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A 2-year measurement campaign of the ZephIR 300 vertical profiling continuous-wave (CW) focusing wind lidar has been carried out by the Royal Netherlands Meteorological Institute (KNMI) at the Cabauw site. We focus on the (height-dependent) data availability of the wind lidar under various meteorological conditions and the data quality through a comparison with in situ wind measurements at several levels in the 213 m tall meteorological mast. We find an overall availability of quality-controlled wind lidar data of 97 % to 98 %, where the missing part is mainly due to precipitation events exceeding 1 mm h−1 or fog or low clouds below 100 m. The mean bias in the horizontal wind speed is within 0.1 m s−1 with a high correlation between the mast and wind lidar measurements, although under some specific conditions (very high wind speed, fog or low clouds) larger deviations are observed. The mean bias in the wind direction is within 2∘, which is of the same order as the combined uncertainty in the alignment of the wind lidars and the mast wind vanes. The well-known 180∘ error in the wind direction output for this type of instrument occurs about 9 % of the time. A correction scheme based on data of an auxiliary wind vane at a height of 10 m is applied, leading to a reduction of the 180∘ error below 2 %. This scheme can be applied in real-time applications in the situation that a nearby freely exposed mast with wind direction measurements at a single height is available.

We report about a new third-party observation, namely, wind measurements derived from hot-air balloon (HAB) tracks. We first compare the HAB winds with wind measurements from a meteorological tower and a radio acoustic wind profiler, both situated at the topographically flat observatory near Cabauw, the Netherlands. To explore the potential of this new type of wind observation in other topographies, we present an intriguing HAB flight in Austria with a spectacular mountain–valley circulation. Subsequently, we compare the HAB data with a numerical weather prediction (NWP) model during 2011–13 and the standard deviation of the wind speed is 2.3 m s−1. Finally, we show results from a data assimilation feasibility experiment that reveals that HAB wind information can have a positive impact on a hindcasted NWP trajectory.

Heat transport through short and closed vegetation such as grass is modelled by a simple diffusion process. The grass is treated as a homogeneous ‘sponge layer’ with uniform thermal diffusivity and conductivity, placed on top of the soil. The temperature and heat-flux dynamics in both vegetation and soil are described using harmonic analysis. All thermal properties have been determined by optimization against observations from the Haarweg climatological station in The Netherlands. Our results indicate that both phase and amplitude of soil temperatures can be accurately reproduced from the vegetation surface temperature. The diffusion approach requires no specific tuning to, for example, the daily cycle, but instead responds to all frequencies present in the input data, including quick changes in cloud cover and day–night transitions. The newly determined heat flux at the atmosphere–vegetation interface is compared with the other components of the surface energy balance at this interface. The budget is well-closed, particularly in the most challenging cases with varying cloud cover and during transition periods. We conclude that the diffusion approach (either implemented analytically or numerically) is a physically consistent alternative to more ad hoc methods, like ‘skin resistance’ approaches for vegetation and bulk correction methods for upper soil heat storage. However, more work is needed to evaluate parameter variability and robustness under different climatological conditions. From a numerical perspective, the present representation of vegetation allows for both slow and rapid feedbacks between the atmosphere and the surface. As such, it would be interesting to couple the present surface parametrization to turbulence-resolving models, such as large-eddy simulations.

An overview is given of 50-year Cabauw observations and research on the structure and dynamics of the atmospheric boundary layer. It is shown that over time this research site with its 200-m meteorological tower has grown into an atmospheric observatory with a comprehensive observational program encompassing almost all aspects of the atmospheric column including its boundary conditions. This is accomplished by the Cabauw Experimental Site for Atmospheric Research (CESAR) a consortium of research institutes. CESAR plays an important role in the educational programs of the CESAR universities. The current boundary-layer observational program is described in detail, and other parts of the CESAR observational program discussed more briefly. Due to an open data policy the CESAR datasets are used by researchers all over the world. Examples are given of the use of the long time series for model evaluation, satellite validation, and process studies. The role of tall towers is discussed in relation to the development of more and better ground-based remote sensing techniques. CESAR is now incorporated into the Ruisdael observatory, the large-scale atmospheric research infrastructure in the Netherlands. With Ruisdael the embedding of the Dutch atmospheric community in national policy landscape, and in the European atmospheric research infrastructures is assured for the coming decade.

This study investigates how wind shear and momentum fluxes in the surface- and boundary layer vary across wind and cloud regimes. We use a 9-year-long data set from the Cabauw observatory complemented by (8.2 × 8.2 (Formula presented.)) daily Large Eddy Simulation (LES) hindcasts. An automated algorithm classifies observed and simulated days into different cloud regimes: (a) clear-sky days, (b) days with shallow convective clouds rooted in the surface layer, with two ranges of cloud cover, and (c) non-convective cloud days. Categorized days in observations and LES do not always match, particularly the number of non-convective cloud days are underestimated in the LES, which likes to develop convection. However, the climatology and diurnal cycle of winds for each regime are very similar in LES and observations, strengthening our confidence in LES’ skill to reproduce certain clouds for certain atmospheric states. Along-wind momentum flux profiles are similar across all regimes, but large cloud cover (convective and non-convective) days have larger total momentum flux distributed over a deeper layer, with up to 30% of the surface flux still present near cloud base. The clear-sky and especially shallow cumulus regime with low cloud cover have notably larger crosswind momentum fluxes in the boundary layer. Surface-layer wind shear at daytime is smallest in the shallow cumulus regimes, having deeper boundary layers and a steady increase in surface layer wind speed during daytime. Compared to clear-sky days at a similar stability, convective cloud regimes have smaller surface-layer wind shear and larger surface friction than estimated by Monin-Obukhov Similarity Theory.

A dataset of 9 years in duration (2009–17) of clouds and radiation was obtained at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. Cloud radiative forcings (CRF) were derived from the dataset and related to cloud cover and temperature. Also, the data were compared with RCM output. Results indicate that there is a seasonal cycle (i.e., winter, spring, summer, and autumn) in longwave (CRF-LW: 48.3, 34.4, 30.8, and 38.7 W m−2) and shortwave (CRF-SW: −23.6, −60.9, −67.8, and −32.9 W m−2) forcings at CESAR. Total CRF is positive in winter and negative in summer. The RCM has a cold bias with respect to the observations, but the model CRF-LW corresponds well to the observed CRF-LW as a result of compensating errors in the difference function that makes up the CRF-LW. The absolute value of model CRF-SW is smaller than the observed CRF-SW in summer, mostly because of albedo differences. The majority of clouds from above 2 km are present at the same time as low clouds, so the higher clouds have only a small impact on CRF whereas low clouds dominate their values. CRF-LW is a function of fractional cloudiness. CRF-SW is also a function of fractional cloudiness, if the values are normalized by the cosine of solar zenith angle. Expressions for CRF-LW and CRF-SW were derived as functions of temperature, fractional cloudiness, and solar zenith angle, indicating that CRF is the largest when fractional cloudiness is the highest but is also large for low temperature and high sun angle.

