On Using Stable Water Isotopes To Better Constrain Convective Simulation

Convective cloud processes play a critical role in Earth's present and future climate,but the simulation of these processes is among the largest uncertainties in climate simulations of the status quo.Atmospheric water-vapor isotopes fractionate during phase changes,and can potentially be used as tracers to track water history,as well as proxies for physics that are difficult to measure directly.In this thesis,I investigate the utility of using atmospheric water-vapor isotopes to constrain our understanding of bulk cloudconvective processes.I focus on entrainment rate and precipitation efficiency among the cloud-convective processes as these are the most uncertain parameters in convective simulation.Previous studies proposed that atmospheric stable water isotopes are promising candidates for revealing sub-grid convective processes as they provide differentiating signatures relative to what ordinary water vapor provides.However,in this thesis I show that the mean tropical profile of HDO in the free troposphere does not usefully constrain the mean convective entrainment rate or precipitation efficiency.This is demonstrated using a single-column analytical model of atmospheric water isotopes.The model has three parameters: the entrainment rate,the precipitation efficiency,and the distance that evaporating condensates fall.At a given relative humidity,the possible range of HDO is small: its range is comparable to both the measurement uncertainty in the mean tropical profile and the structural uncertainty of a single-column model.Therefore,the mean tropical HDO profile is unlikely to add information about the bulk properties of convective processes that cannot already be learned from relative humidity alone.Numerical solutions after relaxing the simplifying assumptions made in the analytical model are also explored.These sensitivity tests confirm that the main conclusions are not sensitive to the simplifying assumptions made to derive the analytical model.This does not preclude,however,the utility of water isotopes in identifying water sources,especially in the tropical tropopause layer(TTL)and in the boundary layer.I suggest that for studies of current climate,atmospheric stable water-vapor isotopes are more valuable for differentiating processes affecting water vapor in the TTL and moisture sources in the boundary layer,rather than for constraining sub-grid convective processes in the troposphere such as entrainment rate and condensate re-evaporation.This dissertation thus challenges the assertions of some previous studies and advises on the most effective allocation of resources for future research on isotopic signatures in atmospheric water vapor.I further investigate changes in summertime extreme daily-maximum temperatures relative to summertime mean daily-maximum temperatures,finding that extreme temperatures warm more than mean temperatures with global warming in relatively moist tropical land regions.I show evidence that this amplified warming of extremes is related to suppressed evapotranspiration and enhanced Bowen ratio,and then discuss potential applications of water isotopes for constraining the modeling of land surface processes and boundary layer moisture sources in current and past climates.Such constraints may aid in evaluating and improving heavily parameterized interactions between the land surface and the atmospheric boundary layer in climate models,including but not limited to partitioning of surface moisture fluxes between evaporation and transpiration.