1 

Simulation of atmospheric turbulence measurements: Fractal turbulence (poster)
A new trend is to observe atmospheric turbulence fields by using scanning Doppler radars and/or lidars. See e.g. Chan (2011) for the retrieval of eddy dissipation rate (EDR) maps at the Hongkong International Airport.

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2 

Factors controlling rapid stratocumulus cloud thinning
The relationship between the inversion stability and the liquid water path (LWP) tendency of a vertically wellmixed, adiabatic stratocumulus cloud layer is investigated in this study through the analysis of the budget equation for the LWP. The LWP budget is mainly determined by the turbulent fluxes of heat and moisture at the top and the base of the cloud layer, as well as by the source terms due to radiation and precipitation.
Through substitution of the inversion stability parameter k into the budget equation, it immediately follows that the LWP tendency will become negative for increasing values of k due to the entrainment of increasingly dry air. Large k values are therefore associated with strong cloud thinning. Using the steadystate solution for the LWP, an equilibriumvalue keq is formulated, beyond which the stratocumulus cloud will thin. The Second Dynamics and Chemistry of Marine Stratocumulus field study (DYCOMSII) is used to illustrate that, depending mainly on the magnitude of the moisture flux at cloud base, stratocumulus clouds can persist well within the buoyancy reversal regime.

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3 

Stochastic parameterization of convective area fractions with a multicloud model inferred from observational data
Observational data of rainfall from a rain radar in Darwin, Australia, are combined with data defining the largescale dynamic and thermodynamic state of the atmosphere around Darwin to develop a multicloud model based on a stochastic method using conditional Markov chains. The authors assign the radar data to clear sky, moderate congestus, strong congestus, deep convective, or stratiform clouds and estimate transition probabilities used by Markov chains that switch between the cloud types and yield cloudtype area fractions. Crosscorrelation analysis shows that the mean vertical velocity is an important indicator of deep convection. Further, it is shown that, if conditioned on the mean vertical velocity, the Markov chains produce fractions comparable to the observations. The stochastic nature of the approach turns out to be essential for the correct production of area fractions. The stochastic multicloud model can easily be coupled to existing moist convection parameterization schemes used in general circulation models.

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4 

Nonlinear infragravity–wave interactions on a gently sloping laboratory beach
A highresolution dataset of three irregular wave conditions collected on a gently sloping laboratory beach is analyzed to study nonlinear energy transfers involving infragravity frequencies. This study uses bispectral analysis to identify the dominant, nonlinear interactions and estimate energy transfers to investigate energy flows within the spectra. Energy flows are identified by dividing transfers into four types of triad interactions, with triads including one, two, or three infragravity–frequency components, and triad interactions solely between shortwave frequencies. In the shoaling zone, the energy transfers are generally from the spectral peak to its higher harmonics and to infragravity frequencies. While receiving net energy, infragravity waves participate in interactions that spread energy of the shortwave peaks to adjacent frequencies, thereby cre ating a broader energy spectrum. In the shortwave surf zone, infragravity–infragravity interactions develop, and close to shore, they dominate the interactions.
Nonlinear energy fluxes are compared to gradients in total energy flux and are observed to balance nearly completely. Overall, energy losses at both infragravity and shortwave frequencies can largely be explained by a cascade of nonlinear energy transfers to high frequencies (say, f . 1.5 Hz) where the energy is presumably dissipated. Infragravity–infragravity interactions seem to induce higher harmonics that allow for shape transformation of the infragravity wave to symmetric.
The largest decrease in infragravity wave height occurs close to the shore, where infragravity–infragravity in teractions dominate and where the infragravity wave is asymmetric, suggesting wave breaking to be the dominant mechanism of infragravity wave dissipation.

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5 

Multifrequency Radar Observations Collected in Southern France during HyMeXSOP1
Article/Letter to the Editor 
Civil Engineering and Geosciences
20150201

Author: 
Bousquet, O.
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Berne, A.
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Delanoe, J.
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Dufournet, Y.
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Gourley, J.J.
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VanBaelen, J.
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Augros, C.
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Besson, L.
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Boudevillain, B.
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Caumont, O.
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Defer, E.
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Grazioli, J.
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Jorgensen, D.J.
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Kirstetter, P.E.
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Ribaud, J.F.
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Beck, J.
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Delrieu, G.
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Ducrocq, V.
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Scipion, D.
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Schwarzenboeck, A.
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Zwiebel, J.

