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Multiscale Hydrologic Remote Sensing: Perspectives and Applications
snow cover patterns are complementary to catchment runoff forecasting in connection with the structure and state of hydrologic processes in various watershed models
(Grayson et al. 2002) and provide a very important source of information in recent
regional climate and global change assessment studies (Pu et al. 2007).
The goal of the chapter is to present an overview of the accuracy, availability, and
recent hydrologic applications of different MODIS snow cover products. The chapter
is organized as follows. Section 9.2 provides a summary of available data sets and
describes basic principles and concepts used in snow cover mapping. The accuracy
of MODIS snow cover products is discussed in Section 9.3. Numerous comparisons of MODIS snow cover images with other satellite-derived snow products and
ground-based snow depth (SD) measurements have confirmed their high accuracy
and consistency, especially for clear-sky conditions. In many parts of the world,
however, cloud obscuration or contamination has been found as the major obstacle
to applying the MODIS snow cover images. Section 9.4, hence, summarizes various approaches used for cloud impact reduction. The main objective here is to discuss numerous filtering techniques, which are remarkably efficient in cloud impact
reduction in image processing. The overview of various hydrologic applications of
MODIS data sets is presented in Section 9.5. This section includes studies focusing
on seasonal, interannual, and subpixel variability of snow cover, validation of snow
(sub-) models, and assimilation of MODIS snow cover into hydrologic simulations.
Final remarks and conclusions are given in Section 9.6.
9.2 MODIS SNOW COVER PRODUCTS
MODIS is an imaging spectroradiometer that employs a cross-track scan mirror,
collecting optics, and a set of individual detector elements to provide imagery of
the earth’s surface and clouds in 36 discrete, narrow spectral bands from approximately 0.4 to 14.4 μm (Barnes et al. 1998). It is a key component of the National
Aeronautics and Space Adminstration (NASA)’s Earth Observing System, and currently (January 2011), it is onboard two satellites, Terra and Aqua. The Terra satellite
has started the observations in February 2000; the Aqua satellite was launched in
July 2002. Both satellites use the same type of MODIS instrument, but the differences in their orbits result in different viewing and cloud cover conditions. The
most noticeable difference between these two satellites is the local equatorial crossing time, that is, approximately 10:30 a.m. in a descending mode for the Terra and
approximately 1:30 p.m. in an ascending mode for the Aqua satellite. The geolocation accuracy of MODIS instrument is about 45 m for Terra and 60 m for Aqua
(George Riggs, personal communication, also see Wolfe et al. 1998). From a variety
of geophysical products derived from MODIS observations, a suite of global snow
cover products are available through the Distributed Active Archive Center located
at the National Snow and Ice Data Center (NSIDC; www.nsidc.org). The products
are available at different spatial and temporal resolutions, and their basic summary
is presented in Table 9.1.
The MODIS snow products are created as a sequence of products beginning with a
swath (MOD10_L2, MYD10_L2) and progressing, through spatial and temporal transformations, to daily, 8-day, and monthly global snow products with a spatial resolution
Multiscale Hydrologic Remote Sensing: Perspectives and Applications
snow cover patterns are complementary to catchment runoff forecasting in connection with the structure and state of hydrologic processes in various watershed models
(Grayson et al. 2002) and provide a very important source of information in recent
regional climate and global change assessment studies (Pu et al. 2007).
The goal of the chapter is to present an overview of the accuracy, availability, and
recent hydrologic applications of different MODIS snow cover products. The chapter
is organized as follows. Section 9.2 provides a summary of available data sets and
describes basic principles and concepts used in snow cover mapping. The accuracy
of MODIS snow cover products is discussed in Section 9.3. Numerous comparisons of MODIS snow cover images with other satellite-derived snow products and
ground-based snow depth (SD) measurements have confirmed their high accuracy
and consistency, especially for clear-sky conditions. In many parts of the world,
however, cloud obscuration or contamination has been found as the major obstacle
to applying the MODIS snow cover images. Section 9.4, hence, summarizes various approaches used for cloud impact reduction. The main objective here is to discuss numerous filtering techniques, which are remarkably efficient in cloud impact
reduction in image processing. The overview of various hydrologic applications of
MODIS data sets is presented in Section 9.5. This section includes studies focusing
on seasonal, interannual, and subpixel variability of snow cover, validation of snow
(sub-) models, and assimilation of MODIS snow cover into hydrologic simulations.
Final remarks and conclusions are given in Section 9.6.
9.2 MODIS SNOW COVER PRODUCTS
MODIS is an imaging spectroradiometer that employs a cross-track scan mirror,
collecting optics, and a set of individual detector elements to provide imagery of
the earth’s surface and clouds in 36 discrete, narrow spectral bands from approximately 0.4 to 14.4 μm (Barnes et al. 1998). It is a key component of the National
Aeronautics and Space Adminstration (NASA)’s Earth Observing System, and currently (January 2011), it is onboard two satellites, Terra and Aqua. The Terra satellite
has started the observations in February 2000; the Aqua satellite was launched in
July 2002. Both satellites use the same type of MODIS instrument, but the differences in their orbits result in different viewing and cloud cover conditions. The
most noticeable difference between these two satellites is the local equatorial crossing time, that is, approximately 10:30 a.m. in a descending mode for the Terra and
approximately 1:30 p.m. in an ascending mode for the Aqua satellite. The geolocation accuracy of MODIS instrument is about 45 m for Terra and 60 m for Aqua
(George Riggs, personal communication, also see Wolfe et al. 1998). From a variety
of geophysical products derived from MODIS observations, a suite of global snow
cover products are available through the Distributed Active Archive Center located
at the National Snow and Ice Data Center (NSIDC; www.nsidc.org). The products
are available at different spatial and temporal resolutions, and their basic summary
is presented in Table 9.1.
The MODIS snow products are created as a sequence of products beginning with a
swath (MOD10_L2, MYD10_L2) and progressing, through spatial and temporal transformations, to daily, 8-day, and monthly global snow products with a spatial resolution