EXPERIMENTAL STUDY ON THE SPECTRAL REFLECTANCE OF SNOW

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1 EXPERIMENTAL STUDY ON THE SPECTRAL REFLECTANCE OF SNOW Tsutmu Nakamura*, Reina Tamura*, Takeshi Ohta* and Osamu Abe *Iwate University and -Shinj Branch f Snw and Ice Studies, NIED ABSTRACT: The spectral reflectance f snw was measured experimentally. Prir t taking the measurements fr snw, the spectral reflectance f glass beads f knwn size distributins was measured frfive grups f beads with different diameters: J,Lm, J,Lffi, 800-1,200J,Lm, 3mm and 6mm. The results with the glass beads shwed n remarkable dependency n incident light angle, but shwed a remarkable size dependency; the smaller the grain size the larger the reflectance. Three different types f snw, artificially made new snw, artificial snw that was cmpacted with age, and natural granular snw, were used fr the measurements. The spectral reflectance f the three types f snw shwed a less cnsistent dependency n size than the glass beads, but in general, the smaller the diameter the larger the reflectance. A ne-time sequential experiment shwed that the spectral reflectance f snw decreased when snw metamrphsed r aged frm new snw t granular snw under the influence f intermittent lighting and freezing. This was due t the grwth f snw grains, which was revealed by measuring grain size n micrscpic phtgraphs taken by the aniline methd. KEYWORDS: Snw reflective prperties, snw ptics, snw physics, snw, snw cver. 1. INTRODUCTION Smetimes we need t knw the physical. -rperties f snw in a very remte lcatin, and in such cases a pwerful remte sensing technique is *Tsutmu Nakamura, Faculty fagriculture, Iwate University, Mrika , Japan; tel.: ; fax: ; tm@iwate-u.ac.jp needed. T btain the necessary infrmatin abut snw remtely requires a fundamental knwledge f the physical prperties f snw and f the magnetic respnse t the snw befrehand. The spectral prperty f snw is ne piece f fundamental infrmatin f which ur knwledge remains inadequate. 214

2 Recently, sme authrs have published infnnatin n the spectral reflectance fsnw. Fr example, Wiscmbe and Warren (1980) presented a methd fr calculating the spectral albed f snw with an adjustable parameter fr the effective grain size. Nakamura and Ikarashi (1993) measured the spectral albed f natural snw packed 0.9m deep, a blck f cmpacted snw, and a blck f granular snw. Aki et al. (1995 and 1998) cnstructed a multiple scattering mdel fr the atmsphere-snw system and cmpared the theretical results with the bserved results using the effective grain size. Hwever, nne f these authrs cnsidered the distributin f the size f snw grains. The grain size distributin shuld be cnsidered because packed snw has a cmplex texture. The purpse f this paper is t measure the spectral reflectance f snw with reference t the snw grain size distributin experimentally. Mst f the measurements were made with artificially prduced snw. 2. EXPERIMENTAL A spectrradimeter (MSR-7000, OPTO Research Crpratin) was used t measure the spectral reflectance f snw. This instrument can fcus n an area 4em in diameter and measure wavelengths ranging frm 280 nm (0.28JllT1) t 2,500 nm (2.5JllT1). Three different light surces were used fr the measurements. The artificial snw was prduced using a new big machine at the Shinj Branch, NIED that prduces dendrite snw crystals. We als met.amrphsed r 'aged' the fresh new snw t btain cmpacted snw. We define the spectral reflectance, a, as the rati fthe reflectin frm the snw surface t that frm a white plate f barium sulfate pwder. The reflectin frm the white plate is nearty 100%. Therefre, ur definitin f spectral reflectance is the same as the nnnal definitin f reflectance. In this paper, we use the wrd reflectance because the paper treats this phenmenn experimentally. In astrnmy, reflectance is the same as albed. Prir t making measurements f snw, the spectral reflectance f five grups f glass beads f different diameters was measured ( IJlT1, Om, 800-1,200JllT1, 3mm and 6mm). Apprximately 8,000 em 3 0f glass beads in a cylinder 20em in diameter with a height f 25em were used fr each measurement. The light surce used fr the measurement was a 500W halgen lamp. The light was placed 30em frm the surface f the glass beads at incident angles f 30, 45, and 60 degrees. All the measurements f the reflectance f the glass beads were carried ut in dry air cnditins, s the beads were nt wet. Three different types f snw, artificially made new snw, artificial snw that was cmpacted with age, and natural granular snw, were used fr the measurements. The cmpacted snw was made by aging artificial snw in a cld rm at -20 C fr abut ne mnth. 215