The agricultural emissions are the dominant sources of N2O and CH4 in the Netherlands. In this study, we have estimated nocturnal surface fluxes of both N2O and CH4 using atmospheric measurements at the Cabauw tall tower (4.927◦ E, 51.971◦ N, - 0.7 m a.s.l.). The nocturnal N2O and CH4 surface fluxes were derived using two different methods, the vertical gradient method (VGM), i.e. the sum of the storage flux and the turbulent flux, and the radon-tracer method (RTM), for the period of March 2017-December 2018 and 2016-2018, respectively. For N2O, we show that a few events occurring between May 30 and June 4 in 2018 dominated the monthly means. Using the VGM, we have estimated the annual mean nocturnal surface flux to be 0.59 ± 0.38 g/m2/yr (1 σ, the same as below) and 0.53 ± 0.19 g/m2/yr with and without events, respectively. The fluxes are high in the summer and low in the winter, with a seasonal amplitude of around 1.0 g/m2/yr and 0.5 g/m2/yr, with and without events, respectively, which is likely caused by the seasonality of agricultural activities. For CH4, the annual mean nocturnal surface flux is 12.1 ± 3.3 g/m2/yr and the amplitude is around 9.9 g/m2/yr. Using the RTM, the mean fluxes of the whole period for N2O and CH4 are estimated to be 1.18 ± 2.25 (1.08 ± 1.29, without the events) g/m2/yr and 26.9 ± 24.8 g/m2/yr, respectively; in contrast to the VGM, no apparent seasonal pattern has been found. However, there is a good linear correlation between the estimated N2O fluxes from the two methods and the monthly means show a similar pattern when the same nights are considered; the R-squared value is around 0.9 with events and 0.6 without events, and the slope varies from 1.9 to 0.8 when different estimates of radon fluxes are used. Furthermore, we found that large N2O fluxes are related to the amount of rainfall occurring days before, with the correlation coefficient of around 0.6 (p value4, there is no correlation between the estimated CH4 fluxes from the two methods. Our findings demonstrate that nocturnal N2O and CH4 fluxes in the Cabauw area are highly variable and vary over different seasons, and that both VGM and RTM are useful to quantify regional N2O and CH4 fluxes.

A two-year measurement campaign of the ZephIR 300 vertical profiling continuous-wave (CW) focusing wind lidar has been carried out by the Royal Netherlands Meteorological Institute (KNMI) at the Cabauw site. We focus on the (height-dependent) data availability of the wind lidar under various meteorological conditions and the data quality through a comparison with in situ wind measurements at several levels in the 213-m tall meteorological mast. We find an overall availability of quality controlled wind lidar data of 97 % to 98 %, where the missing part is mainly due to precipitation events exceeding 1 mm/h or fog or low clouds below 100 m. The mean bias in the horizontal wind speed is within 0.1 m/s with a high correlation between the mast and wind lidar measurements, although under some specific conditions (very high wind speed, fog or low clouds) larger deviations are observed. The mean bias in the wind direction is within 2°, which is on the same order as the combined uncertainty in the alignment of the wind lidars and the mast wind vanes. The well-known 180° error in the wind direction output for this type of instrument occurs about 9 % of the time. A correction scheme based on data of an auxiliary wind vane at a height of 10 m is applied, leading to a reduction of the 180° error below 2 %. This scheme can be applied in real-time applications in case a nearby, freely exposed, mast with wind direction measurements at a single height is available.

This article presents validation results of the grass Reference Evapotranspiration (ETo) product, which is provided operationally by the Satellite Applications Facility on Land Surface Analysis (LSA SAF). ETo is considered to be the evapotranspiration from a hypothetical extensive well-watered field covered with green grass (12 cm high and with 0.23 albedo), under the given down-welling short-wave radiation. The LSA SAF product is estimated from daily solar radiation at the surface via a methodology designed to be applicable to that well-defined reference surface. In line with its definition, the LSA SAF ETo product is based on estimates of the radiative energy available at the surface. Validation of ETo estimates is challenging, given the difficulties of finding reliable in situ measurements that meet the ETo definition. We show that for the site that matches closely the reference surface (Cabauw, The Netherlands), the LSA SAF product outperforms other commonly used methodologies, even when these use in situ observations as input. However, observations taken at other stations, located in areas that do not deviate greatly from the reference surface, put into evidence the high uncertainty in local measurements. It is shown that the LSA SAF ETo product follows well in situ values, with average differences of 0.3 mm/day or less for mid-latitude sites and of the order of 1 mm/day for the Spanish sites near Cordoba and Albacete. Local advection effects cannot be ignored in measurements performed in the latter stations, where summer conditions are mostly warm and dry. In situ ETo measurements performed in those cases with lysimeters within limited fields are higher than satellite estimates due to advection of warm dry air from the vicinity, which acts as an extra source of energy. It is shown that this effect can be parameterized as a function of near surface air temperature. However, it is argued that those local advection effects should not occur in the idealized surface referred above. In contrast to other ETo estimates, the LSA SAF product is not influenced by local aridity or advection effects, and therefore it is particularly appropriate for large scale climate assessments, including drought monitoring (e.g. by considering the ratio of actual and reference evapotranspiration). Additionally, it provides suitable estimates of irrigation requirements in support of water management.

The scaled standard deviations of temperature and humidity are investigated in complex terrain. The study area is a steep Alpine valley, with six measurement sites of different slope, orientation and roughness (i-Box experimental site, Inn Valley, Austria). Examined here are several assumptions forming the basis of Monin–Obukhov similarity theory (MOST), including constant turbulence fluxes with height and the degree of self-correlation between the involved turbulence variables. Since the basic assumptions for the applicability of the MOST approach—horizontally homogeneous and flat conditions—are violated, the analysis is performed based on a local similarity hypothesis. The scaled standard deviations as a function of local stability are compared with previous studies from horizontally homogeneous and flat terrain, horizontally inhomogeneous and flat terrain, weakly inhomogeneous and flat terrain, as well as complex terrain. As a reference, similarity relations for unstable and stable conditions are evaluated using turbulence data from the weakly inhomogeneous and flat terrain of the Cabauw experimental site in the Netherlands, and assessed with the same post-processing method as the i-Box data. Significant differences from the reference curve and also among the i-Box sites are noted, especially for data derived from the i-Box sites with steep slopes. These differences concern the slope and the magnitude of the best-fit curves, illustrating the site dependence of any similarity theory.

Geostrophic wind speed data, derived from pressure observations, are used in combination with tower measurements to investigate the nocturnal stable boundary layer at Cabauw, the Netherlands. Since the geostrophic wind speed is not directly influenced by local nocturnal stability, it may be regarded as an external forcing parameter of the nocturnal stable boundary layer. This is in contrast to local parameters such as in situ wind speed, the Monin–Obukhov stability parameter (z/L), or the local Richardson number. To characterize the stable boundary layer, ensemble averages of clear-sky nights with similar geostrophic wind speeds are formed. In this manner, the mean dynamical behavior of near-surface turbulent characteristics and composite profiles of wind and temperature are systematically investigated. The classification is found to result in a gradual ordering of the diagnosed variables in terms of the geostrophic wind speed. In an ensemble sense the transition from the weakly stable to very stable boundary layer is more gradual than expected. Interestingly, for very weak geostrophic winds, turbulent activity is found to be negligibly small while the resulting boundary cooling stays finite. Realistic numerical simulations for those cases should therefore have a comprehensive description of other thermodynamic processes such as soil heat conduction and radiative transfer.

