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

Precipitation at 2050 m on Blue Glacier, U.S.A., was measured daily from August 1957 through July 1958. Its correlation with a nearby lowland station with a good, long-term (1914-96) record is used to estimate precipitation on the glacier over that entire period. Average annual precipitation on Blue Glacier is 4500 mm we. Snowfall depends on the joint distribution of precipitation and temperature. Over the period 1948-96, for which twice-daily radiosonde observations are available, temperature at any elevation on the glacier is interpolated in the radiosonde profile to partition the precipitation as either rain or snow. Daily partitioning is preferred, especially during spring and autumn storms when averaging over longer periods may substantially under- or overestimate snowfall on the glacier. Prior to 1948. snowfall is estimated from the mean over 1948-96, in a particular month and elevation, of the fraction of the precipitation falling as snow. The standard error in rbe October-May snowfall at 2100 m is estimated to be 250 mm we. during the radiosonde era (1948-96) and 350 mm prior to that. For the first 10 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, increasing snowfall until 1965 and decreasing since, closely parallels measured mass changes of Blue Glacier. When the average vertical profile of total 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.