Eiji Ohtaki

On the similarity in atmospheric fluctuations of carbon dioxide, water vapor and temperature over vegetated fields

This paper describes the similarity between atmospheric fluctuations of carbon dioxide, water vapor and temperature using data which cover a wide range of instability (0.02 < ζΝ < 10). The ζΝ is the Monin-Obukhov stability parameter including the humidity effect.The spectral analysis shows that the coherency between fluctuations of carbon dioxide and water vapor or temperature is very close to unity, and the phase difference is basically out of phase for whole frequency ranges analyzed. The stability dependence of the normalized standard deviation of carbon dioxide is very similar to those of water vapor and temperature. The normalized standard deviation is about 2.5 under near neutral conditions, and it decreases with increasing instability following the -1/3; power law as (-ζΝ)-1/3. The skewness factors of carbon dioxide, water vapor and temperature show a systematic departure with increasing instabilities for 0.02 < s-ζΝ < 1, and level off at high instabilities for 1 < -\s ζΝ < 10. The stability dependence of the flatness factors is not so clear as that noted in the standrard deviation and skewness factors. Dissipation rates of carbon dioxide, water vapor and temperature variance are well related to the spectral peak wavelength. This seems to be real since the local production and local dissipation rates are the main terms, almost balancing one another in the variance budget equations for scalar entities.

Infrared device for simultaneous measurement of fluctuations of atmospheric carbon dioxide and water vapor

An infrared device designed to measure simultaneous fluctuations of atmospheric CO2 and water vapor concentrations is described. The measuring frequency is 30 Hz. The sensing path length is 20 cm. It is compatible with the path length of the standard type of a sonic anemometer. The noise level of the device is equivalent to fluctuations of about 0.8 ppm peak-to-peak for CO2 and 0.02 g kg-1 peak-to-peak for water vapor. Field tests have showed that the device is suitable for simultaneous measurement of turbulent fluxes of CO2 and water vapor in conjunction with a sonic anemometer.

Effects of heat and water vapor transport on eddy covariance measurement of CO2 fluxes

Flux densities of carbon dioxide were measured over an arid, vegetation-free surface by eddy covariance techniques and by a heat budget-profile method, in which CO2 concentration gradients were specified in terms of mixing ratios. This method showed negligible fluxes of CO2, consistent with the bareness of the experimental site, whereas the eddy covariance measurements indicated large downward fluxes of CO2. These apparently conflicting observations are in quantitative agreement with the results of a recent theory which predicts that whenever there are vertical fluxes of sensible or latent heat, a mean vertical velocity is developed. This velocity causes a mean vertical convective mass flux (= ρcw for CO2, in standard notation). The eddy covariance technique neglects this mean convective flux and measures only the turbulent flux ρ′c w′. Thus, when the net flux of CO2 is zero, the eddy covariance method indicates an apparent flux which is equal and opposite to the mean convective flux, i.e., ρ′c w′ = −ρc w. Corrections for the mean convective flux are particularly significant for CO2 because ρcw and ρ′c w′ are often of similar magnitude. The correct measurement of the net CO2 flux by eddy covariance techniques requires that the fluxes of sensible and latent heat be measured as well.

The international at-sea intercomparison of fCO2 systems during the R/V Meteor Cruise 36/1 in the North Atlantic Ocean

The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements.

Application of an infrared carbon dioxide and humidity instrument to studies of turbulent transport

A calibration equation and some results of the field performance of an infrared instrument, which is designed to measure simultaneous fluctuations of atmospheric carbon dioxide and water vapor, are described. Field observations show that the instrument is suitable for simultaneous measurement of turbulent fluxes of carbon dioxide and water vapor in conjunction with a sonic anemometer. Measured values of carbon dioxide and water vapor fluxes show diurnal variations characterized by crop activity with respect to assimilation, respiration and evapotranspiration. Carbon dioxide is transferred downward during the daytime and upward at night, while latent heat and sensible heat are transferred in the opposite sense. The non-dimensional gradient of carbon dioxide is expressed in the following form under weak unstable conditions: Φc= (1 − 16ζv)-1/2. Here, ζv is the Monin-Obukhov stability parameter including the humidity effect. This relation was originally proposed for temperature and humidity. Thus, the results indicate that the turbulent mechanisms of carbon dioxide fluctuations are similar to those of other scalar entities. This is strongly supported by the high correlation coefficient found between fluctuations of carbon dioxide and temperature or humidity in the air layer over crop fields.

