The influence of the pressure velocity covariance term w'p' on eddy covariance co2 fluxes at a high elevation site in southern Wyoming

Measuring CO2 fluxes with open path eddy covariance sensors typically requires correcting the observed flux data for heat and moisture effects (the Webb-Pearman-Leuning or WPL correction). For the heat flux portion, this correction term is proportional to w'T'/T; where w'T' is the kinematic heat flux and T is the mean ambient temperature. However, because the genesis of this correction is fluctuations in density there should also be a term related to pressure fluctuations. Including the pressure covariance term suggests that the WPL correction should be proportional to [w'T'/T - w'p'/P]; where w'p' is the pressure covariance term and P is the ambient pressure. Under conditions of high wind speeds and low heat fluxes the w'p' can dominate the heat flux component of the correction. This presentation evaluates the influence of the pressure covariance term on eddy covariance CO2 fluxes at a high elevation site in southern Wyoming, USA. The site, part of the larger AmeriFlux network, has made continuous CO2, water vapor, heat and momentum flux measurements since early November of 1999. In addition to an open path CO2 instrument, the data also include p' data gathered with a sensitive differential pressure sensor connected to a Quad-Disc probe designed to sample turbulent atmospheric pressure fluctuations. Selected periods during this longer continuous experiment are intensively analyzed to assess the influence of the w'p' covariance on the magnitude of the fluxes.