the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Dust transport and advection measurement with spaceborne lidars ALADIN, CALIOP and model reanalysis data
- 1Department of Marine Technology, College of Information Science and Engineering, Ocean University of China, Qingdao, 266100, China
- 2Laboratory for Regional Oceanography and Numerical Modelling, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao, 266200, China
- 3Institute for Advanced Ocean Study, Ocean University of China, Qingdao, 266100, China
- 1Department of Marine Technology, College of Information Science and Engineering, Ocean University of China, Qingdao, 266100, China
- 2Laboratory for Regional Oceanography and Numerical Modelling, Pilot National Laboratory for Marine Science and Technology (Qingdao), Qingdao, 266200, China
- 3Institute for Advanced Ocean Study, Ocean University of China, Qingdao, 266100, China
Abstract. In this paper, a long-term large-scale Sahara dust transport event occurred during 14 June and 27 June 2020 is tracked with the spaceborne lidars ALADIN and CALIOP observations and the models ECMWF and HYSPLIT analysis. We evaluate the performance of the ALADIN and CALIOP on the observations of dust optical properties and wind fields and explore the capability in tracking the dust events and in calculating the dust mass advection with the combination of measurement data from ALADIN and CALIOP coupled with the products from ECMWF and HYSPLIT. The dust plumes are identified with AIRS/Aqua “Dust Score Index” and with the “Vertical Feature Mask” products from CALIOP. The emission, dispersion, transport and deposition of the dust event are monitored using the data from AIRS/Aqua, CALIOP and HYSPLIT. With the quasi-synchronization observations by ALADIN and CALIOP, combining the wind field and relative humidity, the dust advection values are calculated. From this study, it is found that the dust event generated on 14 and 15 June 2020 from Sahara Desert in North Africa, and then dispersed and transported westward over the Atlantic Ocean, and finally deposited in the Atlantic Ocean, the Americas and the Caribbean Sea. During the transport and deposition processes, the dust plumes are trapped in the Northeasterly Trade-wind zone between the latitudes of 5 °N and 30 °N , altitudes of 0 km and 6 km (in this paper we name this space area as “Saharan dust westward transport tunnel”). From the measurement results on 19 June 2020, influenced by the hygroscopic effect and mixing with other types aerosols, the backscatter coefficients of dust plumes are increasing along the transport routes, with 3.88 × 10−6 ± 2.59 × 10−6 m−1sr−1 in “dust portion during emission phase”, 7.09 × 10−6 ± 3.34 × 10−6 m−1sr−1 in “dust portion during development phase” and 7.76 × 10−6 ± 3.74 × 10−6 m−1sr−1 in “dust portion during deposition phase”. Finally, the advection value at different dust parts and heights on 19 June and on entire transport routine during transportation are computed. On 19 June, the mean dust advection values are about 2.06 mg · m−2 ·s−1 in dust portion during emission phase, 1.47 mg · m−2 ·s−1 in dust portion during development phase and 0.95 mg · m−2 ·s−1 in dust portion during deposition phase. In the whole life-time of the dust event, the mean dust advection values are about 1.50 mg · m−2 ·s−1 on 15 June 2020, 2.41 mg · m−2 ·s−1 on 16 June 2020, 1.47 mg · m−2 ·s−1 on 19 June 2020, 2.01 mg · m−2 ·s−1 on 24 June 2020 and 1.15 mg · m−2 ·s−1 on 27 June 2020. During the dust development stage, the mean advection values gradually increase and reach to the maximum value on 16 June with the enhancement of the dust event. Then, the mean advection values decrease during the transport and the deposition of the dust over the Atlantic Ocean, the Americas and the Caribbean Sea.
Guangyao Dai et al.
Status: closed
-
RC1: 'Comment on acp-2022-53', Anonymous Referee #1, 09 Mar 2022
The combination of two space-based lidars (CALIPSO and Aeolus) is new and deserves attention. The Saharan dust transport across the Atlantic Ocean is a well-known large-scale phenomenon and suited to demonstrate the novel approach. Personally, I welcome the resubmission of the now improved version of the manuscript. However, there are still some major reviews necessary till the final publication.
