the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Aug 2023
Influence of covariance of aerosol and meteorology on co-located precipitating and non-precipitating clouds over Indo-Gangetic Plains
Abstract. Aerosol-cloud-precipitation-interaction (ACPI) plays a pivotal role in the global and regional water cycle and the earth’s energy budget; however, it remains highly uncertain due to the underlying different physical mechanisms. Therefore, this study aims to systematically analyze the effects of aerosols and meteorological factors on ACPI in the co-located precipitating (PCs) and non-precipitating clouds (NPCs) clouds in winter and summer seasons by employing the long-term (2001–2021) retrievals from Moderate Resolution Imaging Spectroradiometer (MODIS), Tropical Rainfall Measuring Mission (TRMM), and National Center for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis-II datasets over the Indo-Gangetic Plains (IGP). The results exhibit a decadal increase in aerosol optical depth (AOD) over Lahore (5.2 %), Delhi (9 %), Kanpur (10.7 %) and Gandhi College (22.7 %) and decrease over Karachi (-1.9 %) and Jaipur (-0.5 %). The most stable meteorology with high values of lower tropospheric stability (LTS) is found in both seasons over Karachi. In summer season the occurrence frequency of clouds is high (74 %) over Gandhi College, 60 % of which are PCs. Conversely, the least number of PCs are found over Karachi. Similarly, in winter season, the frequency of cloud occurrence is low over Karachi and high over Lahore and Gandhi College. The analysis of cloud top pressure (CTP) and cloud optical thickness (COT) indicate high values of cloud fraction (CF) for thick and high-level clouds over all study areas except Karachi. The micro-physical properties such as cloud effective radius (CER) and cloud droplet number concentration (CDNC) bears high values (CER > ~ 15 µm and CDNC > ~ 50 cm-3) for both NPCs and PCs in summer. The AOD-CER correlation is good (weak) for PCs (NPCs) in winter. Similarly, the sensitivity value of the first indirect effect ( FIE ) is high (ranged from 0.2 ± 0.13 to 0.3 ± 0.01 in winter, and from 0.19 ± 0.03 to 0.32 ± 0.05 in summer) for PCs and low for NPCs. Sensitivity value for second indirect effect (SIE) is relatively high (such as 0.6 ± 0.14 in winters and 0.4 ± 0.04 in summer) than FIE. Sensitivity values of the aerosol-cloud interaction (ACI) are low (i.e., -0.06 ± 0.09) for PCs in summers. Furthermore, the precipitation rate (PR) exhibits high values in summer season, and PR values are found high in comparatively thin clouds with fewer CDNC (< ~ 50 cm-3) and intermediate for optically thick clouds with higher CDNC (> ~ 50 cm-3 ).
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Status: final response (author comments only)
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RC1: 'Comment on egusphere-2023-1865', Anonymous Referee #1, 15 Sep 2023
The paper investigates the influence of covariance of aerosol and meteorology on co-located precipitating and non-precipitating clouds over Indo-Gangetic Plains. The investigators analyze the aerosol first/second indirect effects and present the variations in cloud properties in relation to the aerosol and meteorological conditions over the region. Despite its contributions, a few areas might require further refinement:
- The paper asserts that the high loading of aerosols led to a high occurrence of precipitating clouds in summer. While high PR values in summer seem to have some associations with aerosols, the presentation seems to skirt around establishing a firm causality. Is it possible that some factors that influence both aerosols and precipitation simultaneously? Thus, this study may need to discuss the cause-and-effect relationship outlined in the study's conclusion.
- The discussion may have a discussion of the uncertainties tied to the satellite datasets used in this study, as well as how the uncertainties may influence the AOD-CER correlation and the SIE.
- The study only considers a few locations, limiting the scope of the conclusions. The authors should discuss the implications of the relationships between clouds, aerosols, and precipitation over Indo-Gangetic Plains for other regions. For example, whether the high sensitivity value of the FIE in winter can be established to other regions.
- Although the manuscript mentions "the complications of aerosol-cloud-precipitation interactions over complex topography," it doesn't sufficiently explain how specific topographical features impact the ACI. A further discussion of potential topography impacts may strengthen the paper.
Citation: https://doi.org/10.5194/egusphere-2023-1865-RC1 - AC1: 'Reply on RC1', Khan Alam, 08 Dec 2023
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RC2: 'Comment on egusphere-2023-1865', Anonymous Referee #2, 25 Sep 2023
Review “Influence of covariance of aerosol and meteorology on co-located precipitating and non-precipitating clouds over INdo-Gangetic Plains” Gulistan et al.
