OBSERVATIONS OF SUN CRUST FORMATION

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1 OBSERVATIONS OF SUN CRUST FORMATION Tshihir Ozeki*, Eizi Akitaya* and Keisuke Suzuki** ABSTRACT The mechanism f sun crust fnnatin was investigated thrugh field bservatins. During the bservatins, sun crust fnned 12 times. The sun crust is a thin ice layer made f ice particles. With the sun crust fnnatin, the lcal temperature beneath it rse and internal melting ccurred. This led t the fnnatin f cavities under the sun crust. The energy balance calculatin f the sun crust revealed that the shrtwave radiatin absrbed was balanced with the ttal f net lngwave radiative flux, latent heat flux, and sensible heat flux. Further. dwn the snw pack, shrtwave radiatin penetrated thrugh the surface layer, was absrbed and internal melting ccurred. The surces f H20 fr sun crust fnnatin were investigated thrugh the fluctuatins f xygen-18 cncentratin f the sun crust. The istpic cmpsitin changed with sun crust fnnatin: 8 value f sun crust had a. tendency t increase (heavier water) with time, and was larger (heavier water) than that fthe snws beneathit. *Institute flw Temperature Science fhkkaid University Sappr, Japan.**Tky Metrplitan University Hachiuji, Japan 2

2 INTRODUCTION Sun crust, an unusual metamrphism f snw, is a thin grazed ice layer fund at the surface f snw pack n sunny day in winter. It is a very interesting snw surface feature. Hwever, it is difficult t bserve its frmatin because it is fund nly a few times in ne winter seasn in Hkkaid, Japan. The authrs investigated the mechanism f sun crust frmatin thrugh field bservatins and labratry experiments (Ozeki and Akitaya, 1992). The bservatins fund that the sun crust structure and the suitable energy balance cnditin fr sun crust frmatin. These results were verified by labratry experiments. Hwever, they were revealed frm nly ne time bservatin f sun crust, and the surce f H20 fr the sun crust layer was nt knwn. This paper aims t investigate the energy balance and the change f in sun crust frmatin thrugh field bservatins. OBSERVATIONAL SITES AND INSTRUMENTATION The mechanism f sun crust frmatin was investigated thrugh field bservatins. Field bservatins were carried ut fr fur winter seasns (1991, 1992, 1993, 1994) at Mshiri, central part f HOkkaid, and Tikanbetsu, nrthern part f Hkkaid. The site f Mshiri is lcated in the lwest part f the land basin at an altitude f 285m a.s.l. in the Bifukazawa River Watershed, which is surrunded by muntain ridges with an altitude difference f abut 250m (Figure 1). Tw meterlgical statins were set up fr fur seasns: A at the center f the basin; B at the highest part f the surrunding muntains. An additinal meterlgical statin was set up in 1994: C at the 200m suth frm the statin B. The surface cnditin f each statin was cmpsed f thick, flat and smth snw cver. The bserved parameters and the instrumentatin used are summarized in Table 1. These field bservatins were made by autmatic and manual measurements. In additin t these measurements, sampling fcrust layer and 1 cm snw layer beneath it were carried ut when sun crust was frming. The site f Tikanbetsu is lcated n the ridge at an altitude f 240m a.s.l. in the Teshi Experimental Frest f Hkkaid University (Figure 2). Snw sampling and analyses fplarized picture fthin sectin were carried ut fr tw seasns (1993, 1994). 3

3 Fig. 1. Lcatin map f3 meterlgicalstatins in Mshiri. Sappr Tikanbelsu[} 4 Fig. 2. Lcatin map jan bservatry in Tikanbetsu.

