Alexander Makshtas

The recent warming of the Arctic causes degradation of permafrost, release of greenhouse gases due to the decomposition of previously frozen organic matter, increase in the area and diversity of vegetation, and decrease of in the bearing capacity of permafrost soils. In this regard, the evolution of the seasonally thawed soil layer is of particular interest. The paper presents the results of comprehensive studies of energy exchange processes in the atmospheric surface layer and the upper layer of permafrost, carried out in 2016–2020 at the Research Station “Ice Base Mys Baranova” (Bolshevik Island, Severnaya Zemlya Archipelago), supplemented by the results of model calculations of seasonally thawed depth (STD) dynamics. The study examines the role of surface snow albedo decreases due to short-term intrusions of warm air masses, leading to the intensification of snow melting and soil surface heating due to increase in absorbed incoming solar radiation, is analyzed. A version of the Leibenson model, validated by data of observations, is used for assessing the role of landscape factors and meteorological conditions in the dynamics of STD. Despite the simplified formulation of the problem and the approximate assignment of heat and mass transfer of soil properties in the area under study, the model results could be considered satisfactory, and proposed approach can be used for assessing the state of STD.

Large stores of carbon frozen in the Arctic as permafrost are under threat of thawing as temperatures in the Arctic increase at a rate four times that of the global mean. This study uses recent atmospheric data from the North Slope of Alaska and Northeast Siberia to provide the most up-to-date assessment of emissions and trends from these two major high-latitude regions.We use two methods to quantify emissions and assess trends across different seasons: 1) Using 35 years of data from Barrow, Alaska, a wind sector method that quantifies trends in emissions by calculating concentration enhancements over background using wind direction to identify the land sector (first used in Sweeney et al., 2016), and 2) using Barrow data and recent data from three Siberian stations, an inversion method with the high-resolution atmospheric transport model NAME that quantifies both emissions and trends from these regions. We use results from these two approaches to quantify the temperature sensitivity (Q10) of soils based on correlations between surface air and ground temperatures with methane emissions.With the inclusion of atmospheric concentration data after 2015, we now show that land emissions from the North Slope of Alaska have been increasing since 2000, reflecting a change from previous analyses, which showed no significant increase in summertime methane emissions between 1986-2014 (Sweeney et al., 2016). We find significant emissions from the late shoulder season (Autumn-Winter), which has historically been undermeasured and underrepresented in models and emissions inventories, in this region of Alaska as well as two North-eastern Siberian locations, the Taymyr Peninsula and the East Siberian Lowlands. We show that emissions during this late-season have been growing over the past two decades at a rate similar to summer-time emissions.Our results based on long-term atmospheric data can be used to show that important change is happening in the Arctic, with an increasing emissions trend and the presence of late shoulder season emissions.

Measurements of the atmospheric boundary layer (ABL) structure were performed for three years (October 2017–August 2020) at the Russian observatory “Ice Base Cape Baranova” (79.280° N, 101.620° E) using SODAR (Sound Detection And Ranging). These measurements were part of the YOPP (Year of Polar Prediction) project “Boundary layer measurements in the high Arctic” (CATS_BL). ABL measurements and near-surface observations were used for verification of the regional climate model COSMO-CLM (CCLM) with a 5 km resolution for 2017–2020. The SODAR showed a topographical channeling effect for the wind field in the lowest 100 m. The verification of the CCLM with near-surface data of the observatory showed good agreement. The comparison with SODAR data showed a positive bias for the wind speed of about 1.0-1.5 m/s. The CCLM data showed the frequent presence of low-level jets (LLJs) associated with the topographic channeling. Although SODAR wind profiles are limited in range and have a lot of gaps, they represent a valuable data set for model verification. However, a full picture of the ABL structure and the climatology of channeling events could be obtained only with the model data. LLJs were detected in 37% of all profiles and most LLJs were associated with channeling, particularly LLJs with a jet speed ≥15 m/s (which were 29% of all LLJs). The analysis of the simulated 10m wind field showed that the 99%-tile of the wind speed reached 18 m/s and clearly showed a dipole structure of channeled wind at both exits of Shokalsky Strait. The climatology of channeling events showed that this dipole structure was caused by the frequent occurrence of channeling at both exits. Channeling events lasting at least 12 h occurred on about 62 days per year at both exits.