General overview,
The article studies the aerosol-cloud-precipitation interaction at six stations in the Indo-Gangetic Plains. The authors use the synergy of satellite observations and reanalyses to collocate information on cloud, aerosol, precipitation, and meteorological properties in winter and summer. Their analysis is based on the distinction between precipitating and non-precipitating clouds. Several plots show differences between different stations, seasons, and cloud types with different conclusions, e.g. that the lower tropospheric stability is higher for non-precipitating clouds in winter and lower for precipitating clouds. Another interesting result is that the precipitation rate maximum does not occur for the same cloud type when the cloud droplet number concentration is high or low. The study is comprehensive and it is appreciated that the authors analyzed their data set in different ways. Nevertheless, the current study lacks a lot of information on the methodology, uncertainties and I am particularly concerned about a part of the study where the authors retrieve an indirect effect parameter with dependent datasets, therefore it is difficult to give credit to some result as it is now. I have detailed my various concerns below.
From page 14 on, there are no more line numbers, which makes it difficult to refer to the questions I want to ask. From page 14 on, I will refer to the page number, not the line number.
Major revisions:
- Methodology section: The satellite observations are associated with uncertainty, but it does not appear (except briefly mentioned on page 24). The results should be associated with the potential uncertainty. A detailed analysis of the propagation of uncertainty in the results should appear with a discussion of the implications.
- Related to uncertainty, there is no mention of the number of points used in the statistics. For example, Figure 6, there are a lot of regimes and I have some doubt that each regime has a large enough number of pixels to provide significant statistics. Figure 6 is an example, but I have the same concern for all the other results. This is mentioned on page 29, but I would like to see the numbers.
- Methodology: There is no discussion of the collocation of the various products. How are CER and AOD collocated? MODIS does not retrieve AOD when a cloud is detected, did the authors look at the nearest pixel? If so, did they consider the potential 3D effect of clouds? How are MODIS and TRMM data collocated? Same questions with reanalysis (temporal and spatial resolution are not the same).
- Methodology: The authors consider temperature, LTS, RH, vertical velocity as meteorological parameters. It is not clear why they considered these parameters and not others.
- I suspect that the authors considered only liquid clouds for their analysis, but this is not mentioned in the manuscript. Did the authors filter their data set, and if so, how?
- The authors based their study on 6 stations, but they only use satellite and reanalysis datasets, so I wonder why they only focus on 6 stations and not do a full analysis of the region. The eastern part of the region is not considered and this is unfortunate if satellite observations are considered. I expected a comparison with ground observations to explain why the sites were chosen, but there is none. Therefore, I would suggest explaining why the full map of the region is not considered.
- FIE, SIE, and TIE parameters are based on dependent records. CDNC is derived from CER and cLWP, and cLWP is derived from CER and COT. Therefore, it is not possible to infer the different parameters. To study these effects, we should consider only independent datasets. Therefore, Figures 8, 9, 10 and the related discussions and conclusions, I have doubts about them.
- The article lacks quantification in most of the paragraphs, which makes the analysis difficult to follow because I sometimes do not know what is being referred to.
Minor revisions:
- l.11, “different physical mechanisms”, can the authors specify the different physical mechanisms?
- l.11 “systematically analyze”, what does systematically mean?
- keywords, “Aerosol option depth” -> “aerosol optical depth”
- Figure from the abstract, I am not sure if the figure helps to understand the abstract, it is rather a lot of information with parameters not yet defined (LTS, CER, AOD), I suggest removing it.
- Introduction: citations are missing, I suggest adding citations. For example, the first two sentences should have citations.
- l. 66, aerosols can also act as ice nucleating particles but this is never mentioned in the article. Did the authors consider this? I think it should at least be mentioned in the introduction and emphasize that only liquid clouds are relevant for the analysis.
- l. 67, “The decrease in CDNC and increase in CER increases the probability of precipitation rate (PR)”. Stevens and Feingold (2009) have shown that you can actually have the opposite effect: An initial inhibition of precipitation from aerosols can lead to increased precipitation later. The region is affected by large precipitation and this may be an effect that the authors did not consider. I suggest adding a discussion about it.
Stevens, B., & Feingold, G. (2009). Untangling aerosol effects on clouds and precipitation in a buffered system, Nature, 461(7264), 607-613.
- l. 82: “Twomey effect”, I think it would be best to describe the effect before mentioning it.
- l. 83: “anti-Twomey effect”, I do not know this effect, can the authors describe it?
- l.87: “FIE”, the acronym is not defined before.
- l. 87, the Twomey effect refers to the change in cloud radiative properties and not to the cloud droplet size. Also McCoy et al., (2018) may not be the best and the citation from Twomey may be more relevant
Twomey, S. (1977). The influence of pollution on the shortwave albedo of clouds. Journal of the atmospheric sciences, 34(7), 1149-1152.
- l. 111, I suggest including the names of the cities on the map.
- Figure 1 caption: “Geographical map” -> “topography”
- Figure 1: What is the data for topography? Some regions are covered and some are not. I suggest removing Figure 1 and adding the points on Figure 2 (with the names of the stations) since there is no mention of topography in the article.
- l. 122, “resolution of x to study atmospheric…”, change x to the correct value.
- l.125, CDNC and cLWP are not direct products of MODIS. They are defined later but should not appear here.
