City dweller needs umbrella

The rain-soaked dash home from work that the weather forecasters somehow overlooked is one of the pitfalls of urban life. Now meteorologists have been able to reveal the degree to which the unpredicted city downpour is a creation of the urban terrain itself.

Research by an Anglo-French team of experts shows it is now possible to simulate the extent to which a large tract of buildings and roads shape the weather above and around them.

In the latest issue of the journal Atmospheric Research, Jutta Thielen and Alan Gadian, meteorologists at UMIST, applied a complex, fine-scale model to look at rainfall over Paris, though they believe their results will be equally valid for other cities around the world.

The work implies that weather forecasts for city dwellers will often be less predictable - and hence less accurate - than those for the rest of the population as this significant effect is not considered in currently released, official calculations.

"The city of London should have a considerable impact on the rainfall pattern and the UK Met Office does not take this into account yet," said Dr Thielen.

The scientists looked at weather on a meso-scale of between 1km and 100km, much finer than the broad-brush approach modern forecasters generally use. The UK Met Office is developing a meso-model ultimately for the whole country to improve local forecasting.

While the rainfall that comes via the vast frontal systems that sweep over regions are essentially unaffected by the presence of a city, the study found a very different result for convective rain.

During the day, the built-up terrain - asphalt, brick, concrete and tile - absorbs heat, making the overall city warmer than the surrounding countryside, sometimes by as much as 5oC.

The model showed this caused columns of warm air to rise up, destabilising the atmosphere above and pushing what moisture there was high enough for clouds to condense. If there was enough water vapour this could lead to rainfall, often in the form of summer evening thunderstorms.

The "roughness" of the urban terrain also obstructed the flow of air across the city, creating more turbulence and boosting the chances of rain clouds forming. The larger the city and the more high rise its buildings, the more significant the effect.

It appears there is little escape from this urban rain, even for those who choose to commute in from outside - the model showed that much fell downwind of the city, soaking nearby towns and villages.