Major comments:
- The comparison of the 3 cross sections on 19 June 2020 is misleading (Section 4.2). With the 3 cross sections just some hours (<4 h) apart, you get a snapshot of an existing dust plume whose maximum is currently over the central Atlantic. Lower values of the backscatter coefficient above the Sahara and the Caribbean (cross section 1 and 3) can not be directly linked to emission and deposition (named by you “emission phase” and “deposition phase”). Usually, there are several days between emission and deposition and not just some hours. So, there is no benefit in reporting the backscatter values for the 3 cross sections. I would consider removing these values from the abstract and the conclusion.
Your next Section 4.3 is better suited to follow the dust from emission to deposition. - The calculation of the mean mass concentration is not well defined. How do you define your dust layer? Or do you take an average over the whole cross section? You mention some upper and lower threshold values for the mass concentration based on previous observations. However, if you observe such an intense dust event (“Godzilla”), the mass concentration may exceed the upper threshold. To calculate a mean mass concentration, you should define your dust layer, probably with a lower backscatter or extinction coefficient threshold and then take the average over the entire dust layer.
It is positive, that you compare two different methods. In order to judge the differences, you should add uncertainties to both derived mean mass concentrations (Table 1+2).
For the factor method, do you use the extinction coefficient provided by CALIPSO or the extinction coefficient calculated with the adapted lidar ratio (58 sr)? The later would be preferable to be consistent with your advection calculation procedure. - In Section 4.3, you should make sure that the same dust was observed in all the cross sections. The description stays a bit vague. A so-called Lagrangian case study was presented in Weinzierl et al., BAMS 2017, there an aircraft observed the same dust sample at the coast of Africa and some days later over the Caribbean. You have all the trajectory calculations ready, just use them in a more quantitative way to show that you track the same dust event. For example, you could add dots to the trajectories marking intervals of 24 h in Fig. 9. The dots alone won’t be sufficient.
- The CALIPSO examples introduced in Fig. 4 and 5 are later on not used anymore. It would be better to show in Fig. 4 some dates used in Section 4.3. In Fig. 5 you should definitely show the case of 19 June 2020 because it is later on used in the case study of Section 4.2.
- It is a great step forward to use the lidar ratios for (Western) Saharan dust instead of global averages. The lidar ratio of 60 sr at 1064 nm seems a good estimate as recently confirmed by Haarig et al., ACP 2022 (57 – 69 sr). Although, a higher ratio of LR1064/LR532 was reported. Nevertheless, the values used seem to be reasonable.
- Aeolus aerosol products are usually reported on a very coarse horizontal resolution. How do you make sure that your profiles are not influenced by clouds? You are talking about the cloud screening in the case of CALIPSO, but not for Aeolus. Please add some comments on the cloud and aerosol separation in the case of Aeolus.
- Please add uncertainties to all your calculated values, especially to the mean dust advection values. Otherwise, you can’t draw conclusions on changing values.
Minor comments
- Text insides some figures (especially Fig. 3 + 4) is quite small and hard to read.
- Figure 7 and 10 are quite complex and hard to follow. The text is understandable even without these figures. In case of the wind speed and direction, you have the nice Fig. 12, and the other information from Fig. 7 and 10 are not necessary to understand the paper. I would consider removing these figures to make the paper easier to read.
- L55: A reference about SHADOW is missing. What about Veselovskii et al., ACP 2016?
- The technical details about Aeolus could be moved from the introduction to Section 2.1. Just keep the most important facts about Aeolus as you have done it for CALIPSO.
- 4 VFM – please write vertical feature mask
- 4 “west coast of Africa”
- 5 The term “source” might be misleading, because you show a “position” along the CALIPSO track and the corresponding profiles at this position. And then you use this position as source for your trajectories. Reading “source” reminded me on dust sources.
- 6a – it is not a “vertical” view and HYSPLIT trajectories are not shown.
- L286 Explain u and v component of wind vector to readers not familiar with these conventions.