- l. 126 “Data with AOD>1.5”, with the threshold the authors avoid misidentification of clouds as aerosols and not the reverse as stated in the article. Is there a threshold to avoid misidentification of clouds as aerosols?
- Equation 1, square root does not go all the way
- LTS equation (line 144), is not numbered.
- l. 144, \theta_{0} -> \theta_{1000}
- l. 150, PR is defined for precipitation rate but is not an instrument and for Precipitation Radar it is not mentioned.
- l. 150, TMI is not defined
- Methodology section: is both Aqua and Terra for MODIS used?
- l. 187, “is similar”, the authors state the opposite afterwards so I would remove the “similar”
- lines 195 and 196, citations are missing.
- l. 199, Why does Karachi have lower values?
- l. 201, “illustrate pristine atmosphere”, I suggest adding “comparatively”.
- lines 200-206, it would be better to quantify with the median to compare different regimes.
- l. 204, before the new sentence, the authors compare with the other PDF, I think it should be a new paragraph with the description.
- l. 204, “winter season is the wet scavenging”, it contradict with Fig 5 and Fig. 11 for which summer as more precipitation. Can the author explain?
- From Fig 3, AOD in winter is not smaller for Jaipur and Kanpur (it does not look like it). Any reason for this difference?
- Table 1 “Total number of counts”, “counts”, is it pixels?
- Figure 3, it is difficult to distinguish the different points and colors especially between Karachi and Gandhi College,
- l. 219, “estimation of”, I think should be removed.
- l. 221-222, “the potential for vertical convection… precipitation formation”, I do not understand this part, can the author rephrase?
- l. 235, “is relatively high”, is it compared to PCs or to summer?
- l.236 “The increase”, which increase? I am not sure what it refers to. (same for line 242)
- l. 236 “33-57%”, I do not find these values in the Table.
- Sometimes RH is referred to as RH% and sometimes RH.
- l. 242, “suitable thermodynamical conditions” can the authors say more about this and add a citation?
- l 244, Gandhi’s college has a higher value of RH. I was expecting Karachi because it is closer to the coast. Can the author add a discussion about this?
- Table 2, did the author consider the mean? I would suggest considering the median since we do not expect Gaussian distributions.
- page 14 “the frequency of occurrence of precipitable clouds” is it the frequency of occurrence relative to the total or to cloudy pixels?
- page 14, the authors apply filters to avoid overestimation (COT and CF> 5), but I wonder if this does not lead to underestimation.
- page 14, some discussion of the results is missing
- Fig. 6, I think the authors are not showing a joint histogram as stated in the article but rather the CF for different regimes of COT and CTP, can it be explained in more detail what is shown here? Is CF averaged?
- Page 16: Is CF>0.7 a threshold used for the entire paragraph to state that it is “high CF”? if so, it should not be in parentheses but rather explicitly explained.
- Page 16, (23<COT<60, CTP>680), should read (23<COT<60, 800>CTP>680)
- Page 16, “Similarly, in winter season the type of PCs…” why cirrus & cirro-stratus not included with CF>=0.9
- Page 16, “less significant values”, do the authors mean “lower”?
- Page 16: The paragraph starting with “Also, in summer…” it is difficult to follow this paragraph, I suggest changing the presentation of the paragraph.
- Table 3, “>800 to 680” should it be “<800 to 680”?
- Figure 6, Is the mean CF shown? If so, the number of points and the SD should also be shown. I would also suggest showing the median instead of the mean
- Page 19 “depicted an approximately similar values”, did the authors perform a statistical test to infer this conclusion?
- Page 19 “The low number of CDNC”, there is no information on CDNC in Figure 6
- page 21 “detailed linear regressed slopes”, what is meant by “detailed”
- page 21 “correlation is good for PCs and weak for NPC”, what is the criteria and threshold to determine if it is good or not? In both cases, r2 looks low.
- page 21 the positive AOD-CER correlation, what exactly does it mean physically ? Why should droplets be larger in the presence of aerosols?
- Figure 10 “the error bars show the standard deviation” the plot represents sensitivities, so I am not sure to understand what standard deviation is being retrieved here.
- Page 27, I am not sure what the authors mean by “approximation”, is it uncertainty ?
- Page 27 “Fig. 12 shows scatter plots of…” the paragraph lacks quantifications,
- Fig. 11 caption, is the mean shown ?
- l. 30 “Also the frequently occurred…” it should be rephrased
- page 31, the ladsweb website does not work,
- Bibliography: multiple references are not correctly written in the bibliography (some doi are underlined, some are not),
- Bibliography: some names are not outputed correctly, for example: “Thomas, A., Kanawade, V. P., Sarangi, C., & Srivastava, A. K. J. S. o. t. T. E”
- I suggest removing words that are unnecessary (meaningful (179), completely (l. 225), evidently (l. 228)... )
Citation: https://doi.org/10.5194/egusphere-2023-1865-RC2 - AC2: 'Reply on RC2', Khan Alam, 08 Dec 2023
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