Rainfall is one of the central features of Earth’s climate. Understanding the physical mechanisms that control it has deep social impacts on water and food security. In this thesis, we use a series of idealized single column models to reveal mechanisms driving steady-state precipitation both in the tropics and in the global mean. These mechanisms yield a deeper understanding of precipitation in model outputs (Chapter 1), observations (Chapter 2), and projections for a warming climate (Chapter 3). Chapter 1 centers around model development. We use the single column model version of NCAR’s Community Earth System Model (CESM) to better understand its simulation of tropical rainfall under various representations of radiation, convection, and circulation. Using a variety of existing methods – the weak temperature gradient (WTG), damped gravity wave (DGW), and spectral weak temperature gradient (SWTG) method – we parameterize the column’s large-scale dynamics and consider the response of steady-state tropical precipitation to changes in relative sea surface temperature (SST). Radiative cooling is either specified or interactive, and the convective parameterization is run using two different values of a parameter that controls the degree of convective inhibition (CIN) required to cap a convective plume. Under all three methods, circulation strength is decreased when greater CIN is required, that is, when convection is allowed to occur more easily. This effect is shown to come from increased static stability in the column’s reference radiative-convective equilibrium profile and results in decreased rainfall over warm SSTs. This argument can be extended to aquaplanet simulations in CESM, which show that the warmest regions in the tropics rain less when greater CIN is required to cap a convective plume. This suggests that the parameter in CESM which controls the degree of convective inhibition significantly affects the strength of the model’s intertropical convergence zone (ITCZ). In Chapter 2, we use a similar set of idealized models to better understand the observed climatology of tropical rainfall. The distribution of climatological rainfall over tropical oceans can be thought of as primarily the result of two mechanisms: conditional instability in the free troposphere and convergence in the boundary layer. We modify the SWTG method to assess the relative influence of these mechanisms. In its original configuration, the SWTG method applies the weak temperature gradient approximation to the full depth of the troposphere without consideration of the stronger horizontal temperature and pressure gradients in the planetary boundary layer (PBL). To account for convergence in the PBL induced by these stronger pressure gradients, we modify the SWTG method to include an externally-specified vertical mass flux at the PBL top. When forced using the climatological SST and 850 hPa vertical velocity taken from observation-based reanalysis data, the Forced SWTG method reproduces most features of the observed annual mean tropical rainfall climatology. Its predictions remain largely unchanged when it is forced by a spatially uniform SST field. Insofar as the boundary layer convergence field can be interpreted as an external forcing on the column, this would indicate that it controls the precipitation field. However, local column stability likely also plays a role in determining PBL convergence, so this method does not fully untangle the causality behind the climatological precipitation field. In Chapter 3, we shift our perspective from column dynamics to column radiative transfer. Global mean rainfall is known to be constrained by the atmosphere's column-integrated radiative cooling. However, the surface temperature dependence of this radiative constraint on mean rainfall, and the mechanisms which set it, are not well understood. We present a simple spectral model for changes in the clear-sky column-integrated radiative cooling with surface warming. We find that surface warming increases column-integrated radiative cooling – and thus mean rainfall – by decreasing atmospheric transmission in spectral regions with significant longwave emission, that is, by closing the water vapor window. Water vapor's spectroscopy implies a hydrological sensitivity whose magnitude is roughly set by surface Planck emission, and which peaks near tropical surface temperatures. We also examine the role of carbon dioxide and shortwave heating, which primarily act to mute the hydrological response to warming. We validate our findings using line-by-line calculations. Overall, we demonstrate that idealized frameworks, such as those provided by single column models, can elucidate mechanisms controlling tropical and global-mean precipitation. However, the relevance of these results to more complex simulations and observations is tempered by the extent to which our simplifying assumptions neglect important physics.

Indiana University-Purdue University Indianapolis (IUPUI) / U/Pb and (U-Th)/He (ZHe) dating of detrital zircons from glacial till samples in the Ross Embayment, Antarctica records cooling after the Ross/Pan-African orogeny (450-625 Ma) followed by a mid-Jurassic to mid-Cretaceous heating event in the Beacon basin. Zircons were extracted from till samples from heads of major outlet glaciers in East Antarctica, one sample at the mouth of Scott Glacier, and from beneath three West Antarctic ice streams. The Ross/Pan-African U/Pb population is ubiquitous in these Antarctic tills and many Beacon Supergroup sandstones, thus 83 grains were analyzed for ZHe to subdivide this population. Two ZHe age populations are evident in East Antarctic tills, with 64% of grains 115-200 Ma and 35% between 200-650 Ma. The older population is interpreted to be associated with the Ross/Pan-African orogeny including cooling of the Granite Harbour Intrusives and/or exhumation of the older basement rocks to form the Kukri Peneplain. The lag time between zircon U/Pb, ZHe and 40Ar/39Ar ages from K-bearing minerals show cooling over 200 My. Grains in East Antarctic tills with a ZHe age of 115-200 Ma likely reflects regional heating following the breakup of Gondwana from the Ferrar dolerite intrusions, subsidence within the rift basin, and a higher geothermal gradient. Subsequent cooling and/or exhumation of the Transantarctic Mountains brought grains below the closure temperature over a span of 80 My. This population may also provide a Beacon Supergroup signature as most of the tills with this age are adjacent to nunataks mapped as Beacon Supergroup and contain an abundance of vi Beacon pebbles within the moraine. Nine zircons grains from three Beacon Supergroup sandstones collected from moraines across the Transantarctic Mountains yield ages from 125-180 Ma. West Antarctic tills contain a range of ZHe ages from 75-450 Ma reflecting the diverse provenance of basin fill from East Antarctica and Marie Byrd Land. ZHe and U/Pb ages <105 Ma appear to be distinctive of West Antarctic tills. The combination of U/Pb, ZHe and 40Ar/39Ar analyses demonstrates that these techniques can be used to better constrain the tectonic evolution and cooling of the inaccessible subglacial source terrains beneath the Antarctic Ice Sheet.