How Does Humidity Variation Shape Permafrost Dynamics?

The Science

Near-surface air humidity is a basic and crucial meteorological indicator commonly measured in several forms, including specific humidity (SH), relative humidity (RH), and absolute humidity. These different forms can be interderived based on the saturation vapor pressure (SVP). In past decades, dozens of formulae have been developed to calculate the SVP with respect to and in equilibrium with liquid water and solid ice surfaces, but many prior studies use a single function for all temperature ranges without considering the distinction between liquid water and ice.

These different approaches can result in variations in humidity estimates that may impact understanding of surface-subsurface thermal hydrological dynamics in cold regions. However, the degree to which these approaches affect land surface and Earth system model predictions under a changing climate is unknown. In this study, a team of researchers comprehensively analyzed the variations of the relative humidity from SVP with or without over the ice surface and its impact on the predictions of hydrothermal dynamics at a permafrost site using a physics-rich land surface model.

The Impact

RH is a sensitive parameter, and its variations based on the calculation of SVP with or without an over-ice correction meaningfully impact physically based predictions of snow depth, sublimation, soil temperature, and active layer thickness. Under particular conditions when severe flooding (inundation) and cool air temperatures are present, researchers should carefully evaluate how humidity data is estimated for land surface and earth system modeling. These findings have implications in assessing the data quality of humidity variables such as vapor density/diffusivity and simulation performance of surface-subsurface modeling in many other cold regions worldwide.

Summary

This study is among the first to use a physically based permafrost column land surface model to comprehensively examine the impact of often ignored humidity variations due to the (1) different SVP calculations on surface-subsurface thermal hydrology and snow processes and (2) potential effects of global warming and variations in precipitation and surface water levels. Simulation results and findings provide implications for the correct interpretation of humidity data based on SVP formulation and can be applied to improve parameterization schemes for land surface and Earth system modeling studies under a warming climate.

Humidity data based on SVP formulations are not always calculated separately for water and ice surfaces. The SVP calculated in equilibrium with an ice surface yields higher RH values (up to 40%) if the air temperature is below freezing. Snow depth and sublimation vary by up to 30% depending on whether SVP is calculated in equilibrium with an ice or water surface. Active layer thickness and the shape of thaw front propagation are also sensitive to RH data and SVP formulations when water is ponded above the ground (inundation). For example, the difference can be up to 0.2 m in simulated annual maximum thaw depths. Hydrological simulations for permafrost environments are most sensitive to the formulation of SVP in wet climate conditions. Therefore, land surface and Earth system modelers should carefully evaluate their meteorological forcing data and report any assumptions made in converting between SH and RH, especially when surface inundation occurs in nontropical and high-latitude regions with cold climates.

Overall, the study provides directions for future work and suggests that humidity data could significantly control snowpack and active layer thickness in permafrost regions, especially in those with limited drainage resulting in a perched near-surface water table. Future efforts to predict water vapor flow and related solute/nutrient (and microbial) dynamics should directly address the impacts of humidity and water vapor content on the thawing permafrost catchment.

Principal Investigator

Xiang Huang
Los Alamos National Laboratory
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This research was supported by the Biological and Environmental Research program in the U.S. Department of Energy’s Office of Science as a contribution to the Next-Generation Ecosystem Experiments Arctic project.

References