3 T detennine the distributin f grain sizes, reflectance was measured again after the snw micrscpic phtgraphs f each type f snw sample was taken. This prcedure was repeated sample were taken befre and afterthe reflectance measurements using the aniline methd (Kinshita fr a 2 nd and 3 1d radiatin with illuminatin at 475 t 482W/m 2 fr 17 minutes and at 480W/m 2 fr 13 and Wakahama, 1959). The sample thickness fr minutes, respectively. Successive measurements the micrscpic phtgraph was 0.2 t 0.3mm and snw grain size was measured in an area f 20 t were made at the same lcatin n the snw surface. 50 mm 2. The thickness f each branch was measured at lcatins every 0.1 mm alng the T examine the dependency f the length f the branches in the phtgraphs. A reflectance f new snw n the incident angle, the 100W lamp (SOLAX) prducing wavelengths mainly between 370 and 780 nm with a clr temperature f 5,500K was used in this experiment. reflectance was measured at incident angles f 30, 45, and 60 degrees. A halgen lamp was used fr this experiment. It was psitined 90cm frm the snw surface at an incident angle f abut 45 degrees. This lamp 3. RESULTS radiated the weakest radiatin f the three light surces used. 1) Glass beads Figure 1 shws the dependency f the Fr the ne-time sequential experiment, the spectral reflectance f artificial snw was measured after a sample had been metamrphsed by illuminating it intennittently. The successive measurements were carried ut in a cld rm at abut -15 C. After an initial reflectance measurement, the sample was liuminated fr 23 minutes at 500W/m 2 (1 st radiatin). A fixed light surce in the cld rm was used and it was the strngest f the three lamps used in this experiment. The incident angle was apprximately zer. The reflectance was measured 35 minutes later. Snw sample t make a thin sectin was taken ne hur and 45 minutes after the initial illuminatin. The spectral reflectance n the size f the glass beads. Fr this experiment, a halgen lamp was used with. j.. ct::. 1r---r----, ,r------, Wave length A. (nm) Figure 1. Dependency f the spectral reflectance n the size f glass beads. The numbers indicate the glass bead diamaters. 1: tl m, 2: tl m, 3: 800-1,200 tl m, 4: 3mm, 5: 6mm. 216

4 the incident angle at 60 degrees. All the raw curves are smthed fr every 30nm f wavelength t eliminate nise. The spectrradimeter generated mre nise at wavelengths frm 1,100 t 2,500 nm than in the range frm 280 t 1,100 nm. Therefre, the curves in this range shw larger fluctuatins than in the shrter wavelength regin. As seen in this figure, the reflectance fthe glass beads shwed a remarkable size dependency, the smaller the grain. 1 r\.. \...\ N,,\ -1: New Snw 2: Cmpacted Snw 3: Granular Snw size the larger the reflectance. The maximum reflectance fr the m glass beads was Figure 2. Spectral reflectance f the three different types f snw. abut 0.66 at a wavelength f abut 580 nm. As large amunt f nise due t the weak radiatin f the wavelength increased, the reflectance at first the light surce. The spectral reflectance f the increased gradually and then decreased gradually until the reflectance was nearly the same as that at three types f snw at wavelengths frm 350 t 750nm increased gradually t an average value f the shrter wavelengths. This trend was als abut 0.8 t 0.85, and then decreased t abut bserved with the larger glass beads. The zer as the wavelength increased t 1,500nm with minimum reflectance fr the five grups was sme wavy fluctuatins. At wavelengths frm bserved with the 6mm glass beads, which had a 1,500 t 2,500nm, the reflectance ranged frm maximum reflectance value f abut The zer t 0.3 depending n the type f snw. Ofthe results with the glass beads shwed n cnsistent dependency n the incident light angle, which was set at 30, 45, and 60 degrees. three types, new snw had the highest reflectance, althugh the reflectance f bth new snw and granular snw was quite similar in the range 350 t 750nm. The reflectance f cmpacted snw and -;='"2) SPectral reflectance fthree different types f granular snw were similar ver the entire range f wavelengths. The grain size, shape, and size Figure 2 shws the spectral reflectance distributin fr the three types f snw are shwn f three different types f snw: artificially in Figs. 3 t 8. Figure 3 shws a phtgraph f prduced new snw, artificial snw that was metamrphsed frm the new snw, and natural new snw, which was prduced artificially by a special new machine at the Shinj Branch f Snw granular snw. The reflectance at wavelengths and Ice Studies, NIED, and shws branches f frm 745 t 895 nm was eliminated because f the dendritic snw crystals. Figure 4 shws the grain 217

30 25 Cmpacted snw Figure 3. Thin sectin f artificially prduced new snw made by the aniline methd. 80 70 60 50 :::: 40 & 30 2;0 10 New snw Grain size(mm) Figure 4.

