Precipitation accumulation and sublimation

Some aspects of precipitation and accumulation over the ice sheet were outlined in Chapter 6. Accumulation basically represents the net effects of direct precipitation, its redistribution on the surface via wind scour and drifting, and mass losses due to melt and evapo-sublimation. Accumulation is typically assessed via snow pits or ice cores. Based on coastal station observations of precipitation, adjusted for wind speed, and accumulation data from recent ice cores, the annual precipitation averaged over the ice sheet is estimated to be 340 mm (Ohmura etal., 1999). On average, only 40% of the total annual precipitation at the coastal stations is in solid form. However, at Danmarkshavn, this figure rises to 83%. There are zones of maximum precipitation exceeding 2000 mm in the southeast coastal area and 600 mm in the northwest. Amounts in the north-central area are around 100 mm. The southeastern maximum is strongly influenced by orographic uplift of southeasterly flow associated with traveling cyclones. The northwestern maximum is related to flow off northern Baffin Bay and uplift. A trend surface analysis of accumulation data (van der Veen et al., 2001) indicates that 80% of the spatial variance in average accumulation is a result of the large-scale atmospheric circulation and its interaction with the ice sheet topography. From regression analyses, they suggest that the penetration distance for precipitation from cyclones located off the coasts is about 200 km. The low precipitation over the high central parts of the ice sheet is consistent with this basic view.

The analysis of Ohmura et al. (1999) is in general accord with the model-generated precipitation map given in Figure 6.6. While maps of accumulation and precipitation are not directly comparable, Ohmura et al.'s (1999) map is broadly similar to the accumulation map given in Figure 6.5 that shows local peaks >2000 mm. But Figure 6.5 shows differences with the estimate of accumulation discussed by Bales et al. (2001). It is apparent that details of precipitation and accumulation over Greenland are still somewhat uncertain.

Sublimation was briefly introduced in Chapter 5. It refers to the exchange of water vapor between the surface and the overlying atmosphere during sub-freezing conditions (typical of Greenland) in which water molecules are transferred directly from the solid to the gas phase. This contrasts to evaporation, which deals with transfers between the liquid and gas phases. The combination of the two is termed evapo-sublimation.

In the ablation area of the ice sheet, Ohmura et al. (1999) estimate annual evapo-sublimation at between 60 and 70 mm. Over the higher parts of the ice sheet it is probably 20-30 mm during the summer three months. Box and Steffen (2001) estimate the sublimation term over the ice sheet based on application of two methods to GC-Net data. One is from single-level "bulk" estimates of the type commonly used in general circulation models. The second uses measurements of wind and humidity at two levels. Details are provided in that paper.

Sublimation over the ice sheet is highly variable in both space and time. Maximum sublimation rates from the surface to the atmosphere tend to occur when temperatures are close to 0°C and winds are strong. Vertical temperature differences allow for gradients in specific humidity, which in turn drive sublimation. Large vertical temperature differences do not occur under strong winds without a heat source to the surface. Hence, large sublimation rates can occur after melt episodes reduce the surface albedo, promoting increased absorbtion of solar radiation. Under clear skies, sublimation rates are largest toward the middle of the day when solar heating is strongest. Sublimation between even nearby sites may differ greatly. Deposition (vapor to solid) can occur under favorable synoptic conditions with a reversed humidity gradient. Deposition can also occur at nighttime due to radiative cooling.

The annual map from Steffen and Box (2001) from the two-level method shown in Figure 8.4 represents a trend-surface fit to calculated sublimation in terms of elevation and longitude. While results from the single and two-level approaches differ in the magnitude of sublimation rates, the spatial patterns are very similar. They both show sublimation as positive (surface to atmosphere in our adopted convention, see Chapter 5) over most of the ice sheet, and greatest in the warmer lower elevations during the summer season. The highest elevations show a small vapor transfer from the atmosphere to the surface (deposition, negative in our convention). Overall, they estimate that mass losses by sublimation account for at least 12% and possibly up to 23% of annual precipitation, depending on which method of calculation is used. Either way, sublimation emerges as a fairly important term for the Greenland Ice Sheet mass budget.

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