Conceptualization of Arctic Tundra Landscape Transitions Using the Alaska Thermokarst Model

Thermokarst topography forms whenever ice-rich permafrost thaws and the ground
subsides due to the volume loss when excess ice transitions to water. The Alaska
Thermokarst Model (ATM) is a large-scale, state-and-transition model designed to
simulate landscape transitions between landscape units, or cohorts, due to thermokarst.
The ATM uses a frame-based methodology to track transitions and proportion of cohorts
within a 1-km 2 grid cell. In the arctic tundra environment, the ATM tracks landscape
transitions between non-polygonal ground (meadows), low center polygons, coalescent
low center polygons, flat center polygons, high center polygons, ponds and lakes. The
transition from one terrestrial landscape type to another can take place if the seasonal
ground thaw penetrates underlying ice-rich soil layers either due to pulse disturbance
events such as a large precipitation event, wildfire, or due to gradual active layer
deepening. The protective layer is the distance between the ground surface and ice-rich
soil. The protective layer buffers the ice-rich soils from energy processes that take place
at the ground surface and is critical to determining how susceptible an area is to
thermokarst degradation. The rate of terrain transition in our model is determined by the
soil ice-content, the drainage efficiency (or ability of the landscape to store or transport
water), and the probability of thermokarst initiation. Using parameterizations derived
from small-scale numerical experiments, functional responses of landscape transitions
will be developed and integrated into NGEE-Arctic climate-scale (CLM) modeling
efforts.