1.0 K at 300 K 1.0 K at 300 KResolution
Dynamic Range 30 - 330 K 30 - 330 K
Polarization Horizontal Vertical
(a)
ANT (b )SW1 SW2
Sky -- 1-- I _ L /
orn . "
OMT: Orthogonal Mode Transducer
SW: Switch, NS: Noise Source
CNS: Comparison NS, RX: Receiver
Figure 1. (a) Block Diagram of MSR Used in the Experiment,
(b) A Schematic Representation of MSR Fixed on a Frame.
76
As is indicated in the table, 23 GHz horizontal component and 31 GHz vertical component are
observed simultaneously. Although MSR is kept at incident angle 10 degrees onboard MOS-1 with
conical mode of scanning, it was decided to conduct the experiments for various incident angles. The
MSR was mounted on a frame extended from the top of 5 meter height tower with capability of
adjustable elevation angle with a handle.
Polarization was also adjustable by rotating the radiometer about its electrical axis thus it was
possible to measure brightness temperature of snow surface with the incident angle ranging from 0
to 75 degrees in both vertical and horizontal polarization.
In parallel with the brightness temperature observation of snow surface, ground truth data indi-
cated in Table 2 were also acquired.
Table 2
Ground Truth Data Obtained in the Experiments
parameter Instrument Parameter Instrument
Solar
Snow Depth Snow Pole Radiation Pyrheliometer
Radiation
Water Equiv. Snow Sampler Budget Net Radiometer
Density Density Gauge Wind Anemometer
Snow Surface
Thermometer Air Temp. ThermometerTemp.
BRIGHTNESS TEMPERATURE VS INCIDENT ANGLE
Many observations were made for different conditions of snow surface and weather. The
analyses of the data indicate a pronounced effect of incident angle on the brightness temperature. A
few examples are shown in Fig. 2 (a) through (d). It should be stated that it took about 80 minutes
to complete one cycle of observation since rotation of the radiometer about its electrical axis is re-
quired to change polarization from horizontal to vertical and vice versa.
When weather condition was unfavorable longer observation time was required, thus the snow
surface was not strictly constant during the observation. Consequently careful ground truth observa-
tion was made simultaneously.
The figures (a) through (c) are for dry snow while (d) is for wet snow. It can be seen from the
figures brightness temperature decreases more sharply in horizontal component than that of vertical.
It can also be seen that brightness temperature is higher over wet snow surface than over dry snow
surface. This fact indicates the difficulty of interpretation of the data of microwave sensor for
measurement of snow.
77
--o-- 23GHz(H)(a)
--o-- 23GHz(V)
,,, --A-- 3 1GHzt_4)
m_ 240- --*-- 31GHz(V)
,,=,2aoLIJ
03 220 "-o..
}-
(.O SNOW DEPTH:39 cm
Dr" / SNOW SURFACE TEMP: - 9"C _m
AIR TEMP: - 4°C CLOUD COVER: 5
200 JAN 23 1983 (10:44--12:03)
! ! I l I I ! I
0 10 20 30 40 50 60 70
INCIDENCE ANGLE (Degree)
250 ( b )
_ ..... ..._
- 240
D
-_- .___ "'-'D. \
o_<'''23o \
. __" _\\ "_
F-
o9 220
oglLI --O-- 23GHz(H) _.,", ,,
Z __.-- 23GHz(V) _ _.
--,',-- 3 1GHz(H) \,,",,.
•"r" 210
--*-- 3 1GHz(V) ,,\n-
133 \
SNOW DEPTH: 43 cmo AIR TEMP: -1.8 °C \\
200 snow SURFACE TEMP: - 4"C CLOUD COVER: 7 _)
JAN.26 1983 (10:16-11:38)
l I _ I I I I I
0 10 20 30 40 50 60 70
INCIDENCE ANGLE (Degree)
Figure 2. Incident Angle vs Brightness Temperature at 23 & 31 GHz. In the Figure V and H Stand for
Vertical and Horizontal Polarization Respectively. The Combination of 23 GHz H + 31 GHz V or 23 GHz
V and 31 GHz H is Operated Simultaneously. The Figures (a), (b) and (c) are for Dry Snow While (d) is
for Wet Snow.
