Seasonal snow and wintertime carbon emissions in Arctic shrub tundra

The Arctic is at the forefront of global climate change, warming at a rate 4 times faster than the global average. Changes are particularly apparent during winter, which lasts for around 8 months of year. CO2 emissions at this time of year make a considerable contribution to the annual Arctic carbon budget, with enhanced soil CO2 losses due to winter warming exceeding growing season carbon uptake under future climatic conditions. However, high uncertainty surrounds estimates of winter CO2 fluxes across the Arctic region, which vary by a factor of three and a half, with considerable variation between measured and simulated fluxes. Simulations of the Community Land Model (CLM5.0) were examined to address uncertainties impacting simulations of Arctic carbon fluxes; firstly of snow insulation and soil temperatures, and secondly rates of net ecosystem exchange of CO2. Improving simulated snow properties and the corresponding heat flux is important, as wintertime soil temperatures are an important control on subnivean soil respiration and hence impact Arctic winter carbon fluxes. The default model configuration was found to be inappropriate, with a change to the parameterisation of snow thermal conductivity required in order to correct for a cold soil temperature bias and better represent the insulation provided to the soil by the basal depth hoar not physically represented in CLM5.0. Simulated CO2 emissions were then examined through sensitivity testing of the parameterisation of relationships between soil temperature, moisture and respiration as well as snow thermal conductivity. The default value of the minimum soil moisture threshold for decomposition prevented soil respiration for the majority of the winter, with no CO2 emission simulated between November and mid-May, in contrast to observations showing steady CO2 emission throughout the winter. Failure to simulate CO2 emissions for over half of the year has a considerable impact on the simulation of the annual Arctic carbon budget. Changes to the model parameterisation, most crucially a decrease in the minimum moisture required for soil decomposition, allowed emissions to occur throughout the winter. Maintaining continuous wintertime eddy covariance measurements, typically used for model evaluation, in remote Arctic locations is very challenging. Consequently, as other measurement techniques would be advantageous, preliminary investigations with new low-cost CO2 sensors are presented and evaluated. Low-cost sensor measurements are used to examine spatial and temporal patterns in CO2 flux, and how these vary with changes in snow properties across the footprint of the eddy covariance tower. Measurements of CO2 flux using both new and established methods show similar magnitudes of CO2 release during March and April 2022, and asimilar relationship between CO2 and subnivean temperatures as previously shown across the Pan-Arctic. Future development of low-cost sensors should allow uncertainties to be reduced and an improvement in our understanding of the processes governing carbon fluxes from Arctic environments throughout the snow-covered season

Impact of measured and simulated tundra snowpack properties on heat transfer

Snowpack microstructure controls the transfer of heat to, as well as the temperature of, the underlying soils. In situ measurements of snow and soil properties from four field campaigns during two winters (March and November 2018, January and March 2019) were compared to an ensemble of CLM5.0 (Community Land Model) simulations, at Trail Valley Creek, Northwest Territories, Canada. Snow micropenetrometer profiles allowed for snowpack density and thermal conductivity to be derived at higher vertical resolution (1.25 mm) and a larger sample size (n=1050) compared to traditional snowpit observations (3 cm vertical resolution; n=115). Comparing measurements with simulations shows CLM overestimated snow thermal conductivity by a factor of 3, leading to a cold bias in wintertime soil temperatures (RMSE=5.8 ∘C). Two different approaches were taken to reduce this bias: alternative parameterisations of snow thermal conductivity and the application of a correction factor. All the evaluated parameterisations of snow thermal conductivity improved simulations of wintertime soil temperatures, with that of Sturm et al. (1997) having the greatest impact (RMSE=2.5 ∘C). The required correction factor is strongly related to snow depth () and thus differs between the two snow seasons, limiting the applicability of such an approach. Improving simulated snow properties and the corresponding heat flux is important, as wintertime soil temperatures are an important control on subnivean soil respiration and hence impact Arctic winter carbon fluxes and budgets

Impact of measured and simulated tundra snowpack properties on heat transfer

Snowpack microstructure controls the transfer of heat to, as well as the temperature of, the underlying soils. In situ measurements of snow and soil properties from four field campaigns during two winters (March and November 2018, January and March 2019) were compared to an ensemble of CLM5.0 (Community Land Model) simulations, at Trail Valley Creek, Northwest Territories, Canada. Snow micropenetrometer profiles allowed for snowpack density and thermal conductivity to be derived at higher vertical resolution (1.25 mm) and a larger sample size (n = 1050) compared to traditional snowpit observations (3 cm vertical resolution; n = 115). Comparing measurements with simulations shows CLM overestimated snow thermal conductivity by a factor of 3, leading to a cold bias in wintertime soil temperatures (RMSE = 5.8 °C). Two different approaches were taken to reduce this bias: alternative parameterisations of snow thermal conductivity and the application of a correction factor. All the evaluated parameterisations of snow thermal conductivity improved simulations of wintertime soil temperatures, with that of Sturm et al. (1997) having the greatest impact (RMSE = 2.5 °C). The required correction factor is strongly related to snow depth (R2 = 0.77, RMSE = 0.066) and thus differs between the two snow seasons, limiting the applicability of such an approach. Improving simulated snow properties and the corresponding heat flux is important, as wintertime soil temperatures are an important control on subnivean soil respiration and hence impact Arctic winter carbon fluxes and budgets

Simulating net ecosystem exchange under seasonal snow cover at an Arctic tundra site

Estimates of winter (snow-covered non-growing season) CO2 fluxes across the Arctic region vary by a factor of 3.5, with considerable variation between measured and simulated fluxes. Measurements of snow properties, soil temperatures, and net ecosystem exchange (NEE) at Trail Valley Creek, NWT, Canada, allowed for the evaluation of simulated winter NEE in a tundra environment with the Community Land Model (CLM5.0). Default CLM5.0 parameterisations did not adequately simulate winter NEE in this tundra environment, with near-zero NEE (< 0.01 ) simulated between November and mid-May. In contrast, measured NEE was broadly positive (indicating net CO2 release) from snow-cover onset until late April. Changes to the parameterisation of snow thermal conductivity, required to correct for a cold soil temperature bias, reduced the duration for which no NEE was simulated. Parameter sensitivity analysis revealed the critical role of the minimum soil moisture threshold of decomposition (Ψmin) in regulating winter soil respiration. The default value of this parameter (Ψmin) was too high, preventing simulation of soil respiration for the vast majority of the snow-covered season. In addition, the default rate of change of soil respiration with temperature (Q10) was too low, further contributing to poor model performance during winter. As Ψmin and Q10 had opposing effects on the magnitude of simulated winter soil respiration, larger negative values of Ψmin and larger positive values of Q10 are required to simulate wintertime NEE more adequately