Greenland precipitation variability in recent years retrieved by an initialization dynamic method and its relation to atmospheric circulation

Recent airborne laser-altimetry surveys (Krabill et al. 1999; 2000) show ice sheet elevation decreases over the southern coast of Greenland. In order to determine if the climate processes are responsible for these elevation changes, it is necessary to investigate the Greenland precipitation variations and their relation to the circulation. Precipitation variations over Greenland from 1985-99 have been retrieved by a generalized omega-equation method (Chen et al. 1997; Bromwich et al. 2000a,b). There is a high degree of correspondence in the interannual variations between the measured net water equivalent accumulation from ice cores and retrieved precipitation, and the results are better than those of P from the ECMWF Reanalysis (ERA-15). It is inferred that the modeled precipitation changes are responsible for most of the measured ice sheet elevation changes. However, the mesoscale spatial distribution of precipitation needs further improvement.

The data sets (ECMWF TOGA and ERA-15 from NCAR) used currently are large scale with a 2.5^ox2.5^o resolution. An important problem is to use these large-scale analyzed data with the much higher resolution topography of Greenland to obtain a mesoscale precipitation better than that from the omega-equation method. The initialization not only filters the fast gravity waves but also includes the interactions and feedbacks between the large-scale initial conditions and mesoscale topography in the iterative processes, which can produce some mesoscale features of the wind, divergence, moisture and thermal fields. Recently, an application of the equivalent geopotential and geo-streamfunction to the initialization for limited-area models has been developed for use over the steep slopes of mountains and ice sheets. This newly developed initialization has been used to retrieve the precipitation over Greenland. The local precipitation maximum located near (70^oN, 47^oW) is better simulated by this method, and the overall mesoscale spatial distribution of precipitation is greatly improved. The interannual variations of the retrieved precipitation are in agreement with the measured ice sheet elevation decreases over most of Greenland. Thus, the Greenland ice sheet elevation change is primarily caused by the change of precipitation.

It is found that there are many more cyclones in 1985-1988 than in 1996-1999 over the Labrador Sea, which favors more precipitation over Greenland and its southern part. In the multi-year mean sea-level pressure field for winters of 1985-1988, a mean cyclone is located in the Labrador Sea, while in winters of 1996-1999 it is located to southeast of Greenland. In the multi-year mean 500-hPa circulation of 1985-1988, a mean ridge is located over the west coast of North America and a mean trough is over Newfoundland along the east coast, which favors the development of cyclones over the Labrador Sea. By contrast, in 1996-1999 there are a mean trough over the west coast and a mean ridge over the Labrador Sea. The variation of precipitation over Greenland is closely related to changes of cyclonic activity and circulation in recent years. Thus, the Greenland ice sheet elevation changes are a result of variation of Northern Hemisphere circulation, and they can only be explained from the large scale climate variability.