?8
('1<)
25o- (c)
n- 240
I-
<
w 230 a./ ---O'-23GHz(H) -\\ _'_\n
ILl
t- --#--3 1GHz(H) \\ _ \
220 ---AI.--3 1GHz(V) \ _ _ \,,
_nw °xx X -
I"Z _ _ _ \\x X
-r- 210
(.0 SNOWDEPTH:.116cm AIR TEMP:-1.5°C CLOUDCOVER:9 \\
rr SNOWSURFACETEMP:-?.5°C MARCH1 1983 (9:18-10:53)m - \
o
200
I I I I I I I I
0 10 20 30 40 50 60 70
INCIDENCE ANGLE (Degree)
(OK)
uJ 270' (d)
rr
< 260rr" -_
w
I- 250
09 _,
cO
LU
:7 240 --o-- 2BGHzCH) _ x.
__.m 23GHz(V) "_
"1-
_ _za
rr --.-- 3 1GHz(V)
m 230 XSNOWDEPTH:45 cm AIR TEMP:5°C CLOUDCOVER:8
SNOW SURFACETEMP:0.5"C JAN.25 1983 (14:19-15:43)
I I I ! f I I v
0 10 20 30 40 50 60 70
INCIDENCE ANGLE (Degree)
Figure 2. (c) and (d)
79
BRIGHTNESS TEMPERATURE VS SNOW SURFACE CONFIGURATION
Brightness temperature was measured with different incident angle over, (1) snow surface with
parallel ditches of 5 cm depth and 5 cm width of 30 cm and 10 cm spacing respectively, which are
oriented perpendicularly to electrical axis of the radiometer, (2) the snow surface after the ditches
were pressed to level surface.
The incident angle dependence over dry snow under cold air temperature indicates a similar
feature of dry snow while that made under warm air temperature indicates a feature of wet snow.
The influence of surface condition is shown in Fig. 3 (a) and (b) for dry and wet snow surface
conditions respectively. Figure (a) indicates that spacing of ditches does not affect the brightness
temperature except the case of 60 degrees incident angle. The figure also indicates that the
brightness temperature of the pressed snow surface is higher regardless of incident angle which may
lead to a conclusion that a pressed snow surface gives higher brightness temperature.
It should be stated that at the time of brightness temparture measurement, both air and snow
surface temperatures rose significantly as is indicated in the figure, therefore it is considered that the
rise of the brightness temperature is caused at least partly by the rise of both air and snow surface
temperature.
Figure (b), a case of wet snow surface indicates a feature different from that of Fig. (a). It
appears that the brightness temperature over 30 cm spacing is a little higher than that over 10 cm
spacing. It is extremely interesting to notice that different from the previous case, the brightness
temperature over the pressed snow surface is the lowest regardless of the incident angle. It should be
again stressed that both air and snow surface temperature continuously fell as is well recognized in
the figure.
The analysis of these two cases suggests that both air and surface effects must be taken into
account in the interpretation 9f brightness temperature over the snow surface.
POLARIZATION ANGLE DEPENDENCE
The experiments on the polarization angle dependence were made at incident angle of 10, 30
and 45 degrees. As was pointed out in the previous report by Tsuchiya (1982) there was not a large
difference in brightness temperature due to variation of polarization angle. In case of 23 GHz chan-
nel data brightness temperature increased corresponding to the increase of polarization angle while
the opposite tendency was recognized for 31 GHz channel data.
80
--O--23GHz(H) --e--23GHz(V) --A.- 31GHz(H)
--O--23GHz(H) --e--23GHz(V) --z_--31GHz(H)
--A--31GHz(V) --D--AIR TEMP. --xmSURFACE TEMP.
