222
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
earlier, the space-borne passive-microwave sensors Nimbus-7 scanning multichannel microwave radiometer (SMMR) and special sensor microwave imager (SSM/I)
were launched in late 1978 and 1987, respectively (Derksen et al. 2005; Robinson et
al. 1993). Other advantages of passive microwave sensors over the optical sensors
include the ability to observe the earth’s surface when rainfall or darkness is present
and under cloudy conditions (Gao et al. 2010b).
10.3.3 PaSSive MicRowave—Snow dePth and aRea coveRage
As reported in several studies, passive microwave radiometers have the ability to
penetrate clouds and also estimate SCAs under conditions of rainfall and darkness.
While visible imagery provides the highest spatial resolution from satellites, microwave techniques are required to observe the snow fields under low light or obscured
conditions such as at night and/or under cloud cover (Grody and Basist 1996). As
a result of their cloud-penetrating power and various retrieval algorithms, passive
microwave radiometers have the capacity to make virtually all-weather observations of surface parameters, including water vapor, integrated cloud liquid water,
precipitation (surface rain rate and accumulation amount), sea surface wind speed,
sea surface temperature, sea ice concentration, SWE and/or depth, and soil moisture content (Kawanishi et al. 2003). Furthermore, they have the ability to penetrate
snowpacks and also provide information about snow depth (SD) and SWE unlike
the visible and infrared measurements, which can only provide information on the
spatial extent of snow cover (Grody and Basist 1996; Chang et al. 1990; Hallikainen
and Jolma 1986; Chang et al. 1987; Kawanishi et al. 2003). Measurement of SD and
SWE is made possible through the relationship that exists between the microwave
brightness temperature emitted from a snow-covered surface and the mass of snow
deposition that can be represented by either the combined snow density and depth
or the SWE (Kelly et al. 2003). In fact, the scattering effect of snow particles that
redistributes the upwelling radiation according to snow thickness and grain size has
been acknowledged (Chang et al. 1987) as the physical basis for microwave detection
of snow.
Several space-borne microwave imagers on satellites have been developed, including the earlier Nimbus-7 SMMR, the SSM/I, and one of the latest and most advanced
instruments, that is, the advanced microwave scanning radiometer for the Earth
Observing System (AMSR-E) onboard Aqua. The SSM/I, a conically scanning radiometer with channels at 19, 22, 37, and 85 GHz, was launched on the Air Force Block
5D satellites in 1987, 1989, and 1991 (Grody and Basist 1996). The 85-GHz channel of the radiometer provides a higher spatial resolution of 15 km compared to the
25-km resolution of the channels on SMMR. Unfortunately, the spatial resolutions
of the SMMR and SSM/I radiometers tend to limit their effective use to regional
studies (Kelly et al. 2003). On the other hand, AMSR-E is a conically scanning total
power passive microwave radiometer sensing microwave radiation (brightness temperatures) at 12 channels and 6 frequencies ranging from 6.9 to 89.0 GHz (http://
www.ghcc.msfc.nasa.gov/AMSR/instrument_descrip.html). Launched in May 2002
aboard Aqua (Kawanishi et al. 2003), AMSR-E was an improvement over the existing space-borne microwave radiometers in many areas. First, the spatial resolutions
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
earlier, the space-borne passive-microwave sensors Nimbus-7 scanning multichannel microwave radiometer (SMMR) and special sensor microwave imager (SSM/I)
were launched in late 1978 and 1987, respectively (Derksen et al. 2005; Robinson et
al. 1993). Other advantages of passive microwave sensors over the optical sensors
include the ability to observe the earth’s surface when rainfall or darkness is present
and under cloudy conditions (Gao et al. 2010b).
10.3.3 PaSSive MicRowave—Snow dePth and aRea coveRage
As reported in several studies, passive microwave radiometers have the ability to
penetrate clouds and also estimate SCAs under conditions of rainfall and darkness.
While visible imagery provides the highest spatial resolution from satellites, microwave techniques are required to observe the snow fields under low light or obscured
conditions such as at night and/or under cloud cover (Grody and Basist 1996). As
a result of their cloud-penetrating power and various retrieval algorithms, passive
microwave radiometers have the capacity to make virtually all-weather observations of surface parameters, including water vapor, integrated cloud liquid water,
precipitation (surface rain rate and accumulation amount), sea surface wind speed,
sea surface temperature, sea ice concentration, SWE and/or depth, and soil moisture content (Kawanishi et al. 2003). Furthermore, they have the ability to penetrate
snowpacks and also provide information about snow depth (SD) and SWE unlike
the visible and infrared measurements, which can only provide information on the
spatial extent of snow cover (Grody and Basist 1996; Chang et al. 1990; Hallikainen
and Jolma 1986; Chang et al. 1987; Kawanishi et al. 2003). Measurement of SD and
SWE is made possible through the relationship that exists between the microwave
brightness temperature emitted from a snow-covered surface and the mass of snow
deposition that can be represented by either the combined snow density and depth
or the SWE (Kelly et al. 2003). In fact, the scattering effect of snow particles that
redistributes the upwelling radiation according to snow thickness and grain size has
been acknowledged (Chang et al. 1987) as the physical basis for microwave detection
of snow.
Several space-borne microwave imagers on satellites have been developed, including the earlier Nimbus-7 SMMR, the SSM/I, and one of the latest and most advanced
instruments, that is, the advanced microwave scanning radiometer for the Earth
Observing System (AMSR-E) onboard Aqua. The SSM/I, a conically scanning radiometer with channels at 19, 22, 37, and 85 GHz, was launched on the Air Force Block
5D satellites in 1987, 1989, and 1991 (Grody and Basist 1996). The 85-GHz channel of the radiometer provides a higher spatial resolution of 15 km compared to the
25-km resolution of the channels on SMMR. Unfortunately, the spatial resolutions
of the SMMR and SSM/I radiometers tend to limit their effective use to regional
studies (Kelly et al. 2003). On the other hand, AMSR-E is a conically scanning total
power passive microwave radiometer sensing microwave radiation (brightness temperatures) at 12 channels and 6 frequencies ranging from 6.9 to 89.0 GHz (http://
www.ghcc.msfc.nasa.gov/AMSR/instrument_descrip.html). Launched in May 2002
aboard Aqua (Kawanishi et al. 2003), AMSR-E was an improvement over the existing space-borne microwave radiometers in many areas. First, the spatial resolutions