Understanding and modelling fog - WCSSP case study

In many parts of the world, fog is a significant hazard causing economic and societal disruption. Fog is notoriously difficult to simulate in weather forecast models and it is therefore difficult to predict hazardous events. Collaborative research through the Weather and Climate Science for Service Partnership (WCSSP) India is progressing both the understanding of fog formation and the representation of fog in high resolution computer models.

How does fog form?

Fog is made up of lots of tiny water droplets, formed when air close to the ground is cooled, causing the condensation of water vapour. There are different types of fog, which are typically categorised depending on how the air is cooled. In areas with high levels of pollution, fog can become very thick as pollution particles help the water droplets join together and grow. Dense fog significantly limits visibility, resulting in detrimental impacts on transport, particularly in the aviation and road transport industries. In parts of India, dense fog can occur on over half of all winter days, with particularly bad winters, such as that of 2013-2014, resulting in economic losses of approximately 1.78 million USD (120 million Indian rupees).  

WCSSP India is a collaborative project between the Met Office, other UK institutions and the Indian Ministry of Earth Sciences (MoES).

Studying fog occurrence

Research under this project, led by the University of East Anglia and University of Leeds, has studied 20 years of weather observations in India to characterise fog frequency and distribution, aiming to develop a fog climatology for India. This is helping to improve understanding on the causes and processes of fog formation. It shows an increase in fog occurrence over the last 30 years, making it even more important to understand the mechanisms behind fog formation and improve forecasts.

Some of this increase is thought to be related to the rise in human-emitted aerosols, for example from biomass burning and power plants, in the past few decades. However, in some parts of the country, such as Delhi, the research has found that very high aerosol concentrations can limit fog formation. This will have implications for understanding the effects of cutting air pollution. This work has been published as an open-access paper in the International Journal of Climatology.

Fog modelling

WCSSP India scientist Bernard Claxton at the UK Met Office has developed a new model scheme to diagnose fog: “The VERA visibility scheme is designed to produce probabilistic results that help give an indication of how likely the forecast conditions are to occur. The handling of aerosols in VERA is more sophisticated than previous fog schemes, for example capturing variations in particle size.” The implementation of this novel scheme in the regional UK model at the Met Office has helped improve fog forecasting.

Close collaboration between scientists from India and the UK has enabled this visibility scheme, initially developed for the UK, to be implemented in an Indian regional model. VERA has been used to generate fog diagnostics in both the Met Office's Delhi Research model and a model developed by the Indian Institute of Tropical Meteorology. Implementation in both models allows scientists to compare outputs to help further tune and improve the model. This scheme was made operational for the 2021 winter season, providing experimental visibility forecasts for Delhi.

“The visibility forecasts are critical especially for the transportation sector (rail, road, and air) in the northern part of India, as low visibility results in cancellation of flights and increased risk of road and rail accidents. The inclusion of ambient aerosol loading in standard visibility forecasts through VERA helps take into account the aerosol-cloud microphysical interactions in a computationally efficient manner." said Gaurav Govardhan at the Indian Institute of Tropical Meteorology.

Advancing fog forecasts in urban areas

Researchers have also studied how well fog is represented in urban areas. Experiments using the UK Met Office’s Unified Model highlighted deficiencies in the way aerosols are currently represented. For example, the amount and type of aerosols are not representative of different regions, which contributes to biases in fog forecasts. Through WCSSP India, improvements to the way fog is represented in urban areas have been found, resulting in the development of a high-resolution (330 m) visibility and fog forecasting model led by Dr. A. Jayakumar at the National Centre for Medium Range Weather Forecasting (NCMRWF), in collaboration with the Met Office. More realistic city-scale weather forecasts can now be made, helping improve fog forecasts in India, and this approach can potentially be applied to cities around the world.  A recently published study, in the journal Geophysical Research Letters, has applied this model to investigate fog hole formation over cities in the Indo‐Gangetic plains, with findings identifying areas for further model development to better forecast these phenomena.

Agricultural impacts on fog formation

Irrigation of cropland has significantly increased over India in recent years as agriculture has intensified. WCSSP India work, led by the University of Leeds and University of East Anglia, has examined if this could explain the increasing trend in fog formation over the region, and whether including irrigation in modelling helps improve fog simulations. An irrigation scheme has been applied to the model and case studies on past fog events show significantly improved simulations of fog formation and its spatial distribution when compared to satellite observations. The level of improvement is found to be higher during winter than autumn and biases still remain, suggesting that further research and improvements are required, though this advancement in understanding and modelling capability is benefitting NCMRWF for operational forecasting. This research has recently been published as an open-access paper in the journal Communications Earth & Environment.

NCMRWF researcher Dr A. Jayakumar commented; “This collaborative research has made an immense contribution towards the scientific understanding of trends in fog over the Indo-Gangetic Plain and the processes related to its formation. The resultant operational impacts have resulted in significant improvements in the modelling and forecasting of fog, delivering significant benefits.” Fellow NCMRWF researcher Dr T. J. Anurose noted; “A huge thankyou to the whole project team for identifying the benefits of an irrigation parameterisation concept for better modelling instances of dense fog over the Delhi region. We are very much looking forward to continued collaboration to help address further challenges regarding the modelling of fog formation, duration and dissipation over the wider Indo-Gangetic Plain.”