Select strengths and biases of models in representing the Arctic winter boundary layer over sea ice : the Larcform 1 single column model intercomparison
Pithan, Felix ; Ackerman, Andrew ; Angevine, Wayne M. ; Hartung, Kerstin ; Ickes, Luisa ; Kelley, Maxwell ; Medeiros, Brian ; Sandu, Irina ; Steeneveld, Gert Jan ; Sterk, H.A.M. - \ 2016
Journal of Advances in Modeling Earth Systems 8 (2016)3. - ISSN 1942-2466 - p. 1345 - 1357.
Arctic - boundary-layer - intercomparison - inversion - mixed-phase clouds - models
Weather and climate models struggle to represent lower tropospheric temperature and moisture profiles and surface fluxes in Arctic winter, partly because they lack or misrepresent physical processes that are specific to high latitudes. Observations have revealed two preferred states of the Arctic winter boundary layer. In the cloudy state, cloud liquid water limits surface radiative cooling, and temperature inversions are weak and elevated. In the radiatively clear state, strong surface radiative cooling leads to the build-up of surface-based temperature inversions. Many large-scale models lack the cloudy state, and some substantially underestimate inversion strength in the clear state. Here, the transformation from a moist to a cold dry air mass is modeled using an idealized Lagrangian perspective. The trajectory includes both boundary layer states, and the single-column experiment is the first Lagrangian Arctic air formation experiment (Larcform 1) organized within GEWEX GASS (Global atmospheric system studies). The intercomparison reproduces the typical biases of large-scale models: some models lack the cloudy state of the boundary layer due to the representation of mixed-phase microphysics or to the interaction between micro- and macrophysics. In some models, high emissivities of ice clouds or the lack of an insulating snow layer prevent the build-up of surface-based inversions in the radiatively clear state. Models substantially disagree on the amount of cloud liquid water in the cloudy state and on turbulent heat fluxes under clear skies. Observations of air mass transformations including both boundary layer states would allow for a tighter constraint of model behavior.
The Challenge of Forecasting the Onset and Development of Radiation Fog Using Mesoscale Atmospheric Models
Steeneveld, G.J. ; Ronda, R.J. ; Holtslag, A.A.M. - \ 2015
Boundary-Layer Meteorology 154 (2015)2. - ISSN 0006-8314 - p. 265 - 289.
boundary-layer - climate models - prediction - resolution - weather - system - parameterization - detrainment - sensitivity - simulation
The numerical weather prediction of radiation fog is challenging, as many models typically show large biases for the timing of the onset and dispersal of the fog, as well as for its depth and liquid water content. To understand the role of physical processes, i.e. turbulence, radiation, land-surface coupling, and microphysics, we evaluate the HARMONIE and Weather Research and Forecasting (WRF) mesoscale models for two contrasting warm fog episodes at the relatively flat terrain around the Cabauw tower facility in the Netherlands. One case involves a radiation fog that arose in calm anticyclonic conditions, and the second is a radiation fog that developed just after a cold front passage. The WRF model represents the radiation fog well, while the HARMONIE model forecasts a stratus lowering fog layer in the first case and hardly any fog in the second case. Permutations of parametrization schemes for boundary-layer mixing, radiation and microphysics, each for two levels of complexity, have been evaluated within the WRF model. It appears that the boundary-layer formulation is critical for forecasting the fog onset, while for fog dispersal the choice of the microphysical scheme is a key element, where a double-moment scheme outperforms any of the single-moment schemes. Finally, the WRF model results appear to be relatively insensitive to horizontal grid spacing, but nesting deteriorates the modelled fog formation. Increasing the domain size leads to a more scattered character of the simulated fog. Model results with one-way or two-way nesting show approximately comparable results.
Seawater electrodialysis with preferential removal of divalent ions
Galama, A.H. ; Daubaras, G. ; Burheim, O.S. ; Rijnaarts, H. ; Post, J.W. - \ 2014
Journal of Membrane Science 452 (2014). - ISSN 0376-7388 - p. 219 - 228.
cation-exchange membranes - of-the-art - nanofiltration membranes - diffusion-coefficients - reverse-osmosis - calcium-sulfate - boundary-layer - monovalent - desalination - separation
In this work desalination of a ternary salt mixture and at North Sea water is studied with a lab scale electrodialysis stack, which was used in a recycling batch mode. During desalination samples were taken and the ionic composition of the dilute stream was determined. The effect of applied current density (10-300 A/m(2)) On this composition was investigated. A clear effect of applied current density was observed. A lower applied current density leads to a more complete reduction in concentration of divalent ions, in an earlier extent of desalination. This influence of the applied current density could be related to the concentration polarization effects that occur in the diffusional boundary layer and are explained with a model based on the Nernst-Planck flux equation. It was found that the lower initial ion concentration of Ca2+, Mg2+ but also of K+ and SO42- compared to respectively Na+ and Cl-, leads to stronger depletion of these ions in the transport layer adjacent the membrane. These boundary layer effects are more pronounced at higher applied current densities, resulting in reduced transport of ions with a low initial concentration. High monovalent over divalent ion ratios can be found at low applied current. (C) 2013 Elsevier By. All rights reserved.
