Multiscale Interactions in Convection Initiation in the Alps

One of the most visually striking impacts of mountains on the atmospheric circulation is that they favour cloud formation. Clouds develop when water vapour condenses in an air mass whose temperature is decreasing. Mountains favour this process primarily by forcing upward air motion. In fact, rising air masses cool down as they encounter lower atmospheric pressure during their ascent. Meteorologists distinguish between stratiform and cumuliform clouds. Stratiform clouds are developed mostly in the horizontal direction and have a layered appearance. Cumuliform clouds, instead, have a large vertical extent and are caused by convection, a particular form of atmospheric circulation characterized by vigorous vertical motion. The clouds with the largest vertical development, cumulonimbus clouds, may cause heavy rainfall, thunderstorms and other severe weather. The project MICIA aims at improving the knowledge of the processes that cause the formation of cumulus clouds near mountains during summer. In this season, storms and heavy rainfall occur over and near mountains more frequently than over plains. Forecasts of these phenomena are often inaccurate, because storms may form near mountains even if the regional weather conditions seem only marginally supportive. Often, convection initiation is affected by meteorological processes that operate at very different spatial and temporal scales. For instance, even if slowly changing weather conditions remain favourable to storm occurrence continuously for a few days and uniformly over several thousand kilometres, storms can be very localized. In fact, the initial stage of a storm may correspond to a single quickly evolving convective plume with a size of only a few hundred meters. The very wide spectrum of relevant scales poses a challenge both to the scientific understanding and to the forecasting of convective storms. The MICIA project addresses two main knowledge gaps, in particular the possible impacts of mountain waves and of terrain-induced breezes, like valley winds, on storm initiation and development. These topics will be investigated through simulations with state-of-the-art high-resolution numerical weather prediction models. Research plans also include the analysis of a series of summertime storms observed in the eastern Alpine area. In this context, numerical simulation results will be complemented with the analysis of existing observations. Collaboration with professional weather forecasters will ensure that the new knowledge emerging from the project is quickly and effectively incorporated in the routine weather forecasting practice.

We researched the meteorological processes that generate cumulus clouds and thunderstorms over mountains in the summertime. Regions at the boundary between high mountains and surrounding plains are a favourable environment for storms. This happens because breezes induced by the mountains transport humid air towards mountain tops and alter the vertical temperature profile in their vicinity, making cumulonimbus development more likely. In addition, mountains force upward air motion in many ways, which include upslope winds and mountain waves. In a supportive environment, small mountain-induced updrafts amplify leading to storm development. Motivated by the relatively low rate of success in predictions of orographic thunderstorms, our research probed deeply into their underlying mechanisms using numerical simulations of the weather. High-performance computing facilities made it possible for us to run simulations with grid spacing between 100 m and 1 km. We quantified low-level moisture transport by mountain breezes. We studied the conditions in which mountain waves can generate a thunderstorm, examining a few notable events from the recent past. Finally, we elaborated a short-term climatology of the frequency of occurrence of thunderstorms in the Alps, aided by decade-long measurements of lightning flashes and radar observations at high spatial (~2 km) and temporal (10 minute) resolution.