Zhongming Gao

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.

Project Sagebrush: Revisiting the Value of the Horizontal Plume Spread Parameterσy

AbstractThe first phase of an atmospheric tracer experiment program, designated Project Sagebrush, was conducted at the Idaho National Laboratory in October 2013. The purpose was to reevaluate the results of classical field experiments in short-range plume dispersion (e.g., Project Prairie Grass) using the newer technologies that are available for measuring both turbulence levels and tracer concentrations. All releases were conducted during the daytime with atmospheric conditions ranging from neutral to unstable. The key finding was that the values of the horizontal plume spread parameter σy tended to be larger, by up to a factor of ~2, than those measured in many previous field studies. The discrepancies tended to increase with downwind distance. The values of the ratio σy/σθ, where σθ is the standard deviation of the horizontal wind direction, also trend near the upper limit or above the range of values determined in earlier studies. There was also evidence to suggest that the value of σy began to be in...

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.

Enhanced Temperature‐Humidity Similarity Caused by Entrainment Processes With Increased Wind Shear

A number of studies already suggested that entrainment of warm/dry air from aloft degrades the similarity between air temperature (T) and specific humidity (q) in the atmospheric surface layer (ASL). Less is known about entrainment of cool/dry air on the ASL T-q similarity. Using eddy covariance measurements over a large inland water surface, enhanced T-q similarity with increased friction velocity (u*) was measured during daytime unstable conditions. Explaining this enhancement is the main goal of this work. Cool/dry air masses originating and advecting from land set the upper boundary condition on a quasi-equilibrated internal boundary layer embedding the warm/moist ASL over water. With increased u*, the entrainment velocity (dhI/dt ∝ u*) increases, thereby enhancing entrainment of cool/dry air originating from land, where t is time and hI is the internal boundary layer depth. By analyzing the scale-dependent (spectral) correlation coefficients, the enhanced T-q similarity with increased u* was shown to originate from spectral correlation coefficients in the middleto low-frequency ranges (large eddies ≫ z, the distance from the surface). With further increases in u*, similarity between T and qwithin large eddies was further enhanced and phase differences between T and q was reduced. Quadrant analysis showed large increases in flux and time fractions of downward large cool/dry eddies with increasing u*. These results provide evidence that enhanced entrainment of synchronized, large cool/dry eddies was a primary cause for the increased T-q similarity with increasing u*.

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...

Plume Dispersion in Low-Wind-Speed Conditions During Project Sagebrush Phase 2, with Emphasis on Concentration Variability

Eight short-range, open-terrain SF6 tracer tests in low wind speeds were conducted during Phase 2 of Project Sagebrush using continuous releases. Four tests were made during very unstable conditions in July and August 2016, and four during very stable conditions in October 2016. All tests featured 10-min averaging and 1-Hz sampling of tracer concentrations together with an extensive suite of meteorological measurements. We find that the uncertainty in well-mixed daytime measurements of tracer concentrations, using the absolute value of the relative percentage difference in collocated duplicate samplers, approaches a downwind limit of about 7–8%. Concentration variability in collocated sampling, due to stochastic factors and independent of measurement uncertainty, increases the total observational uncertainty closer to the source from about 20% (daytime) to 40% (very stable conditions). Longer averaging periods moderately reduce the concentration variability. The data indicate that the large increase in concentration variability is linked with the suppression of turbulent mixing, small eddy length scales, and meandering in very stable conditions. These results should be considered when comparing observations with model predictions in evaluations.

Interaction Forest–Clearing

In 2011, a special experiment was conducted to investigate turbulent structures at the edge between the Waldstein–Weidenbrunnen forest site and the Kohlerloh clearing. A horizontal moving measuring system was used to detect significant gradients of the radiation fluxes, temperature, moisture, ozone, and carbon dioxide concentrations for different situations at day and night. In agreement with other studies, an increase of the turbulent fluxes and ejections at the forest edge could be found. This means that the energy balance closure was also better than that obtained directly at the Weidenbrunnen site. The vertical coupling by coherent structures was often—mainly at daytime—very good. In contrast, the horizontal coupling between the forest and the clearing at the edge was, in most cases, not apparent. For wind directions coming from the forest, the coherent structures did not touch down at the surface of the clearing. These investigations were made with a wavelet tool. A clear indication of secondary circulations between the forest and the clearing was not possible.