Study of a prototypical convective boundary layer observed during BLLAST: contributions by large-scale forcings
Pietersen, H.P. ; Vilà-Guerau De Arellano, J. ; Augustin, P. ; Boer, A. van de; Coster, O. de; Delbarre, H. ; Durand, P. ; Fourmentin, M. ; Gioli, B. ; Hartogensis, O.K. ; Lohou, F. ; Lothon, M. ; Ouwersloot, H.G. ; Pino, D. ; Reuder, J. - \ 2015
Atmospheric Chemistry and Physics 15 (2015). - ISSN 1680-7316 - p. 4241 - 4257.
large-eddy simulation - turbulence - entrainment - transition - radiation - decay
We study the influence of the large-scale atmospheric 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 approaches, the mixed-layer theory and large-eddy simulation (LES), with a complete data set of surface and upper-air atmospheric 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 account, 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 correspondence of the vertical structure of turbulent variables with observations. We also find that when large-scale advection 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 dynamics 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.
Modelling stable atmospheric boundary layers over snow
Sterk, H.A.M. - \ 2015
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): Gert-Jan Steeneveld. - Wageningen : Wageningen University - ISBN 9789462572263 - 189
sneeuw - atmosferische grenslaag - modelleren - modellen - turbulentie - weersvoorspelling - snow - atmospheric boundary-layer - modeling - models - turbulence - weather forecasting
Modelling Stable Atmospheric Boundary Layers over Snow
Wageningen, 29th of April, 2015
The emphasis of this thesis is on the understanding and forecasting of the Stable Boundary Layer (SBL) over snow-covered surfaces. SBLs typically form at night and in polar regions (especially in winter), when radiative cooling at the surface causes a cooler surface than the overlying atmosphere and a stable stratification develops. This means that potential temperature increases with height and buoyancy effects suppress turbulence. Turbulence is then dominated by mechanical origin. If sufficient wind shear can be maintained, turbulence remains active, otherwise it will cease.
A proper representation of SBLs in numerical weather prediction models is critical, since many parties rely on these forecasts. For example, weather prediction is needed for wind energy resources, agricultural purposes, air-quality studies, and aviation and road traffic. Knowledge on SBLs is also essential for climate modelling. In the Arctic regions, climate change is most pronounced due to stronger changes in near-surface temperature compared to other latitudes. Though this `Arctic amplification' is not yet fully understood, possible responsible processes are the ice-albedo feedback, alterations in cloud cover and water vapour, different atmospheric and oceanic circulations, and the weak vertical mixing in the lower atmosphere. However, many interactions exist between these processes. With positive feedbacks, changes are even further enhanced. This could have worldwide consequences, i.e. due to affected atmospheric circulations and sea level rise with Greenland's melting ice-sheets.
Scientists try to explain the observed climate changes, as well as provide outlooks for future changes in climate and weather. However, the understanding is hampered by the fact that many model output variables (e.g. regarding the 2 m temperature) vary substantially between models on the one hand, and from observations on the other hand. Modelling the SBL remains difficult, because the physical processes at hand are represented in a simplified way, and the understanding of the processes may be incomplete. Furthermore, since processes can play a role at a very small scale, the resolutions in models may be too poor to represent the SBL correctly. Additionally, there are many different archetypes of the SBL. Turbulence can be continuous, practically absent, or intermittent, and vary in strength which affects the efficiency of the exchange of quantities horizontally and vertically.
Processes that are considered critical for the SBL evolution are e.g. turbulent mixing, radiative effects, the coupling between the atmosphere and the underlying surface, the presence of clouds or fog, subsidence, advection, gravity waves, and drainage and katabatic flows. In this thesis, the focus is on the first three processes, as these are most dominant for the evolution of the SBL (e.g. Bosveld et al., 2014b).
In Chapter 3 an idealized clear-sky case over sea-ice was studied based on the GABLS1 benchmark study (e.g. Cuxart et al., 2006), but extended by including radiative effects and thermal coupling with the surface. Hence the following research questions were posed:
Question 1: What is the variety in model outcome regarding potential temperature and wind speed profiles that can be simulated with one model by using different parametrization schemes?
Question 2: Which of the three governing processes is most critical in determining the SBL state in various wind regimes?
Question 3: Can we identify compensation mechanisms between schemes, and thus identify where possible compensating model errors may be concealed?
