Assessment of the accuracy in UV index modelling using the UVIOS2 system during the UVC-III campaign

Fountoulakis, Ilias; Papachristopoulou, Kyriaki; Kazadzis, Stelios; Hülsen, Gregor; Gröbner, Julian; Raptis, Ioannis-Panagiotis; Kouklaki, Dimitra; Masoom, Akriti; Kontoes, Charalampos; Zerefos, Christos S.

doi:https://doi.org/10.5194/egusphere-2024-2964

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https://doi.org/10.5194/egusphere-2024-2964

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04 Dec 2024

| 04 Dec 2024

Assessment of the accuracy in UV index modelling using the UVIOS2 system during the UVC-III campaign

Ilias Fountoulakis, Kyriaki Papachristopoulou, Stelios Kazadzis, Gregor Hülsen, Julian Gröbner, Ioannis-Panagiotis Raptis, Dimitra Kouklaki, Akriti Masoom, Charalampos Kontoes, and Christos S. Zerefos

Abstract. The third campaign for the calibration and intercomparison of solar UV radiometers (UVC III) took place at Davos, Switzerland in June–August 2022. More than 70 radiometers participated in the campaign and measured side-by-side with the portable reference spectroradiometer QASUME. By using inputs from various sources, the UVIOS2 system was used to estimate the UV index (UVI) for the site of the campaign. The UVIOS2 system is a flexible UVI modelling tool that can be exploited for different applications depending on the inputs. Thus, different combinations of satellite, reanalysis, and/or ground-based inputs were used to test the UVIOS2 performance when it is used as a tool for UVI nowcasting or for climatological studies. While UVIOS2 provided quite accurate estimates of the average (for the period of the campaign) UVI levels, larger deviations were found for individual estimates. The average agreement between the UVI from the UVIOS2 and QASUME was better than 1 % for all the different sets of inputs that were used for the study. The range of the variability was of the order of 40 % for instantaneous measurements (15 min), mainly due to the model’s inability to capture the instantaneous effects of cloudiness, especially under broken cloud conditions. Under clear-sky conditions the model was found to perform much better, with the differences between the model estimates and the QASUME measurements being smaller than 12 % for 95 % of the studied cases. Even at the pristine environment of Davos, single scattering albedo (SSA) was found to contribute significantly to the modelling uncertainties under cloudless conditions. For relatively small Aerosol Optical Depth (AOD), of the order of 0.2 – 0.4, the role of the SSA was found to be comparable to the role of AOD in the modelling of the UVI. Radiometers that were not properly maintained and/or calibrated were found to provide UVI measurements with uncertainty that was comparable to the uncertainty of the UVIOS2 estimates, which highlights the significance of systematic maintenance and calibration of the UV radiometers.

Received: 21 Sep 2024 – Discussion started: 04 Dec 2024

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21 Oct 2025

Assessment of the accuracy in UV index modelling using the UVIOS2 system during the UVC-III campaign

Ilias Fountoulakis, Kyriaki Papachristopoulou, Stelios Kazadzis, Gregor Hülsen, Julian Gröbner, Ioannis-Panagiotis Raptis, Dimitra Kouklaki, Akriti Masoom, Natalia Kouremeti, Charalampos Kontoes, and Christos S. Zerefos

Geosci. Model Dev., 18, 7451–7473, https://doi.org/10.5194/gmd-18-7451-2025,https://doi.org/10.5194/gmd-18-7451-2025, 2025

Short summary

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2964', Anonymous Referee #1, 25 Jan 2025

