Heat transfer theory explains sudden cold snaps
The evening boundary layer is an important weather phenomenon that can trap cool air near the ground surface, developing into frost or fog in the morning. A better understanding of this process can improve weather forecasts, potentially saving farmers millions of euro in crop damage and making road and air travel safer. The EU-funded COAT project investigated the conditions driving the evolution of the evening boundary layer. “At night, the ground surface cools and creates a pool of cold air 50 to 200 m deep,” explains Bas van de Wiel, principal investigator on COAT. “This cold air is heavy, and will stratify.” If there is little wind, this static layer of cold air continues to accumulate, eventually producing fog and frost. However, sufficient turbulence can carry away the colder air, preventing fog and frost. This unpredictable nature of the evening boundary layer confounds climate models, producing errors of up to 5 degrees Celsius in forecasts for cold periods. “For that reason, we looked at what causes this collapse of turbulence, when all air mixing disappears,” says van de Wiel.
Feedback loop
With his colleagues at Delft University of Technology in the Netherlands, van de Wiel modelled the heat exchange between ground and air. Paved surfaces such as roads and streets hold more heat than grass, keeping night temperatures higher. However, even the heat lost from unpaved surfaces is eventually replenished from below. This allowed van de Wiel and his colleagues to quantify the total amount of expected heat loss. Using this figure, they could determine the wind speed needed to deliver enough warm air that fog or frost do not form. “We calculated that at windspeeds lower than 3 m/s at nose height, the system collapses,” he adds. “The flow can only transfer a set amount of heat, and if the surface is cooling at a higher rate, you get a positive feedback loop.” The model was validated in three stages. First, the team performed a direct numerical simulation at a high computational power, building the system from first principles. Secondly, they used a weather forecast model with some assumptions built in to see if the predictions were correct. Finally, the model was applied to real-world weather to see if it was accurate. “The predictions still hold very well, here,” says van de Wiel. “We can be confident the theory is valid and that we solved the problem of collapse of atmospheric turbulence.” The model highlights that soil conditions, distance to urban areas and distance to water are key factors in the formation of fog.
Bearing fruit
The research is already being put into practice. As well as improving weather and climate predictions, van de Wiel’s research showed that air movement is a powerful tool against frost formation. A related project used large ventilators to circulate air in fruit orchards, to great success. “The results were so good, temperatures were up 3 to 5 degrees, harvest increased 100 %; we could perfectly show that harvest increased closer to the wind machine,” notes van de Wiel. COAT was supported by the European Research Council. “This funding helped enormously, you have the freedom and opportunity to really be creative with it,” he adds. “Some funding just pays for the researcher’s salary, but science is much more than that.”
Keywords
COAT, boundary layer, air, wind, heat exchange, frost, fog, weather, forecast, climate, orchard, fruit