the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Transported aerosols regulate the pre-monsoon atmosphere over North-East India: a WRF-Chem modelling study
Neeldip Barman
Sharad Gokhale
Abstract. The study differentiates and quantifies the impacts of emissions emitted locally within North-East (NE) India region and those transported from outside this region to ascertain whether local or transported aerosols are more impactful in influencing this region’s atmosphere during the pre-monsoon season (March-April-May). Due to the existence of a declining pre-monsoon rainfall trend in NE India, the study also quantified the role of different aerosol effects w.r.t radiative forcing (RF) and rainfall. The study has been carried out using the WRF-Chem model by comparing simulation scenarios where emissions were turned on and off within and outside the NE region. The impact of all emissions as a whole and Black carbon (BC) specifically was studied. Results show that aerosols transported primarily from the Indo-Gangetic Plain (IGP) were responsible for 93.98 % of the PM10 mass over NE India’s atmosphere and 64.18 % of near-surface PM10 concentration. Transported aerosols contributed >50 % of BC, organic carbon, sulfate, nitrate, ammonium and dust aerosol concentration and hence a major contributor to air pollution. Hence, the aerosol effects were observed to be much greater with transported aerosols. Indirect aerosol effect was found to be the major effect and more impactful with transported aerosols that dominated both rainfall and RF, and suppressed rainfall significantly than the direct and semi-direct effect. However, the increase in direct radiative effects with an increase in transported BC counteracted the rainfall suppression caused by relevant processes of other aerosol effects. Thus, this study shows atmospheric transport to be an important process for this region as transported emissions, specifically from IGP were also found to have greater control over the region’s rainfall. Thus, emission control policies implemented in IGP will reduce air pollution as well as the climatic impacts of aerosols over the NE India region.
- Preprint
(1942 KB) -
Supplement
(1347 KB) - BibTeX
- EndNote
Neeldip Barman and Sharad Gokhale
Status: open (until 07 Apr 2023)
-
RC1: 'Comment on egusphere-2023-88', Anonymous Referee #1, 19 Feb 2023
reply
The study showed the aerosols effect on meteorology, mostly rainfall over North-East (NE) India, through numerical experiments conducted by WRF-Chem. Transported aerosols from the upwind Indo-Gangetic Plain were found to dominate both in concentration and aerosol indirect effects, the latter of which contributes to the suppression of rainfall in NE India. The authors have accomplished tremendous experiments to reveal the role of transported, locally-emitted, absorbing and scattering aerosols. However, the key point of the study is not adequately discussed and proved under the current presentation. Several questions below should be addressed.
The “pre-monsoon atmosphere” in the title seems not specific and less informative. What does the “atmosphere” refer to? Temperature, humidity, stability, precipitation or chemical components? According to the abstract and analysis in the paper, I assume the author intended to focus on the rainfall and explain its decreasing trend. In that case, you may consider narrowing down the title to “the decreasing rainfall”.
The above comment leads to my biggest concern of this study. The author stressed the aerosols effect on rainfall so much yet no figure regarding the rainfall is shown in the main text. The water vapor mixing ratio only indicates the availability or probability of precipitation, and the actual response of rainfall due to transported absorbing and scattering aerosols should be displayed. Is the aerosol indirect effect similar to or different from that over IGP in suppressing rainfall and why? Is there any evidence of water vapor transport from Bay of Bengal? The author may need more figures in the main text to elucidate the point.
As a research greatly based on numerical simulations, the illustration of the model experiments is much deficient with many important details missing. For example, what are the physical and chemical parameterizations of WRF-Chem? How did the author configure the model domain and which area is referred to as NE India? How did the author determine the anthropogenic and natural (i.e., dust and biogenic) emissions? When it comes to experiment with emissions only within NE India does it mean all emissions or mainly anthropogenic emissions? In addition, the evaluation of model performance against MERRA-2 is less convincing since MERRA-2 is also generated by global atmospheric model with assimilation system. In-situ observations concerning chemical components of the atmosphere should be involved in model verification.
Minors:
Line 11: change “and” to “on”.
Line 17: “the aerosols effects were observed to be …” this sentence may cause ambiguity since the aerosols effects in this study are all analyzed by models.
Line 119: what do you mean by “atmospheric distribution”?
Line 144: The BC and sulfate aerosols are all components of PM2.5. Why does the author focused on PM10 instead of PM2.5? How about the spatial distribution of PM2.5?
Figure 1: what do rectangles in panel(a) and (d) mean?