The radar network deployed in southern France during the first special observing period (SOP 1) of the Hydrological Cycle in the Mediterranean Experiment (HyMeX) was designed to precisely document the 3D structure of moist upstream flow impinging on complex terrain as a function of time, height, and alongbarrier distance, and surface rainfall patterns associated with orographic precipitation events. This deployment represents one of the most ambitious field experiments yet, endeavoring to collect highquality observations of thunderstorms and precipitation systems developing over and in the vicinity of a major mountain chain.
Radar observations collected during HyMeX represent a valuable, and potentially unique, dataset that will be used to improve our knowledge of physical processes at play within coastal orographic heavy precipitating systems and to develop, and evaluate, novel radarbased products for research and operational activities. This article provides a concise description of this radar network and discusses innovative research ideas based upon preliminary analyses of radar observations collected during this field project with emphasis on the synergetic use of dualpolarimetric radar measurements collected at multiple frequencies.

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6 

On the Deceiving Aspects of Mixing Diagrams of Deep Cumulus Convection
Mixing processes in deep precipitating cumulus clouds are investigated by tracking Lagrangian particles in a largeeddy simulation. The trajectories of particles are reconstructed and the thermodynamic properties of cloud air are studied using mixing diagrams. The trajectory analysis shows that the incloud mixing is entirely dominated by lateral entrainment and that there is no significant vertical mixing by downdrafts originating from cloud top. Yet the thermodynamic properties of the particles are located close to a line in the mixing diagrams, which appears to be consistent with twopoint vertical mixing. An attempt is made to resolve this paradox using the buoyancysorting model of Taylor and Baker, but it is found that this model does not provide a full explanation for the location of particles in the mixing diagram. However, it is shown that the mixingline behavior can be well understood from a simple analytically solvable model that uses a range of different lateral entrainment rates. Two further factors that determine the location of particles in the mixing diagram are identified: the removal of noncloudy air and precipitation effects. Finally, a thermodynamic argument is given that explains the absence of coherent downdrafts descending from cloud top.

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7 

Stochastic modeling of coherent wave fields over variable depth
Refractive focusing of swell waves can result in fastscale variations in the wave statistics because of wave interference, which cannot be resolved by stochastic wave models based on the radiative transport equation. Quasicoherent statistical theory does account for such statistical interferences and the associated wave inhomogeneities, but the theory has thus far been presented in a form that appears incompatible with models based on the radiative transfer equation (RTE). Moreover, the quasicoherent theory has never been tested against field data, and it is not clear how the coherent information inherent to such models can be used for better understanding coastal wave and circulation dynamics. This study therefore revisits the derivation of quasicoherent theory to formulate it into a radiative transport equation with a forcing term that accounts for the inhomogeneous part of the wave field. This paper shows how the model can be nested within (or otherwise used in conjunction with) quasihomogeneous wave models based on the RTE. Through comparison to laboratory data, numerical simulations of a deterministic model, and field observations of waves propagating over a nearshore canyon head, the predictive capability of the model is validated. The authors discuss the interference patterns predicted by the model through evaluation of a complex crosscorrelation function and highlight the differences with quasihomogeneous predictions. These results show that quasicoherent theory can extend models based on the RTE to resolve coherent interference patterns and standing wave features in coastal areas, which are believed to be important in nearshore circulation and sediment transport.