--A--31GHz(V) ----O--AIR TEMP. _A'----SURFACE TEMP. ( a )
250 250
(°K) INC.0 o /" (°K) INC.10 _ 270 270 ([3)
..o (°K) IN C'0° (°K) INC. 10°
240 (oc)
240 (°C) 260
-__ (oc) 260 (OC)
: 1° 1:230 230 _"* "-.... _° _-., -5 __ 250 250:30cm lOcm Pres. 30cm lOcm Pres. • "
30cm lOcm Pres. 30cm lOcm Pres.
250 INC.20" 250 INC.30 o 260 260
o_ o -
_ 30cm loom Pres. 30cm loom Pres. , , ,
.-, 30cm lOcm Pres. 30cm 10cm Pres.240
250 INC.45 _
260 INC.45 _ 250 _'-',
240 _'-----'-e-'_""-_ .,A230 _- _"
_A
-,r _ 250 240 "" "'"
230 _o 220
INC'60 o \
220 _ 210 x 5
-5 _ -5
, , X_X-------.--..--__x 0 _X_
t I I • f r
30cm 10cm Pres. 30cm 10cm Pres. 30cm 10cm Pres. 30cm 10cm Pres.
Figure 3. Brightness Temperature vs Incident Angle over Different Snow Surface Condition. 30 and 10
are Spacing of Ditches of 5 cm Depth and 5 cm Width. Pres Means Pressed Snow Surface. (a) is Dry
Snow (March 4, 1983) and (b) is Wetsnow (Jan 28 '83).
DIURNALVARIATION
There was a diurnal variation in the brightness temperature with the maximum value in the
daytime with higher value in lower incident angle. The measure of variation depends upon the snow
surface and meteorological conditions. A few typical examples are shown in Fig. 4.
Fig. 4 (a) is a case of comparatively small diurnal variation while the figure (b) is the case with
a sharp rise in the early afternoon. Figure (c) is a case with a large variation which is considered
due to freezing of the snow surface. Comparing the three figures it can be easily seen that when
both the snow surface and air temperature are low brightness temperature variation is also small. It
can be also seen that the brightness temperature is affected by both air and surface temperature.
(°K) (a)
250 --o-- 23GHz(H)
•>.q_Hz(V) _t:l_ AIR TEMP.l.U
rr
::3 __,,.m31GHz(H) IX_ SURFACE TEMP.
_" --_,-- 3 1GHz(V)
< 240r'r"
LLI
U.l
I- 230 _,,__.=_ _ --" ......... (*C)
Z z="-''_" _ _ I.L.
t- 220 _ jx----_.___ _ _ _ x 0
-1-
,-£---x .... -5 _
z
m 210 co
rr
<10 12 14 16 18 20
JAPAN STANDARD TIME
Figure 4. Diurnal Variation of Brightness Temperature is at Incident Angle at
10° on March 2, 1983, (b) is at Incident Angle 45° on Jan 27 '83 and (c) is at
Incident Angle 10° on Jan 25 '83 Respectively. (a) and (b) are for Dry Snow
While (c) is for Wet Snow Respectively.
82
Figure 4. (b) Jan 27 '83. Incident Angle at 45° for Dry Snow. (c) Jan 25
'83 at Incident Angle 10° for Wet Snow.
83
BRIGHTNESS TEMPERATURE VS SNOW DEPTH AND SNOW EQUIVALENT WATER
To obtain the relationship between brightness temperature and snow depth, the brightness
temperature was observed by removing snow at incident angles of 10 and 45 degrees respectively.
The result of the observations for two cases is shown in Fig. 5.
It can be seen that the rate of decrease of the brightness temperature corresponding to the
increase of snow depth is widely different depending on the snow surface and meteorological condi-
tions. The brightness temperature decrease corresponding to the snow depth is recognized up to the
depth of 70 cm for both 23 and 31 GHz horizontal polarization component while in case of vertical
polarization component, decrease of brightness temperature ceased at the depth of 60 cm for both
channels as is indicated in Fig. 5 (b) and (c).
It should be stated that in spite of the fact that as extreme care was taken to remove the snow,
the physical properties of the snow surface must have been affected and the obtained brightness
temperature for the respective snow depth might be a little different from that of natural condition.
