Mamoru Ishikawa

Thermal regimes at the snow–ground interface and their implications for permafrost investigation

Abstract Snow cover is a critical factor determining the presence or absence of permafrost in mid-latitude high mountain regions. This paper discusses the relation between temporal changes in temperature at the snow–ground interface and permafrost distribution. Four fundamental types of temporal change in winter ground surface temperature (GST) are identified: (1) nearly constant at 0 °C, (2) short-term fluctuation, (3) gradual increase without short-term fluctuation, and (4) gradual decrease without short-term fluctuation. The latter three are favorable for permafrost growth, and result from direct cold penetration throughout winter, ground cooling before the onset of seasonal snow cover and cold air funneling and concentrating in voids between coarse blocks, respectively. The fourth thermal effect predominantly contributes to growth of permafrost, and thus should be included in the bottom temperature of snow cover (BTS) concept which involves mainly the third effect. Monitoring GST over a winter provides a new tool for investigating the distribution of permafrost, as well as information on the thermal regimes that control permafrost development.

Thermal regime of sporadic permafrost in a block slope on Mt. Nishi-Nupukaushinupuri, Hokkaido Island, Northern Japan

In this study, we discuss the predominant factors that determine the ground temperature regime of an active layer in a block slope. The distribution of the bottom temperature of snow cover (BTS) measurements, warm funnels at the top of the slope, and ground temperature changes on the block slope indicate continuous air circulation during the winter. In the spring, snowmelt water flows to the valley bottom and refreezes, adding superimposed ice onto the perennial ice that fills the voids between coarse blocks. At the study site, the ground temperatures showed a simultaneous, abrupt increase at all depths in the active layer. These results strongly suggest that air circulation in winter, as well as the ice formation processes in the spring, control the thermal regime of the active layer of the block slope with mean annual air temperature (MAAT) above 0 °C.

Analysis of the Sources of Variation in L-band Backscatter From Terrains With Permafrost

Simultaneous field data collections and Advanced Land Observing Satellite/Phased Array type L-band Synthetic Aperture Radar (PALSAR) full polarimetry observations were performed in Ulaanbaatar (Mongolia) and Alaska (USA). Permafrost is present at the Alaska test sites. Backscattering copolarization ( σco-pol0) values derived from the PALSAR data were compared with those calculated using the integrated equation method (IEM) model, a popular theoretical model describing surface scattering. PALSAR data taken in Ulaanbaatar matched the IEM model results to within a few decibels, whereas data taken in Alaska were 5 to 7 dB lower than those calculated using the IEM model. On the other hand, the σcross-pol0 (σVH0) components estimated from the Oh model were well matched to the PALSAR data in both Ulaanbaatar and Alaska. Moisture levels of the sphagnum moss layer in Alaska were estimated to be about 10% while moisture levels of the underlying organic and mineral layers were 25% to 79%; the moisture values of the organic and mineral layers were factored into the IEM and Oh models. When surface moisture levels of 10% were assumed for Alaska ground conditions, the σco-pol0 values calculated using the IEM model and those derived from the PALSAR data were well matched. From these observations, we conclude that the sphagnum moss layer, which is a seasonally unfrozen layer that occurs above permafrost, plays an important role in radar backscattering processes in permafrost regions and is a main contributor to the σco-pol0 backscattering component; the underlying organic and mineral layers contribute mainly to the σcross-pol0 backscattering component. A two-layer model was applied to the data from a test site in Alaska; the model described the co- and cross-polarization backscatter (σ0) derived from PALSAR data with off-nadir angles of 21.5° and 34.3°.