the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Jul 2023
Impact of urban land use on mean and heavy rainfall during the Indian Summer Monsoon
Renaud Falga
Abstract. Northern India has been subjected to an intense urbanization since the middle of the twentieth century. The impact of such a drastic land-use change on regional weather and climate remains to be assessed. In this work, we study the impact of the modification of land-use – from vegetation to urban – on the Indian summer monsoon rainfall, as well as other meteorological variables. We use the regional meteorology model Meso-NH coupled with an urban module (the Town Energy Balance model) to perform month-long sensitivity simulations centered around Kolkata, the most urbanized area in Northeastern India. Paired simulations, one with and another without urban settings have been performed to identify the impacts related to urbanization through both thermodynamic and kinetic effects. We find that the perturbation induced by urban land-use enhances the mean rainfall over the model domain, principally by intensifying the convective activity through thermodynamical perturbation, leading to an increase of 14.4 % of the monthly mean rainfall. In addition, the modeling results demonstrate that not only does the urban area act in general as a rainfall enhancer, particularly during nighttime, but it also induces the generation of a specific storm in one modeled case that would not have formed in the absence of the urban area. This storm initiation over the city was done primarily by urban terrain’s disturbance of the near surface wind flow, leading to a surge of dynamically produced turbulent kinetic energy (TKE). The thermal production of TKE over the nighttime urban boundary layer, on the other hand, serves as a contributing factor to the storm formation.
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Renaud Falga and Chien Wang
Status: closed
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RC1: 'Comment on egusphere-2023-1445', Anonymous Referee #1, 03 Sep 2023
The manuscript titled"Impact of urban land use on mean and heavy rainfall during the Indian Summer Monsoon" (egusphere-2023-1445) used the meso-scale regional meteorology model coupled with an urban module to explore the impacts of land use change on heavy rainfall during Indian summer monsoon. The authors focused on two heavy rainfall events within a summer month, conducted the sensitivity simulation with both Urban and non-Urban settings. They also revealed the thermodynamic and dynamic mechanisms of the urban effect on rainfall. The manuscript examines an important scientific topic for urban affect the rainfall, which can be complex and challenging. However, there are some issues require additional attention and improvement.
Comments:
1. Line 51-52: "Furthermore, vegetation tends to have a cooling effect on its surrounding environment through evapotranspiration and latent heat release,". Line203-205: "As the release of latent heat through evaporation causes a cooling of the atmosphere, the UHI effect is further strengthened by such a reduction of latent heat flux.". These sentences are misleading, evaporation does not release latent heat, but condensation does. Also, the release of latent heat results the heating of the atmosphere.
2. Line 97: the word "characteristic" and "features" are repeating.
3. Line 161, in subtitle 2.3, "Model comparison with observations". In fact, the model simulated rainfall was compared with the ERA5 reanalysis data in themanuscript. In my opinion, the reanalysis data is not representative of the real rainfall observations, there are some uncertainties when compared to observations. Why the authors used the reanalysis data here instead of the rain gauge observation? What about the uncertainties of the ERA5 rainfall? Line 162, "the model has largely reproduced the evolution of domain-averaged hourly rainfall time series in ERA5 (Fig. 2)."This is a subjective and qualitative description. In Fig.2, the model captured the rainfall well only in part of the period, such as the two major heavy rainfall periods. For those days with small magnitude of precipitation, the simulation bias is evident. Especially for the period after heavy rainfall, the model and ERA5 exhibit almost opposite phases. As we all know, accurately simulating rainfall is challenging, the authors may provide some quantitative evaluation and cite previous studies to demonstrate the plausibility of these simulation results.
4. In this reviewer’s opinion, the inconsistency between the use of a 12-hour clock format in the text (e.g., 2pm, 7am, et.) and a 24-hour clock format in the figures could make reading less convenient.
5. Line 216-217: it seems that the UHI reached the maximum at 9 pm in Figure 3b. Line 219: the UHI intensity starts to decrease after 9 pm in the Figure 3, I guess the author means that the UHI starts to rapidly decrease after midnight.
6. Line 226-227: Indeed, the PBL decreases quickly in both Urban and non-Urban runs, while remains relatively higher in the Urban run.
7. Line 238: "4 and 5 pm"->"5 and 4 pm".
8. Line 242: "This result is another indicator of the residual instability of the nighttime urban boundary layer. ", what does the sentence mean?
