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<p>Aerosols play a pivotal role in modulating Earth’s climate by influencing cloud hydrometeor properties. Nevertheless, the exact nature of this complex interaction is not yet fully understood. This study analyses multi-decadal variations of aerosol optical depth (AOD) and their association with cloud effective radius (CER) over the southern Indian peninsula, focusing on single-layer liquid clouds over the winter (DJF) and pre-monsoon (MAM) seasons from 2003 to 2023, utilizing Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua observations and European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) reanalysis. AOD shows strong spatial variability, with higher values over the northern and eastern regions (≈ 0.45–0.55). Mean AOD is generally higher in the pre-monsoon (0.2–0.55) than in winter (0.1–0.5), with a seasonal increasing trend (≈ 0.01 yr⁻¹). AOD increases consistently during winter (R² = 0.90) but shows greater variability in pre-monsoon (R² = 0.73). The Ångström Exponent (AE ≈ 0.4–1.7) indicates mixed to fine-mode aerosols, with fine-mode dominance in winter and higher coarse-mode influence in pre-monsoon. CER increases with AOD under relatively constant liquid water path (LWP), as revealed by statistically significant linear regression analysis, with a stronger positive correlation (≈ 0.41) in optically thinner clouds (LWP &lt; 100&#xa0;g m⁻²) that gradually weakens as LWP increases. Partial correlation (≈ 0.04–0.46) indicates that the AOD–CER relationship remains significant after controlling for meteorological factors such as humidity and atmospheric stability. Analysis of near-cloud-top relative humidity and lower tropospheric stability shows stronger positive AOD–CER correlations in drier, turbulent clouds, suggesting entrainment mixing as a key mechanism for droplet growth.</p>

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Influence of aerosol optical depth on cloud droplet radius over South India, as observed by MODIS

  • 期刊论文

  • 发表日期：

    2026年7月23日

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  • Vineeth Krishnan Valappil,
  • Dhanya Madhu

摘要

Aerosols play a pivotal role in modulating Earth’s climate by influencing cloud hydrometeor properties. Nevertheless, the exact nature of this complex interaction is not yet fully understood. This study analyses multi-decadal variations of aerosol optical depth (AOD) and their association with cloud effective radius (CER) over the southern Indian peninsula, focusing on single-layer liquid clouds over the winter (DJF) and pre-monsoon (MAM) seasons from 2003 to 2023, utilizing Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua observations and European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA5) reanalysis. AOD shows strong spatial variability, with higher values over the northern and eastern regions (≈ 0.45–0.55). Mean AOD is generally higher in the pre-monsoon (0.2–0.55) than in winter (0.1–0.5), with a seasonal increasing trend (≈ 0.01 yr⁻¹). AOD increases consistently during winter (R² = 0.90) but shows greater variability in pre-monsoon (R² = 0.73). The Ångström Exponent (AE ≈ 0.4–1.7) indicates mixed to fine-mode aerosols, with fine-mode dominance in winter and higher coarse-mode influence in pre-monsoon. CER increases with AOD under relatively constant liquid water path (LWP), as revealed by statistically significant linear regression analysis, with a stronger positive correlation (≈ 0.41) in optically thinner clouds (LWP < 100 g m⁻²) that gradually weakens as LWP increases. Partial correlation (≈ 0.04–0.46) indicates that the AOD–CER relationship remains significant after controlling for meteorological factors such as humidity and atmospheric stability. Analysis of near-cloud-top relative humidity and lower tropospheric stability shows stronger positive AOD–CER correlations in drier, turbulent clouds, suggesting entrainment mixing as a key mechanism for droplet growth.