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8 

Challenges of operational river forecasting
Skillful and timely streamflow forecasts are critically important to water managers and emergency protection services. To provide these forecasts, hydrologists must predict the behavior of complex coupled human–natural systems using incomplete and uncertain information and imperfect models. Moreover, operational predictions often integrate anecdotal information and unmodeled factors. Forecasting agencies face four key challenges: 1)making themost of available data, 2)making accurate predictions usingmodels, 3) turning hydrometeorological forecasts into effective warnings, and 4) administering an operational service. Each challenge presents a variety of research opportunities, including the development of automated qualitycontrol algorithms for the myriad of data used in operational streamflow forecasts, data assimilation, and ensemble forecasting techniques that allow for forecaster input, methods for using humangenerated weather forecasts quantitatively, and quantification of human interference in the hydrologic cycle. Furthermore, much can be done to improve the communication of probabilistic forecasts and to design a forecasting paradigm that effectively combines increasingly sophisticated forecasting technology with subjective forecaster expertise. These areas are described in detail to share a realworld perspective and focus for ongoing research endeavors.
Open Access Content

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9 

Multifrequency radar observations collected in Southern France during HyMeXSOPI
An ambitious radar deployment to collect highquality observations of heavy precipitation systems developing over and in the vicinity of a coastal mountain chain is discussed.

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10 

Surfzone monitoring using rotary wing unmanned aerial vehicles
This study investigates the potential of rotary wing unmanned aerial vehicles (UAVs) to monitor the surfzone. This paper shows that these UAVs are extremely flexible surveying platforms that can gather nearcontinuous moderate spatial resolution and high temporal resolution imagery from a fixed position high above a study site. The rotary wing UAVs used in this study can fly for ;12 min with a mean loiter radius of 1–3.5m and a mean loiter error of 0.75–4.5 m. These numbers depend on the environmental conditions, flying style, battery type, and vehicle type. The images obtained from the UAVs, and in combination with surveyed ground control points (GCPs), can be georectified to a pixel resolution between 0.01 and 1m, and a reprojection error—that is, the difference between the surveyed GPS location of a GCP and the location of the GCP obtained from the georectified image—of O(1 m). The flexibility of rotary wing UAVs provides moderate spatial resolution and high temporal resolution imagery, which are highly suitable to quickly obtain surfzone and beach characteristics in response to storms or for daytoday beach safety information, as well as scientific pursuits of surfzone kinematics on different spatial and temporal scales, and dispersion and advection estimates of pollutants.

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11 

Spectral polarimetric radar clutter suppression to enhance atmospheric echoes
The clutter present in the Doppler spectra of atmospheric targets can be removed by using polarimetry. The purpose is to suppress the Doppler velocity bins where spectral polarimetric parameters have atypical values. This procedure largely improves profiles of moments and polarimetric parameters of atmospheric targets. Several spectral polarimetric clutterreduction techniques, which are based on thresholding and intended for realtime processing, are discussed in this paper. A new method, the double spectral linear depolarization ratio cluttersuppression technique, is proposed. Very satisfactory performances are obtained with this method, which can be used in the full range of elevations (08–908). Spectral polarimetric cluttersuppression techniques for realtime processing were studied for the Sband highresolution Transportable Atmospheric Radar (TARA) profiler. For this study, precipitation, cloud, and clearair scattering are considered examples of atmospheric echoes. After successful testing in 2008, the double spectral linear depolarization ratio filter was implemented in the realtime processing of the Xband scanning drizzle radar (IDRA).

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12 

Influence of the subcloud layer on the development of a deep convective ensemble
The rapid transition from shallow to deep convection is investigated using largeeddy simulations. The role of cold pools, which occur due to the evaporation of rainfall, is explored using a series of experiments in which their formation is suppressed.A positive feedback occurs: the presence of cold pools promotes deeper, wider, and more buoyant clouds with higher precipitation rates, which in turn lead to stronger cold pools. To assess the influence of the subcloud layer on the development of deep convection, the coupling between the cloud layer and the subcloud layer is explored using Lagrangian particle trajectories. As shown in previous studies, particles that enter clouds have properties that deviate significantly from the mean state. However, the differences between particles that enter shallow and deep clouds are remarkably small in the subcloud layer, and become larger in the cloud layer, indicating different entrainment rates. The particles that enter the deepest clouds also correspond to the widest cloud bases, which points to the importance of convective organization within the subcloud layer.