4 Table 1. Observatinal Items andinstruments Items Instruments statins 1. Cntinuus measurements Dry-bulb temperature Wet-bulb temperature Surface temperature Humidity Wind speed All-wave net radiatin Incming and utging f Shrtwave radiatin Air pressure 2. Manual measurements Liquid water cntents Snw density frced ventilated resistance thermmeters frced ventilated resistance thermmeters an infrared thermmeter A,B,C A, B static electric capacitance type B, C hygrmeter a three -cup anemmeter a net pyrradimeter pyranmeter a cylindrical resnatr type barmeter a calrimetric snw-water cntent meter a snw sampler A A,B, C A,B A,B A A, B, C A, B, C ESTIMATION OF HEAT FLUX Sensible heat flux and latent heat flux are calculated at each statin accrding t a bulk methd (Takeuchi and Knd, 1981). An equatin fthe bulk methd fr sensible heat flux QH is the fllwing: QH= pacph VI (1'1- T), (1) where h is the bulk cefficient, TI and T the air temperature at a height f 1 m and snw surface temperature, VI the wind speed at a height f 1 m, pa the air density, Cp the specific heat capacity f air at cnstant pressure. Latent heat flux QE is given by OE = L pa h VI (qi - q), (2) where L is the latent heat f ice n evapratin, qi and q the specific humidity at a height f 1 m and snw surface. The fllwing value fr the 5

bulk cefficient h (Ishikawa and Kdama, 1994) is used in this study: h = 2.3 X 10-3 (3) The amunt f net lngwave radiatin is calculated frm the bserved incming and utging slar radiatin and net radiatin.

5 bulk cefficient h (Ishikawa and Kdama, 1994) is used in this study: h = 2.3 X 10-3 (3) The amunt f net lngwave radiatin is calculated frm the bserved incming and utging slar radiatin and net radiatin. OBSERVATIONAL RESULTS Energy Balance During the bservatins, sun crust frmed 12 times. A crss sectin f the sun crust is shwn in Figure 3. The sun crust was a thin ice layer made f ice particles with the thickness f 1 t 2 mm, and internal melting frmed cavities under it. The thickness decreased as the ice layer extended laterally. The weather cnditins during sun crust frmatin were fine weather, air temperature nearly 0 C, wind speed abut 1 t 5 mis, humidity abut 30 t 60 % at a height f 1 m. In 11 f the 12 times f sun crust frmatin, the surface snw type befre the frmatin was granular snw. In nly ne case did the sun crust frm after new snw had metamrphsed int granular snw. The average density f 3 cm layer frm the surface was 4.2x 10 2 kg/m 3 and the average water cntent was 12.6 % (Table 2). Fig. 3. Vertical crss sectin f the sun crust. 6

6 The cmpnents f heat balance at snw surface are calculated density liquid water cntent during frmatin f sun kg/m 3 % crust. Figure 4 shws the average 4.2x time series f shrtwave max 4.8x and lngwave radiative min 3.7x flux sensible and latent Table 2. Density and liquid water cntent futermst snw layer prir t sun crustjrmatin heat flux during sun crust frmatin at the statinan Apr. 6, In Figure 4, the rdinate shws the amunt f heat flux ging int the snw.layer. The sun crust was frming fr a few hurs near nn. During the sun crust frmatin, absrbed shrtwave radiatin exceeds 300 W1m 2 and net lngwave radiative flux was abut -50 W/m 2 which cls the snw surface. Additinally, sensible heat is transprted frm air t snw surface and ppsitely latent heat frm snw surface t air, and thse tw values are nearly equal. During sun crust frmatin, the thin layer near surface was nt melting and the layer beneath it was melting. In rder t verify it, the amunt f 'E "- 150 >< :::l u II) ::t: Lcal Time Fig. 4. Time series fheat balance cmpnents at statin A n Apr. 6,

