South Cascade Glacier mass balance , 1935 – 2006

A simple model uses daily observations of precipitation and temperature at a nearby weather station to estimate glacier-average seasonal mass-balance components at South Cascade Glacier, Washington, USA, from 1935, 24 years before measurements began at the glacier. This is 13 years earlier than measurements that can be derived using the NCEP–NCAR reanalysis database (begins 1 January 1948). Although the model’s error in estimating winter balance and summer balance over 1959–2006 is greater than that of a model using the reanalysis database, its error in estimating net balance is comparable. The model uses an empirically determined precipitation ratio between the station and the glacier, and a seasonally varying temperature lapse rate determined from 9 years of measurements at the glacier. Temperature is used with a degree-day formulation to estimate ablation and to partition precipitation between rain and snow for estimating accumulation. Both processes are assumed to exist throughout the year, and model results are compared seasonally with adjusted observations of winter and summer balances. The published mass-balance series is adjusted to a constant-topography (1970) series in an attempt to remove the influence of changing topography on the glacier’s response to climate. The reconstructed values prior to 1959 are also with respect to the 1970 glacier topography. Because precipitation is measured at the weather station, rather than being inferred from other meteorological variables, it enables us to distinguish more accurately between wet-day and dry-day conditions, including vertical lapse rates