The Relevance of a Precise In Situ Measurement of the Optically Equivalent Grain Size in a Snowpack
Abstract
The stratigraphy and microstructure of snow cover is very complex. However, for modeling electromagnetic radiative transfer, a simplified description of this complex structure is necessary. Although the specific surface area of snow was already considered in the 1960's as the most relevant parameter (E. LaChappelle), the measurement of the effective optical diameter in the field is often considered to be equivalent to the grain size determined by hand-lens according to the International Classification for Seasonal Snow on the Ground. However, the optical grain size (r_o) derives from the specific surface area whereas the classical grain size inferred from hand lens (r_s) is different, except for spherical particles. Large discrepancies may occur when strongly non-spherical shapes occur, leading to r_s being much larger than r_o and in turn significant errors in optical and microwave radiative transfer modeling. Measurement of the specific surface area of snow may be made without bias for all snow types using model-based stereology on casted samples. In this work, we present reliable and precise methods in the field for inferring optical grain size: (i) to cast snow samples and determine specific surface area in the laboratory using model-based stereology, (ii) calibrated near-infrared photography, and (iii) contact near-infrared spectroscopy, (iv) For sufficiently large and homogeneous snow samples, also the emission and scattering of microwave radiation can be used to determine the specific surface, as shown by Wiesmann et al. 1998. Calibrated near-infrared photography is currently the only technique with a sufficiently high spatial resolution to resolve thinner layers typical for snowpacks, except for extensive sampling of casted snow. A combination of contact spectroscopy, which has its own light source with constant illumination and NIR-photography will provide the most accurate, high resolution characterization of optically equivalent grain size stratigraphy in snowpacks.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.C13A..03S
- Keywords:
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- 0360 Radiation: transmission and scattering;
- 0736 Snow (1827;
- 1863);
- 0758 Remote sensing;
- 0794 Instruments and techniques