7 c: 0... (W 1m 2 ) (W 1m 2 ) "'C c:::: Q)... > (f;? 0 + en "'C Q).c 0CI).c < Lcal Time ::J: p Fig. 5. Cmparisn fthe absrbed shrtwave radiatin in the layers f0-5 mm and 5-10 mm (bxes) and the ttal fsensible heat QH, latent heat QE, lngwave radiatin QL (slid line) at statin A n Apr. 6, shrtwave radiatin absrbed by the snw layers is calculated and cmpared with the result fheat budget at the surface. Shrtwave radiatin 1 attenuate in snw cvers as fllwing: 1 = (1 - rj 10 exp(a Zj, (4) where 10 is the incident shrtwave radiative flux, r the albed f the snw cver, a the attenuatin cefficient and Z the depth. The attenuatin cefficient is assumed t be -0.4 cm- I, which was btained by Fukami and Kjima (1980). Figure 5 shws a case at the statin A n Apr. 6, The bxes in the figure shw the amunt f absrbed shrtwave radiative flux in each layer. The slid line in the figure shws the amunt f the ttal heat flux except net shrtwave radiative flux at the surface (i.e. lngwave radiative flux, sensible and latent heat fluxes). The ttal heat flux is acting t cl the surface and balanced with the amunt f shrtwave radiatin absrbed in the layer frm surface t a few mm in depth during the sun crust frmatin. As a result, near surface thin layer is nt melting and the layer beneath it is melting. 8

8 -l + w + (W/ni) 250,, I sun crust frmatihl t9 : 0: : I I,,,,,,,, I Q:)' 0', : 0 :Ql : j 0 0: 0: : '0 '0' : : 0 0 : q I : 0: 0 0 0:0 0 :...l I,, 80'"i 0 0 0: 0 ' c, %> 0 0' 0 02' 0 : g 0 d 0 8 : 0 0 e: 0 8> 0d"' : : "'0 CO 'a70 I 0 0..', I,I;b,..,,.j 0 Q v, 0 I "0""90 0&0'1 0 do 00 q 0 I , I:h "O B,([)' 0 <0 I 0 ::g:p: 0 c "tl I ' :8 c 0 Ol fie!> 0 CD 0 '0 " '9., 0 0 I. 0' 8, 0..-,0 c;,.9 R,I , \r<' I i doc "'tt!5, 0 OJ c9'. CX> W... '" 8 0 0lP ' te """00 Cb:l OB, e_ ' ()I # I O,p,G\9IO!l 'tf.'. " 000'0 0 'l:) 0: 0.. I.', 0 0" 0..' r;/j, 0<0'.' ',,0 '. ' 0, 0'.' I 0: : : : I, I.' t, I I Absrbed Shrtwave Radiatin 600 (W/ni) Fig. 6. Cmparisn fthe absrbed shrtwave radiatin and the ttal fsensible heat QH, latent heat QE, lngwave radiatin QL thrugh the bservatins..: during sun crustfrmatin, 0: thers. Cmparisn f absrbed shrtwave radiatin and the ttal f net lngwave radiative flux, sensible and latent heat flux is shwn in Figure 6. The circles indicate daytime data averaged every 1 hur thrugh the bservatins and slid nes are fr the data in sun crust frmatin. The slid circles are distributed amng 0 and -140 W1m 2 f the ttal heat flux except fr shrtwave radiative flux. It indicates the surface is surely cling. On the ther hand, the absrptin f shrtwave radiatin exceeds 200 W/m 2 during sun crust frmatin. 9

9 (A) sun crust Fig. 7. Hyptheses fr the surces fh20 fthe sun crust layer. (Aj: cndensatin fwater vapr, (Bj: refreezing fmelt water. ice I iquid water (8) change f8180 The surces fh2o fr fnnatin f sun crust were investigated thrugh the fluctuatins f xygen-18 cncentratin f the sun crust. Surface snw (average density 4.2xI02 kg/m 3 : see Table 2) metamrphsed int an ice layer in sun crust fnnatin. Tw hyptheses are made fr the surces f H2O f the ice layer. One is cndensatin f water vapr. Vapr pressure gradient transprt water vapr frm lwer melting layer t crust layer cled by the heat lss at surface (Figure 7a). Secnd is refreezing f liquid water in the crust layer: the crust layer becmes dense as the thickness decreased, and the heat lss by lngwave radiatin refreezes sme liquid water in it (Figure 7b). Cncentratin f a heavy stable istpe xygen-i8 change with phase change (evapratin r cndensatin) and depends n the temperature f fnnatin (Dansgaard and thers, 1973). Istpic cmpsitin f slid and liquid prtin f wet snw changes during grain carsening: f ice becmes larger than thse fr water (Nakaw and thers, 1993). It was anticipated that istpic cmpsitins wuld change with sun crust fnnatin. The xygen istpic cmpsitin has been expressed in tenns f as: 8180 = (R-RsMOw) / &Mw x 1000 (%0) (5) where R is the.istpic rati (H2 18 0/H2 16 0) f sample and RsMOW is that f Standard Mean Ocean Water. 10