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13 

The Evolution of Inhomogeneous Wave Statistics through a Variable Medium
The interaction of ocean waves with variable currents and topography in coastal areas can result in inhomogeneous statistics because of coherent interferences, which affect wavedriven circulation and transport processes. Stochastic wave models, invariably based on some form of the radiative transfer equation (or action balance), do not account for these effects. The present work develops and discusses a generalization of the radiative transfer equation that includes the effects of coherent interferences on wave statistics. Using multiple scales, the study approximates the transport equation for the (complete) secondorder wave correlation matrix. The resulting model transports the coupledmode spectrum (a form of the Wigner distribution) and accounts for the generation and propagation of coherent interferences in a variable medium. The authors validate the model through comparison with analytic solutions and laboratory observations, discuss the differences with the radiative transfer equation and the limitations of this approximation, and illustrate its ability to resolve coherent interference structures in wave fields such as those typically found in refractive focal zones and around obstacles.

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14 

Parameterization of Surface Roughness Based on ICESat/GLAS Full Waveforms: A Case Study on the Tibetan Plateau
Glaciers in the Tibetan mountains are expected to be sensitive to turbulent sensible and latent heat fluxes. One of the most significant factors of the energy exchange between the atmospheric boundary layer and the glacier is the roughness of the glacier surface. However, methods to parameterize this roughness for glacier surfaces in remote regions are not well known. In this paper, the authors use the data acquired by Ice, Cloud, and Land Elevation Satellite (ICESat)/Geoscience Laser Altimeter System (GLAS) laser altimetry from February 2003 to November 2004 along several tracks over glaciers of the Nyainqentanglha range in central Tibet. The authors make a study of the waveforms measured by the ICESat/GLAS laser system over mountainous and glacial areas. The surface characteristics are evaluated within laser footprints over the glacier outlines based on the glaciological inventory of the Tibetan Plateau constructed by the Cold and Arid Regions Environmental and Engineering Research Institute (CAREERI), Chinese Academy of Sciences. For this purpose, the authors extract waveform parameters: the waveform width, the number of modes, and the RMS width of the waveform. These parameters are compared with surface slope and roughness obtained from the Advanced Spaceborne Thermal Emission and Reflection Radar (ASTER) Global Digital Elevation Model (GDEM). Through this analysis, the impact of morphology on the returned laser waveform is shown for the Nyainq^entanglha range. The roughness and the slope of the surface can be quite significant and may contribute from several meters to tens of meters to the pulse extent. The waveform analysis results indicate that the received waveforms are capable representations of surface relief within the GLAS footprints.

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15 

Continuous singlecolumn model evaluation at a permanent meteorological supersite

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16 

The effects of the internal flow structure on SPM entrapment in the Rotterdam Waterway
Field measurements are presented, which are the first to quantify the processes influencing the entrapment of suspended particulate matter (SPM) at the limit of saltwater intrusion in the Rotterdam Waterway. The estuarine turbidity maximum (ETM) is shown to be maintained by the trapping of fluvial SPM at the head of the salt wedge. The trapping process is associated with the raining out of fluvial SPM from the upper, fresher part of the water column, into the layer below the pycnocline. The dominant mechanisms responsible are baroclinic shear flows and the abrupt change in turbulent mixing characteristics due to damping of turbulence at the pycnocline. This view contrasts with the assumption of landward transport of marine SPM by asymmetries in bed stress. The SPM transport capacity of the tidal flow is not fully utilized in the ETM, and the ETM is independent of a bedbased supply of mud. This is explained by regular exchange of part of the ETM with harbor basins, which act as efficient sinks, and that the Rotterdam Waterway is not a complete fluvial SPM trap. The supply of SPM by the freshwater discharge ensures that the ETM is maintained over time.
Hence, theETMis an advective phenomenon. Relative motion between SPM and saltwater occurs because of lags introduced by resuspension. Moreover,SPM that lags behind the salt wedge after high water slack (HWS) is eventually recollected at the head. Hence, SPM follows complex transport pathways and the mechanisms involved in trapping and transport of SPM are inherently threedimensional.

