Closure of the surface energy budget at 10 OASIS super sites

A historical review of the literature confirms a pattern of underestimating the sum of sensible and latent heat flux (SH+LH) by eddy correlation (EC) when compared to the net available energy of net radiation and ground heat flux (Rn – GH). An underestimation in (SH+LH) leads to an underestimation in closure as defined as C=(SH+LH)/(Rn-GH). Results from FIFE, Monsoon-90, HAPEX-SAHEL, and SGP-97 reflect an underestimation in closure from EC observations.

During 1999 the Oklahoma Atmospheric Surface-layer Instrumentation System (OASIS) Project instrumented 10 Oklahoma Mesonet sites with the capability to monitor the entire surface energy budget in real-time. A four-component net radiometer, the CNR1, measures incoming and outgoing shortwave and longwave radiation, and ground heat flux is estimated from a suite of in-ground sensors. A sonic anemometer and Krypton hygrometer, sampling at 8 Hz, provide measurements of sensible and latent heat flux, respectively. Observations of each component are averaged and recorded every 5-minutes.

A complete one-year data set of surface energy budget measurements collected from the ten OASIS sites are examined in detail. The OASIS data set yields similar results to previous work with a consistent underestimation in closure. Daily estimates of closure range between 70% and 92% at the ten sites. A brief synopsis of closure is presented followed by several hypotheses as to the reasons for the underestimation. Results focus upon the effects of topography, vegetation, and instrument error.