The paper describes the UVC III campaign for calibrating and intercomparing solar UV radiometers, which was held in Davos, Switzerland, from June to August 2022, involving filter radiometers and the portable reference spectroradiometers QASUME and QASUMEII. However, the focus is on incremental improvements of the radiative transfer modeling tool (UVIOS2), which was used to forecast the UV index (UVI) with inputs from satellite, reanalysis, and ground-based sources.
Comparisons with the reference QASUME UVI measurements were used to demonstrate overall good performance of the model for clear skies, i.e., when the sun was not covered by clouds. However, much larger differences were found with instantaneous and daily UVI measurements, which were explained by cloud modeling challenges (Fig.5). Under cloud-free skies enhanced aerosol absorption, i.e., low single scattering albedo (SSA), might have explained model overestimation (Fig. 3 and 4), but there were no SSA measurements in UV to confirm this hypothesis.
There is very brief mention of comparisons between QASUME and filter radiometers in section 3.3 and Figure 10 (previously published) shows that the results mainly depend on application of the consistent calibration factors (PMOD/WRC). This section needs to be either expanded or removed.
UVI references are incomplete.
The paper may be suitable for publication after improving quality of the figures and completeness of the text and addressing technical questions described below.
RT modeling approach.
More details are needed describing extraterrestrial solar irradiance source, e.g., spectral smoothing applied, comparison with the state-of-the-art satellite TSIS-1 hybrid solar reference spectrum [Coddington, et al., https://doi.org/10.1029/2022EA002637 ].
The aerosols are included into the cloudless LUT (Tables 1, 2). This is different to OMI and TROPOMI satellite UVI retrievals, where aerosol and cloud effects are parameterized as a separate scattering (Cc) and absorbing (Ca) correction factors, UV = Ca(SZA, AAOD)*Cc(SZA,COT,…)*UVclear (SZA,TOC,…) [Arola et al., 2021 https://doi.org/10.5194/amt-14-4947-2021]. This explicit absorbing aerosol correction based on aerosol absorption optical depth (AAOD) would be especially important for North Africa and Middle East sites affected by desert dust, e.g. Roshan et al., Atmosphere 2020, 11, 96; doi:10.3390/atmos11010096.
Using aerosol optical thickness in UV (e.g., 340nm or 380nm) would be more appropriate as inputs to UVIOS2 model, because extrapolating visible AE would result in systematic overestimation of AOD in UV, e.g., see Fig 1 in Eck, et al., ‘‘Wavelength dependence of the optical depth of biomass burning, urban and desert dust aerosols,’’ J. Geophys. Res. 104, 31333–31350, 1999.
Using cloud optical thickness in UV would be more accurate, e..g., Krotkov,et al., "Satellite estimation of spectral surface UV irradiance 2. Effects of homogeneous clouds and snow", J. Geophys. Res., http://doi.wiley.com/10.1029/2000JD900721
Measurements:
High mountain site is not ideal for the absolute hemispherical irradiance measurements due to horizon obstruction by mountains. Provide mountain elevation at the measurements site as function of the azimuth (in Figure 1) and estimate horizon blockage correction, which needs to be applied to the model and/or measurements.
Clarify the difference between “clear-sky” (i.e., sun not blocked by clouds [line 275]) and “cloudless” (i.e., “clear sky”, [line 180]) conditions. Provide separate comparisons statistics for completely cloud-free periods.
Describe correction for a non-lambertian angular response of the QASUME and radiometers involved into the UVC III campaign.
Technical comments:
Figure 1: It would be useful to add a panoramic photo of the site and angular horizon elevation table for the observation site at PMOD. Calculate the correction factor in UVIOS2 to account for the horizon blockage effect at different SZAs.
Figures 2, 8-9: Add year in X-axis. Use logarithmic Y-scale. Symbols are difficult to see. Use different and larger symbols and line styles.
55 future climatic changes – climate changes
73-74. limited by the finite width of the satellite pixel – reword
74 weakness of satellite sensors – need clarification
77-78: Copernicus Atmospheric Monitoring Service (CAMS) – Atmosphere
100 information of the public – information to the public
117 reconstructed UVI series - reconstruct
118 The UVIOS (UV-Index Operating System) nowcasting system that its basic features have been already described … - reword sentence
126 summarized as follows – use colon :
147 data were used as a reference
168 serves as a reference
197 atlas plus modtan extraterrestrial spectrum – What was spectral resolution of ETS? Was a spectral smoothing and Sun-Earth distance correction applied? Compare with the TSIS-1 HRRS [Coddington et al., https://doi.org/10.1029/2022EA002637]
201 The US standard atmosphere (Anderson et al., 1986) was used – This model was not developed for a mountainous Davos site.
202 the surface albedo was set to 0.05 – this may not be representative for N. Africa or Middle East sites.
205 A correction for the effect of altitude, assuming an increase of 5% per km – There is a strong spectral dependence of the UV increase with altitude ~5% at 330nm to ~10% at 290nm, e.g., see Fig. 7 in Krotkov et al., JGR, http://doi.wiley.com/10.1029/98JD00233
230 Analyses of different AERONET datasets shows – show
231 around a typical [value]
232 Given that ASY generally increases? with wavelength - ASY should decrease with wavelength
241-244 Table 2: If input parameters are the same (SSA, ASY, surface albedo) they do not need to be included in the table.
245: Re-word the sentence.
267 Level 2 AERONET retrievals were not used because they are not available yet. – They are available with a longer latency and could be used for reanalysis.
268 nearly real time - near real time
275 For the analysis, measurements were classified as clear-sky (i.e., sun was not fully or partially covered by clouds) – This classification is not consistent with the “clear-sky” assumption in UVIOS2 model, where “clear-sky” is defined as “cloudless” conditions (line 182). This leads to inconsistencies in “clear sky” model to measurement comparison results.
287: Under clear-skies – This case includes scattered clouds not blocking the sun. It would be useful to show a separate comparison for the cloud-free periods in Fig 2.
287-288: Remove “both”
Figure 2: Add Year in X-axis. Symbols are difficult to discriminate. Use different and larger symbols and different line styles. It would be useful to show cloud-free periods using different symbols.
Calculating average UVI ratio between DOY 190 and 200 would result in positive bias, while the bias is negative between DOY 200 and 210. Is there an explanation?
295 Figure 2 shows that using highly accurate inputs for TOC, AOD at 500 nm, and AE does not result in a noticeable improvement in the accuracy of the modeled average clear-sky UVI. StDev decrease by less than 10% by using GB inputs
302 Differences in AOD are in all cases within ± 0.1 - There are larger differences in Fig. A1
304-305 on average, TEMIS slightly underestimates TOC – TEMIS TOC is higher than Brewer TOC in Fig. A2
309: differences in AOD – Use Brewer measured AOD.
313: ranging from values smaller than 0.8 (during e.g., events of dust or biomass burning aerosols – These events are not typical for Davos location. Please, provide evidence if such events did occur during UVC-III campaign.
Figure 3. Why show a hypothetical case with SSA=0.8 which is not representative for UVC-III campaign?
327 which denotes that the SSA – which means that the SSA
Figure 4. – Suggest moving this figure to supplement. You can use AERONET SSA retrievals on days 197-199.
354-355: Although we have not corrected the modeled UVI for the effect of limited horizon – This horizon correction should be important for Davos site. Quantify this effect using horizon elevation angle as a function of the azimuthal angle.
Figure 8: analysis of the outliers will be useful.
Figure 9. The campaign average difference is close to zero, but there are certain periods (i.e., 200-210) with larger differences. Again, analysis of the largest outliers would increase the value of the comparisons.
390-400: Section 3.3 is too short. The results in Figure 10 are not discussed. Expand or remove this section.
394: when the PMOD/WRC calibration – explain the difference between USER and POD/WRC calibration. Explain if radiometers were calibrated for the non-lambertian angular response (cosine correction)?
405 Figure 10: Text in the figure is difficult to read. Try to increase the size of the text or move the text to the caption.
426-427: when solar disc is occluded, we do not know the exact COT. – Clarify this sentence.
436. shows the significance of systematic and accurate calibration of such instruments. – This is true regardless of the model performance …
437 discussed in previous studies – add reference to Fioletov, et al., (2004) “UV index climatology over North America from ground-based and satellite estimates”, J. Geophys. Res., 109, D22308, http://doi.wiley.com/10.1029/2004JD004820
440 associated to the assumptions – with the assumptions
451 not available (e.g., Bais et al., 2019). – add these references:
Krotkov, et al., “Aerosol UV absorption experiment (2002- 04): 2. Absorption optical thickness, refractive index, and single scattering albedo”, Opt. Eng., 44(4), 041005, http://doi.org/10.1117/1.1886819 , 2005,
Corr, Chelsia, et al., “Retrieval of aerosol single scattering albedo at ultraviolet wavelengths at the T1 site during MILAGRO (2009)”, Atmos. Chem. Phys., 9, 5813–5827, http://doi.org/10.5194/acp-9-5813-2009
Mok, J., et al., “Impacts of atmospheric brown carbon on surface UV and ozone in the Amazon Basin”, Sci. Rep. (2016); https://doi.org/10.1038/srep36940
Mok, J., et al., “Comparisons of spectral aerosol absorption in Seoul, South Korea”, Atmos. Meas. Tech., 11, 2295-2311, https://doi.org/10.5194/amt-11-2295-2018
Go, et al., “Ground-based retrievals of aerosol column absorption in the UV spectral region and their implications for GEMS measurements”. Remote Sensing of Environment, 245, 2020, 111759, https://doi.org/10.1016/j.rse.2020.111759

