Ground Ice Modeling - Victoria Valley, Antarctica
Why do We Care Ground Ice Stability?
There is increasing interest in subsurface ice due to its paramount role in
controlling the geomorphologic development of the landscapes and potential as an archive for
long-term climate change. According to current ice sublimation and vapor transport models,
no shallow ground ice should be present under a young surface in the Dry Valleys. In
contrary to this, ice is often 5 to 40 cm beneath the surface. It is important to investigate
more thoroughly the ground ice stability in cold, dry areas with respect to the existing
controversy about age and stability of ground ice and its implication for past and modern climate
conditions both on Earth and Mars.
What is the Climate Condition?
We report progress toward a realistic mass balance model through a case study of ice-cemented soils in the Dry Valleys, Antarctica. The study site in Victoria Valley is at an elevation of 450 m. We measured hourly climate parameters (relative humidity, air temperature, wind speed, and wind direction) along with soil temperature at 11 depths to 1.1 m below the ground surface to model vapor diffusion and sublimation rates. Our model accounts for water vapor transport and condensation in both the upper ice-free soil and the underlying porous ice-cemented soil.
Figure 3: Hourly record of (a) relative humidity (blue line) and air temperature (red line), (b) wind speed, (c) wind direction, and (d) soil temperature at various depths from January 2002 to January 2005. Sudden warming events (katabatics or fo¨hns) punctuate the dark winter months and warm the soil to a depth of 1.06 m. From October to February, the soil temperature decreases with depth; during the rest of the year, the upper soil is colder than lower soil. The horizontal dashed white line marks the boundary between dry and ice-cemented soil; the solid white line delineates the 0oC isotherm during the summer.
What did We Get?
The climate data show a consistent seasonal pattern with frequent warm katabatic winds ( fo¨hn) entering Victoria Valley during the dark winter months. The calculated annual ice loss from the soil averages 0.22 mm with most sublimation occurring during the summer. Extension of the diffusion model to the ice-cemented soil reveals that water vapor diffuses downward from the ice cement boundary. The frost point in the atmosphere of -23.4oC is close to the average soil temperature, which suggests that ice could be stable at a depth of ~35 m under current climate conditions if vapor diffusion is the only mechanism that controls the occurrence of ice cement.
Figure 4: Results of the diffusion model for 3 years starting 12 January 2002. (a) Air temperature and vapor concentration in air and at the ice boundary. (b and c) Divergence in vapor flux in dry soil and ice-cemented soil, respectively, expressed as millimeters of ice per day. Positive/negative flux indicates vapor transport directed in/out of the soil. (d) Condensation of vapor due to excess vapor accumulation expressed as mm ice. (e) Total change in ice content of ice-cemented soil over the period of this study. All calculations are made using a tortuosity of 3 and dry-soil porosity of 0.37. Note that water vapor is transported downward into ice-cemented soil thereby increasing the ice content, particularly at a depth of 0.3 m (e).
What is the Effect of Snow Cover?
We explore the effects of winter and summer snow cover on the mass balance of ground ice. The model appears to be incomplete since the modeled sublimation rate is too rapid for the pore ice to persist close to the surface after 10 ka. Incorporating a snow cover into the diffusion model reduces this discrepancy as snow during the summer reverses vapor transport thereby decreasing the annual sublimation rate. In addition, in situ measurements of liquid soil water and direct observations of wetting fronts reveal the potential role of snowmelt in recharge. This snowmelt infiltration, together with slowing of sublimation due to snow cover, would reduce greatly the total amount of ice sublimated.
Figure 5: Sublimation model results assuming year-round thin isothermal snow cover with saturated water vapor pressure. (a) Air temperature and vapor concentration in snow layer and ice boundary. (b–d) The same as in Figure 2. Total gain of ice is 0.13 mm a-1, most of which accumulates below the ice cement boundary.