Mechanisms of water supply and vegetation demand govern the seasonality and magnitude of evapotranspiration in Amazonia and Cerrado
Christoffersen, B.O. ; Restrepo-Coupe, N. ; Arain, M.A. ; Baker, I.T. ; Cestaro, B.P. ; Ciais, P. ; Fisher, J.B. ; Galbraith, D. ; Guan, X. ; Hurk, B. van den; Kruijt, B. - \ 2014
Agricultural and Forest Meteorology 191 (2014). - ISSN 0168-1923 - p. 33 - 50.
land-surface scheme - environment simulator jules - carbon-cycle feedbacks - stomatal conductance - regional evapotranspiration - atmosphere interactions - model description - biosphere model - boundary-layer - climate model
Evapotranspiration (E) in the Amazon connects forest function and regional climate via its role in precipitation recycling However, the mechanisms regulating water supply to vegetation and its demand for water remain poorly understood, especially during periods of seasonal water deficits In this study, we address two main questions: First, how do mechanisms of water supply (indicated by rooting depth and groundwater) and vegetation water demand (indicated by stomatal conductance and intrinsic water use efficiency) control evapotranspiration (E) along broad gradients of climate and vegetation from equatorial Amazonia to Cerrado, and second, how do these inferred mechanisms of supply and demand compare to those employed by a suite of ecosystem models? We used a network of eddy covariance towers in Brazil coupled with ancillary measurements to address these questions With respect to the magnitude and seasonality of E, models have much improved in equatorial tropical forests by eliminating most dry season water limitation, diverge in performance in transitional forests where seasonal water deficits are greater, and mostly capture the observed seasonal depressions in E at Cerrado However, many models depended universally on either deep roots or groundwater to mitigate dry season water deficits, the relative importance of which we found does not vary as a simple function of climate or vegetation In addition, canopy stomatal conductance (gs) regulates dry season vegetation demand for water at all except the wettest sites even as the seasonal cycle of E follows that of net radiation In contrast, some models simulated no seasonality in gs, even while matching the observed seasonal cycle of E. We suggest that canopy dynamics mediated by leaf phenology may play a significant role in such seasonality, a process poorly represented in models Model bias in gs and E, in turn, was related to biases arising from the simulated light response (gross primary productivity, GPP) or the intrinsic water use efficiency of photosynthesis (iWUE). We identified deficiencies in models which would not otherwise be apparent based on a simple comparison of simulated and observed rates of E. While some deficiencies can be remedied by parameter tuning, in most models they highlight the need for continued process development of belowground hydrology and in particular, the biological processes of root dynamics and leaf phenology, which via their controls on E, mediate vegetation-climate feedbacks in the tropics.
Simulation of tropospheric chemistry and aerosols with the climate model EC-Earth
Noije, T.P.C. van; Sager, P. Le; Segers, A.J. ; Velthoven, P.F.J. van; Krol, M.C. ; Hazeleger, W. ; Williams, A.G. ; Chambers, S.D. - \ 2014
Geoscientific Model Development 7 (2014)5. - ISSN 1991-959X - p. 2435 - 2475.
intercomparison project accmip - general-circulation model - dry deposition parameterization - modified band approach - atmospheric chemistry - global-models - transport models - boundary-layer - stratospheric ozone - sulfur emissions
We have integrated the atmospheric chemistry and transport model TM5 into the global climate model EC-Earth version 2.4. We present an overview of the TM5 model and the two-way data exchange between TM5 and the IFS model from the European Centre for Medium-Range Weather Forecasts (ECMWF), the atmospheric general circulation model of EC-Earth. In this paper we evaluate the simulation of tropospheric chemistry and aerosols in a one-way coupled configuration. We have carried out a decadal simulation for present-day conditions and calculated chemical budgets and climatologies of tracer concentrations and aerosol optical depth. For comparison we have also performed offline simulations driven by meteorological fields from ECMWF's ERA-Interim reanalysis and output from the EC-Earth model itself. Compared to the offline simulations, the online-coupled system produces more efficient vertical mixing in the troposphere, which reflects an improvement of the treatment of cumulus convection. The chemistry in the EC-Earth simulations is affected by the fact that the current version of EC-Earth produces a cold bias with too dry air in large parts of the troposphere. Compared to the ERA-Interim driven simulation, the oxidizing capacity in EC-Earth is lower in the tropics and higher in the extratropics. The atmospheric lifetime of methane in EC-Earth is 9.4 years, which is 7% longer than the lifetime obtained with ERA-Interim but remains well within the range reported in the literature. We further evaluate the model by comparing the simulated climatologies of surface radon-222 and carbon monoxide, tropospheric and surface ozone, and aerosol optical depth against observational data. The work presented in this study is the first step in the development of EC-Earth into an Earth system model with fully interactive atmospheric chemistry and aerosols.