Citation: https://doi.org/10.5194/egusphere-2024-2964-RC1
- AC1: 'Reply on RC1', Ilias Fountoulakis, 31 Mar 2025
  
  We thank the reviewer#1 for his/her comments. Analytical replies are provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2964-AC1
RC2:
'Comment on egusphere-2024-2964', Anonymous Referee #2, 03 Feb 2025
GENERAL COMMENTS
The manuscript by Fountoulakis et al. presents the results of a modelling system for estimating the UV Index at the surface (UVIOS2) during an international comparison campaign of broadband UV radiometers in Davos. The system is described, and the results are illustrated for both (i) clear-sky days and (ii) all-sky conditions. The factors influencing the comparison are discussed.
In my opinion, the research topic is important and within the scope of the journal, and the results are in good agreement with the reference instrument. However, the description of the methodology and the presentation of the results could be improved. Hence, I recommend publication after the authors address the issues outlined below.
SPECIFIC COMMENTS
The study evaluates the performance of the UVIOS2 system during a summer campaign (June–August). The authors should be cautious not to extrapolate these results to seasons when snow is present near the measurement area, as surface albedo determination and the estimation of cloud properties from satellite data could be problematic. Additionally, at higher altitudes and latitudes, snow or ice may persist even in summer. A discussion on this topic would be beneficial. A brief reference is given in lines 423–424 ("we expect that the assumption of homogeneity in the satellite pixel would still be the main uncertainty factor"), but the basis for this expectation is unclear. Could the authors provide a bibliographic reference? Moreover, depending on the SEVIRI channel used for retrieving the COT, is there sufficient contrast between snow and clouds?

Role of broadband radiometers in the manuscript: The authors state in the abstract that "Radiometers that were not properly maintained and/or calibrated were found to provide UVI measurements with uncertainty that was comparable to the uncertainty of the UVIOS2 estimates, which highlights the significance of systematic maintenance and calibration of the UV radiometers." They further state (lines 434–436) that "The uncertainty in the UVIOS2 forecasts was found to be comparable to the measurements of filter radiometers when they were not properly calibrated, but ~3 times larger compared to the measurements of accurately calibrated radiometers, which shows the significance of systematic and accurate calibration of such instruments." However, is a model really needed to affirm the importance of systematic and accurate calibration of UV instruments? The comparison of the model accuracy with that of the least reliable broadband radiometers is not particularly insightful. For example, should the conclusion be that UVIOS2 estimates are only as accurate as an uncalibrated or poorly maintained radiometer? In summary, are considerations regarding broadband radiometers necessary in this paper?

Model inputs require a more detailed description:

Extraterrestrial spectrum: A more recent extraterrestrial spectrum (QASUMEFTS) exists compared to "Atlas plus MODTRAN." Given that QASUMEFTS was obtained using the same instrument employed here as a reference, why was it not used for the calculations? What is the expected deviation resulting from this choice?

Cross-sections: These should be presented in a separate table. Are the ozone cross-sections used in the model consistent with Brewer retrievals?

Total Ozone Column (TOC): How was the TOC from the Brewer instrument obtained? Additionally, why was TEMIS ozone used instead of CAMS, given that other parameters are derived from CAMS?

Cloud properties: What is the spatial resolution of the COT and CMFUV products? This information is crucial for understanding the influence of shadows/3D effects and parallax errors in cloud property determination over complex terrain. Furthermore, which SEVIRI channels were used for retrieving cloud optical properties? Do they enable reliable discrimination between clouds and surface snow?

Altitude correction for the station (line 270): How was it determined? Is it just the 5% factor per km mentioned a few lines above?

Results for all-sky simulations: It would be beneficial to compare CMFs in addition to UV Indices to exclude the trivial correlation arising from the diurnal SZA cycle.

Classification of clear/cloudy skies: The classification, based on measured direct irradiance (lines 275–279), appears simplistic. For instance, the situation shown in the upper-right image of Fig. 5 does not seem to represent clear-sky conditions (as defined by the model). The authors could refine their classification to better account for scattered clouds and their effects on surface UV radiation.

Figures: Three different types of plots are used: (i) ratios (e.g., Figs. 2–3), (ii) absolute UVI values (e.g., Figs. 6–7), and (iii) absolute differences (Figs. 8–9). Can these be more homogenised for consistency?

Other services, such as TEMIS and CAMS, already provide UV Index estimates, at least on a European scale. The authors should clarify the advantages offered by UVIOS2, such as improvements in spatial and temporal resolution.

Spectral validation: Have the authors attempted to compare the spectral output of UVIOS2 with spectra measured by QASUME-II? This could provide additional insights into the influence of various factors.