High-resolution large-eddy simulations (LESs) are used to investigate the effect of turbulence as well as the interaction between atmosphere and soil on the life cycle of nocturnal radiation fog. The first part of the article focuses on the validation of the LES model system for simulating radiation fog against measurement data from the meteorological super-site in Cabauw (Netherlands). Differences found in the fog life cycle, depth, and liquid water content between LES and observations can, as in former studies, be largely ascribed to the presence of local advection processes in the observational data and uncertainty in the measurement data used to initialise the model. Moreover it is found that the choice of the droplet number concentration within the cloud microphysical representation has a high impact on the liquid water content within the fog layer, but a rather small effect on its life cycle. In the second part of the article, a set of LES runs is analysed with idealised initial conditions in order to study the effect of turbulent mixing as well as the initial state of the soil on the development of radiation fog. As expected, the results show that turbulent mixing has a strong impact on the time of fog formation, which is complicated by the interaction with both radiative cooling and water vapour removal by dew deposition. Furthermore, it is found that the near-surface soil temperature plays a key role for the exact time of fog formation, whereas near-surface soil moisture is decisive for the lifting and dissipation time of the fog layer.

The 10 m-wind over flat terrain in models is often lower than observations indicate. Here we show that this negative bias is mainly caused by the larger roughness lengths that models use than the World Meteorological Organisation (WMO)-advised roughness length of short grass (0.03 m). The concept of potential wind, which transforms the original wind to the theoretical wind under the assumption of a local roughness of 0.03 m, is well suited to distinguish this effect from other causes. It shows that the yearly averaged bias in Hirlam Aladin Research on Meso-scale Operational NWP (numerical weather prediction) in Euromed (HARMONIE) is largely due to roughness effects. The year-to-year variations are caused by misrepresentations of the weather situations by HARMONIE. This can be concluded from the analysis of the geostrophic wind, which is insensitive for roughness effects. Although the yearly averaged potential wind speed is well represented, HARMONIE reveals a strong seasonal cycle in the bias, being too low in summer, and slightly too high in winter. This is mainly caused by a too strong seasonal cycle in the representation of the leaf area index. The HARMONIE data set points out to underestimate the average wind over The Netherlands up to 10% in the period before 1995. Even after correction for roughness effects, an underestimation of 6% remains. It is likely that the disappearance of the bias after 1995 is caused by the gradually improved quality of the ERA-Interim data set with time.

A conceptual model is used in combination with observational analysis to understand regime transitions of near-surface temperature inversions at night as well as in Arctic conditions. The model combines a surface energy budget with a bulk parameterization for turbulent heat transport. Energy fluxes or feedbacks due to soil and radiative heat transfer are accounted for by a “lumped parameter closure,” which represents the “coupling strength” of the system. Observations from Cabauw, Netherlands, and Dome C, Antarctica, are analyzed. As expected, inversions are weak for strong winds, whereas large inversions are found under weak-wind conditions. However, a sharp transition is found between those regimes, as it occurs within a narrow wind range. This results in a typical S-shaped dependency. The conceptual model explains why this characteristic must be a robust feature. Differences between the Cabauw and Dome C cases are explained from differences in coupling strength (being weaker in the Antarctic). For comparison, a realistic column model is run. As findings are similar to the simple model and the observational analysis, it suggests generality of the results. Theoretical analysis reveals that, in the transition zone near the critical wind speed, the response time of the system to perturbations becomes large. As resilience to perturbations becomes weaker, it may explain why, within this wind regime, an increase of scatter is found. Finally, the so-called heat flux duality paradox is analyzed. It is explained why numerical simulations with prescribed surface fluxes show a dynamical response different from more realistic surface-coupled systems.

Measuring evaporation ( E ) with lysimeters is costly and prone to numerous errors. By comparing the energy balance and the remotely sensed surface temperature of lysimeters with those of the undisturbed surroundings, we were able to assess the representativeness of lysimeter measurements and to quantify differences in evaporation caused by spatial variations in soil moisture content. We used an algorithm (the so called 3T model) to spatially extrapolate the measured E of a reference lysimeter based on differences in surface temperature, net radiation and soil heat flux. We tested the performance of the 3T model on measurements with multiple lysimeters (47.5 cm inner diameter) and micro lysimeters (19.2 cm inner diameter) installed in bare sand, moss and natural dry grass. We developed different scaling procedures using in situ measurements and remotely sensed surface temperatures to derive spatially distributed estimates of R n and G and explored the physical soundness of the 3T model. Scaling of R n and G considerably improved the performance of the 3T model for the bare sand and moss experiments (Nash-Sutcliffe efficiency ( NSE ) increasing from 0.45 to 0.89 and from 0.81 to 0.94, respectively). For the grass surface, the scaling procedures resulted in a poorer performance of the 3T model ( NSE decreasing from 0.74 to 0.70), which was attributed to effects of shading and the difficulty to correct for differences in emissivity between dead and living biomass. The 3T model is physically unsound if the field scale average air temperature, measured at an arbitrarily chosen reference height, is used as input to the model. The proposed measurement system is relatively cheap, since it uses a zero tension (freely draining) lysimeter which results are extrapolated by the 3T model to the unaffected surroundings. The system is promising for bridging the gap between ground observations and satellite based estimates of E .

We present a new methodology, which we call Single Pair of Observations Technique with Eddy Covariance (SPOT-EC), to estimate regional-scale surface fluxes of 222Rn from tower-based observations of 222Rn activity concentration, CO2 mole fractions and direct CO2 flux measurements from eddy covariance. For specific events, the regional (222Rn) surface flux is calculated from short-term changes in ambient (222Rn) activity concentration scaled by the ratio of the mean CO2 surface flux for the specific event to the change in its observed mole fraction. The resulting 222Rn surface emissions are integrated in time (between the moment of observation and the last prior background levels) and space (i.e. over the footprint of the observations). The measurement uncertainty obtained is about ±15 % for diurnal events and about ±10 % for longer-term (e.g. seasonal or annual) means. The method does not provide continuous observations, but reliable daily averages can be obtained. We applied our method to in situ observations from two sites in the Netherlands: Cabauw station (CBW) and Lutjewad station (LUT). For LUT, which is an intensive agricultural site, we estimated a mean 222Rn surface flux of (0.29 ± 0.02) atoms cm−2 s−1 with values > 0.5 atoms cm−2 s−1 to the south and south-east. For CBW we estimated a mean 222Rn surface flux of (0.63 ± 0.04) atoms cm−2 s−1. The highest values were observed to the south-west, where the soil type is mainly river clay. For both stations good agreement was found between our results and those from measurements with soil chambers and two recently published 222Rn soil flux maps for Europe. At both sites, large spatial and temporal variability of 222Rn surface fluxes were observed which would be impractical to measure with a soil chamber. SPOT-EC, therefore, offers an important new tool for estimating regional-scale 222Rn surface fluxes. Practical applications furthermore include calibration of process-based 222Rn soil flux models, validation of atmospheric transport models and performing regional-scale inversions, e.g. of greenhouse gases via the SPOT 222Rn-tracer method.

We examine the dependence on wind speed of the share of the mean turbulent kinetic energy among the three velocity components in the near-neutral surface layer. To contrast the general behaviour and the local effects, four datasets are considered, corresponding to different surfaces and environmental conditions. For high wind speeds (i.e., wind speed $$\approx {10}\,{\hbox {ms}^{-1}}$$ ), the shares are well-defined and about the same for all sites. As wind speed decreases (becoming $$\approx {1}\,{\hbox { ms}^{-1}}$$ ), large record-to-record variability occurs giving, on average, an almost isotropic state for the horizontal velocity components. Through spectral analysis, we relate this behaviour to the low-frequency, submeso motions and to the lack of conditions required by Reynolds averaging. The implications for modelling are also discussed, showing that the wind speed, or a related quantity, must be accounted for, besides stability, in second-order closures.

