Atmospheric water: Perspectives from isotopes and the NCAR climate model

The input of water vapor from the surface and its cycling in the atmosphere is a critical determinant of climate. The isotopic composition of water, in particular, can help understanding and diagnosing each processes of water cycle from evaporation from the ocean to precipitation. In my thesis, I focus on two aspects of atmospheric water. The first investigates the distribution of isotope ratios in the atmospheric water using atmospheric general circulation model. The second seeks to determine the role of root functioning in increasing plant usage of precipitation as a transpiration source of water during the dry season.; The cycling of water isotopes is incorporated into the NCAR atmospheric general circulation model, CAM2. Isotope dynamics follow that of the previous isotope-GCM models, with fractionation associated with evaporation at the surface as well as with cloud processes. A new feature is the direct estimation of the degree of isotopic equilibrium between vapor and raindrops as a function of temperature and rain rate. The model yields a reasonable global pattern of water isotopes in precipitation (deltappt); e.g., the delta ppt decreases with decreasing temperature and increasing precipitation amount. Over the ocean, the balance between precipitation and evaporation determines the isotopic composition of precipitation and vapor, with delta ppt less depleted where evaporation minus precipitation is high. The isotopic composition for the tropical troposphere does not decrease vertically as much as we would expect from the Rayleigh distillation model as a result of transport of heavy isotope-enriched surface vapor by convection.; The application of water isotopes to estimate the glacial-interglacial cycle of temperature (T) assumes the validity of the present-day spatial relationship between T and deltappt to estimate temporal changes of the temperature at a fixed location. We explored how and why the spatial relationship between annual mean T-delta ppt is different from the temporal relationship at one location.; A plant root mechanism that facilitates storage and subsequent use of soil water could influence the seasonal variations of heat and moisture fluxes. Redistribution of soil water by roots (termed hydraulic redistribution; HR)---the nocturnal vertical transfer from moister to drier regions in the soil profile---has now been observed in Amazonian trees. HR was incorporated into the NCAR CAM2 to explore its impact on climate over the Amazon and the globe. Model results show that evapotranspiration does not decrease significantly in the Amazon during the dry season due to HR. (Abstract shortened by UMI.)...