Arctic tundra is facing unprecedented warming, resulting in shifts in the vegetation, thaw regimes, and potentially in the ecosystem–atmosphere exchange of carbon (C). However, the estimates of regional carbon dioxide (CO₂) and methane (CH₄) budgets are highly uncertain. We measured CO₂ and CH₄ fluxes, vegetation composition and leaf area index (LAI), thaw depth, and soil wetness in Tiksi (71^∘ N, 128^∘ E), a heterogeneous site located within the prostrate dwarf-shrub tundra zone in northeastern Siberia. Using the closed chamber method, we determined the net ecosystem exchange (NEE) of CO₂, ecosystem respiration in the dark (ER), ecosystem gross photosynthesis (Pg), and CH₄ flux during the growing season. We applied a previously developed high-spatial-resolution land cover map over an area of 35.8 km² for spatial extrapolation. Among the land cover types varying from barren to dwarf-shrub tundra and tundra wetlands, the NEE and Pg at the photosynthetically active photon flux density of 800 µmol m⁻² h⁻¹ (NEE₈₀₀ and Pg₈₀₀) were greatest in the graminoid-dominated habitats, i.e., streamside meadow and fens, with NEE₈₀₀ and Pg₈₀₀ of up to −21 (uptake) and 28 mmol m⁻² h⁻¹, respectively. Vascular LAI was a robust predictor of both NEE₈₀₀ and Pg₈₀₀ and, on a landscape scale, the fens were disproportionately important for the summertime CO₂ sequestration. Dry tundra, including the dwarf-shrub and lichen tundra, had smaller CO₂ exchange rates. The fens were the largest source of CH₄, while the dry mineral soil tundra consumed atmospheric CH₄, which on a landscape scale amounted to −9 % of the total CH₄ balance during the growing season. The largest seasonal mean CH₄ consumption rate of 0.02 mmol m⁻² h⁻¹ occurred in sand- and stone-covered barren areas. The high consumption rate agrees with the estimate based on the eddy covariance measurements at the same site. We acknowledge the uncertainty involved in spatial extrapolations due to a small number of replicates per land cover type. This study highlights the need to distinguish different land cover types including the dry tundra habitats to account for their different CO₂ and CH₄ flux patterns, especially the consumption of atmospheric CH₄, when estimating tundra C exchange on a larger spatial scale.

Measurements of the atmospheric boundary layer (ABL) structure were performed for three years (October 2017–August 2020) at the Russian observatory “Ice Base Cape Baranova” (79.280° N, 101.620° E) using SODAR (Sound Detection And Ranging). These measurements were part of the YOPP (Year of Polar Prediction) project “Boundary layer measurements in the high Arctic” (CATS_BL) within the scope of a joint German–Russian project. In addition to SODAR-derived vertical profiles of wind speed and direction, a suite of complementary measurements at the observatory was available. ABL measurements were used for verification of the regional climate model COSMO-CLM (CCLM) with a 5 km resolution for 2017–2020. The CCLM was run with nesting in ERA5 data in a forecast mode for the measurement period. SODAR measurements were mostly limited to wind speeds <12 m/s since the signal was often lost for higher winds. The SODAR data showed a topographical channeling effect for the wind field in the lowest 100 m and some low-level jets (LLJs). The verification of the CCLM with near-surface data of the observatory showed good agreement for the wind and a negative bias for the 2 m temperature. The comparison with SODAR data showed a positive bias for the wind speed of about 1 m/s below 100 m, which increased to 1.5 m/s for higher levels. In contrast to the SODAR data, the CCLM data showed the frequent presence of LLJs associated with the topographic channeling in Shokalsky Strait. Although SODAR wind profiles are limited in range and have a lot of gaps, they represent a valuable data set for model verification. However, a full picture of the ABL structure and the climatology of channeling events could be obtained only with the model data. The climatological evaluation showed that the wind field at Cape Baranova was not only influenced by direct topographic channeling under conditions of southerly winds through the Shokalsky Strait but also by channeling through a mountain gap for westerly winds. LLJs were detected in 37% of all profiles and most LLJs were associated with channeling, particularly LLJs with a jet speed ≥ 15 m/s (which were 29% of all LLJs). The analysis of the simulated 10 m wind field showed that the 99%-tile of the wind speed reached 18 m/s and clearly showed a dipole structure of channeled wind at both exits of Shokalsky Strait. The climatology of channeling events showed that this dipole structure was caused by the frequent occurrence of channeling at both exits. Channeling events lasting at least 12 h occurred on about 62 days per year at both exits of Shokalsky Strait.

The estimates of seasonal and interannual variability of surface heat balance components on the Bolshevik Island, Severnaya Zemlya archipelago for period from September 2013 to December 2021 are presented. It is shown that seasonal variability of turbulent sensible and latent heat fluxes is fundamentally different. In winter turbulent sensible heat flux H is directed to the surface due to longwave radiative cooling. In summer H, due to strong heating of surface with low albedo by solar radiation, is directed into atmosphere and reaches up to 25% of incoming short-wave radiation. In opposite turbulent latent heat flux LE in winter is directed from the snow-covered surface to the atmosphere, but its values do not exceed 10% of H. In summer variability of soil surface moisture during the snowless period and the relatively high air humidity determined variable, but mainly from the underlying surface to the atmosphere direction of LE.Data about thermal regime of the soil active layer and the characteristics of surface energy balance allow us to draw a conclusion about the reasons for the abnormally warm state of soil upper layer and extremal permafrost thawing in 2020. It is shown that the timing and intensity of thawing were associated with changes in all components of the surface heat balance, with the main changes occurring in the intensity of incoming long-wave radiation, caused by synoptic processes in troposphere, and absorption of incoming short-wave radiation, determined by earlier onset of snow melting and, accordingly, decrease in surface albedo.

