Air Stagnation in the Nile Delta (1980&ndash;2020): An Assessment of Its Spatio-Temporal Patterns

Badawy, Hesham; Ali, Samia

doi:10.5194/egusphere-2026-3024

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https://doi.org/10.5194/egusphere-2026-3024

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06 Jul 2026

| 06 Jul 2026

Air Stagnation in the Nile Delta (1980–2020): An Assessment of Its Spatio-Temporal Patterns

Hesham Badawy and Samia Ali

Abstract. Air stagnation occurs when weak horizontal ventilation and limited vertical mixing inhibit renewal of the lower atmosphere, allowing heat and pollutants to accumulate within the boundary layer. Despite the climatic and demographic importance of the Nile Delta, the long-term behavior of stagnation across this densely populated Mediterranean–continental transition zone remains poorly documented. This study provides the first multi-decadal assessment of stagnation dynamics over the Nile Delta during 1980–2020 using ERA5 reanalysis data at 0.25° × 0.25° spatial resolution. Stagnation days were identified using the Air Stagnation Index defined by thresholds of near-surface wind speed (U₁₀ < 3.2 m s⁻¹), 500-hPa wind speed (< 13 m s⁻¹), and daily precipitation below 1 mm. ERA5 fields were evaluated against regional meteorological observations, showing strong agreement in wind speed and precipitation variability and confirming the suitability of ERA5 for stagnation diagnostics. The results reveal a pronounced coastal–inland gradient in stagnation frequency. Annual means exceed 150 days in the southern sector (Cairo) and about 135 days in the central Delta (Tanta), whereas coastal locations such as Alexandria experience roughly 30 days per year. Air stagnation frequency increased significantly by about 0.7 day per decade during 1980–2020, with stronger trends inland. Stagnation frequency shows a strong inverse relationship with near-surface wind speed (r = −0.84), indicating that horizontal ventilation appears to exert a primary dynamical control on stagnation variability across the Delta. These findings identify the Nile Delta as one of the most stagnation-prone non-orographic coastal plains in the eastern Mediterranean.

Received: 27 May 2026 – Discussion started: 06 Jul 2026

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Hesham Badawy and Samia Ali

Status: final response (author comments only)

RC1:
'Comment on egusphere-2026-3024', Anonymous Referee #1, 11 Aug 2026

The paper entitled “Air Stagnation in the Nile Delta (1980–2020): An Assessment of Its Spatio-Temporal Patterns” presents an interesting and valuable study of the Nile Delta, a critical region prone to pollutant accumulation. The research provides important insights into the spatial and temporal variability of air stagnation and its potential implications for air quality and environmental management in the area.

1) Introduction: The literature review should be expanded to include air stagnation and pollution accumulation studies from other Mediterranean critical areas. The Po Valley represents a well-known hotspot for stagnant atmospheric conditions and associated air quality issues. Relevant references such as Colombo et al. (2025) and Verratti et al. (2024) should be discussed to better contextualize the Nile Delta within the broader Mediterranean framework.

2) Figure 1(c): Please include the locations of the stations used for validation. Showing these monitoring sites on the map would provide a clearer picture of their spatial coverage and improve the transparency and reproducibility of the validation procedure.

3) Table 1: Please provide the validation statistics separately for the winter and summer seasons, in addition to the annual values. This would help readers evaluate the model's ability to reproduce seasonal variability and assess the level of agreement between observations and simulations during the most relevant periods.

4) Validation procedure: Please clarify whether the stations used for validation were included in the reanalysis assimilation process. If these observations contributed to the generation of the reanalysis product, the agreement between the two datasets may not represent a fully independent validation. A clear statement on this issue should be added to the manuscript.

5) Figure 3: Figure 3 is informative and highlights the spatial distribution of air stagnation; however, I would also like to see a time series showing the annual mean number of stagnation days for each year during the study period (1980–2020). Such a figure would enable a clearer assessment of interannual variability and long-term trends, allowing the authors to discuss whether air stagnation has changed over time and whether any detectable signals may be associated with climate change.

