Understanding the genesis of a dense fog event over Delhi using observations and high-resolution model experiments

The present study aims to understand the three-dimensional structure of a dense fog event during the Winter Fog Experiment (WiFEX) 2016–2017 carried out at the Indira Gandhi International Airport (IGIA). The event is analysed with the help of ground-based measurements and the Weather Research and Forecasting (WRF) model. Sensitivity experiments were conducted using different initial conditions and compared to various observations. We find that higher resolution dynamical downscaling and the initial conditions from the Indian Institute of Tropical Meteorology’s Global Forecasting System (IITM-GFS) database yield satisfactory agreements with the observations carried out during the fog event. We note that a strong surface inversion layer exists in the model and observations before the fog onset. Once the fog forms, the simulations suggest that the fog layer grows vertically, destabilizing the temperature stratification. The intensity of the longwave radiative cooling near the fog top increases as the fog grows vertically and shows maximum liquid water content (LWC) near the fog top. The vertical evolution of fog in the model is well identified by the intensity of turbulent kinetic energy (TKE), which is low before the development of dense fog and endures more TKE as the fog becomes dense and vertically thick. The results demonstrate that the WRF model captures the complexity of the physical process and their interaction during the fog evolution in a manner that can be explored further to improve the accuracy of the operational fog forecast.