The Royal Netherlands Meteorological Institute (KNMI) operates a three-dimensional microbarometer array at the Cabauw Experimental Site for Atmospheric Research observatory. The array consists of five microbarometers on a meteorological tower up to an altitude of 200 m. Ten ground-based microbarometers surround the tower with an array aperture of 800 m. This unique setup allows for the study of infrasound propagation in three dimensions. The added value of the vertical dimension is the sensitivity to wind and temperature in the atmospheric boundary layer over multiple altitudes. In this study, we analyze infrasound generated by an accidental chemical explosion at the Moerdijk petrochemical plant on 3 June 2014. The recordings of the tower microbarometers show two sequential arrivals, whereas the recordings on the ground show one wavefront. This arrival structure is interpreted to be the upgoing and downgoing wavefronts. The observations are compared with propagation modeling results using global-scale and mesoscale atmospheric models. Independent temperature and wind measurements, which are available at the Cabauw Experimental Site for Atmospheric Research, are used for comparison with model output. The modeling results explain the signal arrival times; however, the tower wavefront arrivals are not explained. This study is important for understanding the influence of the atmospheric boundary layer on infrasound detections and propagation.

Conventional in situ observations of meteorological variables are restricted to a limited number of levels near the surface, with the lowest observation often made around 1-m height. This can result in missed observations of both shallow fog, and the initial growth stage of thicker fog layers. At the same time, numerical experiments have demonstrated the need for high vertical grid resolution in the near-surface layer to accurately simulate the onset of fog; this requires correspondingly high-resolution observational data for validation. A two-week field campaign was conducted in November 2017 at the Cabauw Experimental Site for Atmospheric Research (CESAR) in the Netherlands. The aim was to observe the growth of shallow fog layers and assess the possibility of obtaining very high-resolution observations near the surface during fog events. Temperature and relative humidity were measured at centimetre resolution in the lowest 7 m using distributed temperature sensing. Further, a novel approach was employed to estimate visibility in the lowest 2.5 m using a camera and an extended light source. These observations were supplemented by the existing conventional sensors at the site, including those along a 200-m tall tower. Comparison between the increased-resolution observations and their conventional counterparts show the errors to be small, giving confidence in the reliability of the techniques. The increased resolution of the observations subsequently allows for detailed investigations of fog growth and evolution. This includes the observation of large temperature inversions in the lowest metre (up to 5 K) and corresponding regions of (super)saturation where the fog formed. Throughout the two-week observation period, fog was observed twice at the conventional sensor height of 2.0 m. Two additional low-visibility events were observed in the lowest 0–0.5 m using the camera-based observations, but were missed by the conventional sensors. The camera observations also showed the growth of shallow radiation fog, forming in the lowest 0.5 m as early as two hours before it was observed at the conventional height of 2 m.

A thermodynamically based model is presented to estimate daily actual evapotranspiration (ET) of a grass site closely resembling reference grass as defined by the Food and Agriculture Organization of the United Nations (FAO) under nonadvective conditions, from Meteosat Second Generation (MSG) imagery. The model presented here is derived from the thermodynamic theory by Schmidt combined with an atmospheric boundary layer model. Daily net radiation over the (reference) grass surface is parameterized as a function of global radiation, which can be estimated from MSG observations. It is then shown that ET over the grass area can be estimated using remotely sensed daily global radiation and air temperature as input only. The validation relied on observations gathered in Cabauw, a site closely resembling the reference grass, as defined by the FAO. The comparison with in situ data indicated a bias of 2.8 W m−2 and an RMSE of 7.7 W m−2. The possibility of using the approach developed here to provide reference crop evapotranspiration ETo is discussed. Because of the ambiguousness of ETo definition regarding local advection effects, it should be noted that explicitly advection-free conditions are dealt with. It is pointed out that in semiarid regions local advection cannot be ignored.

High-resolution upper-air wind observations are sparse, and additional observations are a welcome source of meteorological information. In this paper the potential of applying balloon flights for upper-air wind measurements is explored, and the meteorological content of this information is investigated. The displacement of a hot-air balloon is a measure for the wind speed and direction and thus a potential source for wind observations in the lower part of the troposphere. The response time of the balloon on the changing wind is fast in the beginning and levels off for smaller relative wind speeds. Four case studies are presented, and the balloon-derived winds are compared with other wind observations and with results from the HIRLAM–ALADIN Research on Mesoscale Operational NWP in Europe (HARMONIE) model. It turns out that hot-air balloon tracks can indeed produce useful wind observations just above and in the atmospheric boundary layer (ABL).

This study evaluates the relative impact of snow-surface coupling, long-wave radiation, and turbulent mixing on the development of the stable boundary layer over snow. Observations at three sites are compared to WRF single-column model (SCM) simulations. All three sites have snow-covered surfaces but are otherwise contrasting: Cabauw (Netherlands, grass), Sodankyla (Finland, needle-leaf forest) and Halley (Antarctica, ice shelf). All cases are characterized by stable, clear-sky, and calm conditions. Part 1 of this study determined the optimal SCM forcing strategy. In this study, the process intensities from that reference are perturbed to study their relative significance and to assess which process could be responsible for the most optimal agreement between model and observation. The analysis reveals a large variability in the modelled atmospheric state and surface parameters. Overall, the modelled gradients of temperature and moisture are under-estimated but decreasing the process intensities improves this. The impact is strongest with reduced mixing, though this then causes the model to overestimate the near-surface wind speed. To study the surface energy balance terms, we use so-called ‘process diagrams’. The achieved variation between the sensitivity runs indicates the model sensitivity to each process. The overall sensitivity is similar for the three sites but the relative offsets in the position of the sensitivity runs with respect to the observations differ, hampering general recommendations for model improvement. Furthermore, sometimes a meaningful interpretation of observations is troublesome, which hampers the comparison with model results. Radiation is relatively more important at Cabauw and Sodankyla, whilst coupling plays a more important role at Halley. The sensitivity analysis is performed with two boundary-layer schemes (MYJ, YSU). YSU generates larger, more accurate gradients of atmospheric temperature and humidity, while wind speeds are predicted better with MYJ. The behaviour of an increase in 2?m temperature with decreased mixing is most obvious with YSU.

We investigated sharp disruptions of local turbulence and scalar transport due to the arrival of sea-breeze fronts (SBFs). To this end, we employed a comprehensive 10-year observational database from the Cabauw Experimental Site for Atmospheric Research (CESAR, the Netherlands). Sea-breeze (SB) days were selected using a five-filter algorithm, which accounts for large-scale conditions and a clear mesoscale-frontal signal associated with the land-sea contrast. Among those days (102 in all, 8.3%), based on the value of the sensible-heat flux at the onset of SB, we identified three atmospheric boundary-layer (ABL) regimes: convective, transition and stable. In the convective regime, the thermally driven convective boundary layer is only slightly altered by a small enhancement of the shear when the SBF arrives. Regarding the transition regime, we found that the ABL afternoon transition is accelerated. This was quantified by estimating the contributions of shear and buoyancy to the turbulent kinetic energy. Other relevant disruptions are the sharp reduction in ABL depth (similar to 250 m/hr) and the sudden increase in average wind speed (> 2 m/s). In the stable regime, the arrival of the SB leads to disturbances in the wind profile at the surface layer. We observed a deviation of more than 1 m/s in the observed surface-layer wind profile compared with the profile calculated using Monin-Obukhov Similarity Theory (MOST). Our findings furthermore reveal the determinant role of the SB direction in the transport of water vapour, CO2 and Rn-222. The return of continental air masses driven by the SB circulation generates sharp CO2 increases (up to 14 ppm in half an hour) in a few SB events. We suggest that the variability in Rn-222 evolution may also be influenced by other non-local processes such as the large-scale footprint from more remote sources.