In spite of this defect a qualitative feature of snow depth vs brightness temperature relationship
obtained in these experiments will be of an extreme value for understanding snow properties and
microwave radiation of the snow surface.
Figure 5. Brightness Temperature vs Snow
Depth.(a)Jan 19 '83 at IncidentAngle10° Over
Wet SnowSurface.(b) and (c) are Dry Snow.
84
MARCH 5 '83,i = i0"
'" (_<)
= (b)
I-- 260 --o-- 23GHz(H)
< __ -----23GHz(V)
W
n _,,_"k___o" --'_--3 1GHz(V)
w 250 - *"_'\--A""_
,,,,
w 240z
t--
212
C
rr 2ao -m
W I ' ; v , I
0 20 40 60 80 100 120
SNOW DEPTH (cm)
Figure 5. (b) and (c). Dry Snow Surface.
Based on the experimental data, an attempt is made to express the brightness temperature as a
function of snow equivalent water for incident angles of 10 and 45 degrees in a form of Eq. (1).
T = a + b exp(-cW) (1)
where W is snow equivalent water in the unit of gr/cm2.
85
Applying the least square method, the coefficients a, b and c are determined. The obtained
values of the coefficients vary in wide range as can be expected. Just for the sake of interest a few
examples are shown in Table 3.
Table 3
The Valuesof the Coefficientsof Eq. (1).
Incident angle: i0" Incident angle: 45"
Jan 29 1983 March 6 1983
a b c a b c
H 257.7 -2.9 0.244 243.8 - 0.007 0.247
23 GHz
V 282.2 -28.9 0.073 251.6 - 0.033 0.187
H 276.9 -21.4 0.109 246.2 - 0.367 0.130
31 GHz
V 271.7 - 20.0 0.107 251.6 - 3.726 0.050
H: Horizontal, V: Vertical
CONCLUDING REMARKS
Foregoing analyses lead to the following conclusion. The brightness temperature of the snow
surface is influenced by various factors which suggest complexities of deducing snow properties
from microwave radiometer data.
Although limited in number and volume, the data obtained during the present experiments are
extremely valuable for obtaining dependable algorithm to deduce geophysical parameters from
MOS-1 MSR data.
ACKNOWLEDGEMENT
The present research has been realized under Japan/US Cooperative Research Program, in this
respect the authors wish to extend their sincere gratitude to all those who have contributed to realize
the program. The authors deep appreciations are due to Messrs Tamotsu Igarashi and Chu Ishida of
National Space Development Agency of Japan for their great effort in conducting the experiments.
The authors are greatly indebted to Prof. Goro Wakahama of Hokkaido University and staff
members of Environmental Research and Technology Institute and Mitsubishi Electric Company in
realizing the experiments. The authors are also thankful to Mr. Katsutoshi Kozai of ERTI for his
help in data handling.
86
REFERENCES
England, A. W., 1975: Thermal microwave emission from a scattering layer. J. of Geophys. Res.,
80, 4484-4496.
Hofer, R. and E. Shanda, 1978: Signature of snow in the 5-94 GHz range. Radio Science, 13,
365-369.
Rango, A., T. C. Chang and J. L. Foster, 1979: Utilization of spaceborne microwave radiometers
for monitoring snowpack properties. Nordic Hydrology, I0, 25-40.
Stiles, W. H. and F. T. Ulaby, 1980: The active and passive microwave response to snow
parameters, 1. Wetness. J. of Geophys. Res., 85, 1037-1045.
Tiuri, M. and H. Schultz, 1980: Theoretical and experimental studies of microwave radiation from a
natural snow field. Proc. Sensing of Snowpack Properties, 10 pp., fortcollins.
Tsuchiya, K., S. Yamamoto, C. Ishida, K. Takeda, Y. Yajima and S. Morizono, 1982: Two fre-
quency microwave radiometer experiment over snow field. Proc. of the 1st Japan/US
Snow/Evapotranspiration Workshop, 127-142 NIR, Science & Technology Agency, Tokyo,
Japan.
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