UC Berkeley

A hierarchy of models is used to connect fundamental mechanisms to impacts in a changing atmosphere. On the subject of forcings, an intermediate-complexity model for the radiative forcing of carbon dioxide is developed. This model is used to provide simple explanations for well-known properties of carbon dioxide (CO2) forcing such as its magnitude, dependence on atmospheric conditions, and logarithmic scaling. In the realm of impacts, climate models are used to investigate the impact of global warming on the kind of severe thunderstorms that produce hail and damaging winds. The results suggest that severe thunderstorms will become more damaging in the future, and that increases in Convective Available Potential Energy (CAPE) are the culprit. This motivates the subsequent use of cloud-resolving simulations to develop of a theory of CAPE and its dependence on temperature, which highlights the importance of the atmospheric saturation deficit. Another result of that theory for CAPE is an explanation for why cloud buoyancy and updraft strength are largest in the upper troposphere, a property that has traditionally been attributed to the release of the latent heat of fusion above the melting line. It is shown that this ice-based explanation is a fallacy: cloud buoyancy and updraft strength are the same in a world with or without ice. Finally, on the topic of feedbacks, the relationship between high clouds and the tropopause is investigated in cloud-resolving simulations. The results support the existence of a Fixed Tropopause Temperature (FiTT) rather than a Fixed Anvil Temperature (FAT), which implies a decoupling of anvil clouds from the tropopause. This decoupling motivates further investigation into the formation mechanisms of anvil clouds; it is found that anvil clouds do not result from enhanced detrainment below the tropopause, as is the traditional view, but from the slow evaporation of cloudy air in the cold upper troposphere.