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Qiu-shi Chen
,
David H. Bromwich
, and
Lesheng Bai

Abstract

In order to calculate the vertical motion over some high mountain regions, such as Greenland, an ω-equation without the quasigeostrophic approximation in σ-coordinates has been developed. A dynamic method for retrieving precipitation over Greenland is based on this ω-equation. The retrieved annual mean precipitation distribution for 1987 and 1988 is in very good agreement with the observed annual accumulation pattern over the Greenland Ice Sheet.

The major weather system producing precipitation over Greenland is the frontal cyclone. Based on the precipitation characteristics, Greenland can be divided into five subregions. Precipitation over the north coastal and central interior regions primarily occurs in summer. For the three other subregions, if the composite monthly mean sea level pressure charts for high and low monthly precipitation amounts are constructed, a clear relationship between precipitation and cyclonic activity emerges. If a mean cyclone exists in the Labrador Sea, heavy precipitation will fall over Greenland during that month. By contrast, if a mean cyclone exists near Iceland, precipitation over Greenland will be reduced. This is an important relationship between Greenland precipitation and cyclonic activity.

The cyclonic tracks near Greenland are established. A synoptic example is used to show the relation between precipitation and a cyclone moving up the west side of Greenland (track B) combined with movement across the southern tip of the island (track C). In this example, lee cyclogenesis is caused by the southern part of the Greenland Ice Sheet. The lee cyclone develops on the east coast along track C. During lee cyclogenesis, heavy precipitation falls over the southern region. The “parent” cyclone moves along track B, and precipitation falls along the west coast of Greenland.

A possible feedback between cyclonic activity and the mass balance of the Greenland Ice Sheet is proposed. On the one hand, cyclonic activity has a significant influence on snow accumulation over the ice sheet. The development of Icelandic cyclones is not favorable for precipitation over Greenland. On the other hand, the Greenland Ice Sheet has an important dynamic effect in producing lee cyclogenesis and affecting the frequency of Icelandic cyclones. This possible feedback may be important for understanding how the mass balance of the Greenland Ice Sheet and the Icelandic low are maintained in the present climate state.

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David H. Bromwich
,
Zhichang Guo
,
Lesheng Bai
, and
Qiu-shi Chen

Abstract

Surface snow accumulation is the primary mass input to the Antarctic ice sheets. As the dominant term among various components of surface snow accumulation (precipitation, sublimation/deposition, and snow drift), precipitation is of particular importance in helping to assess the mass balance of the Antarctic ice sheets and their contribution to global sea level change.

The Polar MM5, a mesoscale atmospheric model based on the fifth-generation Pennsylvania State University–NCAR Mesoscale Model, has been run for the period of July 1996 through June 1999 to evaluate the spatial and temporal variability of Antarctic precipitation. Drift snow effects on the redistribution of surface snow over Antarctica are also assessed with surface wind fields from Polar MM5 in this study. It is found that areas with large drift snow transport convergence and divergence are located around escarpment areas where there is considerable katabatic wind acceleration. It is also found that the drift snow transport generally diverges over most areas of East and West Antarctica with relatively small values.

The use of the dynamic retrieval method (DRM) to calculate precipitation has been developed and verified for the Greenland ice sheet. The DRM is also applied to retrieve the precipitation over Antarctica from 1979 to 1999 in this study. Most major features in the spatial distribution of Antarctic accumulation are well captured by the DRM results. In comparison with predicted precipitation amounts from atmospheric analyses and reanalyses, DRM calculations capture more mesoscale features of the precipitation distribution over Antarctica. A significant upward trend of +1.3 to +1.7 mm yr−2 for 1979–99 is found from DRM and forecast precipitation amounts for Antarctica that is consistent with results reported by other investigators and indicates that an additional 0.05 mm yr−1 is being extracted from the global ocean and locked up in the Antarctic ice sheets. While there is good agreement in this trend among all of the datasets, the interannual variability about the trend on the continental scale is not well captured. However, on the subcontinental scale, the interannual variability about the trend is well resolved for sectors in West Antarctica and the South Atlantic. It is also noted that the precipitation trend is weakly downward over much of the continent.

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