- L310 “Godzilla” – a nice piece of information which could already be placed in the introduction.
- 8 The color plots are shown on the CALIPSO or Aeolus tracks?
- L341 “dust mass” – you’re not showing the dust mass, but “enhanced backscatter and extinction values indicating the presence of dust”
- 9d It is almost impossible to capture the latitudinal component in the plot – I would consider to show it on altitude – longitude plane (this is the interesting information!) and indicate the different positions in latitude by different lines, e.g., position A in dashed lines, position B in dotted lines, …
- The “Saharan dust westward transport tunnel” (L.383) is somehow linked to the “Saharan Air Layer”.
-
AC1: 'Reply on RC1', Kangwen Sun, 11 May 2022
Dear Reviewer,
Many thanks for reviewing our manuscript. We greatly appreciate the substantial amount of time and effort that you dedicated to this review process.
We have revised the manuscript according to your comments point-to-point and the response is presented below as the supplement.
Many thanks and best regards.
Guangyao Dai and Songhua Wu
On behalf of the co-authors
- The comparison of the 3 cross sections on 19 June 2020 is misleading (Section 4.2). With the 3 cross sections just some hours (<4 h) apart, you get a snapshot of an existing dust plume whose maximum is currently over the central Atlantic. Lower values of the backscatter coefficient above the Sahara and the Caribbean (cross section 1 and 3) can not be directly linked to emission and deposition (named by you “emission phase” and “deposition phase”). Usually, there are several days between emission and deposition and not just some hours. So, there is no benefit in reporting the backscatter values for the 3 cross sections. I would consider removing these values from the abstract and the conclusion.
-
RC2: 'Comment on acp-2022-53', Anonymous Referee #2, 19 Apr 2022
This paper describes the combination of satellite borne lidar data from two instruments in combination with model wind field data and back and forward trajectory analyses to investigate the advection of a major dust storm across the Atlantic Ocean. The paper focus on a case study of a major dust storm to assess how the combined CALIPSO and Aeolus satellite products can be combined with ECMWF driven trajectories to describe dust transport and loss. The paper was previously submitted to ACPD and this version has been considerably improved.
I do, however, have a major reservation about section 4.2 and the accompanying statements in the abstract and summary sections. Section 4.2 presents lidar curtains at three locations across the sub-tropical north Atlantic on a day in the middle of the dust storm. The three curtains are close t the source region, over the mid-Atlantic and towards the west, in the far-field of the plume. However, the satellite overpasses presented are taken only 3 hours apart. The advection times between the most easterly lidar curtain and the most westerly are of the order of a week or more. The data in section 4.2 show the overall geographical distribution of dust across the Atlantic as a snapshot on the morning of 19/6/2020. What they do not do is say anything at all about the dynamics of the dust plume as it advects across the Atlantic region. The source region may have changed or emissions of dust varied and the transport pathways may be affected by changing atmospheric conditions over the course of the event. However, section 4.2 assumes the dust plume is time invariant and describes the scene as representing different ages of the plume. This is misleading and in any case is described much better in section 4.3. Either section 4.2 should be rewritten to illustrate geographical variability at a single point in time or removed. Furthermore, the way the results from this section are presented in the abstract and summary should be reframed or removed as they are written as though the data were taken in a pseudo lagrangian way and they were not.
Specific recommendations
Lines 62-63: “Additionally, the CALIOP product Vertical Feature Mask product (VFM)” better to write
“Additionally, the CALIOP Vertical Feature Mask product (VFM)”Line 74 “(e)motion”
Line 170-174 “Based on the dataset consists of the backscatter coefficients and extinction coefficients at the wavelengths of 1064 nm and 532 nm from CALIOP and the extinction coefficients at the wavelength of 355 nm from ALADIN, the aerosol volume concentration distribution can be
calculated based on the regularization method which was performed by generalized cross-validation (GCV) from Müller et al. (1999).” A confusing sentence that needs to be rewritten
lines 240-241: Figure 4a shows the majority of the dust has been lifted to a maximum of around 7km or less south of 20N on 18/6/2020, there is only a small proportion of the dust at the far north end of the overpass that has a maximum close to 10 km. This probably needs rephrasing.