Mega-heatwave temperatures due to combined soil desiccation and atmospheric heat accumulation
Miralles, D. ; Teuling, A.J. ; Heerwaarden, C.C. van; Vilà-Guerau de Arellano, J. - \ 2014
Nature Geoscience 7 (2014). - ISSN 1752-0894 - p. 345 - 349.
boundary-layer - land-surface - summer - europe - evaporation - extremes - moisture - impact - wave
The recent European mega-heatwaves of 2003 and 2010 broke temperature records across Europe1, 2, 3, 4, 5. Although events of this magnitude were unprecedented from a historical perspective, they are expected to become common by the end of the century6, 7. However, our understanding of extreme heatwave events is limited and their representation in climate models remains imperfect8. Here we investigate the physical processes underlying recent mega-heatwaves using satellite and balloon measurements of land and atmospheric conditions from the summers of 2003 in France and 2010 in Russia, in combination with a soil–water–atmosphere model. We find that, in both events, persistent atmospheric pressure patterns induced land–atmosphere feedbacks that led to extreme temperatures. During daytime, heat was supplied by large-scale horizontal advection, warming of an increasingly desiccated land surface and enhanced entrainment of warm air into the atmospheric boundary layer. Overnight, the heat generated during the day was preserved in an anomalous kilometres-deep atmospheric layer located several hundred metres above the surface, available to re-enter the atmospheric boundary layer during the next diurnal cycle. This resulted in a progressive accumulation of heat over several days, which enhanced soil desiccation and led to further escalation in air temperatures. Our findings suggest that the extreme temperatures in mega-heatwaves can be explained by the combined multi-day memory of the land surface and the atmospheric boundary layer
Shallow cumulus rooted in photosynthesis
Vilà-Guerau De Arellano, J. ; Ouwersloot, H.G. ; Baldocchi, D. ; Jacobs, C.M.J. - \ 2014
Geophysical Research Letters 41 (2014)5. - ISSN 0094-8276 - p. 1796 - 1802.
heterogeneous land surfaces - boundary-layer - soil-moisture - gas-exchange - water - formulation - vegetation - clouds - fluxes - co2
We study the interactions between plant evapotranspiration, controlled by photosynthesis (C3 and C4 grasses), and moist thermals responsible for the formation of shallow cumulus clouds (SCu). Our findings are based on a series of systematic numerical experiments at fine spatial and temporal scales using large eddy simulations explicitly coupled to a plant-physiology model. The shading provided by SCu leads to strong spatial variability in photosynthesis and the surface energy balance. This in turn results in SCu characterized by less extreme and less skewed values of liquid water path. The larger water use efficiency of C4 grass leads to two opposite effects that influence boundary layer clouds: more vigorous and deeper thermals due to the larger buoyancy surface flux (positive effect) characterized by less moisture content (negative). We find that under these midlatitude and well-watered soil conditions, SCu are characterized by a larger cloud cover and liquid water path over C4 grass fields.
Variability of the Structure Parameters of Temperature and Humidity Observed in the Atmospheric Surface Layer Under Unstable Conditions
Braam, M. ; Moene, A.F. ; Beyrich, F. - \ 2014
Boundary-Layer Meteorology 150 (2014)3. - ISSN 0006-8314 - p. 399 - 422.
sonic anemometer - water-vapor - aperture scintillometer - heterogeneous surface - local-structure - boundary-layer - sensible heat - fluxes - turbulence - momentum
The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as log(2s )) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of log(C2 s ) depends on instability and on the size of the averaging window over which log(C2 s ) is calculated. If instability increases, differences in log(C2s ) between upward motions (large C2 s ) and downward motions (small C2 s ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of log(C2 s ) decrease. A linear regression of the variances of log(C2 s ) versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in C2 s along a scintillometer path or aircraft flight leg are just within the range of local variability in C2 s or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.