Line 123: what’s the height of level 0 and level 15 of your model? Please be specific.
Line 145: why is NE India separated into four sub-regions? Please explain.
Line 161: “… and However …” is not a complete sentence.
Figure 4 and 5: the title of X-axis is missing in these two plots.
Line 285: it is confusing why the author still discussed the role of locally emitted BC since chapter 3.3 just clarified the role of local aerosols is not important at all.
Line 425: the transported aerosols “exert different impacts” on cloud parameters and rainfall, not “show responses to”.
Citation: https://doi.org/10.5194/egusphere-2023-88-RC1 -
AC1: 'Reply on RC1', Sharad Gokhale, 24 Feb 2023
reply
1. The “pre-monsoon atmosphere” in the title seems not specific and less informative. What does the “atmosphere” refer to? Temperature, humidity, stability, precipitation, or chemical components? According to the abstract and analysis in the paper, I assume the author intended to focus on the rainfall and explain its decreasing trend. In that case, you may consider narrowing down the title to “the decreasing rainfall”.
Response: Rainfall was the primary objective of the study but as was observed from the results, the chemical components affected several components of the atmosphere starting from solar radiation, moisture availability, atmospheric instability and clouds which ultimately influenced rainfall. Hence to underline the importance of effects on a large number of components of the atmosphere as a whole, the word “atmosphere” was used. However, as per suggestion, the title has been changed to “Transported aerosols regulate the pre-monsoon rainfall over North-East India: a WRF-Chem modelling study” and hence “atmosphere” in line 9 also replaced by “rainfall”.
2. The above comment leads to my biggest concern of this study. The author stressed the aerosols effect on rainfall so much yet no figure regarding the rainfall is shown in the main text. The water vapor mixing ratio only indicates the availability or probability of precipitation, and the actual response of rainfall due to transported absorbing and scattering aerosols should be displayed. Is the aerosol indirect effect similar to or different from that over IGP in suppressing rainfall and why? Is there any evidence of water vapor transport from Bay of Bengal? The author may need more figures in the main text to elucidate the point.
Response:
- Figures showing rainfall response calculated from 4NOR-I – NOR-I and No_EMISS_NE_4SO2 - No_EMISS_NE_0.25SO2 has been added as Figure 9. 4NOR-I – NOR-I is not exactly the transported scenario but as seen from the results the local BC component was ineffective in affecting moisture or atmospheric stability and hence majorly represents the effect of transported BC component in it.
- Figure S10 has been moved to main text as Figure 8 which shows increased water vapor transport due to transported BC. A previous study (https://doi.org/10.1007/s00704-017-2057-1), referred in Section 3.4.2 also showed BC to increase moisture transport from Bay of Bengal into North East India and increase rainfall over this region. However, this study adds that the BC outside NE India is mainly responsible for it.
- Water vapor is the primary ingredient for rainfall but its conversion to rainfall is influenced by the aerosols, acting as cloud condensation nuclei (CCN). The presence of large amount of CCN facilitates condensation of water vapor on numerous aerosol particles, producing numerous cloud droplets with smaller radii. This restricts small cloud droplets to grow in size due to reduction in interaction with other cloud droplets which affect its conversion to rain droplet and thus to rainfall. This is termed as aerosol indirect effect. However, BC was found to be inefficient as CCN when its emission was increased from NOR-I to 4NOR-I. Hence the large increase in moisture, caused by the increase in atmospheric instability possibly condensed on relatively a smaller number of CCN particles promoting larger cloud droplets which enhanced rainfall. This has been added to the main text in Section 3.3 and 3.5
- Rainfall suppression due indirect aerosol effect is defined similarly irrespective of location and hence also same for IGP. The mechanism is explained in the above point.
3. As a research greatly based on numerical simulations, the illustration of the model experiments is much deficient with many important details missing. For example, what are the physical and chemical parameterizations of WRF-Chem? How did the author configure the model domain and which area is referred to as NE India? How did the author determine the anthropogenic and natural (i.e., dust and biogenic) emissions? When it comes to experiment with emissions only within NE India does it mean all emissions or mainly anthropogenic emissions? In addition, the evaluation of model performance against MERRA-2 is less convincing since MERRA-2 is also generated by global atmospheric model with assimilation system. In-situ observations concerning chemical components of the atmosphere should be involved in model verification.
Response:
- Better description of simulations has been added in Section 2 for NOR, NOR-I and some other simulation experiments. Remaining simulations can be understood from Table 1 by it comparing with it.
- Physical and chemical parametrizations have been added in Section 2.