Lines 276-282: The narrative in the section assumes a pseudo-langragian language but the lidar passes are on the same day so these are different slices of a dust event that has lasted several days (fig 3) and has a transit time of multiple days between the overpasses shown in fig 5. The wording here needs to better reflect that these are cross sections at different geophysical locations in the plume and do not directly represent plume evolution. This discussion is extended to report values of backscatter and advection for different phases of the dust plume. However, these don’t reflect actual advection of the same air. The underlying assumption is the dust plume does not change with time. Clearly, this is not the case, so the determinations from the 3 different overpasses cant really be compared in the way that is done in the analysis in 4.2. At best this gives a snapshot of the plume at a single point in time across much of the Atlantic. This section needs to be rewritten in my view to make this clear and to convey why this is appropriate, otherwise it is best removed. This same approach is also followed up in the summary (402-406). The analysis is not pseudo-lagrangian and should not be inferred as such, the different phases of the storm were emitted many days apart and may have had very different conditions at source and during advection. This needs to be made explicit. The abstract also has the same errors between lines 22-25. This needs to be removed or corrected.
Line 293: “to calculate(d)”
Line 384: Affected not effected
-
AC2: 'Reply on RC2', Kangwen Sun, 11 May 2022
Dear Reviewer,
Many thanks for reviewing our manuscript. We greatly appreciate the substantial amount of time and effort that you dedicated to this review process.
We have revised the manuscript according to your comments point-to-point and the response is presented below as the supplement.
Many thanks and best regards.
Guangyao Dai and Songhua Wu
On behalf of the co-authors
-
AC2: 'Reply on RC2', Kangwen Sun, 11 May 2022
Status: closed
-
RC1: 'Comment on acp-2022-53', Anonymous Referee #1, 09 Mar 2022
The combination of two space-based lidars (CALIPSO and Aeolus) is new and deserves attention. The Saharan dust transport across the Atlantic Ocean is a well-known large-scale phenomenon and suited to demonstrate the novel approach. Personally, I welcome the resubmission of the now improved version of the manuscript. However, there are still some major reviews necessary till the final publication.
Major comments:
- The comparison of the 3 cross sections on 19 June 2020 is misleading (Section 4.2). With the 3 cross sections just some hours (<4 h) apart, you get a snapshot of an existing dust plume whose maximum is currently over the central Atlantic. Lower values of the backscatter coefficient above the Sahara and the Caribbean (cross section 1 and 3) can not be directly linked to emission and deposition (named by you “emission phase” and “deposition phase”). Usually, there are several days between emission and deposition and not just some hours. So, there is no benefit in reporting the backscatter values for the 3 cross sections. I would consider removing these values from the abstract and the conclusion.
Your next Section 4.3 is better suited to follow the dust from emission to deposition. - The calculation of the mean mass concentration is not well defined. How do you define your dust layer? Or do you take an average over the whole cross section? You mention some upper and lower threshold values for the mass concentration based on previous observations. However, if you observe such an intense dust event (“Godzilla”), the mass concentration may exceed the upper threshold. To calculate a mean mass concentration, you should define your dust layer, probably with a lower backscatter or extinction coefficient threshold and then take the average over the entire dust layer.
It is positive, that you compare two different methods. In order to judge the differences, you should add uncertainties to both derived mean mass concentrations (Table 1+2).
For the factor method, do you use the extinction coefficient provided by CALIPSO or the extinction coefficient calculated with the adapted lidar ratio (58 sr)? The later would be preferable to be consistent with your advection calculation procedure. - In Section 4.3, you should make sure that the same dust was observed in all the cross sections. The description stays a bit vague. A so-called Lagrangian case study was presented in Weinzierl et al., BAMS 2017, there an aircraft observed the same dust sample at the coast of Africa and some days later over the Caribbean. You have all the trajectory calculations ready, just use them in a more quantitative way to show that you track the same dust event. For example, you could add dots to the trajectories marking intervals of 24 h in Fig. 9. The dots alone won’t be sufficient.