Seasonal dependence of the urban heat island on the street canyon aspect ratio
Theeuwes, N.E. ; Steeneveld, G.J. ; Ronda, R.J. ; Heusinkveld, B.G. ; Hove, L.W.A. van; Holtslag, A.A.M. - \ 2014
Quarterly Journal of the Royal Meteorological Society 140 (2014)684. - ISSN 0035-9009 - p. 2197 - 2210.
warmte - steden - stedelijke planning - warmtebalans - seizoenvariatie - meteorologie - nederland - heat - towns - urban planning - heat balance - seasonal variation - meteorology - netherlands - boundary-layer - energy-balance - climate zones - model - temperature - parameterization - simulation - schemes - cabauw - field
In this paper we study the relation between the urban heat island (UHI) in the urban canyon and street geometry, in particular the aspect ratio. Model results and observations show that two counteracting processes govern the relation between the nocturnal UHI and the building aspect ratio: i.e. trapping of longwave radiation and shadowing effects. In general, trapping of longwave radiation supports the UHI, whereas shadowing effects reduce the UHI. The net effect depends on the UHI definition and the amount of available shortwave radiation penetrating the canyon. In summer, autumn and spring the shadowing effects can already reduce the UHI starting at an aspect ratio between 0.5 and 1. The analysis is carried out using several methods. Firstly, the single-column model version of the Weather Research and Forecasting model (WRF) is used extensively. Two separate runs, one rural and one urban, are used to estimate the UHI. Secondly, the urban canyon temperature at the two meter level is introduced, which allows for direct comparison between modelled and observed air temperatures within the urban canyon. Finally, the model is evaluated for all four seasons. The results of this research provide important insights for urban planning on how to use the aspect ratio to mitigate the UHI in the urban canyon
Impact of transport model errors on the global and regional methane emissions estimated by inverse modelling
Locatelli, R. ; Bousquet, P. ; Chevallier, F. ; Fortems-Cheney, A. ; Szopa, S. ; Saunois, M. ; Agusti-Panareda, A. ; Bergmann, D. ; Bian, H. ; Cameron-Smith, P. ; Chipperfield, M.P. ; Gloor, E. ; Houweling, S. ; Kawa, S.R. ; Krol, M.C. ; Patra, P.K. ; Prinn, R.G. ; Rigby, M. ; Saito, R. ; Wilson, C. - \ 2013
Atmospheric Chemistry and Physics 13 (2013)19. - ISSN 1680-7316 - p. 9917 - 9937.
general-circulation model - atmospheric transport - tracer transport - co2 inversions - boundary-layer - vertical profiles - data assimilation - climate-change - growth-rate - part i
A modelling experiment has been conceived to assess the impact of transport model errors on methane emissions estimated in an atmospheric inversion system. Synthetic methane observations, obtained from 10 different model outputs from the international TransCom-CH4 model inter-comparison exercise, are combined with a prior scenario of methane emissions and sinks, and integrated into the three-component PYVAR-LMDZ-SACS (PYthon VARiational-Laboratoire de Meteorologie Dynamique model with Zooming capability-Simplified Atmospheric Chemistry System) inversion system to produce 10 different methane emission estimates at the global scale for the year 2005. The same methane sinks, emissions and initial conditions have been applied to produce the 10 synthetic observation datasets. The same inversion set-up (statistical errors, prior emissions, inverse procedure) is then applied to derive flux estimates by inverse modelling. Consequently, only differences in the modelling of atmospheric transport may cause differences in the estimated fluxes. In our framework, we show that transport model errors lead to a discrepancy of 27 Tg yr(-1) at the global scale, representing 5% of total methane emissions. At continental and annual scales, transport model errors are proportionally larger than at the global scale, with errors ranging from 36 Tg yr(-1) in North America to 7 Tg yr(-1) in Boreal Eurasia (from 23 to 48 %, respectively). At the model grid-scale, the spread of inverse estimates can reach 150% of the prior flux. Therefore, transport model errors contribute significantly to overall uncertainties in emission estimates by inverse modelling, especially when small spatial scales are examined. Sensitivity tests have been carried out to estimate the impact of the measurement network and the advantage of higher horizontal resolution in transport models. The large differences found between methane flux estimates inferred in these different configurations highly question the consistency of transport model errors in current inverse systems. Future inversions should include more accurately prescribed observation covariances matrices in order to limit the impact of transport model errors on estimated methane fluxes.
