particularly over urban regions. This study investigates the temporal and seasonal variability of Atmospheric

Boundary Layer characteristics over New Delhi during the period 2001–2010 using ERA-5 reanalysis data. Key

parameters such as Boundary Layer Height (BLH), surface temperature, and wind speed are analysed on annual

and seasonal scales. The results reveal pronounced seasonal variations, with maximum BLH observed during the

pre-monsoon season and minimum values during winter. Inter-annual variability highlights the influence of

surface heating and urbanization on boundary layer development. The findings provide insight into urban

boundary layer dynamics and their implications for air pollution dispersion over New Delhi.

atmospheric information suitable for climatological studies. The present work aims to analyse and compare the

decadal variability of ABL characteristics over New Delhi from 2001 to 2010.

(ECMWF). ERA-5 provides hourly atmospheric variables at a horizontal resolution of 0.25° × 0.25°.

Boundary Layer Height (BLH)

ERA-5 reanalysis data obtained from ECMWF were used to analyse atmospheric boundary layer characteristics

over New Delhi for the period 2001–2010.

Hourly ERA-5 data were averaged to obtain daily means, which were further processed to compute seasonal and

annual averages. Inter-annual variability was examined using linear trend analysis. The relationship between

BLH and surface meteorological parameters was analysed using correlation statistics.

Although ERA-5 provides high-resolution (0.25°) gridded data from the European Centre for Medium-Range

estimates over large-scale climatological assessments. Therefore, while ERA-5 is suitable for decadal trend

analysis, incorporation of in-situ radiosonde and lidar observations would enhance future studies.

The annual mean BLH over New Delhi during 2001–2010 shows noticeable inter-annual variability. BLH values

generally range between 700 m and 1800 m, with higher values corresponding to years with enhanced surface

heating and stronger winds. No abrupt trend is observed; however, a slight increasing tendency is noted, possibly

associated with urban expansion and increased surface roughness.

A pronounced seasonal cycle in BLH is observed. The pre-monsoon season exhibits the highest BLH due to strong

pre-monsoon and monsoon seasons. Reduced wind speeds and low temperatures during winter lead to shallow

boundary layers, favouring pollutant accumulation.

Figure 1 shows the inter-annual variation of mean Atmospheric Boundary Layer Height (BLH) over New Delhi

during 2001–2010. The BLH exhibits a gradual increase from about 820 m in 2001 to nearly 1250 m in 2010.

This increase may be attributed to enhanced surface heating, urbanization effects, and increasing mechanical

turbulence over the region.

during winter, while post- monsoon values remain moderate.

Figure 3 shows the relationship between surface temperature and BLH. A strong positive association is observed,

indicating that higher surface temperatures lead to enhanced convective mixing and deeper boundary layers.

The variability of Boundary Layer Height (BLH) over New Delhi is not controlled solely by surface temperature.

High aerosol loading over Delhi modifies surface energy balance. Aerosols can:

 Reduce incoming solar radiation (dimming effect),

 Stabilize the lower troposphere,

 Suppress vertical mixing under heavy pollution conditions.

between temperature and BLH.

Western Disturbances during winter and monsoon depressions influence wind shear and stability profiles. Stable

stratification during winter combined with weak synoptic forcing leads to shallow boundary layers.

Reduced vegetation cover and increased built-up area alter moisture fluxes, Bowen ratio, and sensible heat flux,

 Beijing – strong winter suppression due to aerosol loading

 Shanghai – monsoon-modulated boundary layer

 Mumbai – marine influence leading to weaker seasonal amplitude

Delhi exhibits stronger winter inversion intensity compared to coastal cities due to continental climate

dominance.

Shallow winter boundary layers (~600–700 m) significantly reduce vertical pollutant dispersion. Episodes of

severe PM₂.₅ accumulation are strongly associated with suppressed BLH and weak winds.

Thus, BLH can serve as an important meteorological predictor in air-quality forecasting models.

The present study provides a comparative analysis of Atmospheric Boundary Layer

characteristics over New Delhi during 2001–2010 using ERA-5 reanalysis data. The major conclusions are:

4. Shallow wintertime boundary layers have significant implications for air-quality deterioration over the

5. This study demonstrates that Atmospheric Boundary Layer variability over New Delhi is governed by

complex interactions among surface heating, aerosol loading, synoptic meteorology, and urbanization

urban heat island effects significantly modulate boundary layer development.

datasets and mesoscale modeling would enhance urban-scale accuracy. Extending analysis beyond 2010

will provide deeper insights into post-urbanization dynamics and recent air-quality mitigation policies.

“Future work will extend the analysis to 2020 to capture post-2010 urban expansion and air-quality policy

impacts.” The findings contribute to a better understanding of urban boundary layer dynamics and can aid in

improving air-quality management strategies over megacities like New Delhi.

1. Garratt, J. R. (1994). The Atmospheric Boundary Layer. Cambridge University Press.

2. Stull, R. B. (1988). An Introduction to Boundary Layer Meteorology. Kluwer Academic Publishers.

3. Seidel, D. J., Ao, C. O., & Li, K. (2012). Estimating climatological planetary boundary layer heights