The performance of an atmospheric single-column model (SCM) is studied systematically for stably-stratified conditions. To this end, 11 years (2005–2015) of daily SCM simulations were compared to observations from the Cabauw observatory, The Netherlands. Each individual clear-sky night was classified in terms of the ambient geostrophic wind speed with a \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$1\hbox { m} \hbox { s}^{-1}$$\end{document}1ms-1 bin-width. Nights with overcast conditions were filtered out by selecting only those nights with an average net radiation of less than \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$-\,30\hbox { W }\hbox {m}^{-2}$$\end{document}-30Wm-2. A similar procedure was applied to the observational dataset. A comparison of observed and modelled ensemble-averaged profiles of wind speed and potential temperature and time series of turbulent fluxes showed that the model represents the dynamics of the nocturnal boundary layer (NBL) at Cabauw very well for a broad range of mechanical forcing conditions. No obvious difference in model performance was found between near-neutral and strongly-stratified conditions. Furthermore, observed NBL regime transitions are represented in a natural way. The reference model version performs much better than a model version that applies excessive vertical mixing as is done in several (global) operational models. Model sensitivity runs showed that for weak-wind conditions the inversion strength depends much more on details of the land-atmosphere coupling than on the turbulent mixing. The presented results indicate that in principle the physical parametrizations of large-scale atmospheric models are sufficiently equipped for modelling stably-stratified conditions for a wide range of forcing conditions.

The contributions of synoptic- and meso-scales to the boundary layer wind profile evolution in a coastal environment are examined. The analysis is based on observations of the wind profile within the first 200 m of the atmosphere continuously recorded during a 10 year period (2001–2010) at the 213-m meteorological tower at the Cabauw Experimental Site for Atmospheric Research (CESAR, The Netherlands). The analysis is supported by a numerical experiment based on the Weather Research and Forecasting (WRF) model performed at high horizontal resolution of 2 km and spanning the complete observational period (10 years). Results indicate that WRF is able to reproduce the inter-annual wind variability but with a tendency to be too geostrophic. At seasonal scales, we find a differentiated behavior between Winter and Summer seasons with the Spring and Autumn transition periods more similar to the Summer and Winter modes, respectively. The winter momentum budget shows a weak intradiurnal variability. The synoptic scale controls the shape of the near surface wind profile that is characterized by weaker and more ageostrophic winds near the surface than at higher altitudes within the planetary boundary layer (PBL) as a result of the frictional turning. In turn, during summer, mesoscale circulations associated with the differential heating of land and sea become important. As a result, the PBL winds show a stronger intradiurnal component that is characterized by an oscillation of the near surface winds around the geostrophic direction with the maximum departure in the afternoon. Although also driven by thermal land-sea differences, this mesoscale component is not associated with the classical concept of a sea-breeze front. It originates from the thermal expansion of the boundary layer over land and primarily differs from the sea-breeze in its propagation speed resulting in a wind rotation far ahead of any coastal front. We refer to it as the near-coast diurnal acceleration (NCDA). The contribution of the NCDA depends on the specific orientation of the coast (NE-SW at CESAR). Our findings stress the importance of evaluating and understanding the performance of mesoscale models with multi-year observational/simulated data sets in order to provide a statistically robust characterization of the limitations of surface layer and boundary-layer parameterizations and thus compensate for the scarceness of upper level wind observations.

In this paper we evaluated the Weather Research and Forecasting (WRF) mesoscale meteorological model for stable conditions at clear skies with low wind speeds. Three contrasting terrains with snow covered surfaces are considered, namely Cabauw (Netherlands, snow over grass), Sodankyla (Finland, snow over a needle-leaf forest) and Halley (Antarctica, snow over an ice shelf). We used the full 3D model and the single-column versions of the WRF model. The SCM was driven by realistic forcings of the WRF-3D field. Several sets of SCM forcings were tested: A. no advection, B. varying geostrophic wind in time, C. momentum advection in addition to B, D. temperature and moisture advection in addition to C, and E. forcing the SCM field to the 3D field above a threshold height. The WRF-3D model produced overall good results for wind speed, but the near-surface temperatures and specific humidity were overestimated for Cabauw and Sodankyla, and underestimated for Halley. Prescribing advection for momentum, temperature and moisture gave the best results for the WRF-SCM, and simulations deviated strongly from reality without advection. Nudging the SCM field to the 3D field above a threshold height lead to an unrealistic behaviour of the variables below this height and is not recommended. Detailed prescription of the surface characteristics, e.g. adjusting the snow cover and vegetation fraction, improved the 2?m temperature simulation. For all three sites, the simulated temperature and moisture inversion was underestimated, though this improved when prescribing advection. Overall, in clear-sky conditions, the stable boundary layer over snow and ice can be modelled to a good approximation if all processes are taken into account at high resolution, and if land surface properties are carefully prescribed.

We study the influence of boundary layer stability on the near-surface wind speed, especially for high-wind conditions. An analysis of the wind speed ratio between two vertical levels observed at tall masts in the North Sea and The Netherlands demonstrates that over sea non-neutral conditions commonly occur, even when the 10m wind speed is 7Bft or higher (at least 13.9ms � 1 ). Over land, stability conditions are always close to neutral for these strong wind conditions. This is because over land, large vertical temperature differences are rare in these conditions. An analysis of 30years of station data shows that even in storm conditions the ratio of the 10m wind speed between sea and land depends systematically on the difference between the air temperature and the sea surface temperature. The observational results are reproduced by HARMONIE, a state-of-the-art Numerical Weather Prediction model, although the impact of stability is smaller than in the observations. A model sensitivity analysis for a severe storm shows that the near-surface wind speed over sea can vary by 10% depending on the difference between the air temperature and the sea surface temperature. The results presented in this study indicate that even in conditions that are usually classified as ‘(near) neutral’, small variations in stability may have a significant impact on the wind profile. They also indicate that for high wind speeds, the sea-to-land wind speed ratio is dominated by the stability over sea as in these conditions the stability over land is close to neutral. Copyright © 2015 John Wiley & Sons, Ltd.

Field observations and theoretical analysis are used to investigate the appearance of different nocturnal boundary layer regimes. Recent theoretical findings predict the appearance of two different regimes: the continuously turbulent (weakly stable) boundary layer and the relatively “quiet” (very stable) boundary layer. A large number of nights (approximately 4500 in total) are analyzed using an ensemble averaging technique. The observations support the existence of these two fundamentally different regimes: weakly stable (turbulent) nights rapidly reach a steady state (within 2–3 h). In contrast, very stable nights reach a steady state much later after a transition period (2–6 h). During this period turbulence is weak and nonstationary. To characterize the regime, a new parameter is introduced: the shear capacity. This parameter compares the actual shear after sunset with the minimum shear needed to sustain continuous turbulence. In turn, the minimum shear is dictated by the heat flux demand at the surface (net radiative cooling), so that the shear capacity combines flow information with knowledge of the boundary condition. It is shown that the shear capacity enables prediction of the flow regimes. The prognostic strength of this nondimensional parameter appears to outperform the traditional ones like the similarity parameter z/L and the gradient Richardson number Ri as a regime indicator.

