Thunderstorms are known to form preferentially on warm, humid days with unstable air. But predicting where exactly a thunderstorm will develop has so far remained extremely difficult. Heavy rainfall often occurs very suddenly and locally – without warning, yet frequently with severe consequences.
A British-Austrian research team with the participation of TU Wien has now analysed 2.2 million thunderstorm events in Africa. In doing so, the researchers were able to identify a physical explanation for why thunderstorms form at certain locations while failing to develop elsewhere. It is the interplay between wind conditions in the atmosphere and spatial variations in soil moisture that determines whether a harmless cloud grows into a dangerous thunderstorm. The new physical model of thunderstorm development has now been published in the journal Nature .
Soil moisture as a driver of thunderstorms
“Large-scale air-mass movements can be calculated very well today,” says Prof. Wolfgang Wagner from the Department of Geodesy and Geoinformation at TU Wien. “But thunderstorms form on intermediate spatial scales – on the order of a few kilometres. And this is precisely where current models reach their limits.”
For many years, TU Wien has been developing methods to derive soil-moisture information from satellite measurements on a global scale. Improvements in the spatial and temporal resolution of these satellite data have now made it possible to directly relate thunderstorm observations to soil-moisture patterns. This revealed a striking relationship:
Thunderstorm cells tend to grow particularly rapidly and intensely where differences in soil moisture generate near-surface winds, while winds at higher altitudes blow in the opposite direction.
If the soil in one region is wetter than in its surroundings, evaporation cools the near-surface air. Over drier areas, the air warms more strongly and surface pressure is lower. As a result, near-surface air flows from wetter towards drier regions. Wind directions several kilometres above the ground, however, are largely determined by large-scale weather systems and are mostly independent of local soil moisture.
It can therefore happen that clouds at higher altitudes move in exactly the opposite direction to the near-surface airflow. “When this occurs, the relative velocity is at its maximum,” explains Christopher Taylor (UK Centre for Ecology and Hydrology, UKCEH), lead scientist and principal author of the study. “The cloud comes into contact with the greatest possible amount of new, near-surface air per unit time – and this air then feeds the thunderstorm cell from below.”
Under these conditions, convection is strongest: moist air is drawn upward from below, cools as it rises, and can rapidly form large thunderstorm clouds. A self-reinforcing upward motion develops – a local instability turns into a thunderstorm. This sensitivity to small-scale conditions is precisely what makes thunderstorms so difficult to predict.
Millions of thunderstorms confirm the model
The research team validated the model using independent data sets. Satellite imagery shows rapid cloud growth exactly where the model predicts it. Lightning observations confirm that the most intense thunderstorms preferentially form over relatively dry soil areas when near-surface winds and winds at higher altitudes are oppositely directed.
High-resolution satellite measurements of soil moisture were crucial for this analysis. These data were developed at TU Wien and provided by EUMETSAT, the European Organisation for the Exploitation of Meteorological Satellites. The study uses data from the European ASCAT instrument, which orbits the Earth aboard the Metop satellites operated by EUMETSAT. Using sophisticated, tailor-made physically based models, these measurements allow detailed estimates of local soil-moisture conditions.
The insights gained in this study are expected to enable more precise assessments of future thunderstorm development and to improve our understanding of the links between climate change and extreme weather events.
Nature
Observational study
Not applicable
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