ABSTRACT

Mass changes of Blue Glacier, U.S.A. are calculated from topographic maps made from vertical aerial photography in late summer of 1939, 1952, 1957, and 1987, along with laser altimetry flown in June 1996. Changes in elevation between maps were adjusted for seasonal variations in the snow cover, and to account for the ablation between the date of photography and 1 October. Topography obtained from the laser altimetry was adjusted for snow thickness and glacier motion to estimate topography of 1 October 1995. The mass of Blue Glacier has changed less than 7 m (water equivalent) during this 56 year period which is minor compared with other glaciers in the region and elsewhere in the world. Glacier-average annual mass balances, beginning in 1956, have been calculated either from stake measurements and probing of late-season snow, or from a regression analysis using late-season measurements of the equilibrium line altitude. A comparison with the changes derived from surface maps shows values obtained from field measurements are too positive by about 0.4 m/a, indicating that considerable caution is needed when interpreting time series of mass balance. Two alternative time series of mass balance consistent with the long-term mass changes are created by making simple adjustments: (1) a single constant is subtracted from each value so that the series is consistent with the 1957-95 mass change, (2) one constant is subtracted from each value over 1957-87 and another is subtracted from each value over 1987-95 so that the series is consistent with both the 1957-87 and 1987-95 mass changes.

The mass balance of Blue Glacier was generally positive until the mid 1970's and negative since. The fluctuations of mass balance closely resemble those of snowfall on the glacier as estimated from the joint distribution of temperature and precipitation. The climate in western Washington was cooler and wetter during the decade before the mid 1970's, but the trend since has been towards warmer and drier conditions.

ABSTRACT. Precipitation at 2050m on Blue Glacier was measured daily from August 1957 through July 1958. Its correlation with a nearby lowland station with a good, long-term (1914-1996) record is used to estimate precipitation on the glacier over that entire period. Average annual precipitation on Blue Glacier is 4500 mm water equivalent. Snowfall depends on the joint distribution of precipitation and temperature. Over the period 1948-96, when 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 prefered, especially during spring and autumn storms when averaging over longer periods may substantially under- or over-estimate 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 the October-May snowfall at 2100m is estimated to be 250 mm water equivalent during the radiosonde era (1948-96) and 350 mm prior to then. For the first ten or so years after mass balance measurements began at Blue Glacier (1957), precipitation increased and winter temperature at 850mb (about 1450 m) decreased, but since then the trends have reversed. 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 upglacier; the average over the entire glacier is 300 mm water equivalent.

ABSTRACT. A simple flux model using twice-daily measurements of wind, humidity, and temperature from standard upper-air levels in a distant radiosonde estimated winter balance of South Cascade Glacier over 1959-98 with error 0.24 m w.e. Correlation between net and winter balance is strong; the model estimates net balance with error 0.53 m w.e. Over the past 40 years, average net balance of South Cascade Glacier has been strongly negative (-0.46 m w.e.), and it has been shrinking steadily. In comparison, 200 km WSW at Blue Glacier, the average balance has been less negative (-0.13 m w.e.); that glacier has undergone little change over the 40 years. Balance histories of the two glaciers are positively correlated (r =+0.54), and South Cascade has been more out of balance than Blue, presumably because it is still adjusting to climate change since the Little Ice Age. Recent warming and drying has made the net balance of both glaciers strongly negative since 1976 (-0.84 m w.e. at South Cascade, -0.56 m w.e. at Blue). If South Cascade Glacier were in balance with the 1986-1998 climate, it would be about 1/4 of its present area.

ABSTRACT. A simple model uses once-daily upper-air conditions from the NCEP-NCAR Reanalysis database to estimate summer balance of South Cascade Glacier each year over 1959-99. The rms error, 0.30 m water equivalent, (r² = 0.71) is comparable to measurement error. The model relates summer balance linearly to temperature T > 0 C at 2000 m and to snow flux at 1650 m, the altitudes in recent years of the equilibrium line and terminus. The snow flux is the product of the humidity and westerly wind component at 850 hPa when temperature T < +2 C at 1650 m. Temperatures are interpolated linearly between the 850 and 700-hPa levels. Both the positive 2000-m temperature and the snow flux are summed April 26 to October 4. When the summer estimates are combined with those from a winter balance model using the same database, the rms error in estimating net balance is 0.40 m (r² = 0.81). The indicated sensitivities of balance to warming of 1 C are -0.51 m for summer, and -0.24 m for winter. On the assumption that the total -0.75 m/C sensitivity exists at all altitudes, a warming of only 0.7C would be sufficient to overcome the 1986-98 average net balance +0.5 m at the top of the glacier.

ABSTRACT. A simple model using once-daily upper air values in the NCEP-NCAR Reanalysis database estimates seasonal mass balance at two glaciers in southern Alaska, one in western Canada, and one in Washington substantially better than any of several seasonally-averaged, large-scale climate indices commonly used. Whereas sea level pressure and sea surface temperature in the Pacific exert a strong influence on the climate in the region, temperature and moisture flux at 850-hPa have a more direct effect on mass balance processes -- accumulation and ablation -- because their temporal variability better matches that of those processes. The 40-year record of 850-hPa temperature shows 1976-77 winter warming and 1988-89 summer warming throughout the region; mass balance records reflect the summer warming at all four glaciers but winter warming only at the southern two. The only pronounced long-term change in the moisture regime is a decrease of precipitation in the south and an increase in the north. Interannual variations in the location of the moisture flux, however, apparently account for the strong negative correlation between the Alaska glaciers and the other two.