5 30 25 Cmpacted snw Figure 3. Thin sectin f artificially prduced new snw made by the aniline methd :::: 40 & 30 2;0 10 New snw Grain size(mm) Figure 4. Grain size ditributin fr the artificially prduced new snw shwn in Fig.S. 5 (;;)':' t:::> t:::> Grain size(mm) Figure 6. Grain size distributin frthe cmpacted snw shwn in Fig.5., Imm... ", Figure 7. Thin sectin f natural granular snw. 12 r , 10 >. 8 (,) = ;:l 6 cr r.t 4. Granular snw 2 <;) <;)'Y <;) <;)fl;> <;),, 'Y Figure 5. Thin sectin f cmpacted snw metamrphsed frm new snw. Grain size(mm) Figure 8. Grain size distributin fr the natural granular snw shwn in Fig

size distributin f the new snw. In this figure, the grain size, d, ranged frm 0.02 t 0.18mm (0.02 :::; d < O. 18mm) with the majrity f the particles in the range frm 0.06 t 0.1 mm (0.06:::; d < O.

6 size distributin f the new snw. In this figure, the grain size, d, ranged frm 0.02 t 0.18mm (0.02 :::; d < O. 18mm) with the majrity f the particles in the range frm 0.06 t 0.1 mm (0.06:::; d < O.lmm). Figure 5 shws part f a thin sectin f cmpacted snw. The size distributin is shwn in Figure 6. The snw grains were assumed t be elliptical and the grain size was defined by..;;;b, where a and b are the majr and minr diameters f an ellipse. The size ranged frm 0.2 t 1.0mm. The mst numerus grains were 0.4 t 0.6mm in size. Figure 7 shws part f a thin sectin f natural granular snw, and the size distributin is shwn in Figure 8. The size ranged frm 0.2 t 1.6mm and the mst numerus grains were 0.4 t 0.6mm. 3) SUccessive decrease in the spectral reflectance fsnw due t metamrphsis Figure 9 shws the successive decrease in the spectral refledance f snw with metamrphsis caused by melting due t light radiatin and freezing. Curve NO.1 in Figure 9 shws the reflectance f new snw which micrscpic phtgraph is shwn in Figure 10. Branches f new snw are seen in this phtgraph. The grain size distributin befre the snw was expsed t radiatin is shwn in Fig. 11 and ranges frm 0.02 t 0.12mm with mst grains in the range frm 0.02 t 0.06mm. The snw... e:: Wave length A..' (nm) Figure 9. Successive decrease in the spectral reflectance f snw resulting frm metamrphsis. 1: New snw befre light radiatin.2: Immediately after the 1st radiatin. 3: Metamrphsed after the 1st radiatin. 4: After the 2nd radiatin. 5: After the 3rd radiatin. surface was wet and irregular as shwn in Figure 12. The uneven snw surface resulted frm the intense illuminatin and the snw depth decreased frm 26.5 t 22cm, the depth at which the measurements were made. The snw temperature 5cm belw the snw surface was abut -5 t -6 C. In Figure 12, the dark areas 1 t 2cm in diameter are wetter than the white area. The surface was bviusly wet t the tuch and a density was 0.037g/cm 3 Curve NO.2 shws the refledance f snw 35 minutes after the 1 st radiatin. The snw Figure 10. Thin sectin f artificially prduced new snw. 219

120 100 ;>., 80 C) c Q) 60 :l 0- Q)... u.. 40 20!O - Grain size (mm) Figure 11.Grain size distributin fr the artificially prduced new snw shwn in Fig.1 O. Figure 13.

