Rainfall in the climate system: changes under global warming, and challenges

Anyone who has ever been drenched by a sudden downpour that failed to follow the weather forecast has a feeling for the problem that faces climate modelers. Small-scale features like thunderstorms, in which the core might be 5 km across, are much harder to predict in global models for weather and climate then are quantities like temperature that tend to vary on larger scales. Rain and snowfall, together known as precipitation, are one of the most important climate factors affecting human activities. So when planning for adaptation to global warming, one of the questions is always: how will the precipitation change? And because water resource management is done for relatively small regions like states, small countries or watersheds, this information is demanded at the regional scale. This turns out to be an enormous challenge for climate models. Here we give a quick tour of how rainfall is represented mathematically in climate models, what can currently be said, and what some of the challenges are. How is rainfall represented mathematically in climate models? Figure 1 shows a satellite photo in which a myriad of complex cloud systems may be seen. Tropical convective systems, with strong upward motion in thunderstorms, appear as tiny dots at the global scale, despite the fact that these contribute large amounts of rainfall in certain regions