10 Lcal Time = Fig. 8.. Changes with time f in the sun crust V / / (sun crust) v, CD...,.c ' Fig. 9. Cmparisn f in the sun crust and the snw layer belw it. 11

11 The changes f in sun crust with time are shwn in figure 8. The values had a tendency t becme larger with time, thugh there are a few cases the values becme smaller. Cmparisn f the xygen-i8 cntent f the sun crust and the snw layer beneath it is shwn in Figure 9. The f the snw layer is distributed between %0 and %0. These are similar t the value f whle snw layer at Mshiri, %0. Istpic cmpsitins changed with sun crust frmatin. The f sun crust is distributed between -4.7 %0 and %0 and became larger than thse f the snw beneath it. If the main surce f H2O was the cndensatin f water vapr frm the lighter snw beneath it, the values wuld becme smaller fthe sun crust layer changed during refreezing fliquid water by heat lss at the upper surface, frming heavier ice. It is nt sufficient t explain the heavier sun crust layer: the drpping f liquid water was ften bserved during sun crust frmatin, therefre, it als suggests that the lighter residual liquid water drps ut frm the sun crust layer, making the layer heavier. CONCLUSION The mechanism f sun crust frmatin was investigated thrugh field bservatins. During the bservatins, sun crust frmed 12 times. The weather cnditins during sun crust frmatin were fine weather, air temperature nearly 0 C, wind speed abut 1 t 5 mis, humidity abut 30 t 60 % at a height f 1 m. The sun crust is a thin ice layer made f ice particles. With sun crust frmatin, the lcal temperature beneath it rse and internal melting ccurred. As a result, the cavities frmed under it. The energy balance cnditin fr sun crust frmatin was revealed frm the energy balance calculatin fthe surface layer. Lngwave radiative flux and latent heat flux cled the snw surface. Beneath the surface, shrtwave radiatin penetrated thrugh the surface layer, was absrbed and internal melting ccurred. The surces f H20 fr frmatin f sun crust were investigated by cmparing the xygen-i8 cntent f sun crust and the snw layer beneath it. Surface snw (average density 4.2xI02 kg/m 3 ) metamrphsed int an ice layer with sun crust frmatin. Istpic cmpsitins changed with sun crust frmatin. 8 values f sun crust were becming heavier with time. As a result, 8 values fsun crust became larger than thse fthe snw beneath it. 12

12 Acknwledgments The authrs are grateful t the staff f the Uryu Experimental Frests, Teshi Experimental Frests f Hkkaid University and the members f snw damage science and snw hydrlgy sectins f the Institute f Lw Temperature Science, Hkkaid University, fr their lgistic supprt, and Dr. N. Ishikawa, Dr. Y. Kdama and Dr. R. Naruse fr their useful cmments n this investigatin. REFERENCES Dansgaard, W., Jhnsen, S. J., Clausen, H. B., Gundestrup, N., 1973, "Stable istpe glacilgy", Medd. Grnl., 197(2). Fukami, H. and Kjima, K., 1980, "Extinctin measurements fslar radiatin within a snw cver", Lw Temp. Sci., A 39, Ishikawa, N. Kdama, Y., 1994, "Transfer cefficients fsensible heat n a snwmelt surface", Meterl. Atms. Phys., 53, Nakaw, M., Chiba, S., Satake, H., Kinuchi, S., 1993, "Istpic fractinatin during grain carsening fwet snw", Ann. Glacial., 18, Ozeki, T. Akitaya, E., 1992, "An investigatin fsun crust frmatin using field data and labratry experiments", Lw Temp. SCi., A 51, Takeuchi, K., Knd, J., 1981, "Atmsphere near the grund surface", Tky: Tky Univ. Press, 226pp. 13

Chapter 11: Atmosphere

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