the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Comparison of dust optical depth from multi-sensor products and the MONARCH dust reanalysis over Northern Africa, the Middle East and Europe
Michail Mytilinaios
Sara Basart
Sergio Ciamprone
Juan Cuesta
Claudio Dema
Enza Tomaso
Paola Formenti
Antonis Gkikas
Oriol Jorba
Ralph Kahn
Carlos Pérez García-Pando
Serena Trippetta
Lucia Mona
Abstract. Aerosol reanalysis datasets are model-based observationally constrained continuous 3D aerosol fields with relatively high temporal frequency that can be used to assess aerosol variations and trends, climate effects and impacts upon socio–economic sectors, such as health. Here we compare and assess the recently published MONARCH high resolution regional desert dust reanalysis over Northern Africa, the Middle East and Europe (NAMEE) with a combination of ground-based observations and space-based dust retrievals and products. In particular, we compare the total and coarse dust optical depth (DOD) from the new reanalysis with DOD products derived from MODIS, MISR and IASI space-borne instruments. Despite the larger uncertainties, satellite-based datasets provide a better geographical coverage than ground-based observations, and the use of different retrievals and products allows for at least partially overcoming some single-product weaknesses in the comparison. Nevertheless, limitations and uncertainties due to the type of sensor, its operating principle, its sensitivity, its temporal and spatial resolution, and the methodology for retrieving or further deriving dust products, are factors that bias the reanalysis assessment. We, therefore, also used ground-based DOD observations provided by 238 stations of the AERONET network located within the NAMEE region as a reference evaluation dataset. In particular, prior to the reanalysis assessment, the satellite datasets were evaluated against AERONET, showing moderate underestimations in the vicinities of dust sources and downwind regions, whereas small or significant overestimations, depending on the dataset, can be found in the remote regions. Taking into consideration these results, the MONARCH reanalysis assessment showed that total and coarse DOD simulations are consistent with satellite and ground-based data, capturing qualitatively the major dust sources in the area as well as the dust transport patterns. Moreover, the reanalysis reproduces the seasonal dust cycle, identifying the increased dust activity occurred in the NAMEE region during spring and summer. The quantitative comparison between the MONARCH reanalysis DOD and satellite multi-sensor products shows that the reanalysis tends to slightly overestimate the desert dust that is emitted from the source regions and underestimate the transported dust over the outflow regions, implying that the model removal of dust particles from the atmosphere, through deposition processes, is too effective. More specifically, small positive biases were found over the Sahara Desert (0.04) and negative biases over the Atlantic Ocean and the Arabian Sea (−0.04), which constitute the main pathways of the long-range dust transport. Considering the DOD values recorded on average there, such discrepancies can be considered low as the low relative bias in the Sahara Desert (< 0.5) and over the adjacent maritime regions (< 1), certifies. Similarly, over areas with intense dust activity the linear correlation coefficient between the reanalysis simulations and the ensemble of the satellite products is significantly high for both total and coarse DOD, reaching 0.8 over the Middle East, the Atlantic Ocean and the Arabian Sea, and exceeding it over the African continent. Moreover, the low relative biases and high correlations are associated with regions where large amounts of observations are available, allowing for robust model assessment.
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Michail Mytilinaios et al.
Status: final response (author comments only)
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RC1: 'Comment on acp-2022-655', Anonymous Referee #2, 18 Nov 2022
The paper presents a thorough evaluation of the MONARCH regional dust reanalysis over North Africa, Europe, and Middle East in terms of comparisons to satellite and ground-based dust aerosol optical depth products (DOD). The presentation is well written and the figures—although complex—are relatively easy to read. I have only a few minor clarifications and two major points, though I don’t think they require extensive revision.
Line 153 - I am confused by the use of the word “ensemble” here and in the remainder of this paragraph. What ensemble is being referred to? The statistics presented later are data sets versus one another of the reanalysis versus the data sets.
Line 402 - I don’t agree “low RMSE” in the region being described. I see high RMSE in bottom left quadrant of, e.g., Figure 2j
Line 406 - I don’t understand MISR shows smaller differences against AERONET in the regions indicated. It looks worse than MIDAS in terms of MB on the North African coast (comparing Figure 2g and 2h).
Two major comments:
I question the conclusion stated on line 737. The “gold standard” here is AERONET, and the comparisons in Section 3 make plain that MIDAS and AEROIASI underestimate AERONET AOD over source regions in both total and coarse DOD (MIDAS) and coarse DOD (AEROIASI). MONARCH is high AOD over those regions relative to MIDAS and AEROIASI, but still low compared to AERONET in both total and coarse DOD (Figure S3 and S7). Admittedly you don’t have the regional coverage from AERONET that the satellites provide, but still I think my assessment there is fair. Comparing the numbers in Figure S3 and S7 for North Africa, there is also some insight into the coarse/total DOD partitioning in the model and data, as well as interesting aspects in seasonal variability (MONARCH goes from a ratio of 0.14:0.4 (35%) to 0.22:0.31 (71%) from MAM to JJA; AERONET goes from 0.19:0.48 (40%) to 0.26:0.39 (67%)). That’s pretty good and that seasonal change in the coarse:total DOD ratio seems like something to note. The authors are well aware of the recent body of literature on the “under appreciated” dust coarse mode, and MONARCH seems to be doing something robust there.
On the other hand, the gradient from land to ocean is noted as a discrepancy and there are at least two things there to consider. First, it seems notable that the data assimilation only corrects the model over land using the MODIS Deep Blue products. (An interesting aside: how do the data being assimilated compare to the AERONET data? This seems not to be considered here.) So if the data is correcting forward model biases over land that may be conveyed over ocean. Secondly, to those model biases, that models tend to deplete dust over land range transport too efficiently is I think also well known. Without an evaluation of the dust vertical profile it’s hard to tell what’s going on here. It’s also the case that some models are overly aggressive in loss processes, both dry and wet, and some statement of that to the effect of the budget of MONARCH compared to, say, AEROCOM models would be useful.
So I think the authors should address these two major points in their conclusions at least with a couple of paragraphs that expand on these ideas as the pertain to their study.
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RC2: 'Comment on acp-2022-655', Anonymous Referee #3, 27 Jan 2023
The comment was uploaded in the form of a supplement: https://acp.copernicus.org/preprints/acp-2022-655/acp-2022-655-RC2-supplement.pdf
Michail Mytilinaios et al.
Michail Mytilinaios et al.
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