Relations between environmental conditions and the ability to close the energy balance

Any estimates of the transport of mass and energy at the surface must be analyzed to assess reliability and accuracy. A direct approach to this issue is problematic with eddy covariance measurements. However, one approach that offers a measure of self-consistency is to examine the energy balance closure values, defined as the ratio of turbulence energy fluxes over available energy. As reported data continues to grow over a range of surfaces and conditions, it is clear that there is a systematic bias in eddy covariance flux estimates. However, the range of energy balance closure values is rather large, and can even be quite variable at a given location. The implications for this bias are very compelling for specific issues such as water and CO2 budgets. At present there is no general agreement as to the causes of the bias, or what, if anything, to do in response.

This study will examine the variations in energy balance closure for a set of sites in a large soil moisture and atmospheric coupling experiment over an agricultural watershed in Iowa. At each site the complete energy balance was estimated for nearly an entire growing season. The key objective is to determine if there are certain types of atmospheric conditions that are associated with lower and higher values of energy balance closure. The variations in energy balance closure will be documented, and related to stage of growth. The effects of averaging period will be addressed, and connected to the degree of stationary conditions exhibited for wind, temperature, and humidity. Then the connections between short-term changes in energy balance closure and key environmental properties will be examined. These properties include various turbulence statistics, temporal variance in available energy, the development of the atmospheric boundary layer, and any transitional states that often occur during morning and evening periods.