The characteristics of snow crystal growth by vapor diffusion at water saturation and in free fall were quantitatively investigated in a vertical supercooled cloud tunnel for periods up to 30 min at temperatures from -3 to -23*.The results obtained are as follows: 1) the basic growth habits were plates (>-4.0*), columns (-4.0--8.1*), plates (-8.1--22.4*) and columns (<-22.4*), respectively. At about -5.5, -12, -14 .5, -16.5 and -18*, crystal shapes were enhanced with time; 2) for an isometric crystal, the slope of a log-log plot between the crystal mass and the growth time showed the Maxwellian value of 1.5. The mass growth rate of a shape-enhanced crystal was larger than that of the isometric crystal, indicating more effective vapor transfer on the former; 3) in the case of shape-enhanced planar crystals grown at around -12, -14.5 and -16.5*, ventilation effects became recognizable, whereas the effect was not evident for needle crystals grown at about -5.5*. This suggests that the characteristic length of the flow field even around a needle crystal is along the a-axis. The ventilation effect became significant when the Reynolds number exceeded about 2 (sector) and 5 (dendrite); 4) linear relationships between the drag coefficient and the Reynolds number were found in log-log plots.

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AeSTRACT. The forma tion o f superimposed ice in the accumul a tion a rea o f sub-pola r glac ie rs pl ays a n importa nt role in the hea t and m ass bala nce of the glac iers. In order to study the process of superimposed ice form a tion in d etail , field observa ti ons were conduc ted o n M cCall G lacier, a sub-pola r gla cie r in Arcti c Alaska. It was found th a t the a pproximate thickn ess of superimposed ice fo rmed in a whol e summ er was 20 cm in the upper region a nd 30-40 cm in the lower r egion of the accumulatio n a rea of the gl acier. This difference in thi ckness may be a ttributed to the differe nce in the tempera ture of the underlying ice a nd the r a te of supply of melt water. T h e ratio of the a mount of superimposed ice fo rmed in the accumul a tion area from M ay to July in 1972 to the to ta l amount of melt was d etermined. Approxima tely 50 % o f the tota l melt wa ter w as discharged from the gl acier as run-off wate r, a nd the rem a ind er contributed to th e formation of superimposed ice.An experimental study on the a rtifi cia l forma tion of superimposed ice w as conducted in th e cold laboratory to obtain the ratio of superimposed ice, tha t of run-o ff wa ter, a nd that of free wa ter suspe nded between snow gra ins, to the total amount o f melt water produced in the snow. Th e ra ti os obtained in the la bora tory experiment a gree fairl y well with those d eri ved from the observationa l d a ta o n M cCall Glac ie r.Num eri cal calculations wer e conduc ted to exa mine the relationship b e tween the growth rate of superimposed ice, the ra te of snow m elting, the rate of discha rge of excess melt-wa te r , a nd the temper a ture of the u nderl ying ice. C alcula tions wer e m a d e in refere nce to both the labora to r y exp eriment and the field observations on M cC a ll Gla cier. It was found tha t the predomina nt fac tors co ntro lling the growth ra te or the tota l a mount o f superimposed ice in a sub-polar glacie r are the ra te of supply o f melt water to the snow-ice interface a nd the initial tempera ture distribution in the underlying ice. By u sing the prese nt ca lculation, it may be possibl e to estima te the growth rate, the total amount of superimposed ice, a nd the ra tio of superimposed ice to the to tal a mount o f m elting in the accumula tion area of a n y sub-polar glacier, if o bservational d a ta on the initia l tempera ture distribution in ice a nd th e ra te of snow m elting at the snow surface a re ava il a bl e. R ESUME. Observations de terra ills, i tlldes experimentales et thioriques de la glace de sllrimpositioll dll M cCall Glacier, Alaska . La form a tion de glace d e surimposition da ns la zone d 'accumul a ti o n d es glaciers sub-pola ires j oue un role importa nt dans les bilans d e m asse et de cha lcur d es glaciers. P our e tudier en deta il le processus de form a tion d e la gl ace de surimpositio n , d es observa tions d e terra in ont e te co nduites sur le M cC a ll G lacier, un gl acier sub-pol a ire d a ns I' Ar c tique A laska i...

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