Clouds and storms develop due to inflow of warm, moist air from near the ground up into their bases. They also ingest air through their edges (entrainment), which may be substantially cooler and drier. Thus, clouds "inhale" air with a variety of characteristics, influencing their ability to precipitate. Some of this air exits the cloud near its top, but the remainder is "exhaled", descending toward the ground (outflow) and cooled by melting or evaporating precipitation. Depending on these conditions, the cloud/storm outflow may be sufficient to generate new, neighboring clouds or storms. We will present high-resolution (down to 50 m) 3D simulations of lines of clouds, as well as individual deeper thunderstorms and thunderstorm complexes, designed to study these aspects. The ultimate goal is to improve fundamental understanding of entrainment and precipitation that can lead to improved forecasts of convective clouds and storms. Long-standing problems in meteorology include the prediction of (a) how quickly entrainment mixes dry environmental air into clouds/storms, limiting their lifetime and their ability to precipitate, and (b) if/when the outflow from a cloud/storm is sufficient to generate new clouds/storms. A lack of computing power has limited past studies. High-resolution (down to 50 m) 3D simulations of convective lines of clouds, as well as individual deeper thunderstorms and thunderstorm complexes, require a computer like Blue Waters, to properly resolve the smaller turbulent eddies responsible for the entrainment and hold in memory the large arrays of variables that must be computed to evaluate the production of precipitation. The simulations also must be run over large domains (hundreds of kilometers) to represent the larger-scale storm outflows, and over periods 6 hours or more to evaluate their evolution. Such simulations generate very large, complex sets of data, for which Blue Waters is also ideal for processing and analyzing. Our work on Blue Waters has produced key new findings. Clouds/storms spaced by different amounts along a line differ minimally in terms of their entrainment and its effects, a surprising result, but can differ greatly in terms of their outflows and their ability to generate new clouds/storms. Simulations of supercells thunderstorms show that entrainment is significant in their early stages, before the storms start to rotate, and current work is examining how entrainment changes afterward, and the effects on precipitation during those different stages. The outflows of large complexes of storms, capable of generating new storms, have been shown to be predominantly controlled by the melting of large ice particles created inside the storms, which can be linked to the propagation speed of the outflows, holding some promise in deriving a predictive relationship for new storm generation. The scientific impact of our work includes weather forecasting and climate modeling. Entrainment has been implicated as a critical limiting factor for thunderstorm development and concomitant rainfall, hailfall, and/or severe winds. The weather forecasting community fully appreciates that storm outflows can produce severe wind damage, and/or additional storms, and even day-long severe weather outbreaks, but knowledge is lacking to identify when such outbreaks will and won't occur. While other research groups around the world investigate how to parameterize the effects of entrainment and thunderstorm outflows in coarser numerical weather prediction models and global climate simulations, our group is providing new, fundamental knowledge of these topics, providing the scientific underpinning for these parameterization efforts for global weather and climate modeling efforts.
