Wednesday, August 7, 2013

Dry July

Preliminary data show July 2013 to be the driest month on record for the Northern Ice Field, as measured by relative humidity via telemetry (2004-2012). July and August are typically the driest months at the summit, with July relative humidity (RH) averaging 42 +/-4% (1 std. dev.), so last month's mean of 25% was clearly anomalous. Measurements varied by only ~1% between two sensors in two different radiation shields, aspirated and naturally-ventilated. We expect verification of these values upon the next station visit, when 5-minute data from the high-accuracy, CRN-compatible shield will be recovered.

July ablation at the AWS was only ~8 cm. No snowfall, very low humidity and slightly above-average wind speed during the month would all promote sublimation, and thus ablation closer to the 10-year average for July of 11.3 cm might be expected. One explanation for below-average ablation could be that residual snowcover remains from the 2012-13 wet seasons. If so, telemetry suggests that there cannot be much snow/superimposed ice left - and once older, bare glacier ice is exposed, ablation will likely increase dramatically. Re-positioning of the AWS tower may be necessary in October, for the first time in 2 years.