TECHNICAL REMARKS
- Lines 20–21: "The UVIOS2... depending on the inputs" is too general and can be removed.

- Lines 76–84: The distinction between nowcasting and forecasting is unclear. If all inputs are forecasted (as suggested in lines 76–77), what differentiates nowcasting from forecasting?

- Line 118: "system that its basic features" → Incorrect grammar; rephrase.

- Line 134: Remove the typo "as Heading 2."

- Lines 136–139: The stated considerations apply to all radiative transfer models, not specifically to UVIOS2. The description of UVIOS2 should begin with details that are unique to this system.

- Figure 1: What does "% altitude" represent on the colour scale?

- Section 2.2 (QASUME-II vs QASUME): The uncertainty of QASUME is discussed in Sect. 2.2, but QASUME-II is used in the study. Why was QASUME not used, and why is the uncertainty of QASUME-II not discussed?

- Table 2: A similar table for all-sky UV determination might be useful, with different headings.

- Figure 2: The y-axis label should clarify that the ratio represents simulated vs measured values. In addition, would plotting the ratio against solar zenith angle provide useful insights?
- Figure 3: "ratio BG" should be replaced with "UVIOS2 / QASUME" or a similar term to clearly define the ratio. Also, why are the plots not ordered by increasing SSA?

- Lines 330–331: If the aerosol is primarily secondary, could hygroscopicity have influenced the correlation between AOD and SSA?

- Line 342: Cloud-induced irradiance enhancements near the solar disk typically last for a short time. What do short-term variations in the ratio indicate about this effect?
- Lines 355–356: The treatment of limited horizon effects in the model is crucial due to the site complex orography. This information should be presented earlier. Additionally, provide an approximate (cosine-weighted) fraction of the sky obstructed by mountains.

- Figure 10: If retained, specify the UVIOS2 configuration used (in the caption).
- Lines 408–409: The past tense may be more appropriate in the conclusions.

- Table 3: It is unusual to introduce new results in the conclusions. Would it be better placed in the results section?

- Line 434: The term "UVIOS2 forecasts" is ambiguous here, as—if I understand correctly—clouds are not forecasted.
Citation: https://doi.org/10.5194/egusphere-2024-2964-RC2
- AC2: 'Reply on RC2', Ilias Fountoulakis, 31 Mar 2025
  
  We thank the reviewer#2 for his/her comments. Analytical replies are provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2964-AC2

