We examine the dynamics of turbulence subgrid (or sub-filter) scales over a lake surface and the implications for large-eddy
simulations (LES) of the atmospheric boundary layer. The analysis is based on measurements obtained during the Lake-Atmosphere
Turbulent EXchange (LATEX) field campaign (August–October, 2006) over Lake Geneva, Switzerland. Wind velocity, temperature
and humidity profiles were measured at 20 Hz using a vertical array of four sonic anemometers and open-path gas analyzers.
The results indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces, suggesting
that the effect of the lake waves on surface-layer turbulence during LATEX is small. The measurements allowed, for the first
time, the study of subgrid-scale turbulent transport of water vapour, which is found to be well correlated with the transport
of heat, suggesting that the subgrid-scale modelling of the two scalars may be coupled to save computational resources during
LES. 相似文献
During a haze event in Baltimore, U.S.A. from July 6 to 8, 2002, smoke from forest fires in the Québec region (Canada), degraded air quality and impacted upon local climate, decreasing solar radiation and air temperature. The smoke particles in and above the atmospheric boundary layer (ABL) served as a tracer and provided a unique opportunity to investigate the ABL structure, especially entrainment. Elastic backscatter lidar measurements taken during the haze event distinctly reveal the downward sweeps (or wisps) of smoke-laden air from the free atmosphere into the ABL. Visualisations of mechanisms such as dry convection, the entrainment process, detrainment, coherent entrainment structures, and mixing inside the ABL, are presented. Thermals overshooting at the ABL top are shown to create disturbances in the form of gravity waves in the free atmosphere aloft, as evidenced by a corresponding ripple structure at the bottom of the smoke layer. Lidar data, aerosol ground-based measurements and supporting meteorological data are used to link free atmosphere, mixed-layer and ground-level aerosols. During the peak period of the haze event (July 7, 2002), the correlation between time series of elastic backscatter lidar data within the mixed layer and the scattering coefficient from a nephelometer at ground level was found to be high (R=0.96 for z =324 m, and R=0.89 for z=504 m). Ground-level aerosol concentration was at a maximum about 2 h after the smoke layer intersected with the growing ABL, confirming that the wisps do not initially reach the ground. 相似文献
The fair and effective governance of freshwater is an increasingly prominent issue in New Zealand. Emerging from a complex of cultural, economic and biophysical narratives, freshwater geographies are multiple, varied and increasingly acknowledged as worthy of interdisciplinary scrutiny. In this commentary, we reflect on a series of generative spaces that we – as group of postgraduate geographers (plus supporting staff) – created to engage with the multiplicity of freshwater meanings both within and beyond the academy. Through this evolving epistemic‐political project, we significantly reframed our own understandings about what freshwater ‘is’ and how it ought to be governed. By pursuing a deeper understanding of how the world gets made, we expand our ability to know and make it differently. 相似文献
High-elevation mountains often constitute for basins important groundwater recharge sources through mountain-front recharge processes. These processes include streamflow losses and subsurface inflow from the mountain block. However, another key recharge process is from irrigation practices, where mountain streamflow is distributed across the irrigated piedmont. In this study, coupled groundwater fluctuation measurements and environmental tracers (18O, 2H, and major ions) were used to identify and compare the natural mountain-front recharge to the anthropogenically induced irrigation recharge. Within the High Atlas mountain front of the Ourika Basin, Central Morocco, the groundwater fluctuation mapping from the dry to wet season showed that recharge beneath the irrigated area was higher than the recharge along the streambed. Irrigation practices in the region divert more than 65% of the stream water, thereby reducing the potential for in-stream groundwater recharge. In addition, the irrigation areas close to the mountain front had greater water table increases (up to 3.5 m) compared with the downstream irrigation areas (<1 m increase). Upstream crops have priority to irrigation with stream water over downstream areas. The latter are only irrigated via stream water during large flood events and are otherwise supplemented by groundwater resources. These changes in water resources used for irrigation practices between upstream and downstream areas are reflected in the spatiotemporal evolution of the stable isotopes of groundwater. In the upstream irrigation area, the groundwater stable isotope values (δ18O: −8.4‰ to −7.4‰) reflect recharge by the diverted stream water. In the downstream irrigation area, the groundwater isotope values are lower (δ18O: −8.1‰ to −8.4‰) due to recharge via the flood water. In the nonirrigation area, the groundwater has the highest stable isotope values (δ18O: −6.8‰ to −4.8‰). This might be due to recharge via subsurface inflow from the mountain block to the mountain front and/or recharge via local low altitude rainfall. These findings highlight that irrigation practices can result in the dominant mountain-front recharge process for groundwater. 相似文献
The weathering of mantle peridotite tectonically exposed to the atmosphere leads commonly to natural carbonation processes. Extensive cryptocrystalline magnesite veins and stock-work are widespread in the serpentinite sole of the New Caledonia ophiolite. Silica is systematically associated with magnesite. It is commonly admitted that Mg and Si are released during the laterization of overlying peridotites. Thus, the occurrence of these veins is generally attributed to a per descensum mechanism that involves the infiltration of meteoric waters enriched in dissolved atmospheric CO2. In this study, we investigate serpentinite carbonation processes, and related silicification, based on a detailed petrographic and crystal chemical study of serpentinites. The relationships between serpentine and alteration products are described using an original method for the analysis of micro-X-ray fluorescence images performed at the centimeter scale. Our investigations highlight a carbonation mechanism, together with precipitation of amorphous silica and sepiolite, based on a dissolution–precipitation process. In contrast with the per descensum Mg/Si-enrichment model that is mainly concentrated in rock fractures, dissolution–precipitation process is much more pervasive. Thus, although the texture of rocks remains relatively preserved, this process extends more widely into the rock and may represent a major part of total carbonation of the ophiolite. 相似文献
To cope with water scarcity in drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. These stormwater harvesting systems are often accompanied by high evaporation rates and hygiene problems. This is commonly a consequence of low infiltration rates, which are caused by clogging layers that form on top of the soil profile and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using analytical equations, numerical models, and sensitivity analyses. Dams built in the channel of ephemeral streams (wadis) are selected as a study case to construct the numerical simulations. The modelling demonstrated that vadose-zone wells and infiltration trenches contribute to effective bypassing of the clogging layer. By implementing these solutions, recharge begins 2250–8100% faster than via infiltration from the bed surface of the wadi reservoir. The sensitivity analysis showed that the recharge rates are especially responsive to well length and trench depth. In terms of recharge quantity, the well had the best performance; it can infiltrate up to 1642% more water than the reservoir, and between 336 and 825% more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and high recharge rates when there are limitations to the available area. The methods investigated here significantly increased recharge rates, providing practical solutions to enhance aquifer water storage in drylands.