Spatiotemporal variability and evolution of day and night winter fog over the Indo Gangetic Basin using INSAT-3D and comparison with surface visibility and aerosol optical depth

Abstract

Every year during the peak winter months (December–January), dense and prolonged fog envelops the Indo-Gangetic Basin (IGB) over the Indian sub-continent leading to economic loss. Many efforts are being made to understand its characteristics to improve forecasting skills. In the present work Indian National SATellite (INSAT-3D), a geostationary satellite, retrieved fog data available at every 30 min interval throughout the day and night is used to study its evolution, spatial and temporal variability for the winter months of December 2016 and January 2017 in conjunction with surface measurements. Fog data when compared with ground-based horizontal visibility over different stations in IGB showed good relationship, also depicted similar temporal variability in tune with the favorable meteorological parameters of temperature, relative humidity, and wind speed. It is observed that INSAT-3D captured fog very well for visibility ranging from 0 to 500 m representing very dense, dense, and moderate fog, while, the shallow fog with visibility >600 m is captured occasionally. From the maps of spatial and temporal variability of fog, the regions most affected are identified based on fog duration. The intense fog events covering the entire IGB for long duration are observed mostly in the midnight (00:00–04:00) and early morning (04:00–08:00) hours (local time) of December as compared to January. While in January, moderate intense fog spans the entire IGB during day time also. Due to increase in pollution levels, including biomass burning as inferred from MODIS fire counts, and stable atmosphere, high aerosol optical depths (AOD) are observed across IGB. The high AOD regions (> 0.8) covering central and west IGB, are also the hotspot regions of fog/smog onset and slowly expand over a larger area and intensify.