Quantified turbulent diffusion of suspended sediment using acoustic Doppler current profilers
Sassi, M.G. ; Hoitink, A.J.F. ; Vermeulen, B. - \ 2013
Geophysical Research Letters 40 (2013)21. - ISSN 0094-8276 - p. 5692 - 5697.
reynolds stress - boundary-layer - open channels - flow - suspension - transport - fluxes - adcp - sand
Collocated profiles of the Reynolds stress tensor and eddy covariance fluxes are obtained to derive vertical profiles of turbulent momentum and sediment diffusivity in a tidal river, using coupled acoustic Doppler current profilers (ADCPs). Shear and normal stresses are obtained by combining the variances in radial velocities measured by the ADCP beams. The covariances between radial velocities and calibrated acoustic backscatter allow the determination of the three Cartesian components of the turbulent flux of suspended sediment. The main advantage of this new approach is that flow velocity and sediment concentration measurements are exactly collocated, and allowing for profiling over longer ranges, in comparison to existing techniques. Results show that vertical profiles of the inverse turbulent Prandtl-Schmidt number are coherent with corresponding profiles of the sediment diffusivity, rather than with profiles of the eddy viscosity.
Near-bed gradients in particles and nutrients above a mussel bed in the Limfjorden: influence of physical mixing and mussel filtration
Petersen, J.K. ; Maar, M. ; Ysebaert, T. ; Herman, P.M.J. - \ 2013
Marine Ecology Progress Series 490 (2013). - ISSN 0171-8630 - p. 137 - 146.
mytilus-edulis-l - pelagic food-web - boundary-layer - grazing impact - phytoplankton - flow - depletion - plankton - culture - biomass
The aim of this field study was to investigate the role of mussels on near-bed layer characteristics at different hydrodynamic regimes in a micro-tidal system. At Løgstør Broad, the Limfjorden, Denmark, we deployed ‘siphon mimics’ to sample chlorophyll a (chl a), particulate organic carbon (POC) and inorganic nutrients at different distances above the bottom. This was done without disturbing water column gradients and in a manner similar to mussel incurrent flow. Mimics were deployed at 2 sites: a site with a relatively dense mussel bed and a nearby sandy site without mussels. During the 2 wk field campaign, physical conditions in the fjord varied from extremely calm weather with low waves to quite windy with high waves. Results showed that under all conditions, the vertical concentration profiles of chl a were significantly depleted towards the mussel bed due to mussel filtration, whereas the degree of chl a depletion was correlated to wave height. Nutrient profiles consistently showed increasing concentration profiles towards the bed, identifying the mussel bed and the sediment as a source of nutrients with the highest gradients during the period with high waves. In conclusion, the near-bed concentrations of seston and nutrients in this study were temporally variable and closely linked to the physical structure of the water column.
Modeling the influence of open water surfaces on summertime temperatures and thermal comfort in the city
Theeuwes, N.E. ; Solcerova, A. ; Steeneveld, G.J. - \ 2013
Journal of Geophysical Research: Atmospheres 118 (2013)16. - ISSN 2169-897X - p. 8881 - 8896.
urban heat-island - boundary-layer - single-layer - climate-change - canopy model - human health - sensitivity - balance - impact - energy
 Due to the combination of rapid global urbanization and climate change, urban climate issues are becoming relatively more important and are gaining interest. Compared to rural areas, the temperature in cities is higher (the urban heat island effect ) due to the modifications in the surface radiation and energy balances. This study hypothesizes that the urban heat island can be mitigated by introducing open surface water in urban design. In order to test this, we use the WRF mesoscale meteorological model in which an idealized circular city is designed. Herein, the surface water cover, its size, spatial configuration, and temperature are varied. Model results indicate that the cooling effect of water bodies depends nonlinearly on the fractional water cover, size and distribution of individual lakes within the city with respect to wind direction. Relatively large lakes show a high temperature effect close to their edges and in downwind areas. Several smaller lakes equally distributed within the urban area have a smaller temperature effect, but influence a larger area of the city. Evaporation from open water bodies may lower the temperature, but on the other hand also increases the humidity, which dampens the positive effect on thermal comfort. In addition, when the water is warmer than the air temperature (during autumn or night) the water body has adverse effect on thermal comfort. In those cases, the water body eventually limits the cooling and thermal comfort in the surrounding city, and thus diverges from the original intention of the intervention
Surface and atmospheric controls on the onset of moist convection over land
Gentine, P. ; Holtslag, A.A.M. ; Andrea, F. D'; Ek, M. - \ 2013
Journal of Hydrometeorology 14 (2013). - ISSN 1525-755X - p. 1443 - 1462.
