thermal regime of the ground
Link: http://w3id.org/permafrost/v/ipaglossary/t0364
Within: IPA Permafrost Glossary
Definition: A general term encompassing the temperature distribution and heat flows in the ground and their time-dependence (see Figure 2).
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thermal regime of the ground
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Permafrost exists as a result of thermal conditions that maintain a ground temperature that does not rise above 0°C for at least two years. The ground temperature distribution is complicated, varying with depth from the surface and with time. Ground temperatures fluctuate in cyclical patterns on a daily and a yearly basis, in response to the heat losses and gains at the ground surface. The heat balance at the ground surface (which is indicated by the surface temperature) is highly variable with time, being affected by terrain and climate conditions as well by as the thermal properties of the ground itself. Changes in surface temperature impose a response that decays while travelling downwards. The rate of decay depends on the time scale involved, on the soil thermal properties and on the occurrence of freezing and thawing phenomena. The depth beyond which ground temperature fluctuations can be considered insignificant is the depth of zero annual amplitude. The effects of daily temperature variations at the ground surface penetrate to only a few centimetres; those due to weekly variations (e.g., transient weather systems) penetrate to perhaps several metres; and annual variations penetrate to a depth of about 10 to 20 metres. Longer-term changes in surface temperature caused by changes in climate and surface conditions may modify the thermal regime of the ground to much greater depths. Permafrost exists if the mean annual surface temperature is perennially below 0°C. Although the mean annual surface temperature may be below 0°C, the surface temperature will fluctuate during the year, causing a layer of ground immediately beneath the surface to thaw in the summer and freeze in the winter (the active layer). Small changes in the annual range of surface temperature and in the mean annual surface temperature from year to year, or over a period of a few years, may cause a layer of ground between the bottom of the active layer and the permafrost table to remain at a temperature above 0°C, creating a talik or residual thaw layer. The mean annual ground temperature usually increases with depth below the surface. In some northern areas, however, it is not uncommon to find that the mean annual ground temperature decreases in the upper 50 to 100 metres below the ground surface as a result of past changes in surface and climate conditions. Below that depth, it will increase as a result of heat flow from the interior of the earth. The rate of change of temperature with depth in the earth is known as the geothermal gradient. The geothermal gradient at a specific location can be determined from accurate measurements of ground temperature made at several depths to obtain the temperature profile over a period of time. A rough approximation of the mean annual surface temperature can be made by extrapolating the geothermal gradient to the ground surface. In permafrost areas, extrapolation of the gradient downwards to the point where the ground temperature changes from below to above 0°C will provide an estimate of the depth to the permafrost base. The thermal regime of the ground at various locations is often assessed using the mean annual ground temperature at the depth of zero annual amplitude. The geothermal heat flux is the amount of heat moving steadily outward from the interior of the earth by conduction through a unit area in unit time. It is generally calculated as the product of the geothermal gradient and the thermal conductivity of the earth materials at a given depth; its value is very small.
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https://doi.org/10.4224/20386561
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