Interactive discussion

Status: closed

RC1:
'Comment on egusphere-2024-2964', Anonymous Referee #1, 25 Jan 2025

The paper describes the UVC III campaign for calibrating and intercomparing solar UV radiometers, which was held in Davos, Switzerland, from June to August 2022, involving filter radiometers and the portable reference spectroradiometers QASUME and QASUMEII. However, the focus is on incremental improvements of the radiative transfer modeling tool (UVIOS2), which was used to forecast the UV index (UVI) with inputs from satellite, reanalysis, and ground-based sources.
Comparisons with the reference QASUME UVI measurements were used to demonstrate overall good performance of the model for clear skies, i.e., when the sun was not covered by clouds. However, much larger differences were found with instantaneous and daily UVI measurements, which were explained by cloud modeling challenges (Fig.5). Under cloud-free skies enhanced aerosol absorption, i.e., low single scattering albedo (SSA), might have explained model overestimation (Fig. 3 and 4), but there were no SSA measurements in UV to confirm this hypothesis.
There is very brief mention of comparisons between QASUME and filter radiometers in section 3.3 and Figure 10 (previously published) shows that the results mainly depend on application of the consistent calibration factors (PMOD/WRC). This section needs to be either expanded or removed.
UVI references are incomplete.
The paper may be suitable for publication after improving quality of the figures and completeness of the text and addressing technical questions described below.
RT modeling approach.
More details are needed describing extraterrestrial solar irradiance source, e.g., spectral smoothing applied, comparison with the state-of-the-art satellite TSIS-1 hybrid solar reference spectrum [Coddington, et al., https://doi.org/10.1029/2022EA002637 ].
The aerosols are included into the cloudless LUT (Tables 1, 2). This is different to OMI and TROPOMI satellite UVI retrievals, where aerosol and cloud effects are parameterized as a separate scattering (Cc) and absorbing (Ca) correction factors, UV = Ca(SZA, AAOD)*Cc(SZA,COT,…)*UVclear (SZA,TOC,…) [Arola et al., 2021 https://doi.org/10.5194/amt-14-4947-2021]. This explicit absorbing aerosol correction based on aerosol absorption optical depth (AAOD) would be especially important for North Africa and Middle East sites affected by desert dust, e.g. Roshan et al., Atmosphere 2020, 11, 96; doi:10.3390/atmos11010096.
Using aerosol optical thickness in UV (e.g., 340nm or 380nm) would be more appropriate as inputs to UVIOS2 model, because extrapolating visible AE would result in systematic overestimation of AOD in UV, e.g., see Fig 1 in Eck, et al., ‘‘Wavelength dependence of the optical depth of biomass burning, urban and desert dust aerosols,’’ J. Geophys. Res. 104, 31333–31350, 1999.
Using cloud optical thickness in UV would be more accurate, e..g., Krotkov,et al., "Satellite estimation of spectral surface UV irradiance 2. Effects of homogeneous clouds and snow", J. Geophys. Res., http://doi.wiley.com/10.1029/2000JD900721
Measurements:
High mountain site is not ideal for the absolute hemispherical irradiance measurements due to horizon obstruction by mountains. Provide mountain elevation at the measurements site as function of the azimuth (in Figure 1) and estimate horizon blockage correction, which needs to be applied to the model and/or measurements.
Clarify the difference between “clear-sky” (i.e., sun not blocked by clouds [line 275]) and “cloudless” (i.e., “clear sky”, [line 180]) conditions. Provide separate comparisons statistics for completely cloud-free periods.
Describe correction for a non-lambertian angular response of the QASUME and radiometers involved into the UVC III campaign.
Technical comments:
Figure 1: It would be useful to add a panoramic photo of the site and angular horizon elevation table for the observation site at PMOD. Calculate the correction factor in UVIOS2 to account for the horizon blockage effect at different SZAs.
Figures 2, 8-9: Add year in X-axis. Use logarithmic Y-scale. Symbols are difficult to see. Use different and larger symbols and line styles.
55 future climatic changes – climate changes
73-74. limited by the finite width of the satellite pixel – reword
74 weakness of satellite sensors – need clarification
77-78: Copernicus Atmospheric Monitoring Service (CAMS) – Atmosphere
100 information of the public – information to the public
117 reconstructed UVI series - reconstruct
118 The UVIOS (UV-Index Operating System) nowcasting system that its basic features have been already described … - reword sentence
126 summarized as follows – use colon :
147 data were used as a reference
168 serves as a reference
197 atlas plus modtan extraterrestrial spectrum – What was spectral resolution of ETS? Was a spectral smoothing and Sun-Earth distance correction applied? Compare with the TSIS-1 HRRS [Coddington et al., https://doi.org/10.1029/2022EA002637]
201 The US standard atmosphere (Anderson et al., 1986) was used – This model was not developed for a mountainous Davos site.
202 the surface albedo was set to 0.05 – this may not be representative for N. Africa or Middle East sites.
205 A correction for the effect of altitude, assuming an increase of 5% per km – There is a strong spectral dependence of the UV increase with altitude ~5% at 330nm to ~10% at 290nm, e.g., see Fig. 7 in Krotkov et al., JGR, http://doi.wiley.com/10.1029/98JD00233
230 Analyses of different AERONET datasets shows – show
231 around a typical [value]
232 Given that ASY generally increases? with wavelength - ASY should decrease with wavelength
241-244 Table 2: If input parameters are the same (SSA, ASY, surface albedo) they do not need to be included in the table.
245: Re-word the sentence.
267 Level 2 AERONET retrievals were not used because they are not available yet. – They are available with a longer latency and could be used for reanalysis.
268 nearly real time - near real time
275 For the analysis, measurements were classified as clear-sky (i.e., sun was not fully or partially covered by clouds) – This classification is not consistent with the “clear-sky” assumption in UVIOS2 model, where “clear-sky” is defined as “cloudless” conditions (line 182). This leads to inconsistencies in “clear sky” model to measurement comparison results.
287: Under clear-skies – This case includes scattered clouds not blocking the sun. It would be useful to show a separate comparison for the cloud-free periods in Fig 2.
287-288: Remove “both”
Figure 2: Add Year in X-axis. Symbols are difficult to discriminate. Use different and larger symbols and different line styles. It would be useful to show cloud-free periods using different symbols.
Calculating average UVI ratio between DOY 190 and 200 would result in positive bias, while the bias is negative between DOY 200 and 210. Is there an explanation?
295 Figure 2 shows that using highly accurate inputs for TOC, AOD at 500 nm, and AE does not result in a noticeable improvement in the accuracy of the modeled average clear-sky UVI. StDev decrease by less than 10% by using GB inputs
302 Differences in AOD are in all cases within ± 0.1 - There are larger differences in Fig. A1
304-305 on average, TEMIS slightly underestimates TOC – TEMIS TOC is higher than Brewer TOC in Fig. A2
309: differences in AOD – Use Brewer measured AOD.
313: ranging from values smaller than 0.8 (during e.g., events of dust or biomass burning aerosols – These events are not typical for Davos location. Please, provide evidence if such events did occur during UVC-III campaign.
Figure 3. Why show a hypothetical case with SSA=0.8 which is not representative for UVC-III campaign?
327 which denotes that the SSA – which means that the SSA
Figure 4. – Suggest moving this figure to supplement. You can use AERONET SSA retrievals on days 197-199.
354-355: Although we have not corrected the modeled UVI for the effect of limited horizon – This horizon correction should be important for Davos site. Quantify this effect using horizon elevation angle as a function of the azimuthal angle.
Figure 8: analysis of the outliers will be useful.
Figure 9. The campaign average difference is close to zero, but there are certain periods (i.e., 200-210) with larger differences. Again, analysis of the largest outliers would increase the value of the comparisons.
390-400: Section 3.3 is too short. The results in Figure 10 are not discussed. Expand or remove this section.
394: when the PMOD/WRC calibration – explain the difference between USER and POD/WRC calibration. Explain if radiometers were calibrated for the non-lambertian angular response (cosine correction)?
405 Figure 10: Text in the figure is difficult to read. Try to increase the size of the text or move the text to the caption.
426-427: when solar disc is occluded, we do not know the exact COT. – Clarify this sentence.
436. shows the significance of systematic and accurate calibration of such instruments. – This is true regardless of the model performance …
437 discussed in previous studies – add reference to Fioletov, et al., (2004) “UV index climatology over North America from ground-based and satellite estimates”, J. Geophys. Res., 109, D22308, http://doi.wiley.com/10.1029/2004JD004820
440 associated to the assumptions – with the assumptions
451 not available (e.g., Bais et al., 2019). – add these references:
Krotkov, et al., “Aerosol UV absorption experiment (2002- 04): 2. Absorption optical thickness, refractive index, and single scattering albedo”, Opt. Eng., 44(4), 041005, http://doi.org/10.1117/1.1886819 , 2005,
Corr, Chelsia, et al., “Retrieval of aerosol single scattering albedo at ultraviolet wavelengths at the T1 site during MILAGRO (2009)”, Atmos. Chem. Phys., 9, 5813–5827, http://doi.org/10.5194/acp-9-5813-2009
Mok, J., et al., “Impacts of atmospheric brown carbon on surface UV and ozone in the Amazon Basin”, Sci. Rep. (2016); https://doi.org/10.1038/srep36940
Mok, J., et al., “Comparisons of spectral aerosol absorption in Seoul, South Korea”, Atmos. Meas. Tech., 11, 2295-2311, https://doi.org/10.5194/amt-11-2295-2018
Go, et al., “Ground-based retrievals of aerosol column absorption in the UV spectral region and their implications for GEMS measurements”. Remote Sensing of Environment, 245, 2020, 111759, https://doi.org/10.1016/j.rse.2020.111759

