The response of moist convection and the atmospheric general circulation to climate warming

Abstract

In this thesis, the response of atmospheric circulations to changes in surface temperature is investigated. Both cloud-scale and planetary-scale circulations are considered, and a number of different theoretical and numerical tools are employed. First, mechanisms maintaining the thermal stratification of a convecting atmosphere are examined based on simulations of radiative-convective equilibrium (RCE) over a wide range of surface temperatures and radiosonde observations of the tropical troposphere. It is argued that cloud entrainment plays a role in determining the lapse rate of convecting regions of the atmosphere, and that this may explain the large increase in convective available potential energy (CAPE) with warming found in the simulations. Increases in updraft velocity are also found in the RCE simulations as the atmosphere warms. The magnitude of these increases is explained through a conceptual model based on an entraining plume. The results imply that the increase in CAPE identified above is important in determining the behavior of the simulated cloud updrafts in a warming atmosphere. The precipitation distribution in RCE is also examined. The scaling of precipitation extremes with temperature is found to be amplified by increases in the mean fall speed of hydrometeors as the atmosphere warms. The hydrometeor fall speed affects precipitation rates by altering a measure of the efficiency with which condensation within the column is converted to precipitation at the surface. The potential relevance of this mechanism for convective precipitation extremes on Earth is discussed. Finally, the large-scale atmospheric circulation response to increasing surface temperature is investigated through a novel transformation of the governing equations. The transformation allows for an upward shift of the circulation with warming, and it is applied to a hierarchy of numerical simulations, including simulations with both idealized and comprehensive general circulation models. The transformation is found to reproduce some of the features of the simulated circulation responses to warming in the middle and upper troposphere, and it has potential applications in understanding the source of inter-model scatter in climate model simulations of global warming.