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A. Rango, A.E. Walker and B.E. Goodison
operational status in some cases, much snow extent data are readily available to many
users on the world wide web free of charge (see section on Current Applications).
Limitations. There are also several disadvantages to using visible satellite data for
snow and ice mapping. Nothing can directly be learned about snow water equivalent
or ice thickness from the visible data. Snow cover extent is valuable in forecasting of
snowmelt runoff, but clouds can be a significant problem in restricting observations,
especially when the frequency of observation is every 16 days.
Data Availability and Platforms Used. There is no restriction on acquiring visible
or near infrared data for a particular area; however, you must have fmancial resources
available to purchase the more expensive, high resolution data such as Landsat-TM
and SPOT. The primary satellites and sensors employed are SPOT, Landsat-TM,
DMSP, NOAA-A VHRR, and GOES. Airplanes are infrequently, but effectively, used
to provide areal snow cover data.
11.2.3 Thermal Infrared
Thermal infrared remote sensing currently only has minor potential for snow mapping
and snow hydrology. Thermal infrared mapping, like visible mapping, is hindered by
cloud cover. In addition, the surface temperature of snow is not always that much
different from the surface temperatures of other surfaces in rugged mountain terrain
where elevation and aspect differences can cause major changes in temperature. This
makes it extremely difficult to distinguish snow cover from other features, especially
during the snowmelt period. When clouds permit, the big advantage of thermal snow
cover mapping is that the mapping can be done during the nighttime overpasses.
In hydrological forecasting, monitoring the surface temperature of the snowpack
may have some direct application to delineating the areas of a basin where snowmelt
may be occurring. If the snow surface temperature is always below oce, no melt is
occurring. If the snow surface is at oce during the daytime and below oce at night,
the melt-freeze cycle is occurring but it is uncertain whether melt water is being
released at the bottom of the snowpack. If the snow surface stays at oce both day and
night, then it is highly likely that the snowpack is isothermal and that melt is occurring
and being released at the base of the snowpack. Much research needs to be done to
determine whether thermal infrared remote sensing can playa useful role in assisting
in snowmelt runoff modelling and forecasting.
11.2.4 Passive and Active Microwave
Snow Water Equivalent and Associated Applications. Snow on the earth's surface
is, in simple terms, an accumulation of ice crystals or grains, resulting in a snowpack
which over an area may cover the ground either completely or partly. The physical
characteristics of the snowpack determine its microwave properties; microwave
radiation emitted from the underlying ground is scattered in many different directions
by the snow grains within the snow layer, resulting in a microwave emission at the top
of the snow surface being less than the ground emission. Properties affecting microwave response from a snowpack include: depth and water equivalent, liquid water
A. Rango, A.E. Walker and B.E. Goodison
operational status in some cases, much snow extent data are readily available to many
users on the world wide web free of charge (see section on Current Applications).
Limitations. There are also several disadvantages to using visible satellite data for
snow and ice mapping. Nothing can directly be learned about snow water equivalent
or ice thickness from the visible data. Snow cover extent is valuable in forecasting of
snowmelt runoff, but clouds can be a significant problem in restricting observations,
especially when the frequency of observation is every 16 days.
Data Availability and Platforms Used. There is no restriction on acquiring visible
or near infrared data for a particular area; however, you must have fmancial resources
available to purchase the more expensive, high resolution data such as Landsat-TM
and SPOT. The primary satellites and sensors employed are SPOT, Landsat-TM,
DMSP, NOAA-A VHRR, and GOES. Airplanes are infrequently, but effectively, used
to provide areal snow cover data.
11.2.3 Thermal Infrared
Thermal infrared remote sensing currently only has minor potential for snow mapping
and snow hydrology. Thermal infrared mapping, like visible mapping, is hindered by
cloud cover. In addition, the surface temperature of snow is not always that much
different from the surface temperatures of other surfaces in rugged mountain terrain
where elevation and aspect differences can cause major changes in temperature. This
makes it extremely difficult to distinguish snow cover from other features, especially
during the snowmelt period. When clouds permit, the big advantage of thermal snow
cover mapping is that the mapping can be done during the nighttime overpasses.
In hydrological forecasting, monitoring the surface temperature of the snowpack
may have some direct application to delineating the areas of a basin where snowmelt
may be occurring. If the snow surface temperature is always below oce, no melt is
occurring. If the snow surface is at oce during the daytime and below oce at night,
the melt-freeze cycle is occurring but it is uncertain whether melt water is being
released at the bottom of the snowpack. If the snow surface stays at oce both day and
night, then it is highly likely that the snowpack is isothermal and that melt is occurring
and being released at the base of the snowpack. Much research needs to be done to
determine whether thermal infrared remote sensing can playa useful role in assisting
in snowmelt runoff modelling and forecasting.
11.2.4 Passive and Active Microwave
Snow Water Equivalent and Associated Applications. Snow on the earth's surface
is, in simple terms, an accumulation of ice crystals or grains, resulting in a snowpack
which over an area may cover the ground either completely or partly. The physical
characteristics of the snowpack determine its microwave properties; microwave
radiation emitted from the underlying ground is scattered in many different directions
by the snow grains within the snow layer, resulting in a microwave emission at the top
of the snow surface being less than the ground emission. Properties affecting microwave response from a snowpack include: depth and water equivalent, liquid water