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The interactions among several stable-boundary-layer (SBL) processes occurring just after the evening transition of 2 July 2011 have been analysed using data from instruments deployed over the area of Lannemezan (France) during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. The near-calm situation of the afternoon was followed by the formation of local shallow drainage flows (SDFs) of less than 10 m depth at different locations. The SDF stage ended with the arrival of a stronger wind over a deeper layer more associated with the mountain-plain circulation, which caused mixing and destruction of the SDFs. Several gravity-wave-related oscillations were also observed on different time series. Wavelet analyses and wave parameters were calculated from high resolution and accurate surface pressure data of an array of microbarometers. These waves propagated relatively long distances within the SBL. The effects of these phenomena on turbulent parameters (friction velocity and kinematic heat flux) have been studied through multi-resolution flux decomposition methods performed on high frequency data from sonic anemometers deployed at different heights and locations. With this method, we were able to detect the different time-scales involved in each turbulent parameter and separate them from wave contributions, which becomes very important when choosing averaging-windows for surface flux computations using eddy covariance methods. The extensive instrumentation allowed us to highlight in detail the peculiarities of the surface turbulent parameters in the SBL, where several of the noted processes were interacting and producing important variations in turbulence with height and between sites along the sloping terrain.

We study the influence of the large-scale at- mospheric contribution to the dynamics of the convective boundary layer (CBL) in a situation observed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. We employ two modeling ap- proaches, the mixed-layer theory and large-eddy simulation (LES), with a complete data set of surface and upper-air at- mospheric observations, to quantify the contributions of the advection of heat and moisture, and subsidence. We find that by only taking surface and entrainment fluxes into ac- count, the boundary-layer height is overestimated by 70 %. Constrained by surface and upper-air observations, we infer the large-scale vertical motions and horizontal advection of heat and moisture. Our findings show that subsidence has a clear diurnal pattern. Supported by the presence of a nearby mountain range, this pattern suggests that not only synoptic scales exert their influence on the boundary layer, but also mesoscale circulations. LES results show a satisfactory cor- respondence of the vertical structure of turbulent variables with observations. We also find that when large-scale ad- vection and subsidence are included in the simulation, the values for turbulent kinetic energy are lower than without these large-scale forcings. We conclude that the prototypical CBL is a valid representation of the boundary-layer dynam- ics near regions characterized by complex topography and small-scale surface heterogeneity, provided that surface- and large-scale forcings are representative for the local boundary layer.

Template-induced colloidal deposition during solvent evaporation is a promising technique for extending the possibilities of nanosphere lithography and the creation of photonic band gap materials. We investigated the influence of the parameters that determine the surface topography of templates on colloidal crystal structure. On pillar-shaped templates, large defect-free square symmetric monolayers, ordered vacancy arrays, and body-centered cubic (bcc) and simple cubic (sc) colloidal crystals could be grown. Close-packed crystals displayed defects and large defect grains. Our results indicate that this may be avoided when the direction of gravity with respect to the substrate is changed.

Atmospheric scintillations cause difficulties for applications where an undistorted propagation of electromagnetic radiation is essential. These scintillations are related to turbulent fluctuations of temperature and humidity that are in turn related to surface heat fluxes. We developed an approach that quantifies these scintillations by estimating C(n(2)) from surface fluxes that are derived from single-level routine weather data. In contrast to previous methods that are biased to dry and warm air, our method is directly applicable to several land surface types, environmental conditions, wavelengths, and measurement heights (lookup tables for a limited number of site-specific parameters are provided). The approach allows for an efficient evaluation of the performance of, e.g., infrared imaging systems, laser geodetic systems, and ground-to-satellite optical communication systems. We tested our approach for two grass fields in central and southern Europe, and for a wheat field in central Europe. Although there are uncertainties in the flux estimates, the impact on C(n(2)) is shown to be rather small. The C(n(2)) daytime estimates agree well with values determined from eddy covariance measurements for the application to the three fields. However, some adjustments were needed for the approach for the grass in southern Europe because of non-negligible boundary-layer processes that occur in addition to surface-layer processes.

Observations, mixed-layer theory and the Dutch Large-Eddy Simulation model (DALES) are used to analyze the dynamics of the boundary layer during an intensive operational period (1 July 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence campaign. Continuous measurements made by remote sensing and in situ instruments in combination with radio soundings, and measurements done by remotely piloted aircraft systems and two manned aircrafts probed the vertical structure and the temporal evolution of the boundary layer during the campaign. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the models runs are inspired by some of these observations. The research focuses on the role played by the residual layer during the morning transition and by the large-scale subsidence on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night in the development of the boundary layer at the morning. DALES numerical experiments including the residual layer are capable of modeling the observed sudden increase of the boundary-layer depth during the morning transition and the subsequent evolution of the boundary layer. These simulations show a large increase of the entrainment buoyancy flux when the residual layer is incorporated into the mixed layer. We also examine how the inclusion of the residual layer above a shallow convective boundary layer modifies the turbulent kinetic energy budget. Large-scale subsidence mainly acts when the boundary layer is fully developed, and, for the studied day, it is necessary to be considered to reproduce the afternoon observations. Finally, we also investigate how carbon dioxide (CO2) mixing ratio stored the previous night in the residual layer plays a fundamental role in the evolution of the CO2 mixing ratio during the following day.

