Geoscience Reference
In-Depth Information
column water vapour over ocean is mature for climate analysis. In addition, MERIS
observations have shown over time a significant potential for high spatial resolution total
column water vapour over land during daytime in clear sky, e.g., ESA DUE (Data User
Element) GlobVapour.
UV/VIS instruments provide independent means for total column water vapour retri-
evals, but these measurements are biased towards clear sky. Upper tropospheric humidity
(UTH) data sets provide a data source with high value for climate research.
Microwave sounding data sets from AMSU-B, MHS and SSM/T2 hold a great potential
to improve our knowledge on UTH—also allowing estimates of absolute humidity. The
availability of atmospheric temperature and humidity profiles for more than 30 years has
been identified as a critical issue (see, e.g., WMO, 2012 [
http://www.wmo.int/pages/prog/
9 Snow
Snow plays an important role in the regional and global hydrological system, since it acts as
temporary water storage. As a cause of large land surface albedo changes and because it is
highly variable both in time and space, dedicated measurements/monitoring systems are
needed. Satellite products have advantage over point-based measurements of the snow-related
parameters, especially due to their spatial coverage. The mass of seasonal snow (the snow
water equivalent, SWE) accumulated on land surfaces and the extent of the snow-covered area
are the principal variables in hydrology and for water resources applications (Rott
2013
). They
are also essential for determining and modelling surface/atmosphere exchange of mass and
energy, and therefore of great importance for numerical weather prediction. Satellite-based
snow sensing techniques use visible/infrared (optical), active microwave (SAR) and passive
microwave sensing techniques (Frei et al.
2012
;Botteronetal.
2013
).
Operational snow cover products are commonly related to the fractional snow cover
(percentages of the coverage) and snow albedo. They are derived from operational geo-
stationary and numerous polar-orbiting satellite sensors, e.g., AVHRR, MODIS and VIIRS.
The use of active microwave (SAR) on ENVISAT and RADARSAT enables the detection
of wet snow, indicating melting processes. This information can be integrated in regional
hydrological monitoring activities (Bach et al.
2010
). Imaging microwave radiometry
(SSM/I, AMSR-E) allows the global snow mass to be mapped every day or two at a spatial
resolution of about 25 km. SWE retrieval exploits the scattering losses in the (dry) snow
pack of the microwave radiation emitted by the soil below snow. The accuracy of SWE
retrieval is impaired by uncertainty in snow morphology (grain size, stratification).
Satellite scatterometer measurements at Ku-band are also sensitive to morphology of the
snow volume (Nghiem and Tsai
2001
), suggesting that the active microwave measure-
ments support the characterization of snow scattering properties and thus improve the
retrieval of SWE by means of microwave radiometry. The Ku-band scatterometer aboard
flying with the second generation MetOp satellite is also used to measure ice sheet snow
accumulation, for the measurement of land snow mass at medium/low spatial resolutions
and for the characterization of the soil freeze/thaw cycle.
In order to account for improve remote sensing capabilities to measure snow parameters, the
validation with ground-based measurements and regional optimized algorithms to account for
variable landscape and physical properties effects are still a scientific challenge (IGOS
2007
). It
is expected that the Sentinels will be beneficial due to their increased temporal coverage,
accessibility and ability to monitor snow cover and snow melting process.
