A spectral analysis of biosphere-atmosphere trace gas flux densities and meteorological variables across hour to year time scales

The advent of long-term studies on mass and energy exchange provides us with new information on how the atmosphere and biosphere interact. Conventional time-series analysis suggests that temporal fluctuations of weather variables and mass and energy flux densities occur on numerous time scales. The spectra of time scales that characterize the variance of an annual time course of simultaneous records of meteorological variables and scalar flux, however, remains unquantified. To study this problem, we applied Fourier analysis to multiple (4) yearlong records of quantum flux density, air temperature, wind speed, pressure and the flux densities of CO2 and water vapor.

The analysis shows that strong spectral peaks occur in the meteorological and flux density records at periods of 12 and 24 hours. At the synoptic time scale (3 to 10 days) numerous, but ill-distinguished, spectral peaks and gaps are identified. These are a consequence of the passage of weather fronts that alter available sunlight and temperature experienced by the ecosystem. A pronounced spectral gap is associated with periods three to four weeks long. Finally, a broad spectral peak occurs in the spectral band with periods ranging between three and six months due to seasonal changes in weather and plant functionality.

Multi-scale interactions between CO2 flux density and sunlight, air temperature and latent heat flux density were identified using co-spectral and phase angle analysis. Temporal variations in CO2 flux densities are in phase with changes in light, air temperature and latent heat exchange on annual and seasonal time scales. On the monthly to weekly time scales, instances occur when changes in CO2 flux density leads and lags variations of incident sunlight, air temperature and latent heat exchange. On a daily time scale, variations in CO2 flux density are in phase with variations in sunlight. The phase relation between CO2 flux density and air temperature is a function of whether the plants are active or dormant.

Spectral analysis was also used to test the parameterization and application of an air-surface exchange model across a spectrum of time scales. We compared the spectra of field measurements and computations that were generated with the CANOAK model. During a year with adequate soil moisture, there was good correspondence between spectral peaks and gaps detected by measurements and model computations, by a model that considers temporal dynamics in leaf area and photosynthetic capacity.