9. Line 264-265: " This rainfall enhancement by the urban setting appears to be even more intense over the city itself". However,from Fig.S1, it seems that the urban effect on rainfall enhancement is more pronounced in the surrounding areas rather than the urban itself. Is there any possible explanation?
10. Line 287: There is inconsistent city name, "Kolkata area" in manuscript and"Calcutta Area" in Figure.S2.
11. In Section 4, the first two rainfall peaks exhibit less differences between Urban and non-Urban runs, but the third peak shows evident difference. Is this phenomenon solely due to the first two rainfall events occurred during the daytime? Urban effects exist both during the day and at night. Also, the barrier effect of urban dose not disappear during the daytime.Why are the impacts of urban effects on the rainfall associated with the first two rainfall peaks almost negligible, is there further in-depth reasons for this? For example, the urban area blocks the evaporation and transport of water vapor, compared to non-urban areas represented by grasslands in this study. When the path of a storm is parallel to the orientation of a city, especially in the case of the strip-shaped city described in this paper, the barrier effect of the city may be minimal. Additionally, it can be noticed that the rainfall is presented in a regional average manner, while the spatial scale of the impacts generated by urban effects is often relatively small. Within a larger spatial domain, is there a possibility that the rainfallenhancement is offset by an urban effect induced rainfall reduction? In addition, as the geographical specifics of the Indian Summer Monsoon, it is needed to highlight the influences of the Tibetan Plateau and the Indian Ocean Sea on clouds, aerosols, and circulation if possible. Recent studies have focused on these issues (such as, wang et al., 2022, JGR ; Liu et al., 2022,ESR）. The authors may add a little discussions.
12. In Section4.4, a fifth peak of rainfall during the first heavy rain period (July 1st to July 5th), which appears more intense rainfall in the non-Urban run. The authors analyzed the corresponding CAPE time series and concluded that the rainfall during the fourth peakconsumed the CAPE energy, resulting in the opposite rainfall differences during the fifth peak. The CAPE in the Urban run is indeed lower than that in the non-Urban run during the rainfall processes of the fourth and fifth peak.Nevertheless, CAPE is not the only factor, as the CAPE corresponding to the fifth peak is not significantly lower in Urban run compared to non-Urban run. Additionally, during the rainfall on July 1st-2nd, high CAPE in the Urban runeven corresponded to low precipitation in the Urban run.From a mechanistic perspective, the consumption of CAPE does reduce the energy available to trigger convection. However, could there be additional reasons, such as the fourth peak's consumption of the available precipitation leading to insufficient moisture supply for rainfall in the fifth peak?
Citation: https://doi.org/10.5194/egusphere-2023-1445-RC1 -
RC2: 'Comment on egusphere-2023-1445', Madhusmita Swain, 20 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1445/egusphere-2023-1445-RC2-supplement.pdf
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AC1: 'Comment on egusphere-2023-1445', Renaud Falga, 29 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1445/egusphere-2023-1445-AC1-supplement.pdf
Status: closed
-
RC1: 'Comment on egusphere-2023-1445', Anonymous Referee #1, 03 Sep 2023
The manuscript titled"Impact of urban land use on mean and heavy rainfall during the Indian Summer Monsoon" (egusphere-2023-1445) used the meso-scale regional meteorology model coupled with an urban module to explore the impacts of land use change on heavy rainfall during Indian summer monsoon. The authors focused on two heavy rainfall events within a summer month, conducted the sensitivity simulation with both Urban and non-Urban settings. They also revealed the thermodynamic and dynamic mechanisms of the urban effect on rainfall. The manuscript examines an important scientific topic for urban affect the rainfall, which can be complex and challenging. However, there are some issues require additional attention and improvement.
Comments:
1. Line 51-52: "Furthermore, vegetation tends to have a cooling effect on its surrounding environment through evapotranspiration and latent heat release,". Line203-205: "As the release of latent heat through evaporation causes a cooling of the atmosphere, the UHI effect is further strengthened by such a reduction of latent heat flux.". These sentences are misleading, evaporation does not release latent heat, but condensation does. Also, the release of latent heat results the heating of the atmosphere.