Furthermore, the manuscript would benefit from a more explicit discussion of the implications of air stagnation for air quality. Since stagnant atmospheric conditions are generally associated with reduced pollutant dispersion and enhanced pollutant accumulation, it would be valuable to investigate this relationship using available air quality observations (e.g., PM₁₀, PM₂.₅, NO₂, or ozone data). Correlating stagnation days with air pollution levels, at least for selected monitoring stations or representative areas, could provide important insights into the environmental relevance of the identified stagnation patterns and strengthen the practical impact of the study. Such an analysis would help demonstrate whether periods of increased stagnation are effectively linked to deteriorated air quality in the Nile Delta.

Due to these comments, i suggest major revisions to reconsider the paper for pubblications.

Citation: https://doi.org/10.5194/egusphere-2026-3024-RC1
- AC1: 'Reply on RC1', Hesham Badawy, 15 Aug 2026
  
  Responses to Reviewer #1
  We thank the Reviewer for the careful assessment of the manuscript. We have addressed the comments through revisions to the analysis, interpretation and presentation, as detailed below.
  
  1 - Comment: Introduction: The literature review should be expanded to include air stagnation and pollution accumulation studies from other Mediterranean critical areas. The Po Valley represents a well-known hotspot for stagnant atmospheric conditions and associated air quality issues. Relevant references such as Colombo et al. (2025) and Verratti et al. (2024) should be discussed to better contextualize the Nile Delta within the broader Mediterranean framework.
  Response: We have expanded the Introduction to place the Nile Delta within the broader Mediterranean context of air stagnation and air-quality impacts. The revised text now discusses the Po Valley and incorporates Verratti et al. (2023) and Colombo et al. (2025), highlighting the links between stagnant conditions, pollutant accumulation, and atmospheric ventilation.
  2 - Comment: 2) Figure 1(c): Please include the locations of the stations used for validation. Showing these monitoring sites on the map would provide a clearer picture of their spatial coverage and improve the transparency and reproducibility of the validation procedure.
  Response: Response: The locations of the three validation stations have now been added to Fig. 1c alongside the analysis grid points, thereby making their spatial coverage relative to the analysis domain explicit and improving the transparency and reproducibility of the validation procedure.
  
  3 - Comment: Table 1: Please provide the validation statistics separately for the winter and summer seasons, in addition to the annual values. This would help readers evaluate the model's ability to reproduce seasonal variability and assess the level of agreement between observations and simulations during the most relevant periods.
  Response: We have revised Table 1 to report validation statistics for the full period and separately for winter (DJF) and summer (JJA), using the same temporally matched daily ERA5–GSOD pairs. The revised table now provides r, R², RMSE, MBE, d₁, and p for each station, variable, and period, thereby enabling a direct assessment of the seasonal performance of ERA5.
  