large-eddy simulation - fraction diurnal behavior - probabilistic bulk model - coupled mixed-layer - boundary-layer - soil-moisture - evaporative fraction - relative-humidity - hydrologic perspective - spatial variability
The onset of moist convection over land is investigated using a conceptual approach with a slab boundary layer model. We here determine the essential factors for the onset of boundary layer clouds over land, and study their relative importance. They are: 1) the ratio of the temperature to the moisture lapse rates of the free troposphere, i.e. the inversion Bowen ratio, 2) the mean-daily surface temperature, 3) the relative humidity of the free troposphere and 4) the surface evaporative fraction. A clear transition is observed between two regimes of moistening of the boundary layer as assessed by the relative humidity at the boundary layer top. In the first so-called wet soil advantage regime, the moistening results from the increase of the mixed-layer specific humidity, which linearly depends on the surface evaporative fraction and inversion Bowen ratio through a dynamic boundary layer factor. In the second so-called dry soil advantage regime, the relative humidity tendency at the boundary layer top is controlled by the thermodynamics and changes in the moist adiabatic induced by the decreased temperature at the boundary layer top and consequent reduction in saturation water vapor pressure. This regime pertains for very deep boundary layers under weakly stratified free troposphere over hot surface conditions. In the context of the conceptual model, a rise in free-tropospheric temperature (global warming) increases the occurrence of deep convection and reduces the cloud cover over moist surfaces. This study provides new intuition and predictive capacity on the mechanism controlling the occurrence of moist convection over land
Sediment discharge division at two tidally influenced river bifurcations
Sassi, M.G. ; Hoitink, A.J.F. ; Vermeulen, B. ; Hidayat, H. - \ 2013
Water Resources Research 49 (2013)4. - ISSN 0043-1397 - p. 2119 - 2134.
depth-integrated model - open-channel flow - suspended sediment - settling velocity - secondary flow - load transport - boundary-layer - particles - suspension - roughness
 We characterize and quantify the sediment discharge division at two tidally influenced river bifurcations in response to mean flow and secondary circulation by employing a boat-mounted acoustic Doppler current profiler (ADCP), to survey transects at bifurcating branches during a semidiurnal tidal cycle. The ADCP collecting flow velocity and acoustical backscatter data was used to quantify suspended sediment discharge, adopting a recently introduced calibration procedure. Measured profiles of flow velocity and sediment concentration allowed us to compute spatiotemporal distributions of the shear velocity, the roughness length and the Rouse number. Spatiotemporal distributions of the settling velocity were obtained by combining the Rouse number and shear velocity estimates with in situ measurements from a laser particle size analyzer. Bed-load transport rates were inferred from shear stress estimates. The concentration field shows a direct response to bed shear stress, stressing the alluvial context of the system. The flow in the bifurcation regions is characterized by counter rotating secondary-flow cells, which stretch over the full width and depth of the cross sections in the downstream branches, and persist throughout the entire tidal cycle. The pattern of secondary flow suggests the flow approaching the bifurcation is concentrated in two independent threads. A two-cell structure inhibits the exchange of sediment that would occur in case a single cell would stretch over the full channel width. The division of suspended sediment primarily depends on the upstream transverse profile of the suspended sediment concentration, which is in turn dependent on geometrical factors such as upstream curvature.
Measuring H2O and CO2 fluxes at field scales with scintillometry: Part II-Validation and application of 1-min flux estimates
Kesteren, A.J.H. van; Hartogensis, O.K. ; Dinther, D. van; Moene, A.F. ; Bruin, H.A.R. de; Holtslag, A.A.M. - \ 2013
Agricultural and Forest Meteorology 178-179 (2013). - ISSN 0168-1923 - p. 88 - 105.
carbon-dioxide exchange - temperature-humidity correlation - energy-balance closure - surface-layer fluxes - closed c-3 wheat - structure parameters - stomatal responses - moisture fluxes - boundary-layer - solar eclipse
This paper evaluates four methods to obtain accurate averaged flux estimates under conditions of non-stationary turbulence. In Part I (Van Kesteren et al., 2012), we introduced and evaluated these four combined methods for 30-min averaging intervals, notably the flux-variance method, the Bowen-variance method, the structure-parameter method, and the energy-balance method. The aim of this paper, Part II, is to validate the accuracy of the 1-min flux estimates of the CO2 flux, FCO2, and the evapotranspiration/latent-heat flux, LvE. Furthermore, we use the 1-min fluxes to investigate flux and vegetation responses under conditions of non-stationary turbulence. Using several validation methods, we show that both the eddy-covariance method and the energy-balance method are unsuitable for estimating fluxes over 1-min averaging intervals. The three other combined methods are more successful in determining 1-min fluxes. The random error is approximately half that of the eddy-covariance method, but still some issues limit the success. The Bowen-variance method has a +0.09 systematic error and moreover, 30% of the data had to be omitted, because the method requires more stringent conditions. Furthermore, the flux-variance method has a -0.15 systematic error. The structure-parameter method performs best of all methods and accurately resolves 1-min fluxes. With this method, we do a final validation with a different data set and show that also under dry conditions the method accurately resolves FCO2, although LE was more difficult to resolve. In the last part, the structure-parameter method is successfully applied under conditions of non-stationary turbulence. We show that LvE and FCO2 have a different step response upon abrupt changes in solar radiation, because different processes drive these fluxes. Also, we observe a 2-min time lag between solar radiation and 1-min fluxes and show the relevance of taking this into account for determining light-response curves of the plants for both 1-min and 30-min averaging intervals. Furthermore, we show the relevance of 1-min fluxes for studying the light-response curves of plants for conditions with different temperature and humidity. Finally, we show that accurate estimates of 1-min averaged canopy resistances can be determined via the resistance expressions for sensible heat and LvE. As such, we show that vegetation can indeed modify its canopy resistance significantly within several minutes. (c) 2013 Elsevier B.V. All rights reserved.
