the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Jul 2023
A sensitivity study on radiative effects due to the parameterization of dust optical properties in models
Ilias Fountoulakis
Alexandra Tsekeri
Stelios Kazadzis
Vassilis Amiridis
Angelos Nersesian
Maria Tsichla
Emmanouil Proestakis
Antonis Gkikas
Kyriakoula Papachristopoulou
Vasileios Barlakas
Claudia Emde
Bernhard Mayer
Abstract. Most of the dust models underestimate the load of the large dust particles, consider spherical shapes instead of irregular ones, and have to deal with a wide range of dust refractive index (RI) to be used. This leads to an incomplete assessment of the dust radiative effects and dust-related impacts on climate and weather. The current work aims to provide an assessment, through a sensitivity study, of the limitations of models to calculate the dust direct radiative effect (DRE) due to the underrepresentation of its size, RI and shape. We show that the main limitations stem from the size and RI, while the shape plays only a minor role, with our results agreeing with recent findings in the literature. At the top of the atmosphere (TOA) close to dust sources, the underestimation of size issues an underestimation of the direct warming effect of dust of ~18–25 W/m2, for dust aerosol optical depth (DOD) of 1 at 0.5 μm, depending on the solar zenith angle (SZA) and RI. The underestimation of the dust size in models is less above the ocean than above dust sources, resulting in an underestimation of the direct cooling effect of dust above the ocean by up to 3 W/m2, for AOD of 1 at 0.5 μm. We also show that the RI of dust may change its DRE by 80 W/m2 above the dust sources, and by 50 W/m2 at downwind oceanic areas, for AOD of 1 at 0.5 μm at TOA. These results indicate the necessity of including more realistic sizes and RIs for dust particles in dust models, in order to derive better estimations of the dust DRE, especially near the dust sources and mostly for studies dealing with local radiation effects of dust aerosols.
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Ilias Fountoulakis et al.
Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1333', I. Pérez, 07 Aug 2023
This paper presents optical properties of aerosols under three different spectral-resolved refractive indexes. Two ranges for the aerosol size are used, below 10 μm and below 50 μm. Calculations are made over the desert and over the ocean. Moreover, the shape is considered since calculations are made with spherical and spheroidal shapes. The first part of the manuscript presents the spectral dependence of the extinction, scattering and absorption coefficients. The second part refers to a radiative transfer model and irradiances are calculated for specific situations. The authors emphasise the influence of the particle size and shape on the calculated properties.
The paper is quite detailed and complete since an extensive work has been made. Consequently, it could be published in Atmospheric Chemistry and Physics after the introduction of the following minor changes.
The paper structure should be more detailed at the introduction end. Moreover, since varied situations and conditions are considered, the authors should highlight the most relevant ones. For instance, the authors could select the noticeable results that may be followed in further research. Finally, the authors should note the restrictions of their research.
Minor remarks.
- 88. Revise “aa model”.
- 100. One “considered” must be supressed.
- 101. One account must be supressed, “accountthe” must be “account the”, and “RIRI” must be “RI”.
References should follow the journal style. Some of them are quite old.
Citation: https://doi.org/10.5194/egusphere-2023-1333-RC1 -
RC2: 'Comment on egusphere-2023-1333', Anonymous Referee #2, 06 Nov 2023
Review on “A sensitivity study on radiative effects due to the parameterization of dust optical properties in models” by Fountoulakis et. al.
This paper documents a sensitivity study to investigate and compare the sources of uncertainty in the estimate of dust direct radiative effects (DRE). Three major sources of uncertainty are considered, dust spectral refractive index (RI), dust particle size and dust particle shape. The magnitude of each uncertainty source is estimated from the differences of DRE estimated based on different assumptions of RT, size and shape. For example, the uncertainty due to dust RI is estimated by computing and contrasting the DRE of dust based on three sets of RI data with different absorptions. The sensitivity studies are performed for different surface conditions (desert vs. ocean) and solar zenith angles. The results from the sensitivity study suggests that the leading uncertainty in dust DRE computation is the dust RI, followed by dust size. The shape of dust has only negligible effects on dust DRE.