  Section 3.4 in the manuscript now explicitly states that the validation statistics were calculated for the full period and separately for DJF and JJA from the same temporally matched daily observations. The Results section has also been revised to report the principal seasonal patterns identified in the validation (Table 1).
  4 - Comment: Validation procedure: Please clarify whether the stations used for validation were included in the reanalysis assimilation process. If these observations contributed to the generation of the reanalysis product, the agreement between the two datasets may not represent a fully independent validation. A clear statement on this issue should be added to the manuscript.
  Response: We thank the reviewer for raising this important point. Because ERA5 assimilates a broad range of surface observations, including station-based measurements, the GSOD records cannot be assumed to be fully independent of the reanalysis without station-level information on the observations assimilated by the ECMWF system. We have therefore clarified in Section 3.4 that the comparison is treated as a station-based evaluation rather than a strictly independent validation.
  5 - Comment: Figure 3: Figure 3 is informative and highlights the spatial distribution of air stagnation; however, I would also like to see a time series showing the annual mean number of stagnation days for each year during the study period (1980–2020). Such a figure would enable a clearer assessment of interannual variability and long-term trends, allowing the authors to discuss whether air stagnation has changed over time and whether any detectable signals may be associated with climate change.
  Response: We have added the requested annual time series of Delta-wide mean stagnation days for 1980–2020 as new Figure 6, allowing the interannual variability and long-term tendency to be assessed explicitly. The series shows a positive Sen’s slope of +2.41 days decade⁻¹; however, the Mann–Kendall test indicates that this monotonic tendency is not statistically significant (τ = 0.153, p = 0.160), with the 95% confidence interval for the slope (−0.96 to +4.40 days decade⁻¹) encompassing zero. We therefore interpret the result as a positive estimated tendency rather than evidence of a robust long-term regional increase. The trend analysis, including correction for serial autocorrelation, and its interpretation have been incorporated into Sections 3.3, 3.5, and 4.3.1, respectively.
  6 - Comment: Furthermore, the manuscript would benefit from a more explicit discussion of the implications of air stagnation for air quality. Since stagnant atmospheric conditions are generally associated with reduced pollutant dispersion and enhanced pollutant accumulation, it would be valuable to investigate this relationship using available air quality observations (e.g., PM₁₀, PM₂.₅, NO₂, or ozone data). Correlating stagnation days with air pollution levels, at least for selected monitoring stations or representative areas, could provide important insights into the environmental relevance of the identified stagnation patterns and strengthen the practical impact of the study. Such an analysis would help demonstrate whether periods of increased stagnation are effectively linked to deteriorated air quality in the Nile Delta.
  Response: We thank the reviewer for this important suggestion. We have added a focused discussion of the air-quality implications of the identified stagnation regime, explaining how reduced atmospheric ventilation can favor pollutant accumulation while recognizing that responses may differ between particulate matter and ozone. Because pollutant observations were not included in the present dataset, a quantitative stagnation–pollutant analysis is beyond the scope of the present study; we have therefore avoided inferring pollutant exceedances from the stagnation climatology alone. We instead identify the coupling of stagnation-day classifications with observed PM₂.₅, PM₁₀, NO₂ and O₃ concentrations as an important next step for quantifying the air-quality significance of the identified regime. The corresponding discussion has been added immediately after the paragraph describing the strong inverse relationship between stagnation frequency and near-surface wind speed.
  
  Citation: https://doi.org/10.5194/egusphere-2026-3024-AC1
RC2:
'Comment on egusphere-2026-3024', Anonymous Referee #2, 12 Aug 2026
General Comment
The paper presents an analysis of stagnation conditions in the Nile Delta region in the period 1980-2020 from the analysis of ERA5 reanalysis data, in particular 10-m and 500-hPa wind speed and precipitation. In my opinion, the novelty and originality of the work are not sufficient to warrant publication in Weather and Climate Dynamics. Moreover, the analysis is not developed in sufficient depth. For example, the manuscript would benefit from a more thorough investigation of the dynamical mechanisms responsible for the development and persistence of stagnation periods. In addition, the use of the ERA5 dataset limits the ability to investigate local-scale differences in the frequency and duration of stagnation events. Thermal effects, particularly buoyancy, are not taken into account, although they may represent an important mechanism for pollutant removal, especially in warm or hot climates. Furthermore, the Authors refer several times in the manuscript to static stability and boundary-layer development, yet these processes are not explicitly evaluated in the analysis.
Specific Comments
In many parts of the manuscript, the Authors cite thermal stability as a crucial factor for pollutant dispersion. This is of course true, but this aspect is not considered in the paper. See, for example, lines 426, 646, 654, 663, 671, 680, 682, 802… In my opinion, this represents a weakness of the manuscript. The Authors show that, in this geographic context, ASI is largely driven by 10-m wind speed (see Fig. 9). Consequently, the reported results mainly reflect the spatial and temporal variability of near-surface wind speed. However, the relationship between ASI and pollutant concentrations, and therefore the actual added value and usefulness of the index, is not demonstrated in the manuscript.

In several parts of the manuscript, the Authors draw conclusions that are not adequately supported by the analyses presented. This is particularly evident for statements concerning changes in synoptic forcing and boundary-layer development, stability, and turbulence, which are discussed without being directly investigated or quantified.

At line 200, the Authors state that ERA5 is available from 1979. Actually, ERA5 is available from 1940 and ERA5 Land from 1950. Furthermore, it is not clear which fields were extracted from ERA5 Land.