Impact of the Manaus urban plume on trace gas mixing ratios near the surface in the Amazon Basin: Implications for the NO-NO2-O-3 photostationary state and peroxy radical levels
Trebs, I. ; Mayol-Bracero, O.L. ; Pauliquevis, T. ; Kuhn, U. ; Sander, R. ; Ganzeveld, L.N. ; Meixner, F.X. ; Kesselmeier, J. ; Artaxo, P. ; Andreae, M.O. - \ 2012
Journal of Geophysical Research: Atmospheres 117 (2012). - ISSN 2169-897X - 16 p.
volatile organic-compounds - photochemical steady-state - reactive nitrogen-oxides - tropical rain-forest - chemistry box model - boundary-layer - atmospheric chemistry - dry season - airborne measurements - hydroxyl radicals
We measured the mixing ratios of NO, NO2, O-3, and volatile organic carbon as well as the aerosol light-scattering coefficient on a boat platform cruising on rivers downwind of the city of Manaus (Amazonas State, Brazil) in July 2001 (Large-Scale Biosphere-Atmosphere Experiment in Amazonia-Cooperative LBA Airborne Regional Experiment-2001). The dispersion and impact of the Manaus plume was investigated by a combined analysis of ground-based (boat platform) and airborne trace gas and aerosol measurements as well as by meteorological measurements complemented by dispersion calculations (Hybrid Single-Particle Lagrangian Integrated Trajectory model). For the cases with the least anthropogenic influence (including a location in a so far unexplored region similar to 150 km west of Manaus on the Rio Manacapuru), the aerosol scattering coefficient, sigma(s), was below 11 Mm(-1), NOx mixing ratios remained below 0.6 ppb, daytime O-3 mixing ratios were mostly below 20 ppb and maximal isoprene mixing ratios were about 3 ppb in the afternoon. The photostationary state (PSS) was not established for these cases, as indicated by values of the Leighton ratio, Phi, well above unity. Due to the influence of river breeze systems and other thermally driven mesoscale circulations, a change of the synoptic wind direction from east-northeast to south-southeast in the afternoon often caused a substantial increase of ss and trace gas mixing ratios (about threefold for sigma(s), fivefold for NOx, and twofold for O-3), which was associated with the arrival of the Manaus pollution plume at the boat location. The ratio F reached unity within its uncertainty range at NOx mixing ratios of about 3 ppb, indicating "steady-state" conditions in cases when radiation variations, dry deposition, emissions, and reactions mostly involving peroxy radicals (XO2) played a minor role. The median midday/afternoon XO2 mixing ratios estimated using the PSS method range from 90 to 120 parts per trillion (ppt) for the remote cases (sigma(s) <11 Mm(-1) and NOx <0.6 ppb), while for the polluted cases our estimates are 15 to 60 ppt. These values are within the range of XO2 estimated by an atmospheric chemistry box model (Chemistry As A Box model Application-Module Efficiently Calculating the Chemistry of the Atmosphere (CAABA/MECCA)-3.0).
Impact of sound attenuation by suspended sediment on ADCP backscatter calibrations
Sassi, M.G. ; Hoitink, A.J.F. ; Vermeulen, B. - \ 2012
Water Resources Research 48 (2012)9. - ISSN 0043-1397 - 14 p.
