The accumulation regime of Blue Glacier, USA, 1914-96

Precipitation at 2050 m on Blue Glacier, U.S.A., was measured daily from Au gust 1957 through July 1958. Its correlation with a nearby lowland station with a good, long-term (1914-96) record is used to estimate precipitation o n the glacier over that entire period. Average annual precipitation on Blue Glacier is 4500 mm we. Snowfall depends on the joint distribution of preci pitation and temperature. Over the period 1948-96, for which twice-daily ra diosonde observations are available, temperature at any elevation on the gl acier is interpolated in the radiosonde profile to partition the precipitat ion as either rain or snow. Daily partitioning is preferred, especially dur ing spring and autumn storms when averaging over longer periods may substan tially under- or overestimate snowfall on the glacier. Prior to 1948. snowf all is estimated from the mean over 1948-96, in a particular month and elev ation, of the fraction of the precipitation falling as snow. The standard e rror in rbe October-May snowfall at 2100 m is estimated to be 250 mm we. du ring the radiosonde era (1948-96) and 350 mm prior to that. For the first 1 0 years or so after mass-balance measurements began at Blue Glacier (1957), precipitation increased and winter temperature at 850 mbar (about 1450 m) decreased, but since then the trends have re versed. The combined effect, i ncreasing snowfall until 1965 and decreasing since, closely parallels measu red mass changes of Blue Glacier. When the average vertical profile of tota l annual snowfall is subjected to a hypothetical 1 K warming, the resulting reduction in snowfall is greatest at the glacier terminus and decreases up -glacier; the average over the entire glacier is 300 mm w.e.