Optical Properties of Snow in Backscatter

574 Journal of Glaciology, Vol. 52, No. 179, 2006 Optical properties of snow in backscatter S. KAASALAINEN,1,2 M. KAASALAINEN,3 T. MIELONEN,1 J. SUOMALAINEN,1 J.I. PELTONIEMI,1 J. NA¨ RA¨ NEN1,4 1Finnish Geodetic Institute, Geodeetinrinne 2, PO Box 15, FIN-02431 Masala, Finland E-mail: [email protected] 2Finnish Meteorological Institute/Arctic Research Center, Ta¨htela¨ntie 62, FIN-99600 Sodankyla¨, Finland 3Department of Mathematics and Statistics, University of Helsinki, PO Box 68, FIN-00014 Helsinki, Finland 4Department of Astronomy, University of Helsinki, PO Box 14, FIN-00014 Helsinki, Finland ABSTRACT. We present an overview and the first systematic results on the backscatter properties of snow samples with different grain properties. During a 2 year study we investigated the effect of the apparent characteristics of the samples on the intensity enhancement in the direction of exact backscatter (retroreflection), using a specific instrument designed for backscatter measurements. We observed that a sharp peak in intensity (the hot spot) occurs for most types of snow, which is not observable with traditional goniometers. The key factors in the peak properties are the temperature (related to the changes in grain structure) and grain shape and size. These results form the basis of a larger backscatter data archive which is being applied in the systematic study and exploitation of ’hot spots’ in remote sensing of natural targets, as well as in the development of airborne laser intensity measurement. 1. INTRODUCTION instruments that observe at, or close to, exact backscatter (e.g. POLDER and HyMap; see Comacho-de-Coca and The optical properties of snow play a major role in remote others, 2004). The most important observational property of sensing of the cryosphere and mapping of icy regions. exact backscatter is that it cannot be reached with Methods for deriving the key factors of snow- and ice-cover goniometers, as in these the detector necessarily blocks variations, such as albedo, metamorphism and grain the light source. However, the effect at zero-angle vicinity characteristics (Grenfell and others, 1994; Fily and others, undoubtedly contains a significant amount of information on 1997, 1998, 1999), are developed for more effective the target material: as Van Albada and Lagendijk (1985) detection of changes in snow cover. Mapping of snow- note, the effect is simply much too sharp to be observed and ice-covered areas provides important indications of without a beam-splitter. climate change, hydrological processes and water resources There are few ground reference data available on natural (Fily and others, 1998; Magagi and Bernier, 2003). hot spots, even though the backscatter effect has been Remote sensing of snow and other land surfaces is based measured systematically for various substances in laboratory on spectral and directional features of light scattered from experiments. Laboratory experiments on backscattering pat- the surfaces (see Fily and others, 1998; Nolin and Liang, terns have been carried out mostly for powders and particles, 2000). The bidirectional reflectance distribution function and applied in the fields of optics, condensed matter physics, (BRDF) describes the directional properties of light re- astronomy and medicine (for more references see Yoon and flected from a surface, and it is used both in directional others, 1993; Shkuratov and others, 2002; Kaasalainen and corrections of, and as a source of information in, remote- others, 2005a). Small-scale laboratory measurements of sensing data (Verstraete and Pinty, 2001, and references natural targets have been made, for example for clumps of therein). Comparisons with models and in situ measure- grass or needles (Hapke and others, 1996). However, a large ments are important for more reliable information to be number of measurements, particularly field measurements derived from remote instruments. Computational studies of made more or less in situ, would be required to understand the directional reflectance of snow are found in Leroux and and exploit better the hot spot signatures and establish a others (1998) and Mishchenko and others (1999). Among remote-sensing ground reference databank. the most recent goniometric experiments of the bidirec- Snow (back)scattering, exact or general, is still a relatively tional reflectance of snow are those by Aoki and others unexplored field. It has been studied mainly for snow (2000), Painter and Dozier (2004) and Peltoniemi and crystals in the atmosphere (e.g. Borovoi and others, 2000) others (2005). and mapping of snow cover at zero phase angle by radar or The increased reflectance at small phase angles (light- microwave, with connections to physical parameters (Fily source–target–detector angle; see Fig. 1) is observed as a ‘hot and others, 1998; Strozzi and Ma¨tzler, 1998; Magagi and spot’ in remote images for targets such as forest and Bernier, 2003). This is essentially the objective in the optical vegetation (Hapke and others, 1996; Comacho-de-Coca regime as well. In astronomy, strong backscatter peaks and others, 2004). The increase is