Frozen Ground

Kangerlussuaq Ilulissat Sisimiut

Temperature in 1 m depth, calculated with the GIPL permafrost model, which is driven at the surface by data from DMI’s run with the HARMONIE-AROME weather model for Greenland.

The curves show the average temperature in the boxes marked in the figure (Northwest and Southeast Greenland, respectively) at 20 cm and 1 m depth.

The map is updated on a daily basis.

The temperatures shown on the map are not observed, but rather a model product. By clicking on the circles one can see the observed permafrost thermal regime in the three towns of Sisimiut, Kangerlussuaq and Ilulissat in western Greenland. Temperatures are continuously monitored in borehole installations at the three sites, and the data downloaded upon station visits typically once per year.

The figures show the minimum (blue) and maximum (red) observed temperature as well as the average (green) temperature over the course of one year (from August 1 to July 31).

The depth at which the maximum ground temperature (red curve) is 0°C designates the maximum thaw depth of that year, and thus the top of the permafrost. Above this level, there is a layer which is frozen in winter and thawed in summer, the so-called active layer.

The deeper permafrost temperatures are relatively warm at the southern sites (Sisimiut and Kangerlussuaq) and colder at the northern site (Ilulissat).

The temperatures close to the surface are much more variable at the inland station Kangerlussuaq with its continental climate than at Sisimiut, which is close to the coast.

Read more.

 

Temperature calculations in subsurface ground layers

 

The temperature distributions and curves we present here are not observed, but rather a model product that is described in some detail below. We expect to add observed subsurface temperatures to the Polar Portal with one of the next updates. Observed ground temperatures can also be found via the Center for Permafrost (CENPERM) which maintains several field stations in Greenland.

The figures we show here are based on DMI’s run with the HARMONIE-AROME weather model for Greenland, coupled to a dedicated permafrost model, GIPL, which has been developed at the University of Alaska Fairbanks. GIPL is a numerical model that solves the one-dimensional heat conduction equation with phase changes in porous media, first formulated by the Slovenian scientist Josef Stefan in 1891.

GIPL simulates the temperature in the ground down to a depth of 100 m for 24 different ground types. These ground types represent a wide variety of soil properties, including peat-like or muddy soils, silt-rich, clayish or rock-rich types and even partially sealed urban grounds, that differ in the layering of soil textures, soil material, pore space, unfrozen water content and thermal properties. These ground properties strongly determine the heat flow in the ground and, ultimately, if permafrost is present. In addition, insulating layers of snow and vegetation that can change their properties over time are taken into account at the upper boundary.

GIPL receives temperature and precipitation fields from the weather model and outputs vertical temperature profiles so that we can determine the upper boundary of permafrost, which lies below the (seasonally thawed) active layer. The map shows the temperature in 1 m depth, and the curves show spatial averages at 20 cm and 1 m depth.

Since the exact layout of the ground is unknown in large parts of Greenland, we are presenting average temperatures across all typical ground types - if sufficient subsurface data was available, we would be able to compute the actual conditions. Therefore, the soil conditions shown here may deviate from the real conditions in some parts of Greenland.

Since the ground has a very long memory to previous atmospheric conditions, we have to "spin up" the temperature fields for each ground type in the model for several thousand years. For this we have taken atmospheric fields (temperature and precipitation) for present-day climate from our regional climate model HIRHAM and scaled them with temperature variations deduced from ice cores in Greenland to obtain a forcing covering the last 10000 years. 

 

Permafrost observations

 

Permafrost is defined as soil or rock with a temperature that remains below 0°C for two or more consecutive years. The existence of permafrost is related to the climate, as the ground temperature regime is driven mainly by the energy balance at the ground surface (as well as the geothermal heat flux).

Globally, permafrost affects approximately 24% of Earth’s land areas, but in Greenland, more than 75% of the ice free area is affected.

In permafrost areas, the ground typically consists of three layers (see figure). The top layer, which is called the active layer, undergoes seasonal thawing and freezing. Below the active layer, permafrost is encountered, which by definition has a temperature constantly below 0°C. Due to the geothermal heat flux, permafrost has a lower boundary, below which the ground is again unfrozen.

The figure shows a typical ground thermal profile in a permafrost affected area. Both the active layer and the permafrost are affected by seasonal temperature variations down to a depth referred to as the depth of zero annual amplitude (zaa). The gray lines on the plot each represent the temperature profile at a specific point in time. The red curve represents the warmest temperature observed at any given depth over the course of a year, and the blue curve the coldest observed temperature. The green curve represents the average annual temperature. Due to the shape of the blue and red curves, such a plot is often referred to as a trumpet plot.

The geographic distribution of permafrost is related to regional and local climate conditions through parameters such as air temperature, inbound solar radiation, precipitation, and snow cover thickness and duration. The local distribution of permafrost is also affected by geographical properties such as soil types, organic layers and vegetation cover, surface albedo, topography, and drainage conditions.

Permafrost areas are typically classified according to the extent to which the ground is affected by permafrost: continuous (with >90% of the land surface area affected by permafrost), discontinuous (50–90% affected), sporadic (10–50% affected), or isolated patches (less than 10% affected).