Monin–Obukhov similarity theory (MOST) is commonly used to model the wind-speed profile at altitudes relevant to wind-power production (e.g. 10–200 m). Though reasonably accurate for unstable to weakly stable stratification, this approach becomes less accurate under increasingly stable stratification, largely due to the constant-flux surface layer assumed by MOST becoming shallower than the altitude range of interest. Furthermore, above the surface layer, the Coriolis force has a considerable influence on the wind-speed profile (in particular in the formation of low-level jets) that cannot be modelled using similarity theory. Our goal is to compare the accuracy of alternative extrapolation models that are more physically appropriate above the surface layer. Using data from the 213-m Cabauw meteorological tower in the Netherlands between July 2007 and June 2008, it is shown that MOST is accurate only at low altitudes and low stability, and breaks down at high altitudes and high stability. Local similarity is generally more accurate than MOST across all altitudes and stabilities, though the model requires turbulent flux data at multiple altitudes that is generally impractical. In contrast, a two-layer MOST–Ekman model is found to be comparable to the other models at low stability ranges and considerably more accurate in the high stability range, while requiring only a measure of surface stability and the geostrophic wind.

Lake models account for the thermodynamic interaction between fresh water lakes and the atmosphere. During frost periods they predict ice thickness. A stand-alone version of the Fresh water Lake model (FLake) has been compared with the operational ice-growth model at the Royal Netherlands Meteorological Institute. Both models have been driven with various input data like observations and data from the Hirlam Aladin Research on Mesoscale Operational NWP In Europe (HARMONIE) model. The lake ice models are validated with indirect ice thickness measurements at Cabauw during the winter of 1996/1997 and with data from a voluntary observer network in the province of Friesland (The Netherlands) during the winter of 2012. FLake is shown to have predictive power and is competitive with the operational ice-growth model. Keywords : ice thickness, lake models, FLake, Cabauw, validation (Published: 5 February 2014) Citation: Tellus A 2014, 66 , 21244, http://dx.doi.org/10.3402/tellusa.v66.21244 This publication is part of a Thematic Cluster entitled "Parameterization of lakes in numerical weather prediction and climate models". Read the other papers from this thematic cluster here

We derive the turbulent structure parameters of temperature $$C_{T}^2$$ and humidity $$C_q^2$$ from high-resolution large-eddy simulations (LES) of a homogeneously-heated convective boundary layer. Boundary conditions and model forcing were derived from measurements at Cabauw in The Netherlands. Three different methods to obtain the structure-parameters from LES are investigated. The shape of the vertical structure-parameter profiles from all three methods compare well with former experimental and LES results. Depending on the method, deviations in the magnitude up to a factor of two are found and traced back to the effects of discretization and numerical dissipation of the advection scheme. Furthermore, we validate the LES data with airborne and large-aperture scintillometer (LAS) measurements at Cabauw. Virtual path measurements are used to study the variability of $$C_{T}^2$$ in the mixed layer and surface layer and its implications for airborne and LAS measurements. A high variability of $$C_{T}^2$$ along a given horizontal path in the LES data is associated with plumes (high values) and downdrafts (low values). The path average of $$C_{T}^2$$ varies rapidly in time due to the limited path length. The LES results suggest that measured path averages require sufficient temporal averaging and an adequate ratio of path length to height above the ground for the LAS in order to approach the domain average of $$C_{T}^2$$ .

The development of a radiation fog layer at the Cabauw Experimental Site for Atmospheric Research (51.97°N, 4.93°E) on 23 March 2011 was observed with ground-based in situ and remote sensing observations to investigate the relationship between visibility and radar reflectivity. The fog layer thickness was less than 200 m. Radar reflectivity values did not exceed −25 dBZ even with visibilities less than 100 m. The onset and evaporation of fog produce different radar reflectivity–visibility relationships. The evolution of the fog layer was modeled with a droplet activation model that used the aerosol size distribution observed at the 60-m altitude tower level as input. Radar reflectivity and visibility were calculated from model drop size spectra using Mie scattering theory. Since radiative cooling rates are small in comparison with cooling rates due to adiabatic lift of aerosol-laden air, the modeled supersaturation remains low so that few aerosol particles are activated to cloud droplets. The modeling results suggest that the different radar reflectivity–visibility relationships are the result of differences in the interplay between water vapor and cloud droplets during formation and evaporation of the fog. During droplet activation, only a few large cloud droplets remain after successfully competing for water vapor with the smaller activated droplets. These small droplets eventually evaporate (deactivate) again. In the fog dissolution/evaporation stage, only these large droplet need to be evaporated. Therefore, to convert radar reflectivity to visibility for traffic safety products, knowledge of the state of local fog evolution is necessary.

The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.

We introduce a simple, physically consistent method to predict nocturnal wind and temperature profiles from external forcing parameters such as the geostrophic wind. As an indicator of the radiative ‘forcing’ the net longwave radiative cooling is used as a proxy. Surface fluxes are expressed in terms of these parameters by coupling an Ekman model to a rudimentary surface energy balance. Additionally the model assumes validity of Monin-Obukhov similarity in order to predict near-surface wind and temperature profiles up to a height equal to the Obukhov length. The predictions are validated against an independent dataset that covers 11-years of observations at Cabauw, The Netherlands. It is shown that the characteristic profiles in response to external forcings are well-captured by the conceptual model. For this period the observational climatology is in close agreement with ECMWF re-analysis data. As such, the conceptual model provides an alternative tool to giving a first-order estimate of the nocturnal wind and temperature profile near the surface in cases when advanced numerical or observational infrastructure is not available.

In scintillometry Monin–Obukhov similarity theory (MOST) is used to calculate the surface sensible heat flux from the structure parameter of temperature \({(C_{T^2})}\) . 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) \({C_{T^2}}\) is still scaled with the surface flux, whereas in the second (MOSTl) \({C_{T^2}}\) is scaled with the local sensible heat flux. The \({C_{T^2}}\) obtained from both concepts is compared with direct observations of \({C_{T^2}}\) 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 \({C_{T^2}}\) . In the morning (data outside the ASL), our data do not unequivocally support either of the two concepts. First, the peak in \({C_{T^2}}\) 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 \({C_{T^2}}\)) but underestimates \({C_{T^2}}\) 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.

Eddy covariance (EC) technique is often used to determine greenhouse gas exchange at the earth's surface. In general, the instruments involved have a limited high frequency response which reduces the ability to detect the contribution to the flux of small eddies and in addition sensor separation gives high frequency losses. These missing contributions cause an EC flux underestimation which increases for higher values of the stability parameter z/L . Corrections can be performed based on the (empirical) Kaimal co-spectra; however, these were derived using instruments with a limited frequency response. In this study, the validity of the Kaimal spectrum during stable atmospheric conditions is assessed using laser Doppler anemometry (LDA) measurements of the vertical wind velocity at 1 m height during several stable nights at Cabauw in the Netherlands. LDA provides a means to determine the entire turbulent energy spectrum, i.e., from the production scale down to the dissipation scale. Since the measured spectra are found to be in good agreement with the Kaimal spectra, we assume that the Kaimal co-spectra are valid as well. Next, the effect of high frequency correction based on Kaimal co-spectra is assessed using 1 month of EC flux data of CH 4 and N 2 O measured by quantum cascade laser (QCL) spectrometry at Reeuwijk in the Netherlands. After correction, the cumulative emissions increased about 15% for both gases. This underlines the importance of correcting for high frequency losses.