Citation: https://doi.org/10.5194/egusphere-2024-2964-RC1
- AC1: 'Reply on RC1', Ilias Fountoulakis, 31 Mar 2025
  
  We thank the reviewer#1 for his/her comments. Analytical replies are provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2964-AC1
RC2:
'Comment on egusphere-2024-2964', Anonymous Referee #2, 03 Feb 2025
GENERAL COMMENTS
The manuscript by Fountoulakis et al. presents the results of a modelling system for estimating the UV Index at the surface (UVIOS2) during an international comparison campaign of broadband UV radiometers in Davos. The system is described, and the results are illustrated for both (i) clear-sky days and (ii) all-sky conditions. The factors influencing the comparison are discussed.
In my opinion, the research topic is important and within the scope of the journal, and the results are in good agreement with the reference instrument. However, the description of the methodology and the presentation of the results could be improved. Hence, I recommend publication after the authors address the issues outlined below.
SPECIFIC COMMENTS
The study evaluates the performance of the UVIOS2 system during a summer campaign (June–August). The authors should be cautious not to extrapolate these results to seasons when snow is present near the measurement area, as surface albedo determination and the estimation of cloud properties from satellite data could be problematic. Additionally, at higher altitudes and latitudes, snow or ice may persist even in summer. A discussion on this topic would be beneficial. A brief reference is given in lines 423–424 ("we expect that the assumption of homogeneity in the satellite pixel would still be the main uncertainty factor"), but the basis for this expectation is unclear. Could the authors provide a bibliographic reference? Moreover, depending on the SEVIRI channel used for retrieving the COT, is there sufficient contrast between snow and clouds?

Role of broadband radiometers in the manuscript: The authors state in the abstract that "Radiometers that were not properly maintained and/or calibrated were found to provide UVI measurements with uncertainty that was comparable to the uncertainty of the UVIOS2 estimates, which highlights the significance of systematic maintenance and calibration of the UV radiometers." They further state (lines 434–436) that "The uncertainty in the UVIOS2 forecasts was found to be comparable to the measurements of filter radiometers when they were not properly calibrated, but ~3 times larger compared to the measurements of accurately calibrated radiometers, which shows the significance of systematic and accurate calibration of such instruments." However, is a model really needed to affirm the importance of systematic and accurate calibration of UV instruments? The comparison of the model accuracy with that of the least reliable broadband radiometers is not particularly insightful. For example, should the conclusion be that UVIOS2 estimates are only as accurate as an uncalibrated or poorly maintained radiometer? In summary, are considerations regarding broadband radiometers necessary in this paper?

Model inputs require a more detailed description:

Extraterrestrial spectrum: A more recent extraterrestrial spectrum (QASUMEFTS) exists compared to "Atlas plus MODTRAN." Given that QASUMEFTS was obtained using the same instrument employed here as a reference, why was it not used for the calculations? What is the expected deviation resulting from this choice?

Cross-sections: These should be presented in a separate table. Are the ozone cross-sections used in the model consistent with Brewer retrievals?

Total Ozone Column (TOC): How was the TOC from the Brewer instrument obtained? Additionally, why was TEMIS ozone used instead of CAMS, given that other parameters are derived from CAMS?

Cloud properties: What is the spatial resolution of the COT and CMFUV products? This information is crucial for understanding the influence of shadows/3D effects and parallax errors in cloud property determination over complex terrain. Furthermore, which SEVIRI channels were used for retrieving cloud optical properties? Do they enable reliable discrimination between clouds and surface snow?

Altitude correction for the station (line 270): How was it determined? Is it just the 5% factor per km mentioned a few lines above?

Results for all-sky simulations: It would be beneficial to compare CMFs in addition to UV Indices to exclude the trivial correlation arising from the diurnal SZA cycle.

Classification of clear/cloudy skies: The classification, based on measured direct irradiance (lines 275–279), appears simplistic. For instance, the situation shown in the upper-right image of Fig. 5 does not seem to represent clear-sky conditions (as defined by the model). The authors could refine their classification to better account for scattered clouds and their effects on surface UV radiation.

Figures: Three different types of plots are used: (i) ratios (e.g., Figs. 2–3), (ii) absolute UVI values (e.g., Figs. 6–7), and (iii) absolute differences (Figs. 8–9). Can these be more homogenised for consistency?

Other services, such as TEMIS and CAMS, already provide UV Index estimates, at least on a European scale. The authors should clarify the advantages offered by UVIOS2, such as improvements in spatial and temporal resolution.

Spectral validation: Have the authors attempted to compare the spectral output of UVIOS2 with spectra measured by QASUME-II? This could provide additional insights into the influence of various factors.

TECHNICAL REMARKS
- Lines 20–21: "The UVIOS2... depending on the inputs" is too general and can be removed.

- Lines 76–84: The distinction between nowcasting and forecasting is unclear. If all inputs are forecasted (as suggested in lines 76–77), what differentiates nowcasting from forecasting?

- Line 118: "system that its basic features" → Incorrect grammar; rephrase.

- Line 134: Remove the typo "as Heading 2."

- Lines 136–139: The stated considerations apply to all radiative transfer models, not specifically to UVIOS2. The description of UVIOS2 should begin with details that are unique to this system.

- Figure 1: What does "% altitude" represent on the colour scale?

- Section 2.2 (QASUME-II vs QASUME): The uncertainty of QASUME is discussed in Sect. 2.2, but QASUME-II is used in the study. Why was QASUME not used, and why is the uncertainty of QASUME-II not discussed?

