Convective systems and climate change in observations and model simulation using pseudo global warming.

In this presentation we look into observations and results from convection-permitting weather/climate modeling to investigate how (extreme) convective rainfall and convective dynamics could change with global warming. Taking (sub)hourly rainfall observations, surprisingly regular relationships with surface dew point temperatures are found in data from The Netherlands and (central/southern) France, closely following two times the Clausius-Clapeyron relation. Even more so, given the same absolute humidity, lower relative humidity is associated with higher rainfall intensities. This points at important roles of dynamical feedback processes related to both relative and absolute humidity,  for instance due to latent heat release, cold pool dynamics and entrainment processes, affecting rainfall intensities and cloud organisation. Unsurprisingly, convection permitting climate models reproduce the above dependencies much better than conventional climate models. This is, in particular, the case for the dependency on relative humidity, where conventional, convection-parameterized models (12 km) do not capture the observed relationships.  Since climate change will likely lead to increases in absolute humidity, but decreases in relative humidity, it is important to understand these mesoscale/cloud dynamics. Here, we show results of a methodology that allows to study extremes and convective dynamics within the context of climate change using a convection permitting climate model. It is based on a forecasting system, using pseudo global warming as an approximation of climate change, running a small ensemble for various warming levels.  We illustrate the methodology with applications to a recent flood event in Italy (convection embedded into a large-scale system) and a mesoscale convective system/supercell north of the Alps. In particular, it is shown that rainfall extremes tend to concentrate in time and space in a warming world, disproportionately enhancing their impact. Also, the simulation of the super cell shows stronger downward motions in warmer climates (in particular for lower relative humidity futures) causing strong  increases in convective wind gusts. Finally, we  will discuss what can be learned from observed relationships based on day-to-day weather variability, and where  these relationships may deviate from long-term variability due to climate change.