the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 12 Mar 2025
On the accuracy of the measured and modelled surface latent and sensible heat flux in the interior of the Greenland Ice Sheet
Abstract. The latent (LHF) and sensible (SHF) heat fluxes are key components of the surface mass and energy balance in the accumulation area of the Greenland Ice Sheet, making them critical for accurate sea level projections. While Eddy-Covariance(EC) systems provide accurate measurements of the turbulent surface transport of mass and energy in the low and mid-latitudes, frequent stable boundary layer conditions in polar regions introduce uncertainties in the EC method. In addition, as EC measurements are sparse, it is critical to characterise biases in the more common bulk fluxes obtained from automatic weather stations and climate models in polar areas. In this study, we present an intercomparison of three independent EC systems at the EastGRIP site at ∼2700 m a.s.l on the Greenland Ice Sheet to assess the accuracy of LHF and SHF measurements. A comparison of the fluxes by the three systems demonstrates excellent agreement, with a correlation (r) of 0.97 to 0.98, an absolute bias of 0.2 W m-2, an RMSE between 1.2 W m-2 and 1.5 W m-2 and slopes between 1.01 and 1.16 for the LHF, and r = 0.98, an absolute bias of less than 0.5 W m-2, an RMSE between 1.6 and 1.9 W m-2, and slopes of 1.0 for the SHF. A comparison of the validated EC fluxes against the bulk method highlights the sensitivity to the site-specific roughness length z0,m and the limitation of common parameterisations of the humidity and temperature roughness lengths z0,q and z0,t. Using improved values for z0,m, z0,q and z0,t, recomputed bulk fluxes are compared to fluxes simulated by regional climate models MAR, RACMO2.3p2 and RACMO2.4p1. We find an overall good agreement of the summer turbulent flux magnitudes, while all evaluated models simulate stronger near-surface temperature gradients during winter compared to observations from automatic weather stations, leading to consistently larger modelled SHF and LHF values in winter.
Competing interests: At least one of the (co-)authors is a member of the editorial board of The Cryosphere.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this preprint. The responsibility to include appropriate place names lies with the authors.- Preprint
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Status: open (until 24 Apr 2025)
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RC1: 'Comment on egusphere-2025-711', Anonymous Referee #1, 09 Apr 2025
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The present study proposes an analysis of observations of sensible and turbulent heat fluxes during a few months in 2019 within the Greenland Ice Sheet, from three different eddy covariance measurement systems. This is a unique dataset, extremely difficult to obtain, and allows us to explore the importance of latent heat flux on the local mass balance. After presenting an intercomparison between the three measurement systems (which ultimately agree very well), the data are used to train a 'bulk transfer' type relationship in winter and in other years, enabling comparison with turbulent fluxes simulated by two climate models. The multiple challenges of measuring and simulating these fluxes in this type of environment are then discussed.
I really enjoyed reading this paper. The plots are clean and well put together. The text is well written and the sequence of ideas is easy to follow. With the modifications I recommend below, I believe this article has a rightful place in a journal of the caliber of The Cryosphere.
GENERAL COMMENTS
- LHF and SHF in winter
I am puzzled by the 'observed' winter fluxes. The approach used to estimate these fluxes is to apply bulk transfer from a standard weather station based on z0m, z0t and z0q values calibrated in summer. It is clear to me that these values do not hold in winter, partly because the surface does not have the same roughness. More troubling is that the measured temperature difference between the surface and the air about 2.6 m above is very small in winter (at most 1°C in Jan. 2019!!). This seems impossible. Since qsfc depends on Tsfc, both turbulent fluxes are affected.
I know that most of these issues are raised by authors. I would like to see more options explored to improve these winter results:
- Have you tried estimating winter z0m from standard weather station data or other means?
- Have you explored surface emissivity values other than 0.97? Satellite data for Tsfc?
- What about radiative flux divergence? See for instance: https://journals.ametsoc.org/view/journals/apme/46/9/jam2542.1.xml - Wind
While temperature and humidity gradients play a key role in turbulent fluxes, wind is also key. This is barely mentioned in the article. What about katabatic winds at the measurement site? How do they affect the results? This should be covered in the introduction, results and discussion.
SPECIFIC COMMENTS
Abstract: Mention the exact dates of the measurement period.
L22: I know this comes up later, but the introduction should distinguish between surface and blowing snow sublimation.
L33: According to your Fig. 5h, a third of the day in summer experiences unstable conditions. I think we tend to assume that as soon as there is snow or ice, the atmosphere is necessarily stable, which is not the case.
L43: Discuss the strengths and weaknesses of open-path and closed-path gas analyzers.
L54: you need to elaborate on the limitations of the MOST approach - what exactly is at stake during stable atmospheric conditions?
L74-75: I do not understand how this ties in with the main objective. Is it possible to reword the objective stated at the beginning of this paragraph to include climate models?
Figure 1: If possible, increase the resolution of this figure. A view of the footprint of the EC sensors would be useful. Also, there seems to be a fine-wire thermocouple on the IRGASON, but not on the other devices. Is it then the sonic temperature that is used to calculate the sensible heat flux? Please add these clarifications to the text.
L87: Define 'clean-snow' area.
L89-90: Why was the period from 28 May to 31 July 2019 used? Please explain.
L93: At this stage of the paper, it is not clear why the period from 2016 to 2019 is mentioned for the model comparison.
L95: Is it possible to better describe the observed wind regime, beyond the typical values of wind speeds and directions? Are there any katabatic winds?
Section 2.2 and following: For the whole document, always present the three devices in the same order, to make it easier to follow.
L100: How and how often were these devices calibrated? Same question for the radiometer used to calculate the (very crucial) surface temperature.
L111-113: This should have been mentioned earlier.
L125: Mention that this is saturation with respect to ice. What is the validity of this hypothesis?
L147: Presentation of Andreas' (1987) formulation would be useful.
L150-151: I do not understand the changes resulting from the 'physics cycle CY47R.1' update. Is it possible to explain the highlights?
Equation 1a: I suggest removing the minus sign and writing Ts - T.
Equation 1b: I suggest removing the minus sign and writing qs - q.
Equation 4b: why not use the specific humidity q instead of a?
L275-280: again - what was the calibration strategy (zero and span) for this instrument?
Figure 2 and equivalent: add a white box under the performance metric values and add the units on the RMSE.
L298: How have the values of z0m, z0q and z0t been optimized?
L299: 5.7e-7
Citation: https://doi.org/10.5194/egusphere-2025-711-RC1 - LHF and SHF in winter
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