Some sentences are repeated in the manuscript, occasionally in very close proximity (e.g., lines 291–294 and 331–334). More generally, several concepts are reiterated throughout the manuscript, making it unnecessarily lengthy relative to the findings presented.

The comparison between ERA5 and station observations provides good results, but the Authors should specify if the three stations are assimilated into ERA5. In this case, of course, the two datasets are not independent.

The Authors refer to a secondary peak in the ASI days frequency in winter (see line 378), but this cannot be clearly seen from the results presented.

Technical corrections
Line 249: “Airport.ERA5”: space missing.
Line 359: “climatology. These”: capitalize “These”.
Figure 4: please use the same scale for all the panels to help comparability.
Line 412: “that Favour”: “Favour” lower case.
Section “Spatial trends”: The Authors mention several cities whose geographic locations may not be familiar to many readers. I suggest adding these cities to the maps to facilitate interpretation of the spatial patterns.
Line 581: These variables are not the principal components of ASI; rather, they are all the components used to calculate ASI.
Line 786: “perspective, These”: “These” lower case.
Line 801: “stagnation. where”. substitute the full stop with a comma.
Line 941: I would not call ERA5 “high-resolution”.
Citation: https://doi.org/10.5194/egusphere-2026-3024-RC2
- AC2: 'Reply on RC2', Hesham Badawy, 18 Aug 2026
  
  responses to Reviewer # 2
  We thank the Reviewer for the careful and constructive assessment of the manuscript. The comments have helped us clarify the analysis, strengthen the interpretation, and define more precisely the scope of the study. We address each comment below and indicate the corresponding revisions in the manuscript.
  General Comment
  1 - Comment: The paper presents an analysis of stagnation conditions in the Nile Delta region in the period 1980-2020 from the analysis of ERA5 reanalysis data, in particular 10-m and 500-hPa wind speed and precipitation. In my opinion, the novelty and originality of the work are not sufficient to warrant publication in Weather and Climate Dynamics. Moreover, the analysis is not developed in sufficient depth. For example, the manuscript would benefit from a more thorough investigation of the dynamical mechanisms responsible for the development and persistence of stagnation periods. In addition, the use of the ERA5 dataset limits the ability to investigate local-scale differences in the frequency and duration of stagnation events. Thermal effects, particularly buoyancy, are not taken into account, although they may represent an important mechanism for pollutant removal, especially in warm or hot climates. Furthermore, the Authors refer several times in the manuscript to static stability and boundary-layer development, yet these processes are not explicitly evaluated in the analysis.
  Response: Response: We thank the Reviewer for this careful assessment. We have revised the manuscript to strengthen the quantitative analysis and to define more precisely the scope of the study. The revised analysis quantifies the spatial relationship between stagnation frequency and all meteorological components of the ASI. Near-surface wind speed shows the strongest observed association (r = −0.839, p < 0.001), whereas 500-hPa wind speed and dry-day frequency show no statistically significant spatial association. These results, together with the revised characterization of the spatial, seasonal and temporal variability of stagnation, provide a more quantitative basis for the regional climatology.
  We have also revised the dynamical interpretation throughout. Static stability, buoyancy, turbulent mixing, boundary-layer depth and synoptic circulation are no longer presented as demonstrated controls because they were not directly diagnosed. The ASI is instead treated as a climatological diagnostic of meteorological stagnation and ventilation, rather than of boundary-layer structure or pollutant exposure. The potential contribution of these unresolved processes is now identified as a direction for future process-based investigation. We have further clarified the regional scale of the ERA5 analysis, removed the term “high-resolution”, and described the station comparison as a station-based evaluation rather than an independent validation.
  The manuscript has also been streamlined to remove repetition and to distinguish more clearly between empirical results, interpretation and limitations. Its contribution is now defined more precisely as a quantitative regional climatology of air stagnation across the Nile Delta, resolving its spatial, seasonal and temporal variability and identifying near-surface ventilation as the strongest observed correlate, while explicitly delimiting the mechanisms that cannot be established from the present data.
  
