Title: Eddy diffusivities for sensible heat, ozone and momentum from eddy correlation and gradient measurements
Description: Micrometeorological field measurements of the fluxes and the gradients of momentum, sensible heat and ozone are presented and discussed. The eddy-correlation measurement technique was used to obtain the flux data at the heights of three and eight meters. A method to accurately measure mass (ozone) gradients from surface -layer based meteorological towers was developed and used. Both flux and gradient measurements are used for the determination of eddy diffusivities. Exploratory analyses were made with the data to investigate similarity relationships between the eddy diffusivities of momentum K_{ rm m}, sensible heat K_ {rm h}, and mass K_ {rm c}, where ozone was used as the mass tracer. Eddy-diffusivity ratios were computed using dimensionless -gradient ratios classified from the data and from regression models. These ratios were classified by atmospheric stability determined at the geometric mean of the measurement heights. The assumption of similarity between the eddy diffusivities of ozone and sensible heat, K_ {rm c} = K_{ rm h}, based on scalar turbulent transfer theory, was verified for unstable atmospheric conditions. The results for eddy diffusivities of sensible heat and ozone for stable atmospheric conditions however, show that diffusivities of sensible heat are 50% greater than diffusivities of ozone. Chemical reaction of ozone, and/or the need for flux-measurement corrections, decrease the resulting values for ozone diffusivities during stable periods. Established eddy-diffusivity ratios for water vapor and momentum are valid for ozone and momentum under stable-atmospheric conditions over smooth-terrain but not under unstable conditions for flow disturbed by irregular terrain. The relationships between the eddy diffusivities of momentum and the eddy diffusivities of ozone, as well as those between momentum and sensible heat are controlled by free-convection conditions, K_{ rm m} < K_ {rm c} and K_{ rm m} < K_ {rm h}; these results are inconclusive for unstable-atmospheric, smooth-terrain conditions for the Pawnee site during easterly winds. For disturbed-flow conditions at the Pawnee site during westerly winds, the eddy diffusivities for momentum were enhanced and both K_{rm c}/K _{rm m} and K_ {rm h}/K_{rm m} become approximately constant 0.35 and 0.5 respectively for both stable and unstable conditions. Theoretical flux measurement corrections were applied to a subset of the data in an attempt to investigate the concept of a constant flux layer and to validate flux correction theory. The results indicate that flux corrections are warranted. However, improvements to flux correction theory are needed especially for stable atmospheric conditions as demonstrated by the significant increase in data variability as a result of these corrections. The "constant flux-layer" concept holds for momentum flux and sensible heat flux but is questionable for ozone flux.
Subjects: Turbulent diffusion (Meteorology), Eddy flux, Atmospheric turbulence, Micrometeorology
Places: Pawnee National Grassland Region (Colo.)
Time periods: mcrometeorology