Humid air, but no rain - this is the problem many dry coasts face. A computer model now suggests that solar plants could favor precipitation by changing air currents.
Spectrum of Science
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Large-scale photovoltaic plants on the Arabian coast of the Red Sea amplify rainfall in the coastal plain - computer simulations suggest. A team led by Suleiman Mostamandi of the King Abdullah University of Science and Technology in Saudi Arabia calculated the effect of different surface coverings on regional air currents and concluded that a darker surface enhances the influx of moist air. As the team reports in the Journal of Hydrometeorology, the additional precipitation from solar installations could theoretically meet the needs of up to five million people. However, the team points out that the computer model is idealized. Nevertheless, it shows that, in principle, precipitation from moist ocean air could be enhanced on dry coasts by changing land cover.
There is a paradox behind the analysis. The air over the Red Sea contains a lot of water because evaporation is strong. Yet the coast is very dry because little of it rains off. Previous attempts to get the moisture to rain off - for example, by "seeding" clouds with condensation nuclei - have been largely unsuccessful. According to the model developed by Mostamandi's team, the key to more rain is instead the reflectivity of the Earth's surface, known as albedo. A brighter surface radiates more light back and cools, while a lower albedo causes the surface and air to heat up. Helpfully, perhaps surprisingly, it's the latter.
In the models, experts examined how precipitation changes when assuming a higher or lower albedo, or a coastal plain covered with forest. The result: both higher albedo and forest cool the surface. This decreases the temperature difference with the sea and the pressure gradient that drives air currents. The wind blowing from the sea to the land becomes weaker, and so does moisture transport. If, on the other hand, the land surface becomes darker, the effect is reversed. The land becomes warmer, and the rising air lowers the pressure, drawing in more moist air from the sea. In addition, the simulations suggest that more water reaches higher atmospheric layers, where it can condense and rain down.
However, to achieve the albedo of 0.2 used in the experiment, one would need to cover the entire area studied with solar arrays. In this respect, the simulation does not provide a realistic forecast, but only a demonstration of the effect in principle. More detailed analyses would have to clarify what effect such measures actually have on rainfall - for example, forests also have other effects beyond albedo that could favorably influence the regional climate. With regard to the effects of large-scale solar plants, however, the authors point out that they are basically useful. The rain effect could simply be an additional incentive to build photovoltaic plants in dry coastal regions.