The uncertainty in eddy covariance (EC) flux measurements is assessed for CH4 and N2O using data measured at a dairy farm site in the Netherlands in 2006 and 2007. An overview is given of the contributing uncertainties and their magnitude. The relative and absolute uncertainty of a 30 min EC flux are estimated for CH4 and N2O using N = 2185 EC fluxes. The average absolute uncertainty and its standard deviation are 500 +/- 400 ng C m(-2) s(-1) for CH4 and 100 +/- 100 ng N m(-2) s(-1) N2O. The corresponding relative uncertainties have 95% confidence interval ranging from 20% to 300% for CH4 and from 30% to 1800% for N2O. The large relative uncertainties correspond to relatively small EC fluxes. The uncertainties are mainly caused by the uncertainty due to one-point sampling which contributes on average more than 90% to the total uncertainty. The other 10% includes the uncertainty in the correction algorithm for the systematic errors. The uncertainty in a daily and monthly averaged EC flux are estimated for several flux magnitude ranges. The daily and monthly average uncertainty are smaller than 25% and 10% for CH4 and smaller than 50% and 10% for N2O, respectively, based on fluxes larger than 100 ng C m(-2) s(-1) and 15 ng N m(-2) s(-1). (C) 2009 Elsevier B.V. All rights reserved. (Less)

Single-column models (SCMs) are widely employed to evaluate boundary-layer parametrizations under well-controlled conditions. To compare SCM results to observations, these models must be driven by realistic forcings of the three-dimensional (3D) atmospheric state. However, these forcings are inherently uncertain. The central research question is therefore: can observations be used to distinguish between different parametrization schemes in SCM simulations or is the spread due to uncertainties in the forcings too large? This study investigates this question for the nocturnal low-level jet (LLJ) at Cabauw. First, we analyse a single LLJ case that has been used for the third GEWEX Atmospheric Boundary Layer Studies (GABLS3) intercomparison study. To estimate the forcings, for this case a blend of local observations and 3D model output has been used. A sensitivity study to both the forcings and the turbulence formulation is performed by using the SCM version of the ECMWF model. The sensitivity to the turbulence parametrization is largest in the bulk of the stable boundary layer (SBL). The influence of the forcings manifests itself mainly in the upper part of the SBL and above. Second, an ensemble of eight comparable LLJ cases is considered. Using forcings derived from 3D model integrations, SCM results of the separate cases show significant deviations from the observations. However, the mean of the SCM simulations agrees well with the mean of the observations. Based on the eight selected LLJ cases, a composite SCM case is defined from the mean forcings of the 3D model. This improves the signal-to-noise ratio, which enables a better judgement of the quality of boundary-layer parametrizations in a comparison with the observations

In the present work Blackadar’s concept of nocturnal inertial oscillations is extended. Blackadar’s concept describes frictionless inertial oscillations above the nocturnal inversion layer. The current work includes frictional effects within the nocturnal boundary layer. It is shown that the nocturnal wind speed profile describes an oscillation around the nocturnal equilibrium wind vector, rather than around the geostrophic wind vector (as in the Blackadar case). By using this perspective, continuous time-dependent wind profiles are predicted. As such, information on both the height and the magnitude of the nocturnal low-level jet is available as a function of time. Preliminary analysis shows that the proposed extension performs well in comparison with observations when a simple Ekman model is used to represent the equilibrium state in combination with a realistic initial velocity profile.In addition to jet dynamics, backward inertial oscillations are predicted at lower levels close to the surface, which also appear to be present in observations. The backward oscillation forms an important mechanism behind weakening low-level winds during the afternoon transition. Both observational and theoretical modeling studies are needed to explore this phenomenon further.

A novel approach to infer surface soil heat fluxes from measured profiles of soil temperature, soil heat flux, and observations of the vegetation canopy temperature and the incoming shortwave radiation is evaluated for the Cabauw measurement facility in the Netherlands. The approach is a variational data assimilation approach that uses the applied measurements to optimize, on a daily basis, parameter values of a model that describes the heat transport between the vegetation canopy and the surface and within the soil column. Insertion of error characteristics that either are inferred from the field data themselves or are derived from literature leads to valid estimates of the cost function for about 100 days in 2003. The approach gives values of the model parameters that compare well to values derived from the literature, although values for the soil conductivity and the volumetric heat capacity of the soil start to differ from the literature values at the end of 2003, possibly because of specific soil characteristics and the extreme dryness of the summer of 2003. The model gives estimates of the surface soil heat flux that compare well to estimates using the currently operational lambda approach, provided that the latter is adapted to account for the disturbance of the soil heat flux at the locations of the heat flux plates. Only when the surface soil heat flux is very small or very large does the new approach give estimates of the surface soil heat flux that differ from those obtained with the lambda approach.

An intercomparison is made of the Net Ecosystem Exchange of CO2, NEE, for eight Dutch grassland sites: four natural grasslands, two production grasslands and two meteorological stations within a rotational grassland region. At all sites the NEE was determined during at least 10 months per site, using the eddy-covariance (EC) technique, but in different years. The NEE does not include any import or export other than CO2. The photosynthesis-light response analysis technique is used along with the respiration-temperature response technique to partition NEE into Gross Primary Production (GPP) and Ecosystem Respiration (Re) and to obtain the eco-physiological characteristics of the sites at the field scale. Annual sums of NEE, GPP and Re are then estimated using the fitted response curves with observed radiation and air temperature from a meteorological site in the centre of The Netherlands as drivers. These calculations are carried out for four years (2002–2005). Land use and management histories are not considered. The estimated annual Re for all individual sites is more or less constant per site and the average for all sites amounts to 1390±30 gC m−2 a−1. The narrow uncertainty band (±2%) reflects the small differences in the mean annual air temperature. The mean annual GPP was estimated to be 1325 g C m−2 a−1, and displays a much higher standard deviation, of ±110 gC m−2 a−1 (8%), which reflects the relatively large variation in annual solar radiation. The mean annual NEE amounts to –65±85 gC m−2 a−1. From two sites, four-year records of CO2 flux were available and analyzed (2002–2005). Using the weather record of 2005 with optimizations from the other years, the standard deviation of annual GPP was estimated to be 171–206 gC m−2 a−1 (8–14%), of annual Re 227–247 gC m−2 a−1 (14–16%) and of annual NEE 176–276 gC m−2 a−1. The inter-site standard deviation was higher for GPP and Re, 534 gC m−2 a−1 (37.3%) and 486 gC m−2 a−1 (34.8%), respectively. However, the inter-site standard deviation of NEE was similar to the interannual one, amounting to 207 gC m−2 a−1. Large differences occur due to soil type. The grasslands on organic (peat) soils show a mean net release of CO2 of 220±90 g C m−2 a−1 while the grasslands on mineral (clay and sand) soils show a mean net uptake of CO2 of 90±90 g C m−2 a−1. If a weighing with the fraction of grassland on organic (20%) and mineral soils (80%) is applied, an average NEE of 28 ±90 g C m−2 a−1 is found. The results from the analysis illustrate the need for regionally specific and spatially explicit CO2 emission estimates from grassland.

