A parameterization of the synoptic activity at 500 hPa and a simple or
ographic scheme are used to model the spatial and temporal variations
of precipitation over the Greenland ice Sheet for 1963-88 from analyze
d geopotential height fields produced by the National Meteorological C
enter (NMC). Model coefficients are fitted to observed accumulation da
ta, primarily from the summit area of the ice sheet. All major spatial
characteristics of the observed accumulation distribution are reprodu
ced apart from the orographic accumulation maximum over the northweste
rn coastal slopes. The modeled time-averaged total precipitation amoun
t over Greenland is within the range of values determined by other inv
estigators from surface-based observations. A realistic degree of inte
rannual variability in precipitation is also simulated. A downward tre
nd in simulated ice sheet precipitation over the 26 years is found. Th
is is supported by a number of lines of evidence. It matches the accum
ulation trends during this period from ice cores drilled in south-cent
ral Greenland. The lower tropospheric specific humidities at two south
coastal radiosonde stations also decrease over this interval. A syste
matic shift away from Greenland and a decrease in activity of the domi
nant storm track are found for relatively low precipitation periods as
compared to relatively high precipitation periods. This negative prec
ipitation trend would mean that the Greenland Ice Sheet, depending on
its 1963 mass balance state, has over the 1963-88 period either decrea
sed its negative, or increased its positive, contribution to recently
observed global sea level rise. Superimposed on the declining simulate
d precipitation rate for the entire ice sheet is a pronounced 3-5-yr p
eriodicity. This is prominent in the observed and modeled precipitatio
n time series from Summit, Greenland. This cycle shows some aspects in
common with the Southern Oscillation. Some deficiencies in the NMC an
alyses were highlighted by this work. A large jump in simulated precip
itation amounts at Summit around 1962, which is not verified by accumu
lation data, is inferred to be due to an artificial increase in cyclon
ic activity at 500 hPa associated with the NMC change from manual to n
umerical analyses. The activity of the storm track along the west coas
t of Greenland appears to be anomalously low in the NMC analyses, perh
aps due to mesoscale cyclogenesis that is not resolved by the NMC anal
ysis scheme.