Brief Communication: Effects of different saturation vapor pressure calculations on simulated surface-subsurface hydrothermal regimes at a permafrost field site
Abstract. Air saturation vapor pressure (SVP) can be calculated using different formulas, with or without over-ice correction. These different approaches result in variability that affects the simulation of surface-subsurface thermal-hydrological processes in cold regions; however, this topic has not been well documented to date. In this study, we compared the relative humidity (RH) downloaded and calculated from four data sources in Alaska based on five commonly used SVP formulas. RH, along with other meteorological indicators, was used to drive physically-based land surface models. Results show that RH is a sensitive parameter, and its biases from SVP with or without over-ice correction meaningfully impact model-based predictions of snow depth, sublimation, soil temperature, and active layer thickness.
Xiang Huang et al.
Xiang Huang et al.
Xiang Huang et al.
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The authors of this paper have tested the sensitivity of a land surface model to different uses and formulations of saturation vapor pressure equations. The title of this work is appropriate for the work contained. The results are important to users of these models, and similar or equivalent models in meteorology. More much work is required to clarify their point, but I think the authors are more than capable of it. If this were not submitted as a brief communication it could be accepted with major revisions. Even so, there are important clarifications to be made in language (e.g. SVP over liquid water or ice), experimental design (e.g. comparison to snow depth without snow density information), and statement of the problem (this is a common issue in meteorology). Specific comments are below. I wish the authors luck in revising this work for resubmission.
1. The problem addressed by the authors is overstated. The choice of saturation vapor pressure formula is an important one, but one that many already take seriously. I agree that authors in our disciplines ought to list the vapor pressure formula in their published work, especially if results might be sensitive to the choice. However, this information is almost always available upon further requests for information from authors about their models, or investigation into open source model code itself.
2. Nevertheless, the authors here have clearly demonstrated a sensitivity of their model to choice of vapor pressure formulation. Some important questions remain about what exactly was done as the language used here is ambiguous. In this work, there is confusion about what the 'over-ice' correction is. It seems that the 'over-ice' correction is really just the SVP over a plane surface of ice, not a correction of an existing formula of SVP over ice. In revising this work, it is critical that the authors use more standard language about saturation vapor pressure over 'liquid water' or 'ice'.
3. In any case, there are implicit procedural assumptions that the authors make here in their sensitivity test that should further be clarified. It seems that this work is not just about a choice of which formula to use, but also when their is saturation with respect to liquid water or saturation with respect to ice in the air and soil/snow. The default assumption in this work, outline in the formulas in Section 2, is that saturation with respect to liquid water does not exist below 0oC, after which all vapor pressures are saturated with respect to ice. This may be true in interstitial pore spaces of soil and snow, but it is not true for the near-surface atmosphere. Air can be saturated and even super-saturated with respect to liquid water below 0oC. In clean Arctic air, liquid water fogs can exist down below -30oC. I wonder how your narrative, or even the results, would change in testing this assumption explicitly, rather than implicitly when suggesting that some authors are not using the appropriate formulas below 0oC? How often this sort of RH occurs can be found in the BEO or similar data sets. It could be that the differences between obs from BEO and the polygon center/trough are the result of assumptions about liquid/ice saturation threshold.
4. I recommend that this work be rejected from the Brief Communications. The results are interesting and important, but require reframing and an expansion to separate assumptions around the use of which formula, as well as when to choose liquid vs ice saturation. I recommend the authors focus their future work on Arctic permafrost hydrology, not the broad misuse of vapor formulations (this problem is not as rampant as is implied here) . I suggest using multiple scenarios to test assumptions around temperature thresholds of liquid vs ice saturation in the near-surface atmosphere, as well as which formula is most appropriate. Better curation and assessment of the observations is also important here. The observations of snow depth that are used as ground truth here require more processing. Specifically in this work, we are left wondering which observations to trust in 2013. But, critically if snow depth is the ground truth, then we need to know the snow density, also, if we are truly tracking RH and latent heat impacts on the snow surface as the result of RH formulations. Finally, I would wish that the authors make clear, strong, and evidence-based recommendations on which formulas are best.