Instruments and MethodsContact spectroscopy for determination of stratigraphy of snowoptical grain sizeThomas H. PAINTER,1Noah P. MOLOTCH,2Maureen CASSIDY,1Mark FLANNER,3Konrad STEFFEN41National Snow and Ice Data Center/World Data Center for Glaciology, CIRES, University of Colorado, Boulder,Colorado 80309-0449, USAE-mail: [email protected] of Civil and Environmental Engineering, University of California, Los Angeles, California 90095-1593, USA3Department of Earth System Science, University of California, Irvine, California 92697-3100, USA4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado 80309-0216, USAABSTRACT. We present a technique for in situ measurement of the vertical and spatial stratigraphicdistribution of snow optical grain size with a coupled contact illumination probe and fieldspectroradiometer. Accurate measurements of optical-equivalent grain size are critical for modelingradiative properties of snow such as spectral albedo and microwave emission. Measurements of thespectral reflectance of the snow-pit surface are made at 2 cm intervals in the vertical plane underconstant illumination and view geometries. We invert the integral of the continuum normalization ofthe ice absorption feature with maximum at 1.03 mm wavelength for optical-equivalent grain size usingthe validated model of Nolin and Dozier (2000) that has accuracy of 10–50 mm across the grain-sizerange 50–900 mm. Results are presented for an alpine site in southwest Colorado, USA, across theablation season and for a Greenland ice-sheet site at the onset of snowmelt. These results suggest thattraditional measurements of grain size using a hand lens are nearly accurate only for rounded grains(R2¼ 0.41, rmse ¼ 160 mm); for polycrystals and faceted grains the hand-lens approach is veryinaccurate (R2¼ 0.03 and 0.24, rmse ¼ 1206 and 1010 mm, respectively). We demonstrate an order-of-magnitude improvement in modeling of shortwave spectral albedo and net shortwave flux with contactspectroscopy measurements of grain-size stratigraphy over those from a hand lens.INTRODUCTIONThe grain size of snow controls the optical properties of thesnowpack (Warren, 1982) and indicates the advancement ofmetamorphism processes (Davis and others, 1993). Thestratigraphy of snow grain size is critical for mechanical,thermodynamic and radiative properties of the snowpack(Colbeck, 1991; Pielmeier and Schneebeli, 2003). In situobservations of snow properties for hydrology and avalanchestudies frequently include manual, hand-lens estimates ofgrain-size stratigraphy. The NASA/US National WeatherService (NWS) Cold Land Processes Experiments (CLPX) of2002–03 in the western United States included a protocolfor estimates of grain-size stratigraphy using a hand lens toprovide modeling constraints for microwave radiativetransfer models (D. Cline and others, http://nsidc.org/data/nsidc-0176.html). However, these manual measurementslack repeatability and do not infer the optical-equivalentgrain size used for radiative transfer modeling. Quantitativeand repeatable measurements of optical-equivalent grainsize may be made through stereological techniques (Davisand others, 1987; Painter and Dozier, 2004), but thesemethods require extensive procurement of samples, delicatetransport and many laboratory hours that necessarily spanseveral days.Accurate detection of snow grain size is particularlyimportant for modeling applications where snow grain size istreated explicitly (e.g. Crocus (Brun and others, 1992),SNOWPACK (Bartelt and Lehning, 2002) and SNTHERM(Jordan, 1991)). The logistical constraints and, in some cases,subjective nature of the aforementioned measurement tech-niques limit their applicability in supporting the developmentof physically based snow process models and thereforetoward improving the land-surface, hydrologic and generalcirculation models that require representation of snowpackprocesses. In this regard, Jin and others (1999) found thatcomplex models of snowpack processes do not improveupon more simplistic algorithms, largely due to inadequaterepresentation of key snowpack states such as grain size.Accurate measurements of snow grain size are also criticalfor the development of direct inversion techniques forestimating snow water equivalent (SWE) from microwaveremote sensing (Armstrong and others, 1993; Kelly andothers, 2003) and for estimating snow albedo and fractionalsnow-covered area (Li and others, 2001; Painter and others,2003; Flanner and Zender, 2006). An improved representa-tion of snow grain size within these direct retrieval algorithmsis critical for solving the many-to-one relationships betweenSWE and brightness temperature that currently constrainretrieval accuracy (Foster and others, 2005). With regard toboth improving representation of grain size as a model stateand within direct inversion techniques, information aboutthe variability in snow grain size and uncertainty in grain-sizeparameterizations has considerable utility within data-assimilation schemes aimed at bringing together measure-ments and models (Durand and Margulis, 2006).Journal of Glaciology, Vol. 53, No. 180, 2007 121Here, we refer to grain size inferred from traditionalhand-lens measurements as TGR (traditional grain radius)and optical-equivalent grain radius as OGR (optical grainradius). Given the subjective nature of TGR measurements, itis poorly defined and as such is not repeatable from observerto observer (personal communication from S. Colbeck,2006). The OGR is well defined as the spherical grain radiusrequired to give the same spectral or spectrally integratedalbedo. OGR may also be represented by the specificsurface area (surface area per unit volume ice) in the case offluxes or albedo for snow (Dozier, 1989; Flanner andZender, 2006; Matzl and Schneebeli, 2006) or clouds(Grenfell and Warren, 1999).Matzl and Schneebeli (2006) presented a digital near-infrared (NIR) photographic technique for estimating snowspecific surface area (SSA ¼ 6/d where d is the spherediameter). The photographic technique inverts based on arelationship between SSA, as determined with stereologicalanalysis of select regions of the snow-pit face, and reflect-ance, and therefore requires a planar face for uniformillumination. Much higher spatial resolution imagery(1 mm spatial resolution) of SSA can be derived from NIRphotography than from the technique described in thepresent work (2 cm).