Microwave radiometer and its application
to snow measurement
The microwave dielectric property of snow
The dielectric constant of material is complex number. The real part
gives the capacity of energy storing whereas the imaginary part
represents the energy dissipation. The microwave dielectric property
of snow is closely related to the liquid water content in the snow
density
rd and the permittivity of
ice, the real part of the dry snow permittivity
ed', like ice,
is independent of frequency and temperature. And the imaginary part
ed" is approximately zero
[3,4].
Studies have shown that the real of the dry snow permittivity depends
only on its density
rd[g. cm
-3].
They are related by
[3]
ed‘ = 1+1.7
rd + 0.7
r2
d...........(5)
The imaginary part of the dry snow permittivity is a function of frequency, temperature and density. Its property can be studied by measuring the loss tangent of the dry snow. The relation is
[5]
tan
dd =
e "
d /
e '
d =
1.59 X 10
6 X (0.52
rd + 0.62
r2d ) / ( 1+1.7
rd + 0.7
r2d )
X ( f
-1 + 1.23 X 10
-14Öf)e
0.36T.............6
where T is the temperature of snow (°c and f denotes frequency (H
z). From (6), the tangent loss of dry snow can be calculated at different frequencies and different temperatures.
Wet snow is a mixture of air, ice partides and a certain amount of liquid water. The real and imaginary part of wet snow is much larger compared with that of dry snow. This is due to the relatively large permittivity of water. The permittivity increases with increasing wetness of snow Wv. In 4.14 GHz frequency range, the relation is given by
[6]
e ’ws = 1+2
rs + bW
3/2v
e ”ws= (1.0994/f) .
aÖe 'ws
.............7
where
rs is the density of wet snow
(g cm
-3], f, is frequency (GH
z],
while the wetness Wv represents fractional volume of the liquid water
in snow,
a is the attenuation coefficient
for certain frequency and water content
[6].
b is a constant when frequency is given
b = 5.87 X 10
-2-3.10 X 10
-4 (f-4)
2 .................(8)
In 500-1000GHz frequency range, the permittivity of wet snow is given by
[5]
e ’ws = 1+1.7
rd+0.7
r2d+8.7W
v+70W
2
v
e ”ws = f/10
9 (0.9W
v
+ 7.5W
2v)
..................9
rd is the density of snow when the liquid water of wet snow is replaced by air.
The measurement of snow radiation in microwave band
In China, snow measurements have been carried out since 1986 using self-developed airborne multifrequency microwave radiometers. The null-lalancing Dicke type radiometer and the two reference temperature radiometer. with automatic gain compensation were adopted. The data collecting, storing, displaying and processing of the system is controlled by a micro computer.
On Feburary 25, 1987, an experiment was carried out at Changchun Nanhu to measure the snow cover on the lake ice surface using 8mm microwave radiometer. Snow of different depth was piled up above the lakd ice which was 80cm thick. Fig.2 corresponds the brightness temperature to the snow depth. It shows that the brightness temperature decreases exponentially with increasing snow depth. The regression equation is given
TB(d) = 252.4 X e
-0.0114d ..................(10)
The brightness temperature is 170K when the snow is piled to 40cm deep, 90K lower than the bare Lake ice. The work site photo is shown on Fig. 1.
On February 23, 1990, once again a snow measurement was conducted at Changchun Nanhu using 10cm microwave radiometer on the lake ice surface. The ice depth was 90cm with water underneath. Snow was piled up on the lake. Fig. 3 gives the relation curve between the brightness temperature increases with increasing snow depth.
On February 22, 1989 at Changchun Jingyue Tan Remote Sensing Experimental Site, 5cm microwave radiometer was used to measure the brightness temperature of different snow depth which was piled up on the cement ground. The result is shown in Fig. 4
Fig. 1 Work site for snow measurement