I have several main concerns and reservations about this study, which are summarized below. As a result, I have a hard time seeing how this paper advances our understanding of dust DRE and therefore do not think the paper should be accepted for publication in ACP.
- My first main concern is that the sensitivity studies seem too simple to capture the variability of dust aerosols in reality, and the complex environments in which the dust aerosols are found. The objective of this study is to understand the uncertainty in dust DRE (i.e., the term). In my view, this objective can only be achieved after a reasonable estimation of the mean DRE (i.e., the term). In this study, the DRE computations are performed in a quite simple and idealized set up. For example, only two types of surfaces (desert vs. ocean) and three values of solar zenith angles are considered. The surface temperature and atmospheric profiles are assumed to be constant without considering the diurnal cycles which can be quite strong in the desert regions. As a result of this rudimentary case set up, the meaningfulness of the mean DRE and therefore DRE uncertainty is quite questionable. For example, one of the main conclusions is that “At the top of the atmosphere (TOA) close to dust sources, the underestimation of size issues an underestimation of the direct warming effect of dust of ~18 - 25 W/m2, for dust aerosol optical depth (DOD) of 1 at 0.5 μm”. What is the meaningfulness of the DRE 18 - 25 W/m2? Is it a regional mean value? Is it instantaneous value or diurnally averaged? How could a modeler compare their DRE simulations to such DRE values reported in this study?
As a suggestion for revision (should the authors consider resubmission), I would recommend the study to set up the case studies in a more realistic and meaningful context that hopefully can be compared to the model simulations or other observations studies quantitatively.
- In comparison with several previous modeling studies (e.g., a recent one by Li et al. 2022) or the observations studies (e.g., Song et al. 2022), this study seems to be rudimentary and lack novelty. As mentioned above, the setup of the case studies is too simple. Moreover, the main conclusion from his study seems to be identical to previous ones (e.g., Li et al. 2021, 2022, Song et al. 2022). So, the novelty and significance of this study are quite questionable. They should be clearly explained and justified.
- Several places of the paper are confusing and/or misleading. For example, in most previous studies (e.g., Kok et al. 2017, all of Ryder et al. papers) dust particle size distribution was described in terms of dust geometrical diameter whereas this study uses radius. Although this is not physically wrong, making it difficult to compare this study with previous ones. In Fig. 13 to Fig. 16, the comparison of LW DRE with SW DRE at different SZA is confusing because the LW DRE is independent of SZA. A more meaningful comparison is the diurnally averaged DRE.
- Finally, the overall quality of the paper seems to fall short of the ACP standard. The paper has many typos and many references are missing (e.g., Ryder et al. 2021, Gutleben et al., 2019; 2020, Dubovik et al., 2006 to name a few)
References:
Li, L., Mahowald, N. M., Kok, J. F., Liu, X., Wu, M., Leung, D. M., Hamilton, D. S., Emmons, L. K., Huang, Y., Sexton, N., Meng, J., and Wan, J.: Importance of different parameterization changes for the updated dust cycle modeling in the Community Atmosphere Model (version 6.1), Geosci. Model Dev., 15, 8181–8219, https://doi.org/10.5194/gmd-15-8181-2022, 2022.
Li, L., Mahowald, N. M., Miller, R. L., Pérez García-Pando, C., Klose, M., Hamilton, D. S., Gonçalves Ageitos, M., Ginoux, P., Balkanski, Y., Green, R. O., Kalashnikova, O., Kok, J. F., Obiso, V., Paynter, D., and Thompson, D. R.: Quantifying the range of the dust direct radiative effect due to source mineralogy uncertainty, Atmos. Chem. Phys., 21, 3973–4005, https://doi.org/10.5194/acp-21-3973-2021, 2021.Kok, J. F. et al. Smaller desert dust cooling effect estimated from analysis of dust size and abundance. Nat Geosci 10, 274–278 (2017).
Citation: https://doi.org/10.5194/egusphere-2023-1333-RC2 - RC3: 'Comment on egusphere-2023-1333', Anonymous Referee #3, 09 Nov 2023
Ilias Fountoulakis et al.
Ilias Fountoulakis et al.
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