Determining optimal model height to represent mountain observations in atmospheric transport and inversion studies

Dahl, Lilja; Vojta, Martin; Pisso, Ignacio; L. Thompson, Rona; Bigi, Alessandro

doi:10.5194/egusphere-2026-2734

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https://doi.org/10.5194/egusphere-2026-2734

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11 Jun 2026

| 11 Jun 2026

Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Determining optimal model height to represent mountain observations in atmospheric transport and inversion studies

Lilja Dahl, Martin Vojta, Ignacio Pisso, Rona L. Thompson, and Alessandro Bigi

Abstract. Accurate modelling atmospheric transport over complex terrain is challenging due to processes interacting with the orography and the use of finite resolution wind fields in Atmospheric Transport Models (ATMs). An unresolved issue is the representation of mountain stations within ATMs, and particularly the choice of model height that best represents mountain observations. This study proposes a methodology to determine optimal heights for representing mountain sites in ATMs, using FLEXPART driven by ECMWF ERA5 wind fields at two spatial resolutions. Three different release height methods were evaluated: i) sampling inlet height (S-rh), ii) an intermediate height based on model orography (P-rh), and iii) a varying release height determined by matching ERA5 potential temperature with observations (T-rh). A sensitivity analysis is presented, determining the optimal particle release height for three mountain sites and its influence on the simulated mixing ratios, using SF₆ as a tracer and two prior fluxes across different seasons and diurnal cycles. Results show that the model performance strongly depends on the release height choice, spatial resolution of the wind fields, station characteristics and atmospheric condition. The varying T-rh, generally provided a good representation of the observations across seasons and sites, although summer advection was not well captured. S-rh systematically underestimated observed variability, leading to a weaker source-receptor relationship (SRR) and biased posterior emissions. These findings highlight the importance of representing mountain stations in ATMs to reduce the model-data mismatch and that an inappropriate release height can lead to systematic bias of posterior flux estimates.

Received: 13 May 2026 – Discussion started: 11 Jun 2026

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Lilja Dahl, Martin Vojta, Ignacio Pisso, Rona L. Thompson, and Alessandro Bigi

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RC1:
'Comment on egusphere-2026-2734', Anonymous Referee #1, 19 Jul 2026 reply
General Comment
The paper proposes a new methodology to determine the optimal height for representing mountain sites in atmospheric transport models. The methodology is tested using the Lagrangian particle dispersion model FLEXPART driven by ERA5 reanalysis, performing a sensitivity analysis for three mountain sites in the European Alps, using SF₆ as a tracer.
The topic is interesting, potentially valuable to the scientific community, and well aligned with the scope of Atmospheric Chemistry and Physics. However, in my opinion, the manuscript in its current form does not meet the journal’s quality standards, particularly with regard to the methodology and the presentation of the results. The latter is difficult to follow and consists largely of an extensive comparison of statistical metrics, with limited physical interpretation.
Specific Comments
The Authors decided to use ERA5 reanalysis fields to drive FLEXPART. The reasons for this choice are weakly explained in the manuscript. Moreover, it is unclear how the Authors derived the ERA5 fields at two different horizontal grid spacings (0.5°x0.5° and 0.2°x0.2°). As far as I know, ERA5 reanalysis presents an original horizontal grid spacing of about 31 km (https://cds.climate.copernicus.eu/datasets/reanalysis-era5-complete?tab=overview). Therefore, it is unclear, particularly for the higher-resolution 0.2° × 0.2° grid, how the fields were obtained, as this is not explained in the manuscript. This lack of information hinders the interpretation of the results.

The Authors in Section 2.3 try to explain the rationale for the choice of the grid points closest to the three mountain sites. I think I have not completely grasped the problems they encountered in matching ERA5 coordinates with station coordinates. However, from Table 2, it seems strange to me that the altitude of the two high-altitude stations is lower in the higher-resolution dataset than in the coarser-resolution dataset. Moreover, it is still unclear how the higher-resolution orography was derived, starting from the original ERA5 orography.

It is not completely clear to me how the proxy release height (P-rh) was calculated

In Section 2.2.2, it is written “at mountain sites, θ increases monotonically with height and can therefore be used as a vertical coordinate”. Actually, this is not always true. It is true that the free atmosphere is most of the time stably stratified and therefore the potential temperature increases with height, but sometimes unstable stratification can be present. I suppose that this can occur, for example, at the low-elevation station (Schauinsland). In the same section, it is written that the pressure in ERA5 “fluctuated beyond acceptable limits”. What exactly does this mean, and how were these acceptable limits defined?

In my opinion, the presentation of the results is difficult to read and is limited to a long list of evaluation metrics, without an in-depth discussion or physical interpretation of the results. Moreover, by the end of the results section, it remains unclear, at least to me, what advantages or added value the proposed new methodology offers for determining the optimal height at which mountain sites should be represented.

Technical corrections
Lines 58-59: the second part of the sentence “and ATM might not have a suitable turbulence parameterization…” is not related to the first part.
Lines 63-64: potential temperature is conserved in adiabatic conditions, not always.
Line 88: “stochastic turbulent and diffusive transport”
Lines 91-93: The pronouns and verbs should be plural rather than singular, as the subject is “LPDMs.”
Lines 135-136: “Atmospheric observations…were collected”
Line 151: Why is the wind field at 0.2°x0.2° spatial resolution missing in December?
Line 165: for consistency, it should be “the geopotential height of Zt and Zs”
Line 171: actually, 1000 hPa is typically used as the reference pressure in the definition of potential temperature (https://glossary.ametsoc.org/wiki/potential-temperature/).
Line 196: Figure 2 does not show “a contour of the ERA5 orography of the three mountain sites”, but rather the ERA5 orography of the Alps.
Line 225: “we run 30 iterations…”
Line 244: something is missing at the end of the sentence.
Line 271: the lowest MBE.
Lines 282-283: “T-rh showed greatest…in comparison to T-rh”. The initial or final T-rh is wrong in this sentence.
Line 311: the reference to the section is missing.
Line 316: “exhibited slightly higher σ_n values”.
Lines 329-333: this is a long sentence difficult to follow.
Line 347: “The results of the atmospheric inversions presented in Fig. 12 show…”
Line 349: “at two spatial resolutions”.
Line 368: “and choosing an inappropriate…”
Lines 380-381: “using” is repeated.

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Citation: https://doi.org/10.5194/egusphere-2026-2734-RC1

Lilja Dahl, Martin Vojta, Ignacio Pisso, Rona L. Thompson, and Alessandro Bigi

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Short summary

This study addresses the challenge of accurately representing mountain observations in Atmospheric Transport Models and inverse modeling studies. We introduce a new varying release height method based on potential temperature to better represent mountain observations in the ATM. We evaluated three release height methods at three mountain sites and the results show that the model performance strongly depends on the release height choice, meteorological resolution, and atmospheric conditions.


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