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In radiative-convective equilibrium (RCE), radiative cooling of the troposphere is roughly balanced by the vaporization enthalpy set free by precipitating moist convection. Many earlier studies restricted the investigation of RCE to the dynamics of the atmosphere with constant boundary conditions including prescribed surface temperature. We investigate a GCM setup where a slab ocean is coupled to the atmosphere, and we explore a wide range of CO2 concentrations. We obtain reliable statistical quantities from thousand-year-long simulations. For moderate CO2 concentrations, we find unskewed temporal variations of 1–2 K in global mean surface temperature, with an almost constant climate sensitivity of 2 K. At CO2 concentrations beyond four times the preindustrial value, the climate sensitivity decreases to nearly zero as a result of episodic global cooling events as large as 10 K. The dynamics of these cooling events are investigated in detail and shown to be associated with an increase in large-scale low-level stratiform cloudiness in the subsiding region, which is a result of penetrative shallow convection being capped by an inversion and thus not ventilating the lower troposphere. These dynamics depend on the CO2 concentration: both through the effect of temperature on stratification and through the changing spatial scale of organization of the flow, which determines the spatial scale and temporal coherence of the stratiform cloud sheets.

The response of extreme precipitation to climate change is elusive and evidently does not simply scale at around 7%/K following Clausius-Clapeyron (CC) scaling. Understanding extreme precipitation is challenging: for example, observational studies have shown opposite signs of change with local warming in different regions. The question is further complicated by the highly variable nature of extreme precipitation, such that events at different timescales may behave differently. This thesis combines observations and a variety of model results to understand how extreme precipitation changes in response to warming and endeavours to explain why the responses can deviate from CC scaling.First, this work resolves important inconsistencies in tropical extreme precipitation scaling among observational studies, finding that extreme daily precipitation in the warmest tropics is unlikely to decrease with warming as previously derived from a binning method, due to flawed assumptions in the method. Instead, regional-downscaling simulations project that extreme daily precipitation tends to increase almost everywhere in Australia, likely exceeding CC scaling, between 5.7-15%/K depending on model details, for the strongest events.This work also identifies convective organisation as important for extreme precipitation scaling, in a suite of idealised simulations. Extreme daily precipitation at a point location is found to increase faster than CC scaling if and when convection becomes more organised with warming. This is because convection is constrained in a relatively fixed region once it becomes organised, which increases daily accumulated precipitation at the rainiest locations. However, convective organisation has a negligible impact on extreme instantaneous precipitation, because of the balance between a negative dynamical contribution (weakened updraft with organisation) and a positive microphysical contribution (increase in precipitation efficiency), both of which are induced

The atmosphere can self-organize into long-lasting large-scale overturning circulations over an ocean surface with uniform temperature. This phenomenon is referred to as convective self-aggregation and has been argued to be important for tropical weather and climate systems. Here we present a boundary layer centric framework based on the available potential energy budget of convective self-aggregation. We show that boundary layer diabatic processes dominate the available potential energy production and are, therefore, essential to convective self-aggregation. We further show that the enhanced virtual effect of water vapor can lead to convective self-aggregation.

The atmosphere can self-organize into long-lasting large-scale overturning circulations over an ocean surface with uniform temperature. This phenomenon is referred to as convective self-aggregation and has been argued to be important for tropical weather and climate systems. Here we present a boundary layer centric framework based on the available potential energy budget of convective self-aggregation. We show that boundary layer diabatic processes dominate the available potential energy production and are, therefore, essential to convective self-aggregation. We further show that the enhanced virtual effect of water vapor can lead to convective self-aggregation.