Interactions between diurnal winds and floodplain mosaics control the insect boundary layer in a river corridor

Insect flight along river corridors is a fundamental process that facilitates sustainable succession and diversity of aquatic and terrestrial insect communities in highly dynamic fluvial environments. This study examines variations in the thickness of the insect boundary layer (i.e., the pre-surface atmosphere layer in which air velocity does not exceed the sustained speed of flying insects) caused by interactions between diurnal winds and the heterogenous habitat mosaics in the floodplain of a braided river. Based on advective–diffusive theory, we develop and test a semi-empirical model that relates vertical flux of flying insects to vertical profiles of diurnal winds. Our model suggests that, in the logarithmic layer of wind, the density of insect fluxes decreases exponentially with the altitude due to the strong physical forcing. Inside the insect boundary layer, the insect fluxes can increase with the altitude while the winds speed remains nearly constant. We suggest a hypothesis that there is a close correspondence between the height of discontinuity points in the insect profiles (e.g. points with abrupt changes of the insect flux) and the displacement heights of the wind profiles (e.g. points above which the wind profile is logarithmic). Vertical profiles were sampled during three time-intervals at three different habitat locations in the river corridor: a bare gravel bar, a gravel bar with shrubs, and an island with trees and shrubs. Insects and wind speed were sampled and measured simultaneously over each location at 1.5-m intervals up to approximately 17 m elevation. The results support our working hypothesis on close correspondence between discontinuity and displacement points. The thickness of the insect boundary layer matches the height of the discontinuity points and was about 5 m above the bare gravel bar and the gravel bar with shrubs. Above the island, the structure of the insect boundary layer was more complex and consisted of two discontinuity points, one at the mean height of the trees’ crowns (ca. 15 m), and a second, internal boundary layer at the top of the shrubs (ca. 5 m). Our findings improve the understanding of how vegetation can influence longitudinal and lateral dispersal patterns of flying insects in river corridors and floodplain systems. It also highlights the importance of preserving terrestrial habitat diversity in river floodplains as an important driver of both biotic and abiotic (i.e., morphology and airscape) heterogeneit

Interactions between diurnal winds and floodplain mosaics control the insect boundary layer in a river corridor

Insect flight along river corridors is a fundamental process that facilitates sustainable succession and diversity of aquatic and terrestrial insect communities in highly dynamic fluvial environments. This study examines variations in the thickness of the insect boundary layer (i.e., the pre-surface atmosphere layer in which air velocity does not exceed the sustained speed of flying insects) caused by interactions between diurnal winds and the heterogenous habitat mosaics in the floodplain of a braided river. Based on advective–diffusive theory, we develop and test a semi-empirical model that relates vertical flux of flying insects to vertical profiles of diurnal winds. Our model suggests that, in the logarithmic layer of wind, the density of insect fluxes decreases exponentially with the altitude due to the strong physical forcing. Inside the insect boundary layer, the insect fluxes can increase with the altitude while the winds speed remains nearly constant. We suggest a hypothesis that there is a close correspondence between the height of discontinuity points in the insect profiles (e.g. points with abrupt changes of the insect flux) and the displacement heights of the wind profiles (e.g. points above which the wind profile is logarithmic). Vertical profiles were sampled during three time-intervals at three different habitat locations in the river corridor: a bare gravel bar, a gravel bar with shrubs, and an island with trees and shrubs. Insects and wind speed were sampled and measured simultaneously over each location at 1.5-m intervals up to approximately 17 m elevation. The results support our working hypothesis on close correspondence between discontinuity and displacement points. The thickness of the insect boundary layer matches the height of the discontinuity points and was about 5 m above the bare gravel bar and the gravel bar with shrubs. Above the island, the structure of the insect boundary layer was more complex and consisted of two discontinuity points, one at the mean height of the trees’ crowns (ca. 15 m), and a second, internal boundary layer at the top of the shrubs (ca. 5 m). Our findings improve the understanding of how vegetation can influence longitudinal and lateral dispersal patterns of flying insects in river corridors and floodplain systems. It also highlights the importance of preserving terrestrial habitat diversity in river floodplains as an important driver of both biotic and abiotic (i.e., morphology and airscape) heterogeneity

Dynamics of shallow lateral shear layers: Experimental study in a river with a sandy bed

Shallow lateral shear layers forming between flows with different velocities, though essential for mixing processes in natural streams, have been examined only in laboratory settings using smooth, fixed?bed channels. This paper reports the results of an experimental study of a shear layer in a straight reach of a natural river where the layer, in contrast to the two?dimensional structure observed in the laboratory, is highly three?dimensional. The study included pronounced transverse pressure gradients, which influenced shear layer structure compared to flume experiments. It also introduces an analysis that complements conventional theory on mixing layers. The lateral velocity gradient between the flows downstream from a splitter plate placed in the river, the principal controlling factor, was adjusted for three experimental runs to determine the influence of different gradients on shear?layer dynamics. In each run, detailed three?dimensional measurements of mean and turbulent characteristics were obtained at five cross sections downstream from the splitter plate. Although experimental results agreed with conventional mixing?layer theories with respect to turbulence, the dynamics of the shear layers were dominated by the mean lateral fluxes of momentum. After re?examining the governing equations, we developed a parabolic equation describing the shear layer evolution and several scaling relations for essential terms of the energy budget: mean and turbulent lateral fluxes of momentum, turbulent kinetic energy, and dissipation rates. The study also provides insight into the spectral dynamics of turbulence in the shear layer and clarifies previous observations reported for confluences in natural streams.Hydraulic EngineeringCivil Engineering and Geoscience

