Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications

During the years 2010–13, atmospheric eddy covariance measurement of oxygen was performed at the marine site Östergarnsholm in the Baltic Sea. The fast response optode Microx TX3 was used with two different types of tapered sensors. In spite of the increased lifetime, the optical isolated sensor is limited by the slower response time and is unsuitable for ground-based eddy covariance measurements. The sensor without optical isolation shows a −⅔ slope within the inertial subrange and attains sufficient response time and precision to be used in air–sea applications during continuous periods of 1–4 days. Spectral and cospectral analysis shows oxygen measured with the nonoptical isolated sensor to follow the same shape as for CO2 and water vapor when normalized. The sampling rate of the Microx TX3 is 2 Hz; however, the sensor was found to have a limited response and resolution, yielding a flux loss in the frequency range f > 0.3 Hz. This can be corrected for by applying cospectral similarity simultaneously using measurements of latent heat as the reference signal. On average the magnitude of the cospectral correction added 20% to the uncorrected oxygen flux during neutral atmospheric stratification.

Jet Interaction and the Influence of a Minimum Phase Speed Bound on the Propagation of Eddies

The feedback between planetary-scale eddies and analogs of the midlatitude eddy-driven jet and the subtropical jet is investigated in a barotropic β-plane model. In the model the subtropical jet is generated by a relaxation process and the eddy-driven jet by an imposed wavemaker. A minimum zonal phase speed bound is proposed in addition to the established upper bound, where the zonal phase speed of waves must be less than that of the zonal mean zonal flow. Cospectral analysis of eddy momentum flux convergence shows that eddy activity is generally restricted by these phase speed bounds. The wavenumber-dependent minimum phase speed represents a turning line for meridionally propagating waves. By varying the separation distance between the relaxation and stirring regions, it is found that a sustained, double-jet state is achieved when either a critical or turning latitude forms in the interjet region. The interjet turning latitude filters eddies by zonal wavenumber such that shorter waves tend to be reflected off of the relaxed jet and are confined to the eddy-driven jet. The interjet region is transparent to long waves that act to blend the jets and may be associated with barotropic instability. The eddy-driven and relaxed jets tend to merge owing to the propagation of these long waves through the relaxed jet waveguide.

Cospectral analysis of high frequency signal loss in eddy covariance measurements

The cospectra of momentum (M), sensible heat (H), latent heat (LE), and carbon dioxide (Fc) fluxes measured by eddy covariance (EC) over a shortgrass steppe are calculated for over 800 time intervals spanning a range of wind, surface heating, evaporative, and photosynthetic conditions. The power spectrum of the vertical wind clearly shows that the inertial subrange is not sufficiently captured. The cospectra of the different fluxes show that the lack of measurement resolution in the high frequency results in a loss of flux, especially as stability approaches neutral. A procedure is outlined to use statistics from the cospectrum to estimate the amount of high-frequency flux that remains unmeasured for each time interval. The greatest loss of flux was for H (14% on average for 0>z/L>0.001 where z/L is the dimensionless stability), consistent with other studies which indicate temperature fluctuations actively produce turbulence at high frequencies. LE and Fc showed less than half as much loss of flux as H. This differential loss of flux has direct implications for addressing energy balance closure in EC studies, as well as reconciling biases of fluxes measured by EC with the Modified Bowen Ratio technique. It is recommended that the cospectra of fluxes be examined while setting the height of instrumentation in order to insure that high frequency eddies are resolved.

Turbulent penetration of a thermally stratified interfacial layer in a wind tunnel

A stably stratified interface, with strong turbulence below and quiescent air above, is studied in a wind tunnel with the aim of simulating the conditions at the inversion cap at the top of the atmospheric boundary layer. The interfacial layer was generated by means of a composite grid, with small mesh size above and a large one below (Veeravalli & Warhaft 1989). A temperature step generated in the plenum of the wind tunnel, was located at the centre of the layer. There is no shear and thus turbulence interactions, usually masked by turbulent production in traditional mixing layers, are highlighted. Close to the grid where the velocity fluctuations are strong, buoyancy effects are insignificant, but as the turbulence decays they become dominant. The bulk Richardson number, N2B/(〈u2〉2/L2u), where NB is the Brunt—Väisälä frequency across the layer, and 〈u2〉2 and Lu are the velocity variance and integral lengthscale, respectively, of the turbulence on the lower side of the layer, varied from approximately zero close to the grid to 80 far downstream. The stratification inhibited the turbulent penetration into the layer, reducing the high skewness and kurtosis of the velocity field for the neutral case, to Gaussian values. The layer, which initially thickened with downstream distance, thinned when buoyancy became pronounced, owing to the collapse of the heat flux. Significant regions of countergradient heat flux, and reversals in sign of the triple moment transport terms were observed in the upper part of the layer. An analysis of the value of the heat flux conditioned on the temperature fluctuations, showed that the large temperature fluctuations associated with weak turbulence became affected by stratification first. Cospectral analysis shows that these fluctuations are associated with large scales. We also show that although the joint normal approximation between velocity and temperature fluctuations is sound for a passive scalar field, it becomes less good with the onset of stratification, failing completely when the stratification is strong.