Monitoring of Seasonal Variations in Ground Temperature

Frolov Denis Maksimovich Scientific Associate, Faculty of Geography, M. V. Lomonosov Moscow State University 119991, Russia, g. Moscow, ul. Leninskie Gory, 1, of. 1904B denisfrolovm@mail.ru Other publications by this author     Rzhanitsyn German Anatol'evich Lecturer, Department of Cryolithology and Glaciology, Lomonosov Moscow State University 119991, Russia, Krupskoy 19 47, Leninskie Gory str., 1, of. C-01 german-r@mail.ru Koshurnikov Andrei Viktorovich PhD in Geology and Mineralogy Leading Scientific Associate, Department of Geocryology, M. V. Lomonosov Moscow State University 119234, Russia, Moscow, Leninskie Gory str., 1, office 205 koshurnikov@msu-geophysics.ru Other publications by this author

Gagarin Vladimir Evgen'evich PhD in Geology and Mineralogy Scientific Associate, Department of Geocryology, M. V. Lomonosov Moscow State University 119234, Russia, Moscow, Leninskie Gory str., 1, room C23 gagar88@yandex.ru Other publications by this author

According to media reports (https://nia.eco/2022/11/28/52357/), the government supported the Ministry of Natural Resources’ bill for monitoring permafrost. As the head of the department, Alexander Kozlov, noted, the creation of a monitoring system is a national task, which President Vladimir Putin previously spoke about.

It was also said that background monitoring, on the creation of which scientists are already working, will be done based on the Roshydromet observation network. Experimental polygons have already been made at Cape Baranov and the Svalbard archipelago. 25-meter wells were drilled, and thermometric scythes were installed in them, data from which, via satellite channels, is continuously transmitted to the institute. According to the minister, there will be 140 such well stations in the monitoring system. The system will provide data on the degradation of permafrost soils. This will make it possible to develop adaptation measures for the relevant sectors in the economic and social spheres. Monitoring seasonal changes in soil temperature in mountainous regions is also very important in light of modern climatic changes [1–8].

MATERIALS AND METHODS

In the territory of the Moscow State University Meteorological Observatory, in order to study the influence of natural cover (primarily snow cover) on the distribution of the thermal field in the ground, observations are being made of air temperature, snow cover thickness, and soil freezing depth using exhaust thermometers and permafrost meters of the Danilin and Ratomsky systems on a bare site and under natural cover. The meteorological observatory staff has conducted observations since its foundation, of which the date of construction of Moscow State University's main building is about 1953. Recently, work has also been carried out to study the spatial and temporal heterogeneity of the snow column, as well as modeling is being carried out to assess the effect of snow cover on the depth of soil freezing within the city of Moscow and the Moscow region [9–12]. In the autumn of 2021, a thermometric well of a depth of 18 meters with full core sampling was also passed at the meteorological site. There are plans to install a thermal mower with a logger in the well for monitoring and recording air temperature, snow cover, and soil at different depths. This system is a prototype for monitoring the state of permafrost soils used in the Arctic and mountainous areas.

When drilling a well, a soil sample was taken to control the soil temperature at the meteorological site of Lomonosov Moscow State University at a depth of 18 m. The soil sample was examined in the laboratory. The results are shown in Table 1.

Table 1. Well, 2021 at the MSU Meteorological Observatory

The core soil samples' thermal conductivity, heat capacity, and thermal conductivity were determined along the entire sample length with a step of about 15–20 cm. The distribution of the measured values of thermal conductivity, heat capacity, and thermal conductivity of the soil over the depth of the sample is shown in the graphs in Figure 1.

Fig. 1. Distribution of measured values of thermal conductivity, heat capacity, and thermal conductivity of soil by the core depth.

It can be seen that the upper two meters of sod, humus, and the manufactured layer are characterized by a rather low thermal conductivity of about 1.5 W/m K. The next moraine is the Moscow, rock, and Dnieper moraine which has a high thermal conductivity of about 2.5 W/m K. The next layer of paleosoil has a low thermal conductivity of 1.5-2 W/m K. Then follows the Dnieper moraine with a thermal conductivity of 2.5 W/m K.

RESULTS AND DISCUSSION

The described data and meteorological data, such as air temperature and snow thickness, make it possible to calculate the penetration of a wave of seasonal temperature fluctuations in the ground with the Fourier equation using the effective heat capacity method. This method differs from the simplified scheme for calculating the depth of soil freezing for a meteorological site and also for the Caucasus and Tien Shan localities given in most of the author's recent works [9–18] in that the calculations there were based on the problem of thermal conductivity of a three-layer medium (snow, frozen and thawed soil) with a phase transition at the border of frozen and thawed soil. The heat balance equation included the energy of the phase transition, the inflow of heat from the thawed soil and outflow into the frozen soil and, in the presence of snow cover, into the atmosphere through it. The heat flow was calculated according to Fourier's law as the product of thermal conductivity and temperature gradient. It was assumed that the temperature in each medium varies linearly (for example, ^[19]). For snow cover and frozen ground, the formula of thermal conductivity of a two-layer medium was used. Figure 2 shows the penetration of a heat wave into the soil thickness under the influence of seasonal changes in air temperature calculated according to the full methodology and design scheme [20–23].

Fig. 2. Penetration of the wave of seasonal temperature fluctuations in the ground at the MSU meteorological site in 2014–2017.

The initial calculation data were the result of measuring the temperature of the soil surface. Calculations were carried out using an explicit difference scheme based on the finite difference method for the Fourier heat equation (in partial derivatives, second order). The sample cell was divided into i=1,n (=250) parts. A rectangular grid was created for the calculation area, and the equation obtained based on the difference approximation was written on the calculation grid according to an explicit calculation pattern. The time step was selected, and the values were calculated on a new time layer using the boundary conditions of zero flow at the lower boundary and setting the measured temperature at the upper boundary of the computational domain.

CONCLUSION

Thus, the results of the numerical simulation of the penetration of a wave of seasonal temperature fluctuations in the ground at the MSU meteorological site in 2014–2017 in the MATLAB environment considered in the paper are consistent with thermometry data and, consequently, the developed calculation scheme shows fairly good simulation results. This makes it possible to use the calculation scheme to assess the thermal state of frozen soils and assess the stability of foundations and buildings and linear structures located on them in the conditions of the north and mountainous territories. Therefore, the presented methodology can serve as a good aid for monitoring and preventing the destruction of the studied structures in the conditions of climate warming during the development of the northern territories and maintaining these facilities in proper condition.

The work was carried out in accordance with the state budget theme "Danger and risk of natural processes and phenomena" (121051300175-4) and "Evolution of the cryosphere under climate change and anthropogenic impact" (121051100164-0).