7 ;>., 80 C) c Q) 60 :l 0- Q)... u !O - Grain size (mm) Figure 11.Grain size distributin fr the artificially prduced new snw shwn in Fig.1 O. Figure 13. Thin sectin f the partially metamrphsed snw surface frm Fig.12. The metamrphsis was prduced by radiatin and freezing (after the initial radiatin) Figure 12. Metamrphsed snw surface melted by radiatin. The dark area is much wetter than the surrunding white area (12:00). small film f liquid water culd be seen in the dark areas by the naked eye. Curve N.2 was measured under these cnditins. A thin sectin f the snw surface is shwn in Figure 13. Snw 30 G25 c J: ,Im.m m <:) <:)'" <:) <:)'1> <:)":- "", "I).,,,,'I> <:)'Y ",'" <:). <:). Grain size (mm) Figure 14. Grain size distributin fr the metamrphsed snw shwn in Fig.13. sample t make the thin sectin was taken ne hur and 45 minutes after the measurement f as large ice grains in Fig.13. Partially Curve NO.2. By then, the water n the snw metamrphsed snw grains are als bserved surface had frzen cmpletely. The wettest under the ice grains. The size distributin is prtin, which appears black in Figure 12, is seen shwn in Figure 14. The grains range in size frm 220

The snw depth was reduced t 18.5cm. Curve N.5 was measured after the 3 rd radiatin. As seen in Figure 15, the snw surface became much rugher and was mre granular than that shwn in Figure 12.

8 The snw depth was reduced t 18.5cm. Curve N.5 was measured after the 3 rd radiatin. As seen in Figure 15, the snw surface became much rugher and was mre granular than that shwn in Figure 12. The phtgraph f the thin sectin f the snw surface and the size distributin are shwn in Figures 16 and 17, respectively. The Figure 15. Rugh metamrphsed snw surtace prduced by melting with further illuminatin (Immediately after the 3 rd radiatin at abut 15:50). >, () c 2 Q) ::l 1.5 CL '- "" '" "' Ib Ib Grain size (mm) Figure 17. Grain size distributin fr the metamrphsed snw shwn in Fig.16. Figure 16. Thin sectin f the metamrphsed snw. surface prduced with further illuminatin (Immediately after the 3 rd radiatin). '''i).02mm, which is the same as the minimum size f the new snw befre radiatin, t abut 2mm (althugh this part is nt shwn in Figure 14) with grains f 0.04 t 0.1 mm being the mst numerus. Curve N.3 in Figure 9 shws the reflectance f the snw ne hur and 50 minutes after Curve N.2 was recrded. Curve N.4 was taken at the same lcatin after the 2 nd radiatin. size ranges frm 0.5 t 5.0mm and the mst numerus grains are 1.5 t 2.0mm. Figs.16 and 17 shw that the ice grain sizes increased cmpared t the sizes seen in Figures 13 and 14. 4) Incident angle dependeney. Figure 18 shws the dependency f the reflectance f new snw n the incident angle. Curves N.1 t 3 shw the spectral reflectance at incident angles f 30, 45, and 60 degrees, respectively. The reflectance had the minimum value at 30 degrees. At wavelengths frm 1,500 t 2,500 nm, the reflectance was largest fr 60 degrees, intermediate fr 45 degrees and smallest 221

. 1 ----------- 2--- 3--- fr 30 degrees. The thin sectin f the new snw and the size distributin are hwn in Figs.19 and 20, respectively. The new snw is quite similar t the new snw shwn in Fig.3. 4.

9 fr 30 degrees. The thin sectin f the new snw and the size distributin are hwn in Figs.19 and 20, respectively. The new snw is quite similar t the new snw shwn in Fig CONCLUSIONS AND DISCUSSION Wave length :l (nm) Figure 18. Dependency f the reflectance f new snw n the incident light angle. 1: 30,2: 45,3: >. 70 g 60 gj 50 C' 2: 40 Cl Figure 19. Thin sectin f new snw. C\ C\"- C\'>- C\IO C\CO C\':-,"-,'>-,10,c C\':I- Grain size (mm) Figure 20. Grain size distributin fr the new snw shwn in Fig.19. The glass beads shwed a rather flat reflectance with a small peak arund 600nm. This means that, unlike snw, the glass beads have n specific spectral absrptin f light at the larger wavelengths. The reflectance shwed size dependency at wavelengths frm 300 t 2,500nm, i.e., the smaller the glass beads, the larger the reflectance. On the ther hand, the snw had a higher reflectance at shrter wavelengths than at lnger wavelengths. This means that snw has smaller absrptin at the shrter wavelengths than at the lnger nes. This tendency must be a native prperty f ice itself (Hbbs, 1974). The new, cmpacted, and granular snw all had similar reflectances. Of these three snw types, new snw had the largest spectral reflectance at all wavelengths, althugh the reflectance fr all three snw types was nearly the same (0.8 t 0.9) at wavelengths f 300 t 800nm. The cmpacted snw had the secnd largest reflectance and the granular snw the smallest reflectance. The difference in the reflectance between the cmpacted snw and the granular snw was small. This was prbably due t the similar grain size distributin f the cmpacted snw and granular snw. The large amunt f 222