2. Line 97: the word "characteristic" and "features" are repeating.
3. Line 161, in subtitle 2.3, "Model comparison with observations". In fact, the model simulated rainfall was compared with the ERA5 reanalysis data in themanuscript. In my opinion, the reanalysis data is not representative of the real rainfall observations, there are some uncertainties when compared to observations. Why the authors used the reanalysis data here instead of the rain gauge observation? What about the uncertainties of the ERA5 rainfall? Line 162, "the model has largely reproduced the evolution of domain-averaged hourly rainfall time series in ERA5 (Fig. 2)."This is a subjective and qualitative description. In Fig.2, the model captured the rainfall well only in part of the period, such as the two major heavy rainfall periods. For those days with small magnitude of precipitation, the simulation bias is evident. Especially for the period after heavy rainfall, the model and ERA5 exhibit almost opposite phases. As we all know, accurately simulating rainfall is challenging, the authors may provide some quantitative evaluation and cite previous studies to demonstrate the plausibility of these simulation results.
4. In this reviewer’s opinion, the inconsistency between the use of a 12-hour clock format in the text (e.g., 2pm, 7am, et.) and a 24-hour clock format in the figures could make reading less convenient.
5. Line 216-217: it seems that the UHI reached the maximum at 9 pm in Figure 3b. Line 219: the UHI intensity starts to decrease after 9 pm in the Figure 3, I guess the author means that the UHI starts to rapidly decrease after midnight.
6. Line 226-227: Indeed, the PBL decreases quickly in both Urban and non-Urban runs, while remains relatively higher in the Urban run.
7. Line 238: "4 and 5 pm"->"5 and 4 pm".
8. Line 242: "This result is another indicator of the residual instability of the nighttime urban boundary layer. ", what does the sentence mean?
9. Line 264-265: " This rainfall enhancement by the urban setting appears to be even more intense over the city itself". However,from Fig.S1, it seems that the urban effect on rainfall enhancement is more pronounced in the surrounding areas rather than the urban itself. Is there any possible explanation?
10. Line 287: There is inconsistent city name, "Kolkata area" in manuscript and"Calcutta Area" in Figure.S2.
11. In Section 4, the first two rainfall peaks exhibit less differences between Urban and non-Urban runs, but the third peak shows evident difference. Is this phenomenon solely due to the first two rainfall events occurred during the daytime? Urban effects exist both during the day and at night. Also, the barrier effect of urban dose not disappear during the daytime.Why are the impacts of urban effects on the rainfall associated with the first two rainfall peaks almost negligible, is there further in-depth reasons for this? For example, the urban area blocks the evaporation and transport of water vapor, compared to non-urban areas represented by grasslands in this study. When the path of a storm is parallel to the orientation of a city, especially in the case of the strip-shaped city described in this paper, the barrier effect of the city may be minimal. Additionally, it can be noticed that the rainfall is presented in a regional average manner, while the spatial scale of the impacts generated by urban effects is often relatively small. Within a larger spatial domain, is there a possibility that the rainfallenhancement is offset by an urban effect induced rainfall reduction? In addition, as the geographical specifics of the Indian Summer Monsoon, it is needed to highlight the influences of the Tibetan Plateau and the Indian Ocean Sea on clouds, aerosols, and circulation if possible. Recent studies have focused on these issues (such as, wang et al., 2022, JGR ; Liu et al., 2022,ESR）. The authors may add a little discussions.
12. In Section4.4, a fifth peak of rainfall during the first heavy rain period (July 1st to July 5th), which appears more intense rainfall in the non-Urban run. The authors analyzed the corresponding CAPE time series and concluded that the rainfall during the fourth peakconsumed the CAPE energy, resulting in the opposite rainfall differences during the fifth peak. The CAPE in the Urban run is indeed lower than that in the non-Urban run during the rainfall processes of the fourth and fifth peak.Nevertheless, CAPE is not the only factor, as the CAPE corresponding to the fifth peak is not significantly lower in Urban run compared to non-Urban run. Additionally, during the rainfall on July 1st-2nd, high CAPE in the Urban runeven corresponded to low precipitation in the Urban run.From a mechanistic perspective, the consumption of CAPE does reduce the energy available to trigger convection. However, could there be additional reasons, such as the fourth peak's consumption of the available precipitation leading to insufficient moisture supply for rainfall in the fifth peak?
Citation: https://doi.org/10.5194/egusphere-2023-1445-RC1 -
RC2: 'Comment on egusphere-2023-1445', Madhusmita Swain, 20 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1445/egusphere-2023-1445-RC2-supplement.pdf
-
AC1: 'Comment on egusphere-2023-1445', Renaud Falga, 29 Oct 2023
The comment was uploaded in the form of a supplement: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1445/egusphere-2023-1445-AC1-supplement.pdf
Renaud Falga and Chien Wang
Renaud Falga and Chien Wang
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