Xuhui Lee

Sara Shallenberger Brown Professor of Meteorology

Research Overview

My research concerns the movement of momentum, energy, water, greenhouse gases, and air pollutants between the earth’s surface and the lower atmosphere. The principles that govern such movement are drawn from several scientific disciplines including forest meteorology, boundary layer meteorology, ecosystem ecology, and atmospheric chemistry. The following is a brief summary of my current research projects.

Modeling study of land-atmosphere interactions: The objective of this project is to establish a mechanistic understanding of the interplay among air flow in the lowest portion of the atmosphere, spatial variation in land use, and fluxes of energy, water and carbon dioxide. In a typical weather and climate model, vegetation covers within a model grid are averaged to a single type and air over the grid space is treated as a one-dimensional column. This simplification may grossly underestimate the impact of biology on the atmospheric state and processes. The project will deploy a modeling methodology that couples the NCAR’s large eddy simulation model to a land surface model. It will determine biases in climate and weather prediction models when assimilating measurements taken in the presence of land surface heterogeneity.

Ecosystem-atmosphere oxygen isotopic fluxes and discrimination mechanisms: The goal of this project is to investigate processes that control the exchange of oxygen isotopes in carbon dioxide and water vapor between terrestrial vegetation and the atmosphere. Specifically, the project aims to establish a body of new knowledge on the oxygen isotope discrimination mechanisms of the whole-ecosystem photosynthesis and respiration, quantify the interplay between water and carbon dioxide isotopic exchanges at the ecosystem scale, and compare and contrast the discrimination mechanisms between C3 and C4 photosynthesis modes. The project will use soybean and corn as model systems for C3 and C4 plants. The ability to determine the ecosystem-scale oxygen isotope discrimination is essential to a number of fundamental science questions relevant to atmospheric carbon budget and ecosystem processes. In the former case, atmospheric oxygen isotopes are important tracers of carbon uptake on land. In ecosystem studies, quantification of the discrimination mechanisms can be brought to bear on the question of how to utilize atmospheric oxygen isotopes as indicators of environmental changes such as increased biospheric productivity and land use change.

CO2 emissions from forest soil during rainstorm:  In this project, we use in-situ eddy covariance observation, field manipulative experiment and laboratory simulation to investigate the influence of rainstorm on soil CO2 emission. Early results from our Great Mountain forest site show that CO2 emission responds rapidly and instantaneously to the onset of rain. The pulse-like emission could amount to 5-10% of the annual net carbon sequestration by the forest in a single intensive storm. If precipitation becomes more variable in a future warmer world, the rain pulse should play an important part in the transient response of the ecosystem carbon balance to climate, particularly for ecosystems on ridge-tops with rapid water drainage.