- The NE India region description has been added in Section 2 .The model domain is shown in Fig. 1(a) and the NE India is the part of India within the region bounded by the blue box. The region within the box is bounded by 22° N and 29° N latitudes and 89° E and 97° E longitudes.
- During the study period the near surface wind flow was from the Bay of Bengal towards NE India, which gradually changed to westerly wind flow carrying aerosols from IGP towards NE India. Hence the domain was selected by keeping the NE India region near the upper-right corner of the domain. This has been added in Section 2.
- Details regarding emissions and model inputs have been added in Section 2.
- All emissions (natural and anthropogenic) were considered with emissions only within NE India and mentioned in Section 2.
- Comparison against in-situ observations have been included in Section 2 and performance statistics included in Table S3.
Minors:
4. Line 11: change “and” to “on”.
Response: Changed “and” to “on”.
5. Line 17: “the aerosols effects were observed to be …” this sentence may cause ambiguity since the aerosols effects in this study are all analyzed by models.
Response: Removed “observed to be”.
6. Line 119: what do you mean by “atmospheric distribution”?
Response: It meant the three-dimensional distribution. For better understanding, it has been replaced by “vertical distribution”.
7. Line 144: The BC and sulfate aerosols are all components of PM5. Why does the author focused on PM10 instead of PM2.5? How about the spatial distribution of PM2.5?
Response: This study is mainly a climatic study where aerosols of all size ranges have climatic impacts. Also, the maximum size supported in the model is 10 µm. Hence PM10 mass was reported in order to later relate with the impacts. The spatial and vertical distribution of PM2.5 was found to be more or less similar to the PM10 with NOR-I, No_EMISS_NE and Only_EMISS_NE, albeit with lower concentration values.
8. Figure 1: what do rectangles in panel(a) and (d) mean?
Response:
- Rectangle in Figure 1(a) includes the North-East region of India. See response #3 to comment #3.
- The North-East India region was divided into 4 sub-regions, which is shown in Figure 1(d) and better description regarding it has been added in Section 3.1. See response to comment #10.
9. Line 123: what’s the height of level 0 and level 15 of your model? Please be specific.
Response: The height of the model levels vary spatially according to the height of the terrain as the model levels are terrain-following. Hence any model level does not have any specific height. The height of the model level is the geopotential height and the spatial distribution of it are provided in the supplementary Figure S1. The height of concentration values for Figure 1(a), 1(b) and 1(c) is Figure S1(a) and similarly the height of concentration values for Figure 1(d), 1(e) and 1(f) is Figure S1(b).
10. Line 145: why is NE India separated into four sub-regions? Please explain.
Response: The NE India region was divided into four sub-regions based on the proximity from the IGP. Region 1 and region 2 fall along the Brahmaputra River valley, but with region 1 being closest to IGP. Region 3 is mostly a mountainous region. Region 4 is the southern region closer to the Bay of Bengal. Better description regarding it has been added in Section 3.1
11. Line 161: “… and However …” is not a complete sentence.
Response: Correction done.
12. Figure 4 and 5: the title of X-axis is missing in these two plots.
Response: Titles have been added.
13. Line 285: it is confusing why the author still discussed the role of locally emitted BC since chapter 3.3 just clarified the role of local aerosols is not important at all.
Response: The analysis with the locally emitted BC provided a further verification of the results in Section 3.3. Moreover, the analysis in Section 3.4 provides a comparison of the impacts of local and transported BC which showed that increase in locally emitted BC emissions has much lesser impact than transported BC and hence more focus can be given on the regions responsible for the transported BC, i.e. IGP.
14. Line 425: the transported aerosols “exert different impacts” on cloud parameters and rainfall, not “show responses to”.
Response: Correction done.
Citation: https://doi.org/10.5194/egusphere-2023-88-AC1
-
AC1: 'Reply on RC1', Sharad Gokhale, 24 Feb 2023
reply
Neeldip Barman and Sharad Gokhale
Neeldip Barman and Sharad Gokhale
Viewed
| HTML | XML | Total | Supplement | BibTeX | EndNote | |
|---|---|---|---|---|---|---|
| 287 | 57 | 11 | 355 | 20 | 5 | 4 |
- HTML: 287
- PDF: 57
- XML: 11
- Total: 355
- Supplement: 20
- BibTeX: 5
- EndNote: 4
Viewed (geographical distribution)
| Country | # | Views | % |
|---|
| Total: | 0 |
| HTML: | 0 |
| PDF: | 0 |
| XML: | 0 |
- 1