- Table 2: A similar table for all-sky UV determination might be useful, with different headings.

- Figure 2: The y-axis label should clarify that the ratio represents simulated vs measured values. In addition, would plotting the ratio against solar zenith angle provide useful insights?
- Figure 3: "ratio BG" should be replaced with "UVIOS2 / QASUME" or a similar term to clearly define the ratio. Also, why are the plots not ordered by increasing SSA?

- Lines 330–331: If the aerosol is primarily secondary, could hygroscopicity have influenced the correlation between AOD and SSA?

- Line 342: Cloud-induced irradiance enhancements near the solar disk typically last for a short time. What do short-term variations in the ratio indicate about this effect?
- Lines 355–356: The treatment of limited horizon effects in the model is crucial due to the site complex orography. This information should be presented earlier. Additionally, provide an approximate (cosine-weighted) fraction of the sky obstructed by mountains.

- Figure 10: If retained, specify the UVIOS2 configuration used (in the caption).
- Lines 408–409: The past tense may be more appropriate in the conclusions.

- Table 3: It is unusual to introduce new results in the conclusions. Would it be better placed in the results section?

- Line 434: The term "UVIOS2 forecasts" is ambiguous here, as—if I understand correctly—clouds are not forecasted.
Citation: https://doi.org/10.5194/egusphere-2024-2964-RC2
- AC2: 'Reply on RC2', Ilias Fountoulakis, 31 Mar 2025
  
  We thank the reviewer#2 for his/her comments. Analytical replies are provided in the attached pdf file.
  
  Citation: https://doi.org/10.5194/egusphere-2024-2964-AC2

Peer review completion

AR: Author's response | RR: Referee report | ED: Editor decision | EF: Editorial file upload

AR by Ilias Fountoulakis on behalf of the Authors (31 Mar 2025) Author's response Author's tracked changes Manuscript

ED: Referee Nomination & Report Request started (13 Apr 2025) by Xiaohong Liu

RR by Anonymous Referee #1 (29 Apr 2025)

Suggestions for revision or reasons for rejection

Comments on revised version (see also purple comments in the Replay_RC1_comments.pdf).
The text has been clarified and the figures have been improved.
Most previous comments have been addressed.
Adding Table 2 is useful for comparison of the available UVI operational models.
The revised manuscript can be published with additional technical corrections:

192: -> dose rates
207: Table 1: explain abbreviations in table caption and wavelength pair for AE
Since the surface pressure and albedo are not included as inputs to the cloudless sky LUT explain how they are accounted for?
217: -> cloudless simulations
218: (mainly O3, SO2, and NO2) – use subscripts
225: Explain which default values were used?
230: profiles -> optical properties
231: indicate wavelengths in the Table caption
244: explain CMF abbreviation
246: correct this sentence as following: Using wavelength dependent CMFUV to simulate UV would be more accurate (Krotkov et al., 2001), but ...
258: the cloudless LUT

Table 2: Total ozone: Aura/OMI instrument is at the end of its lifetime. Consider switching TOC inputs to OMPS Nadir Mapper or TROPOMI operational products.

294. Do not need "ahead"
311. parameters are the same
350. which denotes -> means
350 formulation -> modeling
361. justify -> explain
369: polluted -> pollution
371 used -> input
372 Figure 3, which can be compared with a similar Figure 12 for Toronto location (Krotkov et al 1998).
383. Figure 3: I think that SSA=0.99 is more representative of the average aerosols at DAVOS - see discussion of Figure 12 in Krotkov et al (1998)
433. correction -> correcting
459. remove "that" is occluded by clouds
490. should be Table 4
503. inability of the model to predict solar disc occlusion by clouds?
510. usual -> standard
Figure 11. Suggest to remove all text on the left which is unreadable.

565 "Capturing the instantaneous impact of clouds using satellite images is challenging" - It is impossible since satellite derived CMF represents spatially averaged cloud transmittance over several kilometers, while ground instrument measures instantaneous irradiance at a given location. Time averaging of the ground measurements can only partly mitigate this difference.

Referee Report: PDF

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RR by Anonymous Referee #3 (17 May 2025)

Suggestions for revision or reasons for rejection

General comments

I would first like to note that I have not reviewed the original manuscript. My review below refers to the revised version (file egusphere-2024-2964-manuscript-version3.pdf). I have also checked whether the response of the authors to the comments of the two reviewers of the original manuscript were appropriate. Accordingly, I confirm that the authors have modified their manuscript appropriately in response to these comments, with the exception of two issues: their choices of (1) the extraterrestrial spectrum (ETS) and (2) the ozone absorption cross section (OCS) used to build the look-up tables (LUTs) for the calculation of the erythemal irradiance. I agree with the comments of the previous reviewers that these choices were unfortunate (as explained in more detail below). I encourage the authors to recalculated the LUTs and process their data with revised LUTs. After all, this paper is about the advancement of their previous model (UVIOS) to the version described in this manuscript (UVIOS2). Rather than improving this model again in the future, it would be better if the model’s obvious shortcomings are corrected now. In the “minor comments” section below, I also point out other issues that should be resolved before the manuscript is published.

Major comments

L205: The two reviewers of the original version of the manuscript asked why the “atlas plus modtan” extraterrestrial spectrum (ETS) was used in the calculations of the look-up tables and not the newer “QASUMEFTS” extraterrestrial spectrum. This ETS has an expanded uncertainties of 2% between 310 and 500 nm and 4% at 300 nm according to Gröbner et al. (2017; https://doi.org/10.5194/amt-10-3375-2017.) This is the lowest uncertainty in the UV range of any ETS that I am aware of. Without having read the comments by the other reviewers first, I was also wondering why the “atlas plus modtan” (ETS) was used when reading the revised manuscript. This is particularly puzzling considering that this new ETS was produced by several of the paper’s co-authors. In their response letter, the authors pointed out that “using a different ETS would require either recalculation of the LUTs or to run libRadtran directly” and then state: “Using a different ETS might result to differences in the simulated erythemal irradiances, as for example was shown in the study of (Gröbner et al., 2017). Based on the results of the latter study we estimate that the simulated irradiances might differ by up to 5% if a different ETS was used, making the used ETS spectrum a major uncertainty factor in UVIOS II simulations.“ A 5% difference is rather large and this “uncertainty” could be greatly reduced if the QASUMEFTS spectrum were used, assuming that the uncertainty assessment of this spectrum described by Gröbner et al. (2017) is correct. Hence, in my opinion, the right thing to do would be to recalculated the LUTs with this spectrum. Changing the ETS in the UVSPEC input configuration is rather trivial and scripts to set up the UVSPEC input files for the parameters used in the LUTs should still be available. So it should not be too burdensome to re-calculated the LUTs.

