Modeled Variations of Precipitation over the Greenland Ice Sheet

A parameterization of the synoptic activity at 500 hPa and a simple orographic scheme are used to model the spatial and temporal variations of precipitation over the Greenland Ice Sheet for 1963–88 from analyzed geopotential height fields produced by the National Meteorological Center (NMC). Model coefficients are fitted to observed accumulation data, primarily from the summit area of the ice sheet. All major spatial characteristics of the observed accumulation distribution are reproduced apart from the orographic accumulation maximum over the northwestern coastal slopes. The modeled time-averaged total precipitation amount over Greenland is within the range of values determined by other investigators from surface-based observations. A realistic degree of interannual variability in precipitation is also simulated. A downward trend in simulated ice sheet precipitation over the 26 years is found. This is supported by a number of lines of evidence. It matches the accumulation trends during this period from ice cores drilled in south-central Greenland. The lower tropospheric specific humidifies at two south coastal radiosonde stations also decrease over this interval. A systematic shift away from Greenland and a decrease in activity of the dominant storm track are found for relatively low precipitation periods as compared to relatively high precipitation periods. This negative precipitation trend would mean that the Greenland Ice Sheet, depending on its 1963 mass balance state, has over the 1963–88 period either decreased its negative, or increased its positive, contribution to recently observed global sea level rise. Superimposed on the declining simulated precipitation rate for the entire ice sheet is a pronounced 3–5-yr periodicity. This is prominent in the observed and modeled precipitation time series from Summit, Greenland. This cycle shows some aspects in common with the Southern Oscillation. Some deficiencies in the NMC analysts were highlighted by this work. A large jump in simulated precipitation amounts at Summit around 1962, which is not verified by accumulation data, is inferred to be due to an artificial increase in cyclonic activity at 500 hPa associated with the NMC change from manual to numerical analyses. The activity of the storm track along the west coast of Greenland appears to be anomalously low in the NMC analyses, perhaps due to mesoscale cyclogenesis that is not resolved by the NMC analysis scheme.