Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1979. / Microfiche copy available in Archives and Science. / Bibliography : leaf 80. / by John Lee Lillibridge III. / M.S.

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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.