Turbulent transport of carbon dioxide over a paddy field

Turbulent fluctuations in CO2 concentrations over a paddy field are measured by a fastresponse device with an open sensing path. This IR device coupled with a sonic anemometer constitutes an eddy correlation instrument to measure CO2 fluxes. Three experiments were conducted in the surface layer over paddy 90 cm high. The stability (z − d)/L ranged from -0.14 to 0.20, where L denotes the Monin-Obukhov length.CO2 power spectra show the range of applicability of the -2/3 power law to be between f = 0.2 and f = 2, where f is the frequency normalized by wind speed and height. The cospectral estimate between CO2 and vertical component of wind speed ranging from f = 0.005 to f = 2 shows a peak at about f = 0.15 under near-neutral stratification.Hourly means of CO2 flux measured by the eddy correlation method increase with intensity of net radiation. The maximum value of downward flux of CO2 rises to 0.6 mg cm-2 hr-1 over the paddy field at the stage of ear emergence.Some turbulence statistics relating to the CO2 transport are evaluated: the correlation coefficient between CO2 and vertical velocity is about -0.3, and that between CO2 and humidity attains -0.7 ~ -0.8 under unstable stratification; nondimensional gradients Φc for CO2 and Φm for wind speed are 0.89 and 0.99, respectively.

Controlling factors on the interannual CO2 budget at a subarctic black spruce forest in interior Alaska

The eddy covariance method was applied to measure net ecosystem CO2 exchange (NEE) at a subarctic black spruce forest in interior Alaska during 2003 and 2004. To clarify the budget of CO2, we divided photosynthesis and respiration by applying the Carbon Budget Analysis Tool, in which the potential photosynthetic rate, the light use efficiency, and the suppression factors on photosynthesis were evaluated. The potential photosynthetic rate and the light use efficiency were related with the understory leaf area index. The determined optimal temperature of photosynthesis was higher than 15 ◦C, which was higher than that of other boreal forests. Drought in midsummer of 2004 suppressed both photosynthesis and respiration, but the suppression was more effective in photosynthesis, resulting in a slightly decreased NEE. Cumulative respiration and photosynthesis were 2.29 and -2.50 kg CO2 m-2 in 2003, and 2.37 and -2.44 kg CO2 m-2 in 2004, resulting in calculated annual CO2 sink budgets of -210 and -70 g CO2 m-2 in 2003 and 2004, respectively.

Turbulent transport processes of momentum and sensible heat in the surface layer over a paddy field

A sonic anemometer-thermometer was used to measure turbulent fluxes of momentum and sensible heat and related turbulence statistics just above plant canopies in unstable conditions. The stability dependence of transport processes is presented, using observational data in a wide range of instability. The analysis of joint probability distributions of w − u, w − T, w − wu, and w − wT confirmed that just above plant canopies, downdrafts were remarkably efficient for vertical transport of momentum and scalar quantities in near neutral conditions. Furthermore, it was shown that updrafts became more efficient than downdrafts for vertical transport, especially of scalar quantities, in very unstable conditions.

An Independent Method to Determine the Height of the Mixed Layer

A method for the independent evaluation ofmixed-layer (ML) height, zi, has beenproposed. The ML height is determined bythe functional relationshipszi = 0.75 z/nv max or 0.53 z/nu max in which, z is a measuring height; nv max and nu max are normalized peak frequencies of lateral velocity component, v, and longitudinal velocity component, u, spectra at height z in the surface layer respectively. Using Doppler sodar data, the technique was shown to be feasible; it is easy to apply to micro-meteorological field experiments and works even for the ML top above the range of the sodar.

A Study of Correlations of Scalar Quantities in the Atmospheric Surface Layer

Standard deviations for vertical velocity and scalar quantities, such as temperature, T, and specific humidity, q, were analyzed on the basis of Monin-Obukhov (M-O) similarity theory in the atmospheric surface layer. The correlation coefficient between scalar quantities T and q, ∣RTq∣, was derived from the similarity functions and can be expressed as the ratio of BT/Bq (BT≤ Bq), where parameter B is the value of the normalized standard deviation of any scalar quantity at neutral conditions.