5.3 Freeze thaw fronts
Freeze-thaw processes in soils, including changes in FTFs, are very
sensitive to warming. However, the latest climate models do not predict
changes in FTFs directly. Implementation of the FTF module into
CAS-FGOALS was performed by generally following the prescription of Gao
et al. (2019). We used the maximum thaw depth in permafrost regions as
the active layer depth, where its spatial distribution is shown in Fig.
11(a). Although a direct comparison with observations was difficult
because no subgrid-scale permafrost representation was currently used in
the model, the spatial distribution of the model simulation was still
comparable with the maps of permafrost provided by the International
Permafrost Association (IPA; Brown et al. 1998) (Figs. 11(a–b)).
In terms of the FTFs, Fig. 12 shows their spatial distribution for
February and August, and the seasonal cycle for two points, one in
seasonally frozen ground, and another in a permafrost region. The thaw
depth was usually the shallowest in February and deepest in August. The
simulated thaw depths in February were thus all near the surface, except
for a few places near the borders of seasonally frozen ground and
permafrost (Fig. 12(a)). In August, the simulated thaw depths presented
a decreasing trend as the latitude increased (Fig. 12(b)). The simulated
frost depths, on the other hand, showed an opposite trend as seen for
the thaw depths. It was seen that the frost depth extended to the south
in February and retreated north in August (Figs. 12(c–d),
respectively). Figures 12(e) and 12(f) show the weight-averaged FTF
seasonal cycle of two regions (seasonally frozen ground and permafrost,
respectively). For the seasonally frozen ground (Fig. 12(e)), the frost
depth was deeper in winter and gradually became shallower as time
advanced to summer. The thaw depth, on the other hand, starts to melt
down in Spring, and gets deeper in May until both the frost and thaw
depths meet, then the thaw depth gets shallower again in summer to fall.
For the permafrost region (Fig. 12(f)), as the frost depth below is
basically frozen all the time, there is only thaw depth that starts to
melt in spring and gets deeper through summer, and gets shallower until
winter when the thaw depth basically stay on the surface. Overall, the
FTFs tended to simulate the seasonal variation of the freeze and thaw
process, and the FTF-derived permafrost extent was comparable to that
seen in the observations. Thus, the inclusion of FTF was apt for
simulations of the seasonal freeze thaw process in seasonally frozen
ground and permafrost regions.