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Over snow and ice, the surface mass balance determines whether the surface loses mass to or gains mass from the atmosphere. The mass balance is the sum of accumulation and ablation. In the case of dry snow surfaces, melt and runoff can be ignored, which means that ablation can take place only through sublimation and wind erosion. Surface sublimation represents the transport of water vapour directly from the snow surface into the atmosphere. Wind erosion occurs when winds are stronger than the threshold for snowdrift initiation, and snow particles become mobile. The snow taken up by the wind either can be redeposited downflow (in which case there is no net effect on the overall mass balance) or be sublimated when in the air. The latter is commonly referred to as snowdrift sublimation. The sum of both sublimation components represents the net ablation, with a magnitude that is strongly related to the near-surface atmospheric profiles of wind, temperature and especially humidity. I will illustrate this relation for a case study in Antarctica.

In 1997-98, a meteorological expedition to Dronning Maud Land, Antarctica, studied the details of the Antarctic boundary layer (Bintanja, 2001a). An extensive array of techniques was used at various locations, of which just one will be described here. Wind speed, temperature and relative humidity were measured at five levels (0.5 to 9 m above the surface) at a horizontally homogeneous location on the Antarctic ice sheet, near Swedish research station Svea (74°11'S, 10°13'W, 1150m above sea level). This site is subject to katabatic winds and prevailing synoptic easterlies, with an unobstructed fetch of at least 10km. The measurements took place from late December 1997 to early February 1998, with mean values of wind speed, temperature and relative humidity of 4.3 ms-1, -10.2°C and 70%, respectively. Sensors were sampled every 2min, and half-hourly means were calculated. For the purpose of this paper we will focus on a period with particularly strong winds (8-10 January 1998).

Most snowdrift-related processes are of highly non-linear nature. The amount of particles in the air increases exponentially with wind speed (e.g. Kobayashi, 1979), and Bintanja et al. (2001) showed that snowdrift transport rates at the study site concur with this. The floating particles are continuously subject to snowdrift sublimation, which is found to be a very efficient way to pump moisture into the atmosphere (Schmidt, 1972). This is because the total exposed surface area of all snowdrifting particles is many orders of magnitude larger than that of the surface. Hence, once particles are swept into the undersaturated atmosphere, snowdrift sublimation starts, which decreases their size and mass. The lowest atmospheric layers quickly become saturated, because these are already close to saturation. Hence, snowdrift sublimation decreases again, because its value is proportional to the undersaturation of the ambient air (note that snowdrift sublimation rates are maximum near the surface, where most particles reside). This constitutes a negative feed-back (e.g. Dery et al., 1998). Figure 33.1 illustrates this effect. It shows observed profiles of relative humidity in a stormy case with snowdrift and in a quiet case without snowdrift. In the snowdrift case, snowdrift sublimation has significantly moistened the lowest layers, with levels below a certain height (zsat; here about 12 cm) being entirely saturated. Thus, the lowest 12 cm of the atmosphere were saturated completely through the action of snowdrift sublimation. This means that surface sublimation (which is proportional to the vertical moisture gradient at the surface) is essentially shut off.

The temporal variation of zsat and other variables over a 5-day period including the strong wind period is shown in Fig. 33.2. During 3 days (8-10 January) winds are sufficiently strong to generate drifting and blowing snow with peak transport rates of 2000kgm-1s-1. Snowdrift sublimation steadily increases to more than 100Wm-2 on 8 January, after which it drops because relative humidities increase and a saturated layer of several centimetres thick is established (notice that surface sublimation vanishes simultaneously). In the early morning of 9 January, advection of relatively dry air enables snowdrift sublimation rates to increase to 250Wm-2 (equal to 8mm of ice per day). Also, the stronger winds lift the particles to higher levels where the air is still under-saturated and snowdrift sublimation can occur. The increased moisture flux to the atmosphere enhances the thickness of the saturated layer to its peak value of almost 18 cm (in a similar study at coastal station Halley during winter, saturated layers of several metres have been observed during strong wind events (Mann et al., 2000)). When wind speeds drop, relative humidity decreases and surface sublimation becomes non-zero again. Over the 5-day