- The CALIPSO examples introduced in Fig. 4 and 5 are later on not used anymore. It would be better to show in Fig. 4 some dates used in Section 4.3. In Fig. 5 you should definitely show the case of 19 June 2020 because it is later on used in the case study of Section 4.2.
- It is a great step forward to use the lidar ratios for (Western) Saharan dust instead of global averages. The lidar ratio of 60 sr at 1064 nm seems a good estimate as recently confirmed by Haarig et al., ACP 2022 (57 – 69 sr). Although, a higher ratio of LR1064/LR532 was reported. Nevertheless, the values used seem to be reasonable.
- Aeolus aerosol products are usually reported on a very coarse horizontal resolution. How do you make sure that your profiles are not influenced by clouds? You are talking about the cloud screening in the case of CALIPSO, but not for Aeolus. Please add some comments on the cloud and aerosol separation in the case of Aeolus.
- Please add uncertainties to all your calculated values, especially to the mean dust advection values. Otherwise, you can’t draw conclusions on changing values.
Minor comments
- Text insides some figures (especially Fig. 3 + 4) is quite small and hard to read.
- Figure 7 and 10 are quite complex and hard to follow. The text is understandable even without these figures. In case of the wind speed and direction, you have the nice Fig. 12, and the other information from Fig. 7 and 10 are not necessary to understand the paper. I would consider removing these figures to make the paper easier to read.
- L55: A reference about SHADOW is missing. What about Veselovskii et al., ACP 2016?
- The technical details about Aeolus could be moved from the introduction to Section 2.1. Just keep the most important facts about Aeolus as you have done it for CALIPSO.
- 4 VFM – please write vertical feature mask
- 4 “west coast of Africa”
- 5 The term “source” might be misleading, because you show a “position” along the CALIPSO track and the corresponding profiles at this position. And then you use this position as source for your trajectories. Reading “source” reminded me on dust sources.
- 6a – it is not a “vertical” view and HYSPLIT trajectories are not shown.
- L286 Explain u and v component of wind vector to readers not familiar with these conventions.
- L310 “Godzilla” – a nice piece of information which could already be placed in the introduction.
- 8 The color plots are shown on the CALIPSO or Aeolus tracks?
- L341 “dust mass” – you’re not showing the dust mass, but “enhanced backscatter and extinction values indicating the presence of dust”
- 9d It is almost impossible to capture the latitudinal component in the plot – I would consider to show it on altitude – longitude plane (this is the interesting information!) and indicate the different positions in latitude by different lines, e.g., position A in dashed lines, position B in dotted lines, …
- The “Saharan dust westward transport tunnel” (L.383) is somehow linked to the “Saharan Air Layer”.
-
AC1: 'Reply on RC1', Kangwen Sun, 11 May 2022
Dear Reviewer,
Many thanks for reviewing our manuscript. We greatly appreciate the substantial amount of time and effort that you dedicated to this review process.
We have revised the manuscript according to your comments point-to-point and the response is presented below as the supplement.
Many thanks and best regards.
Guangyao Dai and Songhua Wu
On behalf of the co-authors
- The comparison of the 3 cross sections on 19 June 2020 is misleading (Section 4.2). With the 3 cross sections just some hours (<4 h) apart, you get a snapshot of an existing dust plume whose maximum is currently over the central Atlantic. Lower values of the backscatter coefficient above the Sahara and the Caribbean (cross section 1 and 3) can not be directly linked to emission and deposition (named by you “emission phase” and “deposition phase”). Usually, there are several days between emission and deposition and not just some hours. So, there is no benefit in reporting the backscatter values for the 3 cross sections. I would consider removing these values from the abstract and the conclusion.