  Specific Comments
  2 - Comment: In many parts of the manuscript, the Authors cite thermal stability as a crucial factor for pollutant dispersion. This is of course true, but this aspect is not considered in the paper. See, for example, lines 426, 646, 654, 663, 671, 680, 682, 802… In my opinion, this represents a weakness of the manuscript. The Authors show that, in this geographic context, ASI is largely driven by 10-m wind speed (see Fig. 9). Consequently, the reported results mainly reflect the spatial and temporal variability of near-surface wind speed. However, the relationship between ASI and pollutant concentrations, and therefore the actual added value and usefulness of the index, is not demonstrated in the manuscript.
  Response: We agree with the Reviewer that thermal stability is physically relevant to stagnation and pollutant dispersion, but it is not directly diagnosed in the present analysis. We have therefore revised the manuscript to separate observed relationships from mechanisms that remain unresolved. The revised analysis shows that near-surface wind speed is the strongest observed correlate of stagnation frequency (r = −0.839), whereas 500-hPa wind speed and dry-day frequency show no statistically significant spatial association. We now identify reduced near-surface ventilation as the clearest observed dimension of stagnation variability, while explicitly acknowledging that the contributions of static stability, buoyancy, turbulent mixing and boundary-layer depth cannot be quantified from the present diagnostics.
  We have also clarified the interpretation of the ASI with respect to air quality. The index is used here as a climatological diagnostic of meteorological stagnation, not as a direct measure of boundary-layer structure or pollutant concentrations. Accordingly, we no longer interpret stagnation as evidence of elevated pollution. We instead state that its air-quality relevance remains to be tested against collocated observations of PM₂.₅, PM₁₀, NO₂ and O₃. Establishing that relationship would require a dedicated analysis beyond the objectives of the present climatological study.
  These revisions have been incorporated in Sections 4.3.3, 4.3.4 and 5, and in the Conclusion.
  3 - Comment: In several parts of the manuscript, the Authors draw conclusions that are not adequately supported by the analyses presented. This is particularly evident for statements concerning changes in synoptic forcing and boundary-layer development, stability, and turbulence, which are discussed without being directly investigated or quantified.
  Response: We thank the reviewer for this important point. We have revised the manuscript throughout to align the interpretation with the analyses actually performed. Statements invoking synoptic forcing, boundary-layer stability, turbulence and vertical mixing have been removed or qualified where these processes were not directly assessed.
  We now identify near-surface wind speed as the strongest observed correlate of stagnation frequency (r = −0.839, p < 0.001), while the association with 500-hPa wind speed is weak and non-significant (r = −0.216, p = 0.07); dry-day frequency likewise shows no significant spatial association. We therefore interpret the results in terms of observed ventilation conditions rather than infer untested mechanisms.
  The revised manuscript also makes explicit that static stability, buoyancy, turbulent mixing, boundary-layer depth and broader synoptic circulation were not directly diagnosed. The Air Stagnation Index is consequently treated as a climatological diagnostic of meteorological stagnation and ventilation
  4 - Comment: At line 200, the Authors state that ERA5 is available from 1979. Actually, ERA5 is available from 1940 and ERA5 Land from 1950. Furthermore, it is not clear which fields were extracted from ERA5 Land.
  Response: We have corrected the temporal coverage of ERA5 in Section 3.1 to 1940 onward and clarified that ERA5 was the only reanalysis dataset used in this study. The revised text now explicitly lists all ERA5 fields used: 10-m zonal and meridional wind components, 500-hPa zonal and meridional wind components, and total precipitation. No ERA5-Land fields were used.
  5 - Comment: Some sentences are repeated in the manuscript, occasionally in very close proximity (e.g., lines 291–294 and 331–334). More generally, several concepts are reiterated throughout the manuscript, making it unnecessarily lengthy relative to the findings presented.
