- 著者
- Takata Kumiko
- 巻号頁・発行日
- 1998
The thermal and hydrological impacts of frozen ground in the climate system are investigated using an atmospheric general circulation model(AGCM)and a one-dimensional land-surface model. The AGCM experiments are performed with and without the soil freezing,i.e.,including and excluding the latent heat of fusion and impermeability of frozen soil. The inclusion of soil freezing leads to higher surface temperature in summer in mid-and high-latitudes over the land. This leads to larger water vapor fluxes and associated precipitation induced by stronger summer monsoons. The higher temperature over the land in summer results from the smaller evaporation caused by the smaller surface soil moisture.The small surface soil moisture is caused by the additional runoff in spring due to impermeability of frozen soils and the small available soil liquid water due to underlying frozen ground. The positive temperature anomaly is large in the regions where the potential evaporation is large even if the negative soil moisture anomaly is small,and vice versa.Moreover,the precipitation is smaller in the middle of the continent,owing to the smaller evaporation there. In winter,although the deep soil temperature in frozen ground ergions is figher with the soil freezing than without due to latent heat of fusion,the surface temperature anomaly is more likely to be governed by the atmospheric dynamical responses.Consequently,the frozen ground has the impacts on the continental scale climate.Nevertheless,the land-surface model used in the AGCM is so simple that the reproductivity of the land-surface processes and the magnitude of the frozen ground impact may be different with respect to the schemes and parameters of the soil model.Then sensitivity experiments using a one-dimensional land-surface model are performed. The effects of altering soil moisture flux schemes,adding the frozen soil permeability,adding the surface “ponding”and changing the snow infiltration parameter are examined. The atmospheric conditions are specified to the measurement data at three ststions in frozen ground regions of Russia. A sigle forcing is repeated until an equilibrated annual cycle is obtained. The equilibrinm,but that of soil moisture in permafrost,does not depend on the initial soil moisture. The initial soil moisture should,therefore,be carefully chosen when soil moisture in permafrost becomes a subject of discussion,e.g.,global warming. The freezing on thawing depth agrees with the measurements. It is however different by a few factors related to the soil moisture:the larger the soil moisture content is,the shallower the freezing or thawing depth is,since the larger soil moisture requires the larger latent heat fusion. The annual cycle of soil moisture is captured by the model. The inclusion of frozen soil permeability increases the infiltration in spring and autumn. The wet zones above the thaw front in spring and the water uptake towards the freezing front from unfrozen zones beneath in winter are also produced by including the permeability. These lead to the larger seasonal change in soil moisture that is closer to the measurements. The surface“ponding”increases the soil moisture in spring by holding snowmelt water. Changing the partial snow infiltration into the total infiltration also increases the soil moisture in spring.but its magnitude is smaller than the surface“ponding”effect. The combined effect of these two effects is sometimes not equal to the sum due to runoff processes. The differences in soil moisture change the annual heat and water budget at a considerable range. The frozen soil permeability and the surface“ponding”are fund to increase the annual evaporation. Finally,the sensitivity of the soil freezing impact to the parameterization is inverstigated using the one-dimensional model. The similar anomalies in summer are obtained as in the AGCM experiments,irrespective of the schemes and paramenters,although they may be reduced by considering the frozen soil permeability and the surface“ponding”. In conclusion.the frozen ground has continental scale impacts on the energy and water cycles. The measured soil moisture variations are reproduced better by considering the frozen soil permeability and the surface“ponding”. These also have effects on the annual heat and water budget. Further studies are needed to evaluate the surface“ponding”quantitatively against measurements,and to evaluate the model in a two-dimensional way,for example,using the river runoff data.