-
RC2: 'Comment on acp-2022-53', Anonymous Referee #2, 19 Apr 2022
This paper describes the combination of satellite borne lidar data from two instruments in combination with model wind field data and back and forward trajectory analyses to investigate the advection of a major dust storm across the Atlantic Ocean. The paper focus on a case study of a major dust storm to assess how the combined CALIPSO and Aeolus satellite products can be combined with ECMWF driven trajectories to describe dust transport and loss. The paper was previously submitted to ACPD and this version has been considerably improved.
I do, however, have a major reservation about section 4.2 and the accompanying statements in the abstract and summary sections. Section 4.2 presents lidar curtains at three locations across the sub-tropical north Atlantic on a day in the middle of the dust storm. The three curtains are close t the source region, over the mid-Atlantic and towards the west, in the far-field of the plume. However, the satellite overpasses presented are taken only 3 hours apart. The advection times between the most easterly lidar curtain and the most westerly are of the order of a week or more. The data in section 4.2 show the overall geographical distribution of dust across the Atlantic as a snapshot on the morning of 19/6/2020. What they do not do is say anything at all about the dynamics of the dust plume as it advects across the Atlantic region. The source region may have changed or emissions of dust varied and the transport pathways may be affected by changing atmospheric conditions over the course of the event. However, section 4.2 assumes the dust plume is time invariant and describes the scene as representing different ages of the plume. This is misleading and in any case is described much better in section 4.3. Either section 4.2 should be rewritten to illustrate geographical variability at a single point in time or removed. Furthermore, the way the results from this section are presented in the abstract and summary should be reframed or removed as they are written as though the data were taken in a pseudo lagrangian way and they were not.
Specific recommendations
Lines 62-63: “Additionally, the CALIOP product Vertical Feature Mask product (VFM)” better to write
“Additionally, the CALIOP Vertical Feature Mask product (VFM)”Line 74 “(e)motion”
Line 170-174 “Based on the dataset consists of the backscatter coefficients and extinction coefficients at the wavelengths of 1064 nm and 532 nm from CALIOP and the extinction coefficients at the wavelength of 355 nm from ALADIN, the aerosol volume concentration distribution can be
calculated based on the regularization method which was performed by generalized cross-validation (GCV) from Müller et al. (1999).” A confusing sentence that needs to be rewritten
lines 240-241: Figure 4a shows the majority of the dust has been lifted to a maximum of around 7km or less south of 20N on 18/6/2020, there is only a small proportion of the dust at the far north end of the overpass that has a maximum close to 10 km. This probably needs rephrasing.
Lines 276-282: The narrative in the section assumes a pseudo-langragian language but the lidar passes are on the same day so these are different slices of a dust event that has lasted several days (fig 3) and has a transit time of multiple days between the overpasses shown in fig 5. The wording here needs to better reflect that these are cross sections at different geophysical locations in the plume and do not directly represent plume evolution. This discussion is extended to report values of backscatter and advection for different phases of the dust plume. However, these don’t reflect actual advection of the same air. The underlying assumption is the dust plume does not change with time. Clearly, this is not the case, so the determinations from the 3 different overpasses cant really be compared in the way that is done in the analysis in 4.2. At best this gives a snapshot of the plume at a single point in time across much of the Atlantic. This section needs to be rewritten in my view to make this clear and to convey why this is appropriate, otherwise it is best removed. This same approach is also followed up in the summary (402-406). The analysis is not pseudo-lagrangian and should not be inferred as such, the different phases of the storm were emitted many days apart and may have had very different conditions at source and during advection. This needs to be made explicit. The abstract also has the same errors between lines 22-25. This needs to be removed or corrected.
Line 293: “to calculate(d)”
Line 384: Affected not effected
-
AC2: 'Reply on RC2', Kangwen Sun, 11 May 2022
Dear Reviewer,
Many thanks for reviewing our manuscript. We greatly appreciate the substantial amount of time and effort that you dedicated to this review process.
We have revised the manuscript according to your comments point-to-point and the response is presented below as the supplement.
Many thanks and best regards.
Guangyao Dai and Songhua Wu
On behalf of the co-authors
-
AC2: 'Reply on RC2', Kangwen Sun, 11 May 2022
Guangyao Dai et al.
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