Modelling the storm

A new mathematical model indicates that dust devils, water spouts, tornadoes, hurricanes and cyclones will intensify as climate change warms the Earth's surface.

The new model, developed by University of Michigan atmospheric and planetary scientist Nilton Renno, could allow scientists to more accurately calculate the maximum expected intensity of a spiralling storm based on the depth of the troposphere and the temperature and humidity of the air in the storm's path. The troposphere is the lowest layer of Earth's atmosphere.

The model improves upon current methods, Renno said, because it takes into account the energy feeding the storm system and the full measure of friction slowing it down. Current thermodynamic models make assumptions about these variables, rather than include actual quantities.

'This model allows us to relate changes in storms' intensity to environmental conditions,' Renno said. 'It shows us that climate change could lead to increases in how efficient convective vortices are and how much energy they transform into wind. Fuelled by warmer and moister air, there will be stronger and deeper storms in the future that reach higher into the atmosphere.'

Renno and research scientist Natalia Andronova used the model to quantify how intense they expect storms to get based on current climate predictions. For every 3.6F that the Earth's surface temperature warms, the intensity of storms could increase by at least a few percent, the scientists said. For an intense storm, that could translate into a 10 per cent increase in destructive power.

Renno's model is what scientists call a 'generalisation' of Daniel Bernoulli's 18th-century theorem that states that the pressure in a fluid decreases as its velocity increases. But that theorem left out variables that were considered difficult to deal with such as friction and energy sources (which, in the case of a whirling storm, is warm air and condensation of water vapour.)

But by including these additional variables, Renno was able to broaden Bernoulli's equation to apply it to more general phenomena such as atmospheric vortices.

The new model helps to explain the formation of spiral bands and wall clouds, the first clouds that descend during a tornado. It's clear now that they are the result of a pressure drop where the airspeed has increased.

Renno said that unifying convective vortices from dust devils to cyclones will help scientists understand them better.

'This is the first thermodynamic model that unifies all these vortices,' he said. 'When you unify them, you can see the big picture and you can really understand what makes them form and change.'

A co-investigator on NASA's Mars Phoenix Lander mission, Renno has used his new model to calculate the intensity of dust storms in Mars' polar regions. He found that at the Phoenix landing site dust storms can have winds in excess of 200mph.