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17 

Automatic curve extraction for digitizing rainfall strip charts
Amethod has been developed that largely automates the laborintensive extraction work for large amounts of rainfall strip charts and paper rolls. The method consists of the following five basic steps: 1) scanning the charts and rolls to highresolution digital images, 2) manually and visually registering relevant meta information from charts and rolls and preprocessing rolls to locate day transitions, 3) applying automatic curve extraction software in a batch process to determine the coordinates of cumulative rainfall lines on the images, 4) postprocessing the curves that were not correctly determined in step 3, and 5) aggregating the cumulative rainfall in pixel coordinates to the desired time resolution. The core of the method is in step 3. Here a color detection procedure is introduced that automatically separates the background of the charts and rolls from the grid and subsequently the rainfall curve. The rainfall curve is detected by minimization of a cost function. In total, 321 station years of locations in the Netherlands have successfully been digitized and transformed to longterm rainfall time series with 5min resolution. In about 30% of the cases, semiautomatic postprocessing
of the results was needed using a purposebuilt graphical interface application. This percentage, however, strongly depends on the quality of the recorded curves and the charts and rolls. Although developed for rainfall, the method can be applied to other elements as well.

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18 

Analysis of tidal straining as driver for estuarine circulation in wellmixed estuaries
Tidal straining, which can mathematically be described as the covariance between eddy viscosity and vertical shear of the alongchannel velocity component, has been acknowledged as one of the major drivers for estuarine circulation in channelized tidally energetic estuaries. In this paper, the authors investigate the role of lateral circulation for generating this covariance. Five numerical experiments are carried out, starting with a reference scenario including the full physics and four scenarios in which specific key physical processes are neglected. These processes are longitudinal internal pressure gradient forcing, lateral internal pressure gradient forcing, lateral advection, and the neglect of temporal variation of eddy viscosity. The results for the viscosity–shear covariance are correlated across different experiments to quantify the change due to neglect of these key processes. It is found that the lateral advection of vertical shear of the alongchannel velocity component and its interaction with the tidally asymmetric eddy viscosity (which is also modified by the lateral circulation) is the major driving force for estuarine circulation in wellmixed tidal estuaries.

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19 

Convective boundary layers driven by nonstationary surface heat fluxes
In this study the response of dry convective boundary layers to nonstationary surface heat fluxes is systematically investigated. This is relevant not only during sunset and sunrise but also, for example, when clouds modulate incoming solar radiation. Because the time scale of the associated change in surface heat fluxes may differ from case to case, the authors consider the generic situation of oscillatory surface heat fluxes with different frequencies and amplitudes and study the response of the boundary layer in terms of transfer functions. To this end both a mixed layer model (MLM) and a largeeddy simulation (LES) model are used; the latter is used to evaluate the predictive quality of the mixed layer model. The mixed layer model performs generally quite well for slow changes in the surface heat flux and provides analytical understanding of the transfer characteristics of the boundary layer such as amplitude and phase lag. For rapidly changing surface fluxes (i.e., changes within a time frame comparable to the large eddy turnover time), it proves important to account for the time it takes for the information to travel from the surface to higher levels of the boundary layer such as the inversion zone. As a followup to a 1997 study by Sorbjan, who showed that the conventional convective velocity scale is inadequate as a scaling quantity during the decay phase, this paper addresses the issue of defining, in (generic) transitional situations, a velocity scale that is solely based on the surface heat flux and its history.

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20 

Can wind lidars measure turbulence?
Modeling of the systematic errors in the secondorder moments of wind speeds measured by continuouswave (ZephIR) and pulsed (WindCube) lidars is presented. These lidars use the conical scanning technique to measure the velocity field. The model captures the effect of volume illumination and conical scanning. The predictions are compared with the measurements from the ZephIR, WindCube, and sonic anemometers at a flat terrain test site under different atmospheric stability conditions. The sonic measurements are used at several heights on a meteorological mast in combination with lidars that are placed on the ground. Results show that the systematic errors are up to 90% for the vertical velocity variance, whereas they are up to 70% for the horizontal velocity variance. For the ZephIR, the systematic errors increase with height, whereas for the WindCube, they decrease with height. The systematic errors also vary with atmospheric stability and are low for unstable conditions. In general, for both lidars, the model agrees well with the measurements at all heights and under different atmospheric stability conditions. For the ZephIR, the model results are improved when an additional lowpass filter for the 3s scan is also modeled. It is concluded that with the current measurement configuration, these lidars cannot be used to measure turbulence precisely.

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