In a study, researchers evaluate how the TERRA_URB scheme helps to represent heat islands, an important phenomenon also in terms of civil protection, in the COSMO meteorological model. This is an important result to be able to improve the forecasting of urban microclimate
Being able to correctly predict the so-called “urban heat island effect” in cities is important to understand how, especially during heat waves, the temperature behaves in these areas. To do so, however, it is necessary to provide the correct indications to the weather model. This is what an international group of researchers has done: in a paper published in February in Atmosphere, they describe how the scheme called “TERRA_URB” can be used to improve the reproduction of urban heat islands in weather models.
Urban heat islands
The phenomenon known as “urban heat island” produces higher nocturnal temperatures in large cities rather than in their surrounding rural areas. “The reason lies essentially in the buildings that, being made of concrete, absorb a lot of solar radiation during the day. The heat accumulated is released at night, thus preventing the temperature from dropping,” explains Massimo Milelli, researcher in the Meteorology and Climate Department of CIMA Research Foundation and co-author of the study.
The higher temperatures experienced in urban environments may contribute to the heat stress for the population, an effect that could even worsen. In fact, according to projections by the Intergovernmental Panel on Climate Change, climate change may lead to an increase in the duration, number and intensity of heat waves, which in turn would also proportionally increase the heat islands effect. Moreover, as the researchers explain in the article, some studies suggest that urban heat islands may also affect convective precipitation in the area. The effect, especially evident in the tropics, makes thunderstorms in cities more intense than in their surrounding rural areas.
Being able to predict, through weather models, the presence and characteristics of heat islands is therefore important to be able to alert the population and allow them to take the appropriate precautionary measures. But how to do it, and especially how to do it in an operational context?
Schemas and weather models
“In weather models, a series of ‘instructions,’ called schemes or parameterizations, explain how to represent a given phenomenon,” says Milelli. “ A parameterization is nothing more than an approximation of the phenomenon which, if described with all the necessary equations, significantly increases the calculation time, making the operational use of the model difficult. In short, we must be able to find a balance between speed and accuracy of representation”.
This is particularly complex for the representation of urban heat islands, which in fact imply considering all the heat fluxes exchanged between atmosphere, ground and buildings. It is therefore crucial to know the urban geometry (height of buildings, width of roads, materials…). The researchers focused on the COSMO meteorological model, in which the representation of urban environments is very simple, applying and testing the scheme called TERRA_URB, developed by Belgian researchers.
The results have been evaluated over three European cities (Turin, Naples and Moscow), and show that the TERRA_URB scheme succeeds in effectively reproducing the urban heat island effect and improves air temperature forecasts for all the cities studied, even though each one has morphological and climatic characteristics very different from the others. “The scheme has shown to be able to combine computational lightness (allowing forecasts to be made quickly) with a good representation of reality,” says Milelli.
Although more studies will be needed on the calibration of TERRA_URB and on the urban geometry, this first result represents an important advancement to improve urban climate modeling. “In short, it gives us a first hint for a correct prediction and representation of urban temperatures,” the researcher concludes.