Ph.D. Meteorology, University of Melbourne, 2001
Atmospheric and Oceanic Sciences
Global climate modeling, cycles of water and carbon, polar climate variability, large-scale dynamics of the atmospheres and oceans
Current Research: Water and Carbon Cycle Studies in Colorado at the NO AA Tall Tower
The exchange of water and carbon between the atmosphere and the land surface remains poorly understood, particularly in regions of complex terrain and in the case of stable nighttime boundary layers. We are using the stable isotope chemistry of water to evaluate hydrological air-land exchanges of water and energy and to use this to trace the transport of CO2 in the boundary layer. Limitations in knowledge of CO2 transport in the boundary layer is the leading source of error in global model estimates of CO2 fluxes, and understanding water is critical to resolving regional water recycling and the surface energy balance.
Variations in isotope rates arise because the abundance of heavy isotopologues (HDO and H218O) relative to normal water (H216O) changes during evaporation and condensation. Lighter H2O molecules preferentially evaporate, heavier HDO and H218O molecules preferentially condense, and lighter molecules diffuse faster than the heavier ones. In this regard, the isotopic composition can be used for identifying surface water sources and tracking the fate of that water as it moves though the boundary layer.
High-resolution profile measurements of water, the isotopic composition of water, and CO2 concentration were made at the NOAA Boulder Atmospheric Observatory (BAO) tall-tower facility in Colorado (Figure 1). Measurements were made by placing instruments on the elevator and controlling the ascent and decent every 15 minutes, leading to a total of 311 profiles (Figure 2). During an observation period that followed a storm, melt and evaporation of about one inch of snow shows the isotopic composition unambiguously tracks the fate of the water as it was transported from the surface though the surface layer during the daytime and that there is a significant advective sink. Nighttime conditions were very stable, leading to strong surface trapping of water vapor and also restricting transport of near-surface CO2. The experimental results are of particular interest because of the extremely weak transport during times of the very stable nighttime boundary layer, which is poorly modeled even in state-of-the-art climate models. Our analysis shows that this is associated with a broad failure of simple turbulence theory, which is of the type typically found in global- and regional-scale models. To this end, improving the representation of the evolution of our observed water isotope and CO2 profiles provides a clear way to improve models.