L210: Similar to my last comment, I am also puzzled why the authors use the Molina and Molina (1986) ozone absorption cross section (OCS). This OCS is dated. Current Dobson and Brewer networks and satellite observations use the Bass and Paur OCS, which is also part of the libRadtran/UVSPEC distribution. This OCS should be used if LUTs are recalculated. (Although I like to point out that the Bass and Paur (1985) OCS can now also be considered dated (Voglmeier et al., 2024; https://amt.copernicus.org/articles/17/2277/2024/amt-17-2277-2024.html), but ozone data stored in the WOUDC database have not been updated yet to the best of my knowledge. Since the OCS by Voglmeier et al. (2024) is likely more accurate than the Bass and Paur OCS, it would also be appropriate to use this spectrum instead of Bass and Paur.)

Minor comments

L29: I would not consider AODs in the order of 0.2 – 0.4 at 550 nm small. Corresponding AODs could easily be twice as large in the UV-B. Please delete “small”.
L185: QASUME is a well-known instrument. Up to now reading to L185, I had assumed that QASUME and not QASUMEII was used as the reference instrument. If in fact QASUMEII was used, QASUME should be replaced with QASUMEII throughout the manuscript.
L201: Please specify the version of UVSPEC.
L228: The publication by Webb et al. (2011) compares the old and new CIE action spectrum for erythema. It should be specified here what CIE norm was actually used (e.g., CIE 1987 or CIE 1998) for calculating the LUTs.
L260: What does “UVI.2.4” mean?
Figure 3: Title "ratio GB" of the Y-axis should be changed to “simulated / measured UVI”. The information that ground-based data were used as inputs to the simulations only needs to be mentioned in the caption or perhaps somewhere else on the plot.
L370: Results for SSA=0.99 are even more symmetrical about the mean. So from these results I conclude that SSA=0.99 is the actual value for Davos. Why do the authors think that it is 0.9 instead?
Sentence starting in L412: It is not clear from this sentence that UVSPEC results obtained with the LUT plus altitude correction were compared with similar calculations where the altitude (or air pressure) were explicitly specified. The wording should be improved.
L427: Regarding “is much faster”: Why is it much faster? It should not matter whether a LUT (once it is prepared) is based on CMFUV or COT.
L450: assessed > assess; “UVI when measured or reanalysis timeseries” > “UVI based either on AOD and TOC measured at Davos or taken from reanalysis data.”
L467: “Table 3” is not correctly numbered. It should be Table 4 as Table 3 starts on line 276. Also, please Include a row below the title, indicating that columns 2–5 are statistics based on the absolute UVI value while columns 6–9 indicate relative differences. And finally, change QSUME to QASUMEII.
Figure 11:Please explain red, orange and green datasets. (Only the blue and cyan data are described in the text.)
L513: Regarding: “the role of SSA was found to be equally important” I am really surprised that SSA at Davos is as low as reported in the manuscript. Has this been confirmed with independent measurements?
L526: Regarding: “When AOD is 0.3 – 0.4”: Please specify wavelength. In general, when AOD is specified, the wavelength should also be provided. And an AOD of 0.3 – 0.4 (presumably at 500 nm) is not low. These are not pristine conditions.

Technical corrections

L260: What does “UVI.2.4” mean?
L290, L298, L300: The numbers (1), (2) and (3) should also be added to Table 3 that it is clear what column of the table is described here.
L339: formulation > calculation
L354: justify > explain
L376: “air masses that transfer polluted aerosols from lower altitudes at Germany.” > “polluted air masses originating from low altitudes over Germany.”
L379–381: “in DOY” > on DOY”, “in these days” > “on these days”
Figure 5: The upper broken blue line is missing in the figure.
L404: “in the ratio,” > “to the ratio,”
L407: Regarding “The SSA has been estimated using Kinne,”: Shouldn’t “SSA” replaced with “ASY” here?
L497: “to the campaign.” > “in the campaign.”
L507: “when solar disc” > when the solar disc”
L511: “reanalysis AOD and TOC that were” > “reanalyzed AOD and TOC data that were”

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ED: Publish subject to minor revisions (review by editor) (17 May 2025) by Xiaohong Liu

AR by Ilias Fountoulakis on behalf of the Authors (14 Aug 2025) Author's response Author's tracked changes Manuscript

ED: Publish as is (08 Sep 2025) by Xiaohong Liu

AR by Ilias Fountoulakis on behalf of the Authors (12 Sep 2025) Author's response Manuscript

Journal article(s) based on this preprint

21 Oct 2025

Assessment of the accuracy in UV index modelling using the UVIOS2 system during the UVC-III campaign

Geosci. Model Dev., 18, 7451–7473, https://doi.org/10.5194/gmd-18-7451-2025,https://doi.org/10.5194/gmd-18-7451-2025, 2025

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The UVIOS2 model has been evaluated at Davos, Switzerland during the UVCIII campaign. The accuracy in the modelled UV indices has been assessed for different combinations of model inputs. A good overall agreement between UVIOS2 and the world reference spectroradiometer QASUME was found (average ratio of ~1 between the modelled and measured UV index), although the variability in the ratio can be large under cloudy conditions.


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