Arctic tundra is facing unprecedented warming, resulting in shifts in the vegetation, thaw regimes, and potentially in the ecosystem-atmosphere exchange of carbon (C). The estimates of regional carbon dioxide (CO₂) and methane (CH₄) budgets, however, are highly uncertain. We measured CO₂ and CH₄ fluxes, vegetation composition and leaf area index (LAI), thaw depth, and soil wetness in Tiksi (71° N, 128° E), a heterogeneous site located within the prostrate dwarf-shrub tundra zone in northeastern Siberia. Using the closed chamber method, we determined net ecosystem exchange (NEE) of CO₂, dark ecosystem respiration (ER), ecosystem gross photosynthesis (Pg), and CH₄ fluxes during the growing season. We applied a previously developed high-spatial-resolution land-cover map over an m area of 35.8 km². Among the land-cover types varying from barrens to dwarf-shrub tundra and tundra wetlands, the light-saturated NEE and Pg scaled with the LAI of vascular plants. Thus, the graminoid-dominated tundra wetlands, with high LAI and the deepest thaw depth, had the highest light-saturated NEE and Pg (up to −21 (uptake) and 28 mmol m⁻² h⁻¹, respectively) and were disproportionately important for the summertime CO₂ sequestration on a landscape scale. Dry tundra, including the dwarf-shrub-dominated vegetation and only sparsely vegetated lichen tundra, had only small CO₂ exchange rates. While tundra wetlands were sources of CH₄, lichen tundra, including bare ground habitats, consumed atmospheric CH₄ at a substantial rate. On a landscape scale, the consumption by lichen tundra and barrens could offset ca. 10 % of the CH₄ emissions. We acknowledge the uncertainty involved in spatial extrapolations due to a small number of replicates per land-cover type. This study, however, highlights the need for distinguishing different land-cover types including the dry tundra habitats to account for their consumption of the atmospheric CH₄ when estimating tundra C-exchange on a larger spatial scale.

In 2014/2015 a one-year field campaign at the Tiksi observatory in the Laptev Sea area was carried out using Sound Detection and Ranging/Radio Acoustic Sounding System (SODAR/RASS) measurements to investigate the atmospheric boundary layer (ABL) with a focus on low-level jets (LLJ) during the winter season. In addition to SODAR/RASS-derived vertical profiles of temperature, wind speed and direction, a suite of complementary measurements at the Tiksi observatory was available. Data of a regional atmospheric model were used to put the local data into the synoptic context. Two case studies of LLJ events are presented. The statistics of LLJs for six months show that in about 23% of all profiles LLJs were present with a mean jet speed and height of about 7 m/s and 240 m, respectively. In 3.4% of all profiles LLJs exceeding 10 m/s occurred. The main driving mechanism for LLJs seems to be the baroclinicity, since no inertial oscillations were found. LLJs with heights below 200 m are likely influenced by local topography.

Investigations of active soil layer on the Research station “Ice Base Cape Baranova’’ had been started in February 2016 after installation on the meteorological site sensors of Finnish Meteorological Institute: thermochain with IKES PT00 temperature sensors at depths of 20, 40, 60, 80 and 100 cm, soil heat flux sensor HFP, and two ThetaProbe type ML3 soil moisture sensors. Based on the results of measurements annual cycle of soil temperature changes was revealed with amplitudes 10 - 15 ° C less than the amplitudes of surface air layer temperature (Ta) and especially the temperature of the soil upper surface (Tsrad), in great degree determined by short-wave radiation heating and long-wave radiation cooling. Approximation by linear fittings shows average rates of increase Ta - 0.4°C/year, Тsrad - 0.3°C/year, and temperatures of active soil layer - 0.2°C/year.The data on thermal regime of active soil layer and characteristics of energy exchange in atmospheric surface layer make it possible to draw the conclusion about the reason for the abnormally warm state of the upper meter soil layer in summer 2020, despite in March during the whole period under study active soil layer was the warmest in 2017. Comparison in temperatures of the underlying surface and characteristics of surface heat balance during period under study showed that in 2020 the temperature of the soil surface at the end of May for a short time reached the temperature of snow melting. It is happened 25 days earlier than in 2017 as well as other years and led to radical decrease in surface albedo, sharp increase of heat flux to the underlying surface, and increased duration of active soil layer heating.Additionally, permafrost thawing studies using a manual contact method were carried out on the special site, organized according to CALM standards. These studies showed significant variety of soil active layer thicknesses in the relatively small area (~0.12 km2), which indicates significant spatial variability of microrelief, structure and thermophysical properties of soil, as well as vegetation, typical for Arctic desert. Calculations carried out with version of the well-known thermodynamic Leibenzon model for various parameterizations of vegetation and soil properties partly described peculiarities of spatial variability of observed thawing depths.