Using upper-air conditions to estimate South Cascade Glacier (Washington, USA) summer balance

A simple model uses once-daily meteorological values in the U.S. National Centers for Environmental Prediction and U.S. National Center for Atmospheric Research (NCEP-NCAR) re-analysis database to estimate summer balance of South Cascade Glacier, Washington, U.S.A., each year over 1959-99. The rms error, 0.30 m w.e. (r(2) = 0.71), is comparable to measurement error. The model relates summer balance linearly to temperature T > 0degreesC 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 < +2degreesC at 1650 m. Temperatures are interpolated linearly between the 850 and 700 hPa levels. Both the positive 2000 in temperature and the snow flux are summed from 26 April to 4 October. 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 in (r(2) = 0.81). The indicated sensitivities of balance to warming of 1degreesC are -0.51 m for summer and -0.24 m for winter. On the assumption that the total -0.75 mdegreesC(-1) sensitivity exists at all altitudes, a warming of only 0.7degreesC would be sufficient to overcome the 1986-98 average net balance +0.5m at the top of the glacier.