  Response: We thank the reviewer for highlighting this important issue. We have revised the manuscript throughout to remove both repeated wording and broader conceptual redundancy. The revision goes beyond deleting closely repeated sentences. We have clarified the distinct function of each section: the Results now report the empirical patterns, the Discussion interprets them without restating the results, and the Conclusion synthesizes the principal findings without repeating the preceding discussion. The Introduction has likewise been tightened so that climatic context and motivation are established without anticipating interpretations developed later. Particular attention was given to the repeated treatment of the coastal–inland gradient, seasonal variability, near-surface wind, the long-term tendency, boundary-layer processes, and air-quality implications. Material that duplicated these points has been removed or recast where necessary, while the references and scientific distinctions essential to the argument have been retained. The revised manuscript is therefore substantially more focused, with a clearer separation between observation, interpretation, limitation, and implication, and is no longer unnecessarily extended by repetition.
  6 - Comment: The comparison between ERA5 and station observations provides good results, but the Authors should specify if the three stations are assimilated into ERA5. In this case, of course, the two datasets are not independent.
  Response: We thank the reviewer for this important clarification. The three GSOD stations cannot be assumed to be independent of ERA5, because the reanalysis assimilates a broad range of surface observations, including station measurements (Hersbach et al., 2020), while station-level information identifying the observations assimilated into ERA5 is not available to us. We have therefore avoided describing the comparison as a fully independent validation and now refer to it explicitly as a station-based evaluation. This clarification has been added to Section 3.4 (Validation) of the revised manuscript.
  7 - Comment: The Authors refer to a secondary peak in the ASI days frequency in winter (see line 378), but this cannot be clearly seen from the results presented.
  Response: We agree that Figure 4 does not support describing winter as a distinct secondary peak. We have therefore revised Section 4.2 to remove this characterization and to describe the seasonal distribution directly from the spatial patterns shown in Figure 4. Summer is now identified as the clear maximum, while winter is described as a weaker but spatially distinct phase of the seasonal cycle. We also removed the repeated winter interpretation in Section 4.3.1 to avoid redundancy.
  Technical corrections
  8 - Comment: Line 249: “Airport.ERA5”: space missing.
  Response: Thank you for noting this typographical error. The missing space has been inserted in the revised manuscript: “Airport. ERA5”.
  9 - Comment: Line 359: “climatology. These”: capitalize “These”.
  Response: Corrected. “These” is now capitalized in the revised manuscript.
  10 - Comment: Figure 4: please use the same scale for all the panels to help comparability.
  Response: Response: Revised. All four seasonal panels now use a common scale and identical colour breaks, with a single shared colour bar, and the accompanying text has been revised accordingly.
  11 - Comment: Line 412: “that Favour”: “Favour” lower case.
  Response: Corrected in the revised manuscript.
  12 - Comment: Section “Spatial trends”: The Authors mention several cities whose geographic locations may not be familiar to many readers. I suggest adding these cities to the maps to facilitate interpretation of the spatial patterns.
  Response: The referenced cities have been added to Fig. 8a to provide geographic context and facilitate interpretation of the spatial patterns.
  13 - Comment: Line 581: These variables are not the principal components of ASI; rather, they are all the components used to calculate ASI.
  Response: Corrected as suggested. The sentence has been revised to describe these variables as the meteorological components used to calculate the Air Stagnation Index.
  14 - Comment: Line 786: “perspective, These”: “These” lower case.
  Response: The cited wording has been revised in the current manuscript, and the reported capitalization error is no longer present.
  15 - Comment: Line 801: “stagnation. where”. substitute the full stop with a comma.
  Response: The cited passage has been revised in the current manuscript, and the reported punctuation error is no longer present.
  16 - Comment: Line 941: I would not call ERA5 “high-resolution”.
  Response: The term “high-resolution” has been removed in the revised manuscript.
  
  Citation: https://doi.org/10.5194/egusphere-2026-3024-AC2

Hesham Badawy and Samia Ali

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Short summary

Fifty million people in the Nile Delta breathe air that barely moves. More than one hundred fifty days each year. In Cairo, twenty eight stagnant days per summer month. Getting worse by half a day each decade. We tracked forty years of weather data to uncover this. Weak winds, crowded cities, and rising heat now trap pollution over one of the world's most vulnerable deltas. This work gives Egypt its first tool to forecast dangerous air and protect its people. No one had done this before.


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