A study of flow dynamics and implications for benthic fauna in a meander bend of a lowland river

Channel curvature and riffle-pool bathymetry in meandering streams control complex hydrodynamic and morphodynamic processes. This study investigates how spatial and temporal heterogeneities in flow hydraulics influence benthic fauna in a meander bend of a lowland sand-bed river. Spatial heterogeneity of riverbed morphology and secondary flow, induced by channel curvature, make pools hydraulically more diverse compared with riffles. Numerical simulations demonstrate that velocity reversal between riffles and pools in this meander bend produces spatially variable flow with complex temporal variations. Patterns of macro-invertebrates indicate an increase in population density from riffle to pool, reflecting an increase in diversity of abiotic factors. For most invertebrate species the observed patterns persisted during temporal variations of the flow. Considerable changes were observed only in some groups with specific preferences.Fil: Sukhodolov, Alexander N.. Leibniz-Institute of Freshwater Ecology and Inland Fisheries; AlemaniaFil: Blettler, Martin Cesar Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto Nacional de Limnología. Universidad Nacional del Litoral. Instituto Nacional de Limnología; ArgentinaFil: Zhang, Jingxin. Shanghai Jiao Tong University; ChinaFil: Sukhodolova, Tatiana. Leibniz-Institute of Freshwater Ecology and Inland Fisheries; AlemaniaFil: Nützmann, Gunnar. Leibniz-Institute of Freshwater Ecology and Inland Fisheries; Alemani

The Infrared Solar Spectrum Measured by the SOLSPEC Spectrometer Onboard the International Space Station

International audienceA solar spectrum extending from the extreme ultraviolet to the near-infrared is an important input for solar physics, climate research, and atmospheric physics. Ultraviolet measurements have been conducted since the beginning of the space age, but measurements throughout the contiguous visible and infrared (IR) regions are much more sparse. Ageing is a key problem throughout the entire spectral domain, but most of the effort extended to understand degradation was concentrated on the ultraviolet spectral region, and these mechanisms may not be appropriate in the IR. This problem is further complicated by the scarcity of long-term data sets. Onboard the International Space Station, the SOLSPEC spectrometer measured an IR solar spectral irradiance lower than the one given by ATLAS 3, e.g. by about 7 % at 1 700 nm. We here evaluate the consequences of the lower solar spectral irradiance measurements and present a re-analysis of the on-orbit calibration lamp and solar data trend, which lead to a revised spectrum

SOLSPEC onboard the International Space Station: Absolute Solar Spectral Irradiance in the Infrared Domain and Comparison with Recent Solar Models

International audienceOnboard the SOLAR payload of the International Space Station (ISS), the SOLSPECspectrometer measures the solar spectral irradiance (SSI) from 170 to 2900 nm. This instrument uses lamps to monitor its behavior in orbit. In particular, it employs two tungsten ribbon lamps in the IR domain (1000-2900 nm). The infrared absolute irradiance scale was determined from preflight laboratory calibration measurements and the in-flight measurements gathered at first light in April 2008. We reported a systematic discrepancy between SOLAR-ISS measurements and the ATLAS 3 spectrum obtained from SOLSPEC observations onboard the shuttle-ATLAS missions with a discrepancy reaching 10 % at 1800 nm. If confirmed such a discrepancy would have strong implications for the Total Solar Irradiance (TSI) and the brightness temperature of the lower solar photosphere. However, the onboard lamp and solar data time series show that the IR spectrometer did not reach its permanent regime at first light but only after several months of operation. The solar data at first light and in permanent regime show a difference, which is wavelength dependent. Using that difference (or the data in permanent regime), we show that the SOLSPEC-ISS IR spectrum is consistent with the ATLAS 3 spectrum within their combined uncertainties. We present the properties of that corrected spectrum in terms of its contribution to TSI, the photospheric temperature, and comparisons with independently measured IR spectra from ground-based and on-orbit platforms.The absorption coefficient of the negative ion of hydrogen has its minimum around 1600 nm so that measurements at this wavelength provide a unique opportunity to probe the deepest layers of the solar photosphere. Thus the comparison between the IR measurements and model predictions is of particular interest for understanding the structure of the solar photosphere. We present a comparison of the corrected spectrum with theoretical spectra calculated with radiative transfer codes COSI and ATLAS9 and discuss different physical mechanisms which can affect the absolute level of the IR irradiance