10 nise seen at wavelengths between 750 and 880nm bserved in the measurement f all three types f snw was prbably due t the light used fr the measurement. The SOLAX lamp used fr this measurement had the weakest light intensity, and wuld prduce mre nise than the ther tw light surces used. The spectral reflectance f snw decreased successively with metamrphsis caused by melting due t light radiatin and freezing. The successive decrease in the spectral reflectance crrespnded t an increase in the grain size f the snw revealed by the measurements f the grain size frm micrscpic phtgraphs f a thin sectin f snw. The decrease in the spectral reflectance at wavelengths f abut 380, 1,020 and 1,250nm must be a prperty f ice as these minimum values crrespnd t maximum values fr the absrptin curve f ice (Hbbs, 1974), althugh there are sme differences in the crrespnding wavelengths. In the successive measurements a clear size dependency was bserved in the reflectance at wavelengths arund 1,000 and 1,400nm. These wavelengths can be used t identify the snw type and can therefre be used fr the remte sensing f snw in the future. Incident angle dependency was bserved in new snw at 30, 45 and 60 degrees. The experimental results shwed that the larger the incident angle, the larger the reflectance. This relatin was clearly bserved at wavelength frm abut 1,470 t 2,500 nm. This relatinship must be partly due t the prperties f ice because the reflectance f ice remains cnstant at incident angles frm 0 t nearly 50 degrees, after which the reflectance suddenly increases t 0.45 (Hbbs, 1974). The general pattern f the spectral reflectance btained in this experiment agrees well with the results btained by Wiscmbe and Barren (1980), Nakamura et al. (1993) and Aki et a!. (1995 and 1998). Hwever, a detailed quantitative cmparisn shuld be made. A cmparisn between the effective grain size and the grain size distributin as determined in this paper must als be carried ut. ACKNOWLEDGMENTS The authrs thank The Institute f Snw and Ice Studies, NIED, fr lending us the ptical instrument. We thank Dr. Teru AOKI, Meterlgical Research Institute, and Dr. Atsushi SAITO, Thku University, frtheir kindness in giving us recent publicatins and the directr, Dr. Atsushi SATOW, and staff f the Shinj Branch f Snw and Ice Studies, NIED, fr their help. Thanks are als given t Mrs. Kazuyshi SUZUKI and Tmhir HASEGAWA fr their technical help. REFERENCES Aki, Teru, Tada Aki, and M. Fukabri, 1995: Optical prperties f snw with a multiple scattering mdel fr the atmsphere-snw system (Text in Japanese). Prceedings f the Meterlgical Research Institute 223

11 sympsium, Aki, Teru, Tada Aki, M. Fukabri, Y. Tachibana, Y. Zaizen, F. Nishi and T. Oishi, 1998: Spectral albed bservatin n the snw field at Barrw, Alaska. Prc. NIPR Symp. Plar Meterl. Glacil. Vl. 12 (in print). Hbbs, P.v., 1974: Ice Physics. Oxfrd U. 837, p.214. Kinshita, S. and G. Wakahama, 1959: Thin sectins f depsited snw made with the use f aniline (Text in Japanese with English resume). Lw Temperature Science, Ser. A, 18, Nakamura, T. and T. Ikarashi, 1993: Scattering characteristics f the slar radiatin frm the snw surface. Prc. JUST Wrkshp n the Applicatin f Remte Sensing Technlgy t Natural Disaster Reductin. p WlScmbe W.J. and S.G. Warren, 1980: A mdel fr the spectral albed f snw. J. Atms. Sci., 37,

Interference is when two (or more) sets of waves meet and combine to produce a new pattern.

Interference is when two (or more) sets of waves meet and combine to produce a new pattern. Interference Interference is when tw (r mre) sets f waves meet and cmbine t prduce a new pattern. This pattern can vary depending n the riginal wave directin, wavelength, amplitude, etc. The tw mst extreme