Evaluations of analytical footprint models using data from several stations located in different land use types are still scarce, but valuable for defining the spatial context of the measurements. Therefore, we evaluated two analytical footprint models by applying a ‘forward’ and an ‘inversion’ method. We used eddy covariance measurements from a flat agricultural landscape in western Germany in the summer of 2009, with seven eddy covariance systems over three different land use types with contrasting sensible heat fluxes. We found that the model of Hsieh et al. (2000. Adv. Water Resour. 23, 765–772) and of Kormann and Meixner (2001. Boundary Layer Meteorol. 99, 207–224) are both overestimating the distance of the peak contribution of the footprint. In our evaluation, the former model performs slightly better, independent of whether the crosswind dispersion was used from the latter model, or from the proposed model by Detto et al. (2006. Water Resour. Res. 42, 1–16).

Monin Obukhov Similarity Theory (MOST) is one of the cornerstones of surface layer meteorology and as such it is widely applied in models and for data analysis. One major disadvantage of using MOST for describing land-atmosphere interactions is that all turbulence properties are described as a function of the same limited set of key parameters. Therefore, MOST is in particular not valid over heterogeneous terrain (Andreas et al., 1998). To study the limits of MOST, deviations of similarity functions from their homogeneous versions need to be linked to the degree of heterogeneity (which may be different for different scalars, i.e. water vapor, CO2 and heat (Moene and Schüttemeyer, 2008)). In particular, the degree to which observations at a given point are influenced by different sources should be known. Footprint models can be used to partition EC (eddy covariance) measurements in scalar fluctuations originating from the different land use types in the measurement area. Calculations are made within footprint models to estimate the contribution of each unit element of the upwind surface area to a measured vertical flux. However, validations of footprint models with natural tracer experiments using several EC-stations and land use types are still needed. We will therefore present validation results of two analytical footprint models, using two different validation methods and several eddy covariance stations in two different landscapes. The first landscape is flat, with very small spatial variability in roughness lengths and soil properties. This mosaic-like land use pattern, together with entrainment, are expected to be the only challenge to MOST-based footprint models. The second landscape, in contrast, additionally includes slight slopes, spatial patterns in soil moisture, and some tree groups. The two sites should thus enable a characterization of the importance of different types of MOST violation. The analytical model of Kormann and Meixner (2001) and the approximately analytical model of Hsieh et al. (2000) were validated first using flux data from the Transregio experiment in Merken in August 2009. The eddy covariance observations were obtained in three adjacent fields with contrasting fluxes, a wheat, barley and a sugar beet field, as well as close to the edge between two of the fields. Two validation methods were applied: forward validation and validation by inversion. For the forward method, fluxes from the wheat, barley and sugar beet field were used together with the footprint model to estimate the flux measured at an EC-station in the barley field near the border with the sugar beet field. The inverted validation method is based on decomposed fluxes of seven eddy covariance stations in the three fields, to estimate the surface fluxes of the three underlying land use types and therewith perform a cross validation with left-out eddy covariance measurements. We secondly used EC-measurements from a more heterogeneous and complex campaign called BLLAST (in southern France in June 2011) to validate both footprint models using flux measurements from a wheat and a grass field, and from measurements at the border between these fields. Validation results will be shown and compared for both models, validation methods and datasets. Using the validated footprint models in a later stage of our study, the degree of heterogeneity at the point of observation can be quantified. Subsequently, deviations from published MOST relationships can be linked to the degree of heterogeneity.