The Penman–Monteith equation is extended to describe evaporation of intercepted rain, transpiration and the interaction between these processes in a single explicit function. This single-layer model simulates the effects of heat exchange, stomatal blocking and changed humidity deficit close to the canopy as a function of canopywater storage. Evaporation depends on the distribution of water over the canopy and the energy exchange between wet and dry parts. Transpiration depends on the dry canopy surface resistance that is described with a Jarvis-type response. The explicit functions obtained for water vapour fluxes facilitate a straightforward identificationof the various processes. Canopy water storage amounts and xylem sapflow were measured simultaneously during drying episodes after rainfall in a dense, partially wet, Douglas-fir forest. Estimates of evaporation and transpiration rates are derived from these observations. The analysis shows that evaporation induced transpirationreduction is mainly caused by energy consumption. Changes in water vapour deficit have a minor effect due to a compensating stomatal reaction. The remaining difference between observed and modelled transpiration reduction can be attributed to partial blocking of stomata by the water layer.

We describe a novel methodology on the selection and composition of a single-case observational dataset from the comprehensive measurement program at the Cabauw observatory field site located in the Netherlands. The case can be regarded as the basis of the third case study conducted within the framework of the GEWEX (Global Energy and Water Exchange) Atmospheric Boundary-Layer Study (GABLS) and is meant to be used for the evaluation of single-column models. The ideal case is supposed to cover a period of at least 24 h with clear skies, moderate near-surface winds and a stable stratification during nighttime. From the multi-year data archive with Cabauw observations data for 1–2 July 2006 were found to best match the requirements, and were consequently selected for analysis. The dates contains a 24-h period with a nearly constant geostrophic wind of $$\approx $$ 7 m $$\hbox {s}^{-1}$$ , and a considerable wind shear in the vertical. It is also characterized by a pronounced low-level jet related to an inertial oscillation that developed around sunset when the atmosphere had decoupled from the surface. Detailed initial conditions, surface conditions and dynamical forcings are derived on the basis of local observations and the outcome of a conceptual and a three-dimensional atmospheric model. It appears that a very precise prescription of the forcings is a prerequisite to enable a meaningful evaluation of models against observational data.

The morning transition between the stable nocturnal situation and the daytime convective boundary layer (CBL) is of interest both for basic understanding and for initializing prognostic models. While the morning growth phase of the CBL has been studied in detail, relatively little has been published on the transition itself. In this paper, conventional observations of surface temperature, humidity, and turbulent fluxes,and data from a meteorological tower, are combined with measurements of the onset of convection by boundary-layer wind profilers to explore the timing and behaviour of the transition period. The transition is defined here as the period between sunrise and the time at which the depth ofconvection reaches about 200 m AGL. Diagnostic relationships based on surface heat flux, the temperature difference between 2 m and 200 m, and bulk Richardson number are explored. The transition is foundto be enabled by surface heating relaxing the surface stability, while the warming of the layerbetween 2 m and 200 m is in large part due to shear-driven entrainment.

It is shown that sampling rate has no influence on the expected value of an eddy-covariance flux estimate. The uncertainty in the flux estimate, however, increases with decreasing sampling rate. This sampling induced uncertainty is compared with the natural uncertainty in the flux estimate due to the stochastic nature of turbulence. It is shown that the sampling induced uncertainty becomes important when the sampling interval is larger than the integral time scale of the flux time series. Theoretical expressions are evaluated by using 9 days of turbulence data taken over a coniferous forest. For sampling intervals smaller than the integral time scale the sampling induced uncertainties are underestimated by the model because the actual auto-correlation function for the flux time series deviates from the exponential form assumed in the model.

Hourly observations of eddy-covariance water vapour fluxes obtained over a Douglas-fir forest are used to evaluate three transpiration models. The models are (1) the Penman–Monteith model with a Jarvis type of formulation for the canopy resistance, including an explicit function for specific humidity deficit, (2) the Penman–Monteith model in which the specific humidity deficit response is replaced by a response to transpiration rate itself as described by Monteith (1995) , and (3) a modified Priestley–Taylor formula. Model parameters are optimised against the observations of 43 dry days during the growing season. Systematic differences between model and observed transpiration could be related to wind direction. These deviations correspond with deviations found in the observed energy balance for the same wind directions. The mean square of the residuals is approximately two times larger than the value found for the uncertainty in the eddy-covariance measurements due to atmospheric statistics. Distinct responses to the specific humidity deficit, solar radiation, soil matrix potential and shoot growth are found. No temperature response is found. The response of transpiration to an increase of leaf area index during shoot growth suggests that the transpiration from new shoots is higher as that from older shoots. However, other physiological changes at the start of the growing season may play a role as well. An analysis of residuals shows that the Jarvis model gives good results for all conditions encountered. It is shown that the Monteith model can be formulated such that it is almost equal to the Jarvis model for this aerodynamically rough forest. Despite its simple formulation a modified Priestley–Taylor formula (including LAI and soil matrix potential response) gives reasonable results, although at moderate irradiation and high specific humidity deficit deviations are significant. A comparison with results from another coniferous forest (Thetford forest) shows that transpiration rates are only slightly larger despite the much larger leaf area index.

Infrared radiative surface temperatures as observed over a dense Douglas fir forest during stable atmospheric conditions are analyzed. It is shown that the concept of a single surface temperature to describe both the thermal coupling and the radiative coupling between atmosphere and forest fails when longwave cooling is large and wind speeds are low. In such cases a decoupling of the radiative surface temperature from the air temperature aloft is observed. Conditions for decoupling are formulated in terms of an appropriate Richardson number. It is shown that a convective surface temperature comes into play that is coupled to the forest interior air temperature. Observed radiative surface temperature is then composed of this convective surface temperature and the aerodynamic surface temperature. Forest interior air temperature during nighttime is, in general, lower than air temperature above the canopy. A simple two-layer canopy model is used to explain this phenomenon in terms of the distribution of atmospheric sensible heat flux and storage heat flux over the two canopy layers.

Infrared crown radiation temperatures as observed over a dense Douglas fir forest are analyzed in the context of similarity theory and the concept of transport resistances. As such we obtain a rather high value of the roughness length for heat, which is about equal to the roughness length for momentum. This value can be explained by the more efficient transport of heat relative to momentum in the roughness sublayer of the forest. Correcting for this effect we arrive at the classic value for homogeneous terrain of about 0.1 times the roughness length for momentum. For unstable cases the presence of enhanced mixing of heat in the roughness sublayer leads to a modified integral stability function for the dimensionless potential temperature difference between the surface and the top of the roughness sublayer. The observations give some evidence for this different stability behaviour. The analysis suggests that during daytime the radiative surface temperature and the aerodynamic surface temperature are not significantly different when used to estimate fluxes. Daytime trunk space air temperature is satisfactory parameterized with the concept of gusts and with surface renewal analysis. As such it is related to the sensible heat flux and the storage heat flux. Night time radiation temperatures at times strongly deviate from the expected behaviour based on similarity theory and the roughness length for heat, suggesting that the concept of a single surface temperature is too simple for such cases.