Fog Droplet Relative Dispersion in Polluted Urban Environment in India: study from WiFEX, India.

A critical understanding of fog droplet relative dispersion is crucial for accurate fog forecasting, intensity estimation, and duration prediction, particularly in polluted urban environments. Aerosol-fog interactions significantly influence aerosol indirect effects and fog radiative properties. Our analysis of eleven fog episodes in New Delhi during the winters of 2017-2018 revealed a notable shift in the correlation between relative dispersion (ε) and fog droplet number concentration (N_d) from positive to negative as Nd increased at constant liquid water content (LWC). This correlation pattern exhibited distinct behaviours during the development and maturation phases of fog.

We identified activation, condensation, deactivation, evaporation, and sedimentation as the dominant microphysical processes driving these diverse dispersion effects. Threshold values for the first bin strength (0.3-0.4) and volume-mean diameter (10-14 μm) were established to classify the different dispersion regimes. Based on the observed ε-Dv correlations, we developed a non-monotonic parameterization of ε using the Nelder function.

The wide range of droplet sizes during fog events reflects complex fog dynamics, where multiple microphysical processes contribute to droplet formation. The presence of different droplet sizes during dissipation suggests that visibility can remain low even as fog dissipates due to the persistence of larger droplets.

This study provides new insights into the complex dispersion effects observed in polluted urban fog environments. By elucidating the roles of key microphysical processes, our findings contribute to reducing uncertainties in the simulation of aerosol-cloud interactions.