Eric S. Russell

Large eddies modulating flux convergence and divergence in a disturbed unstable atmospheric surface layer

The effects of large eddies on turbulence structures and flux transport were studied using data collected over a flat cotton field during the Energy Balance Experiment 2000 in the San Joaquin Valley of California in August 2000. Flux convergence (FC; larger fluxes at 8.7m than 2.7m) and divergence (FD) in latent heat flux (LE) were observed in a disturbed, unstable atmospheric surface layer, and their magnitudes largely departed from the prediction of Monin-Obukhov similarity theory. From our wavelet analysis, it was identified that large eddies affected turbulence structures, scalar distribution, and flux transport differently at 8.7m and 2.7m under the FC and FD conditions. Using the ensemble empirical mode decomposition, time series data were decomposed into large eddies and small-scale background turbulence, the time-domain characteristics of large eddies were examined, and the flux contribution by large eddies was also determined quantitatively. The results suggest that large eddies over the frequency range of 0.002Hz<f<0.02Hz (predominantly 300-400m) enhanced the vertical velocity spectra more significantly at 8.7m than 2.7m, leading to an increased magnitude of the cospectra and thus LE at 8.7m. In the FD case, however, these large eddies were not present and even suppressed in the vertical velocity spectra at 8.7m. Consequently, the cospectra divergence over the low-frequency ranges primarily caused the LE divergence. This work implies that large eddies may either improve or degrade the surface energy balance closure by increasing or decreasing turbulent fluxes, respectively.

Turbulence dependence on winds and stability in a weak‐wind canopy sublayer over complex terrain

The daytime and nighttime turbulence profiles within a weak-wind forest canopy were investigated using data collected within a temperate mixed conifer canopy in northern Idaho, USA. Turbulence measurements made at three heights on a single tower within a Douglas fir canopy were compared. Data were split between the daytime and nighttime to determine the relationships among the local temperature gradient, wind direction, wind speed, and turbulence levels. The total flow field distributions and vertical statistical profiles were determined for the overnight and daytime periods to observe how the overall flow changed with time of day. During the day, the wind probability distribution function was consistent between heights but depended on the canopy depth overnight. The skewness changed with the dominant wind direction. The kurtosis increased with depth into the canopy and from during the day to overnight. The range of wind speeds observed was higher under unstable conditions than stable conditions. Daytime turbulence had no dependence on wind direction. Overnight, the relationship between turbulence and wind speed changed with wind direction and canopy depth. The highest turbulence values were associated with downslope winds near the canopy top but the wind direction for the highest turbulence was variable within the trunk space.

Evidence for Gap Flows in the Birch Creek Valley, Idaho

AbstractA field study was conducted of flows in the Birch Creek Valley in eastern Idaho. There is a distinct topographic constriction in the Birch Creek Valley that creates two subbasins: an upper and lower valley. The data were classified into one of three groups based on synoptic influence (weak/absent, high wind speeds, and other evidence of synoptic influence). Gap flows commonly developed downwind of the constriction in association with the weak/absent group but also occurred in association with the two synoptic groups suggesting the potential for more diverse origins. In general, the frequency and strength of gap flows appeared to be linked to the development of the requisite thermal regime and minimization of any synoptically driven southerly winds that would suppress outflows. Gap flows were characterized by high wind speeds with jetlike vertical profiles along the axis of the lower valley. For all three groups the morning transition in the upper valley and western sidewall usually proceeded sligh...

Wavelet spectra within a forest subcanopy under different wind directions and stabilities

Vertical structure of turbulence within a forest canopy is complex as a result of interactions between the flow dynamics, stability, and vegetation. It remains unclear as to how the combination of the changing stability, flow conditions, and upstream obstacles alter vertical variations in turbulence structures within canopy. Here, we used data from three sonic anemometers within the forest canopy to study the interplay between these factors. The turbulence structure depended upon the wind direction and stability similarly as particular stability conditions were primarily associated with certain wind directions over the sloped terrain. The general shape of the spectra varied with the depth into the canopy, creating a perceived spectral gap near the surface that was not observed near the top of the canopy. The strength of the division between smaller scales and larger scales increased under more stable conditions as the different layers were unable to couple, limiting the passage of energy through the canopy.

Effects of climatic conditions and management practices on agricultural carbon and water budgets in the inland Pacific Northwest USA

Cropland is an important land cover influencing global carbon and water cycles. Variability of agricultural carbon and water fluxes depends on crop species, management practices, soil characteristics, and climatic conditions. In the context of climate change, it is critical to quantify the long-term effects of these environmental drivers and farming activities on carbon and water dynamics. Twenty site-years of carbon and water fluxes covering a large precipitation gradient and a variety of crop species and management practices were measured in the inland Pacific Northwest using the eddy covariance method. The rain-fed fields were net carbon sinks while the irrigated site was close to carbon neutral during the winter wheat crop years. Sites growing spring crops were either carbon sinks, sources, or neutral, varying with crops, rainfall zones, and tillage practices. Fluxes were more sensitive to variability in precipitation than temperature: annual carbon and water fluxes increased with the increasing precipitation while only respiration increased with temperature in the high-rainfall area. Compared to a nearby rain-fed site, irrigation improved winter wheat production but resulted in large losses of carbon and water to the atmosphere. Compared to conventional tillage, no-till had significantly lower respiration but resulted in slightly lower yields and water use efficiency over four years. Under future climate change, it is expected that more carbon fixation by crops and evapotranspiration would occur in a warmer and wetter environment.