Guided and constrained by a complete data set of surface and upper-air observations taken during the fifth Intensive Observational Period (IOP-05, 25th June 2011) of the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) experiment, we reconstruct the evolution of the atmospheric boundary layer (ABL) using mixed-layer theory. The model results are corroborated by UHF radar and multiple profiles done by unmanned and manned aircraft platforms as well as by radiosondes. During the first morning hours, the ABL is mainly controlled by surface forcing, reaching a depth between 500 and 600 meter. At these levels, large-scale subsidence motions become similar in magnitude to the entrainment velocity and the ABL-growth becomes nearly zero. This equilibrium is maintained until the afternoon transition where mixed-layer model results and observations show a decrease of the boundary layer depth in spite of well mixed observed vertical profiles of heat and moisture. We discuss potential explanations to the development of subsidence motions associated to mesoscale flows driven by the proximity of the Pyrenees mountain range and to large-scale forcing. The heat and moisture budgets are also further analyzed with the mixed layer model. We find strong indications of the active role of heat and moisture advection required to reproduce the observed diurnal variability in temperature and specific humidity. By quantifying the budget terms, we are able to obtain a first estimation of the contribution of advection to the heat and moisture budgets. To complete the analysis, we extend the study to examine the turbulent characteristics of the ABL by using aircraft measurements combined with large-eddy simulations. In so doing, we are able to study the role of wind (directional) shear in the development and maintenance of the ABL. IOP05 was characterized by easterly flows within the ABL and westerly winds aloft. Our findings stress the important role of large scale forcing in understanding the ABL development at various stages during BLLAST. The proposed method can also be very useful to support the further interpretation of observations and mesoscale model experiments.

Mixed-layer theory and large-eddy simulations are used to analyze the dynamics of the boundary layer on two intensive operational periods during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign: 1st and 2nd of July 2011, when convective boundary layers (CBLs) were observed. Continuous measurements made by several remote sensing and in situ instruments in combination with radiosoundings, and measurements done by unmanned aerial vehicles and an aircraft probed the vertical structure and the temporal evolution of the boundary layer. Mixed layer theory (Tennekes and Driedonks, 1981) and the Dutch Atmospheric Large-Eddy Simulation model (DALES, Heus et al., 2011) are set to reproduce and analyze the dynamics of the atmosphere during these two days. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the model runs are inspired by the observations taken at the two supersites that concentrated most of the instrumentation during the campaign. For DALES the initial profile (at 7 UTC on 1st July and 5 UTC on 2nd July) takes into account the existence of a residual layer above the nocturnal stable layer observed during the early morning. The mixed layer model is initialized when a well-developed convective boundary layer was observed, from 10 UTC. Due to the surface heterogeneity of the area, the models were run separately to analyze the boundary layer conditions at the two different supersites. First, the research focuses on the role-played by the residual layer (RL) on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night to the development of the boundary layer at the morning. DALES, which takes into account the residual layer, is capable to model the observed sudden increase of the boundary layer depth and of the potential temperature occurred during the morning transition. There are different sources, which can be the responsible of the overshooting for instance surface and entrainment fluxes or large-scale phenomena. Analyzing the entrainment buoyancy heat flux, a large increase is obtained by the simulation when the residual layer is incorporated in the mixed layer by the growing convective boundary layer. Consequently, a precise definition of the characteristics of the residual layer is fundamental even though it is complex because the evolution of the main variables in the residual layer during the previous night depends on different factors such as radiation divergence or advection. Once the boundary layer is fully developed around midday, both models are used to study the dynamics of the boundary layer during the studied days. The models are able to reproduce the evolution of the boundary layer depth, potential temperature and specific humidity from late morning. They are also used to analyze the role played by subsidence and large-scale advection during the afternoon transition. During 1st of July, heat and moisture advection are negligible contributions to the heat or moisture budget, respectively, but subsidence plays an important role in the turbulence decay during the afternoon when a clear decrease of boundary layer depth was observed. On 2nd of July the main boundary layer variables follow similar evolution to the previous day, but with a lower maximum of the boundary layer depth. However, in the last part of the day, a front was approaching advecting moisture over the area and increasing the cloud cover. To summarize, the residual layer and large-scale phenomena play a crucial role in the development of the boundary layer during some days of the BLLAST campaign.

Eddy covariance measurements are the method of choice to determine net land surface – atmosphere fluxes of energy and matter, but in some cases cannot be applied in a representative way. In particular, requirements on the measurement height and the principles underlying the fetch or footprint of the sensor inhibit straightforward application in small (

BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) aims at better understanding the thermodynamical processes that occur during the late afternoon in the lower troposphere. In direct contact with the Earth surface, the atmospheric boundary layer is governed by buoyant and mechanical turbulence, with a strong diurnal cycle. The late afternoon transition, from the daytime dry convection to the night-time stable boundary layer, still raises a lot of issues and is poorly represented in the meteorological models. Yet, it plays an important role in the transport and diffusion of trace gases, like water vapour, carbon dioxide, pollutants, dusts... How does the afternoon decay in the lower troposphere happen, when the surface sensible heat flux start to sharply decrease ? How do the scales of the motions and transfers change ? What is the impact on the chemical closure and transport of trace gases and aerosols ? How to properly represent those processes in the meteorological models ? An international group is working on those issues by use of observations and numerical simulations, in order to improve our understanding and representation of the turbulent processes of the boundary-layer late afternoon transition. The roles of surface heterogeneity, entrainment at the boundary layer top, large scale subsidence, radiative effects, advection and gravity waves are studied. Due to the large lack of observations during this phase, a field campaign was organized in the vicinity of a 60-m instrumented tower of Laboratoire d'Aérologie, near the Pyrenees ridge in Southwest France, from 14 June to 8 July 2011. This experiment puts together complementary observation resources, in order to obtain an exhaustive description of the boundary-layer dynamical processes, its vertical structure, and the spatial variability related to surface heterogeneity. Continuous measurements (UHF radar and sodar wind profilers, lidars, ground stations), and intensive observations with aircraft, unmanned aerial vehicles, tethered balloons and radiosoundings were used. Particular emphasis was placed to bridge the different spatial scales with an integrated analysis of the observations and a complete numerical model hierarchy. This presentation will give an overview of the field experiment, with an emphasis on specific and innovative instrumental aspects, and with some preliminary results, introducing various contributions of BLLAST participants that will be shown along the 20th BLT conference.

To study turbulent flows in the atmospheric boundary layer dimensional analysis is widely applied. For flows in the lower part of the atmospheric boundary layer the cornerstone of analysing experiments and modelling is Monin-Obukhov Similarity Theory (MOST). The central assumption is that once all relevant variables are identified, the number of dimensionless groups can be determined. Those dimensionless groups should be related in a universal way, irrespective of the magnitude of the individual (dimensional) variables. For MOST the list of relevant variables is limited, which is both its strength and its weakness. The main assumptions needed to limit the number of relevant variables are stationarity, horizontal homogeneity and irrelevance of processes outside the atmospheric surface layer. These conditions are easily violated in realistic situations. Although violations of MOST conditions in relation to non-universality of the MOST relationships are widely discussed in literature, this is nearly always done in a qualitative sense only. Here we will focus on the similarity relationships for standard deviations of temperature and humidity (sT and sq) and study how they deviate from universal behaviour due to heterogeneity of the underlying surface. The spatial scale of the heterogeneity we address is the scale of contrasts between individual fields (order of 100 meter). In the analysis we will use the framework presented in Moene and Schüttemeyer (2008). This study presents a detailed analysis of a dataset gathered during the BLLAST field campaign near Lannemezan (Southern France) in the summer of 2011. Three eddy-covariance systems were set up: one in a meadow (green throughout the campaign), one in a field with a mixture of cereals and peas (ripening during the campaign) and one right at the edge between the two fields ('edge station'). This setup enables us to obtain surface fluxes and other turbulence statistics both for two contrasting landuse types with homogeneous fetch as well as for a station with a heterogeneous fetch. The objective is to quantify the impact of surface heterogeneity on the dissimilarity between the similarity relationships of different scalars. The degree to which the edge station experiences a heterogeneous fetch depends on two aspects. The potential heterogeneity is determined by the contrast in surface fluxes between the two fields: the mid-day Bowen ratio of the meadow is approximately constant at 0.3, whereas the Bowen ratio of the cereal field increases from 1 to 1.5 over the course of the experiment. The midday CO2 flux is -7·10-7 kg m-2s-1 for the meadow, whereas for the cereals it increases from -2·10-7 kg m-2s-1 to +0.6·10-7 kg m-2s-1. The actual heterogeneity is determined by the wind direction, or more exactly, the location of the source area. Only if the source area of the observations is located in different fields, then the contrast in fluxes between fields will be actually relevant and lead to non-universal behaviour of MOST relationships. In order to quantify the relative contribution of the two fields in the signal of the edge station the footprint model of Hsieh et al. (2000), augmented with the crosswind distribution of Kormann and Meixner (2001), is used. The development of the fluxes of both surfaces indicates that the potential heterogeneity indeed increases in time. Furthermore, it turns out that for a significant part of the time footprint contributions come from both fields. Hence, the actual heterogeneity increases in time as well. This we express in a new heterogeneity parameter that combines the flux contrast between the fields and the heterogeneity of the land use in the footprint. The actual heterogeneity is clearly visible in the midday correlation coefficient between temperature and humidity (rTq) which decreases from 0.75 to 0.5 in the course of the experiment (rTq is of the order of 0.8 for both homogeneous fields). The mean Bowen ratio measured at the edge station is generally below 0.5, which implies that, according to Moene and Schüttemeyer (2008), one would expect that sT/¿* > sq/q* (equivalent to a relative transport efficiency ¿=rwT/rwq less than 1). This is indeed what is observed. Midday values for ¿ gradually decrease from 0.9 to 0.7. Apart from the similarity relationships for sT and sq, the relationships for CO2 standard deviation will also be discussed briefly.