Snow accumulation, surface height change, and firn densification at Summit, Greenland: Insights from 2 years of in situ observation

Weekly measurements of surface height change were made at an accumulation forest of 100 stakes at Summit, Greenland, over a 2-year period (17 August 2000 to 8 August 2002). On average, the surface height relative to the stakes increased 64 (+/-4.8) cm in the first year and 65 (+/-5.3) cm in the second, identical to the average (65 +/- 4.5 cm yr(-1)) previously reported for the period 1991-1995 in a similar forest 28 km to the southwest. The continuous 2-year data set indicates that the rate of surface rise was not constant, with the summers of 2001 and 2002 both showing markedly slower increases. On-site weather observations suggest that more new snow fell during the summer months than in any other season, consistent with results from previous snow pit and modeling studies yet apparently at odds with the slow rate of height increase. Density profiles from a series of 1-m-deep snow pits sampled monthly reveal that the thickness of the most recent year of accumulated snow (25 cm water equivalent) decreased rapidly between late May and early July, and the layers remained thin through early September. The thinning of the top year is clearly due to compaction in the snowpack. Combining the observed variations in annual layer thickness with a linear height increase based on assumed constant accumulation at 0.18 cm d(-1) explains much of the variation in surface height found in the stake measurements. Estimated surface height changes can be forced to exactly match the stake measurements by combining changes in annual layer thickness with a variable accumulation rate over the intervals between pits. This exercise suggests that during the 2 years of this study a consistent seasonal pattern in accumulation was not apparent, rather the intervals indicated to have had enhanced accumulation in the first year (August-October and March-April) apparently had reduced accumulation in the second year.