Energy and mass fluxes over dry snow surfaces 175

0.01

Snowdrift

, , , i , , ,\

70 75 80 85 90

Relative humidity (%)

70 75 80 85 90

Relative humidity (%)

Figure 33.1 Vertical profiles of relative humidity. Measured values represent half hourly averages. The line labelled 'Snowdrift' was taken at 9 January, 1800-1830 hours, during the peak of the storm. The other, labelled 'No drift', was taken at 11 January, 2030-2100 hours, during a period with weak winds and no snowdrift. The level zsat is indicated at the right vertical axis for the 'Snowdrift' profile.

period, average values of surface and snowdrift sublimation are 7.8 and 44.1 Wm-2, respectively, illustrating the importance of blowing snow sublimation. Note that verification of snowdrift sublimation estimates remains one of the great challenges of snowdrift research (Pomeroy & Essery, 1999; Bintanja, 2001a).

This case study highlights the complex interaction between surface sublimation, snowdrift sublimation and moisture budget of the lower atmosphere. The key observation is that the effectiveness of the snowdrift sublimation process limits its own value as well as that of surface sublimation. In high winds, the total moisture flux into the atmosphere is dominated by snowdrift sublimation, as shown in Fig. 33.3. For winds stronger than about 13ms-1, the formation of a saturated layer close to the surface effectively diminishes surface sublimation, leaving snowdrift sublimation as the only source of moisture to the atmosphere. Whether or not snowdrift increases the total moisture flux over a situation without loose surface particles (with only surface sublimation) depends strongly on the prevailing atmospheric conditions. In a katabatic wind region, horizontal advection and vertical entrainment of dry air continuously supply the lowest atmospheric layers with relatively dry air, rendering the negative feed-back process less effective. The feed-back is more effective and sublimation values are more reduced if the moisture produced by snowdrift sublimation is somehow able to remain near the surface (Mann et al., 2000). In such situations, model experiments demonstrate that the total moisture flux attains maximum values at intermediate winds (Bintanja, 2001b).

Averaged over the entire measuring period of 37 days, surface and snowdrift sublimation rates were about equal at this

Date (January 1998)

Figure 33.2 Temporal variation of relative humidity, in per cent (at 5 cm and at 2 m), wind speed, in ms-1 (2 m), zsat, in cm, transport rate, in 102kgm-1 s-1, snowdrift and surface sublimation, in Wm-2, during a 5-day period in January 1998.

Figure 33.2 Temporal variation of relative humidity, in per cent (at 5 cm and at 2 m), wind speed, in ms-1 (2 m), zsat, in cm, transport rate, in 102kgm-1 s-1, snowdrift and surface sublimation, in Wm-2, during a 5-day period in January 1998.

Figure 33.3 Dependence of snowdrift and surface sublimation rates on wind speed (2m). Symbols represent grouped data, and the error bars represent the standard deviations of the average values.

z sat

location (6.4 and 6.7Wm-2, respectively). This is because in below-threshold winds (which occurred 79% of the time) snowdrift sublimation rates are obviously equal to zero whereas surface sublimation is non-zero. In windier regions, snowdrift sublimation will be more important. Sublimation rates peak in summer owing to warm conditions, in spite of the relatively modest winds. As a rough estimate of annual values we assume that the calculated rates are valid for the three summer months, and zero for the rest of the year. In that case, sublimation would remove 20-30% of the annual snow accumulation. This value may change considerably over the Antarctic continent, depending on the prevailing climate conditions (wind, temperature, humidity). Even though the magnitude of wind erosion is unknown, it seems likely that sublimation is the primary ablation mechanism at this site. This contrasts sharply with the situation in Greenland and other glacier environments, where ablation is dominated by surface melt. This case study demonstrates that for a proper evaluation of the surface mass-balance of dry snow regions it is imperative that the contribution of snowdrift sublimation is taken into account, and to do so one needs to consider explicitly the interaction between snowdrift and the lowest atmospheric layers.

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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