doppler-current-profiler - particle-size - acoustic measurements - laboratory evaluation - particulate matter - settling velocity - beam attenuation - boundary-layer - transport - scattering
Although designed for velocity measurements, acoustic Doppler current profilers (ADCPs) are widely being used to monitor suspended particulate matter in rivers and in marine environments. To quantify mass concentrations of suspended matter, ADCP backscatter is generally calibrated with in situ measurements. ADCP backscatter calibrations are often highly site-specific and season dependent, which is typically attributed to the sensitivity of the acoustic response to the number of scatterers and their size. Besides being a joint function of the concentration and the size of the scatterers, the acoustic backscatter can be heavily affected by the attenuation due to suspended matter along the two-way path to the target volume. We aim to show that accounting for sound attenuation in ADCP backscatter calibrations may broaden the range of application of ADCPs in natural environments. The trade-off between the applicability and the accuracy of a certain calibration depends on the variation in size distribution and concentration along the sound path. We propose a simple approach to derive the attenuation constant per unit concentration or specific attenuation, based on two water samples collected along the sound path of the ADCP. A single calibration was successfully applied at five locations along the River Mahakam, located up to 200 km apart. ADCP-derived estimates of suspended mass concentration were shown to be unbiased, even far away from the transducer.
On Monin–Obukhov Scaling in and Above the Atmospheric Surface Layer: The Complexities of Elevated Scintillometer Measurements
Braam, M. ; Bosveld, F.C. ; Moene, A.F. - \ 2012
Boundary-Layer Meteorology 144 (2012)2. - ISSN 0006-8314 - p. 157 - 177.
large-aperture scintillometer - boundary-layer - structure parameters - sonic anemometer - refractive-index - sensible heat - water-vapor - temperature - fluxes - humidity
In scintillometry Monin–Obukhov similarity theory (MOST) is used to calculate the surface sensible heat flux from the structure parameter of temperature (CT2)(CT2) . In order to prevent saturation a scintillometer can be installed at an elevated level. However, in that case the observation level might be located outside the atmospheric surface layer (ASL) and thus the validity of MOST questioned. Therefore, we examine two concepts to determine the turbulent surface sensible heat flux from the structure parameter at elevated levels with data obtained at 60-m height on the Cabauw tower (the Netherlands). In the first concept (MOSTs) CT2CT2 is still scaled with the surface flux, whereas in the second (MOSTl) CT2CT2 is scaled with the local sensible heat flux. The CT2CT2 obtained from both concepts is compared with direct observations of CT2CT2 using a sonic anemometer/thermometer. In the afternoon (when the measurement height is located within the ASL) both concepts give results that are comparable to the directly observed values of CT2CT2 . In the morning (data outside the ASL), our data do not unequivocally support either of the two concepts. First, the peak in CT2CT2 that occurs when the measurement height is located in the entrainment zone disqualifies the use of MOST. Second, during the morning transition, local scaling shows the correct pattern (zero flux and a minimum in CT2CT2) but underestimates CT2CT2 by a factor of ten. Third, from the best linear fit a we found that the slope of MOSTl gave better results, whereas the offset is closer to zero for MOSTs. Further, the correlation between the direct observations and MOST-scaled results is low and similar for the two concepts. In the end, we conclude that MOST is not applicable for the morning hours when the observation level is above the ASL.
On the temporal upscaling of evapotranspiration from instantaneous remote sensing measurements to 8-day mean daily-sums
Ryu, Y. ; Baldocchi, D.D. ; Black, T.A. ; Moors, E.J. - \ 2012
Agricultural and Forest Meteorology 152 (2012). - ISSN 0168-1923 - p. 212 - 222.
carbon-dioxide exchange - net ecosystem productivity - co2 exchange - energy fluxes - water-vapor - heterogeneous landscape - temperate grassland - daily evaporation - surface fluxes - boundary-layer
The regular monitoring of evapotranspiration from satellites has been limited because of discontinuous temporal coverage, resulting in snapshots at a particular point in space and time. We developed a temporal upscaling scheme using satellite-derived instantaneous estimates of evapotranspiration to produce a daily-sum evapotranspiration averaged over an 8-day interval. We tested this scheme against measured evapotranspiration data from 34 eddy covariance flux towers covering seven plant functional types from boreal to tropical climatic zones. We found that the ratio of a half-hourly-sum of potential solar radiation (extraterrestrial solar irradiance on a plane parallel to the Earth’s surface) between 10:00 hh and 14:00 hh to a daily-sum of potential solar radiation provides a robust scaling factor to convert a half-hourly measured evapotranspiration to an estimate of a daily-sum; the estimated and measured daily sum evapotranspiration showed strong linear relation (r2 = 0.92) and small bias (-2.7%). By comparison, assuming a constant evaporative fraction (the ratio of evapotranspiration to available energy) during the daytime, although commonly used for temporal upscaling, caused 13 underestimation of evapotranspiration on an annual scale. The proposed temporal upscaling scheme requires only latitude, longitude and time as input. Thus it will be useful for developing continuous evapotranspiration estimates in space and time, which will improve continuous monitoring of hydrological cycle from local to global scales.