particularly pronounced (called opposition effect in this context) have been observed at zero phase and its immediate vicinity, which is often for numerous ice-covered surfaces (including those with referred to as ‘exact backscattering’ to distinguish it from water ice), such as rings and satellites (Domingue and backward scattering in the same hemisphere in which the Verbiscer, 1997; Poulet and others, 2002). Optical back- light source is placed (also termed retrosolar scattering in scattering of snow on the ground has been addressed in only remote sensing). Backscattering has recently gained more a few studies thus far (Piironen and others, 2000; Shkuratov importance in terrestrial remote sensing, since there are new and others, 2002; Kaasalainen and others, 2003), with a few Downloaded from https://www.cambridge.org/core. 02 Oct 2021 at 18:25:48, subject to the Cambridge Core terms of use. Kaasalainen and others: Optical properties of snow in backscatter 575 peaks observed for some samples. These studies have focused mainly on the effects of impurities. Systematic backscattering experiments are lacking, and no description of snow backscatter, or the key parameters related to it, is available. This lack of information is one of the main incentives for undertaking the present study. Another key factor is that this study makes use of a standard experimental set-up that has been used widely in backscatter studies of materials other than snow. Therefore its use here is a natural extension of these applications. On the theoretical side, there are a number of studies on exact backscattering in both optics and light scattering, so the need to access the zero phase-angle vicinity properly is also important for developing the theoretical methods. Indeed, much of current backscatter theory has remained Fig. 1. The small-angle laser goniometer. Retroreflection from the tentative due to a lack of the systematic experiments needed sample is observed through the beam-splitter by the charge- for building any hypotheses into a theory. In fact, current coupled device (CCD) camera (left). Non-zero phase angles theoretical results and recent experiments have several (source–target–detector) are obtained by rotating the beam-splitter mutual inconsistencies or uncertainties (Nelson and others, (right). The distance from beam-splitter to sample was 12 cm, and from the CCD to the sample 120 cm. 2002), so there is an obvious need for new measurements. Although the backscatter effects are extremely strong, even their basic physical interpretation requires both experimental and theoretical studies. The current physical interpret- charge-coupled device (CCD) detectors in the measurement ation of hot spots is based on shadow hiding in rough of (backscattered) laser intensity (e.g. Yoon and others, surfaces and coherent backscatter, which is a microscopic- 1993; Wiersma and others, 1995; Nelson and others, 2000; scale interference effect (Hapke and others, 1996; Shkuratov Shkuratov and others, 2002). The measurement presented and others, 2002). here is based on these techniques, and has been used This paper presents the results of a 2 year campaign of widely in laser-based backscatter measurement in optics snow backscattering measurements, the first extensive study and related fields for laser intensity measurement and with systematic variation of snow parameters. The existing, standard CCD photometry used in the laboratory and, for well-established techniques for measurement of back- example, in astronomical studies. scattering were applied to snow surfaces to obtain informa- The measurements were made using the small-angle tion on the backscattering properties of snow and to search goniometer constructed particularly for backscattering for the main contributors to snow hot spot. The potential measurements, as shown in Figure 1. A 632.8 nm He–Ne applications to glaciology and snow studies via remote laser was used as a light source. The backscatter geometry is sensing are obvious as airborne laser intensity measurements worked out using a beam-splitter. The sample was placed are under development (Geist and others, 2003; Kaasalainen on a rotator to smooth out the laser speckle and obtain a and others, 2005b). The laser altimetry measurement deals larger sample area as the spot (about 4 mm in diameter) with the retroreflected laser beam, but the intensity moved on the surface. Transmittance filters of 10% and 3% information has not been discussed thus far. The search for were used in front of the light source to keep the signal in a tool for change detection is of primary importance to the linear range in the CCD frame. More detailed glaciology in this context. It is essential to realize that this descriptions of the instrument are provided by Na¨ra¨nen approach is quite feasible even when exact quantitative and others (2004) and Kaasalainen and others (2005a, b). inference of the physical properties of the target is uncertain We modified the original laboratory set-up to be robust due to insufficient theoretical modelling, noisy data, or even enough for field conditions.

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