From the analysis with different parametrization schemes performed with the WRF single-column model (SCM, Question 1), it followed that quite different types of SBLs were found. Some schemes forecasted a somewhat better mixed potential temperature profile where stratification increased with height, while another scheme produced profiles with the strongest stratification close to the surface and stratification decreased with height. After only 9 h of simulation time, a difference in temperature of almost 2 K was found near the surface. Regarding the wind speed profile, some variation was found in the simulated low-level jet speed and height. Mainly the difference in atmospheric boundary-layer (BL) schemes which parametrize the turbulent mixing are responsible for these model output variations. A variation in long wave parametrization schemes hardly affected the model results.
Question 2 addresses the problem whether other processes than turbulent mixing may be responsible for a similar spread in model results. A sensitivity analysis was performed where for one set of reference parametrization schemes the intensity of the processes was adjusted. The relative sensitivity of the three processes for different wind regimes was analysed using `process diagrams'. In a process diagram, two physically related variables are plotted against each other, which in this case represent either a time average or a difference over time of the variable. A line connects the reference state with the state for which the process intensity is modified. By comparing the length and orientation of the lines, the relative significance of the individual processes for the different wind speeds can be studied. Overlapping line directions identify possible compensating errors.
Geostrophic wind speeds of 3, 8 and 20 m s-1 were selected representing low, medium and high wind speeds, capturing the range of wind speeds frequently occurring in the Arctic north of 75oN according to the ERA-Interim reanalysis dataset. Overall, a shift in relative importance was detected for the various wind regimes. With high geostrophic wind speeds, the model output is most sensitive to turbulent mixing. On the contrary, with low geostrophic wind speeds the model is most sensitive to the radiation and especially the snow-surface coupling. The impact of turbulent mixing is then minor, unless when mixing in both boundary layer and surface layer is adjusted. This stresses that proper linking between these two layers is essential.
Also with one set of parametrization schemes different SBL types were simulated. Potential temperature profiles were better mixed (increasing stratification with height) for high geostrophic wind speeds, and this tended to develop to profiles with the strongest stratification near the surface (decreasing stratification with height) for low geostrophic wind speeds. However, a variety in types was also found when keeping the same wind regime, but by varying the mixing strength. With enhanced mixing, the profile became better mixed, also when the reference profile showed the strongest stratification near the surface. With decreased mixing, profiles with a stronger stratification were found, again shaped with the strongest stratification near the surface. Thus a different mixing formulation has a strong impact on the vertical profiles, even when it may not necessarily strongly affect the surface variables. Therefore, it is recommended that when a model is evaluated and optimized, the vertical structure is also regarded in this process, since near-surface variables may be well represented, strong deviations aloft are still possible.
Furthermore, the process diagrams showed overlap in sensitivity to some processes. Therefore errors within the parametrizations of these processes could compensate each other and thus remain hidden (Question 3), making the model formulation possibly physically less realistic. This study did not reveal an unambiguous indication for the compensating processes regarding the various sets of variables, though overlap for single variables is seen.
This study also revealed a non-linear behaviour regarding the 2 m temperature, which is also found in observations (e.g. Lüpkes et al., 2008) and in a model study by McNider et al. (2012). Here the 2 m temperature decreased with enhanced mixing strength and increased with a lower mixing intensity. This counter-intuitive behaviour is explained by that mixing only occurs in a shallow layer close to the surface. Cold air that is mixed up by the enhanced mixing, is insufficiently compensated by the downward mixed relatively warm air. This behaviour was found mostly for low wind speeds or with decreased mixing at the medium wind regime, when the potential temperature profile showed the strongest stratification near the surface.
The study proceeds with a model evaluation against observations in low wind speed regimes. Three stably stratified cloud-free study cases with near-surface wind speeds below 5 m s-1 were selected with each a different surface: Cabauw in the Netherlands with snow over grass, Sodankylä in northern Finland with snow in a needle-leaf forest, and Halley in Antarctica with snow on an ice shelf.
Chapter 4 presents the evaluation of the WRF-3D and SCM for these cases. In this study, the WRF-3D model was used to determine the forcings, as often not all the required observations at high resolution in time and space are available. Hence the following questions were formulated:
Question 4: What is the performance of WRF in stable conditions with low wind speeds for three contrasting snow-covered sites?
Question 5: How should we prescribe the single-column model forcings, using WRF-3D?
The standard WRF-3D simulation had an incorrect representation of the snow-cover and vegetation fraction, which deteriorates the conductive heat flux, the surface temperatures and the SBL evolution. Indeed, Chapter 3 highlighted the critical role of the land-surface coupling representation. Adjusting the settings with site specific information, improved model simulations compared with the observations.
In general, the performance of WRF-3D was quite good for the selected cases, especially regarding the wind speed simulations. The temperature forecast proved to be more challenging. For Cabauw and Sodankylä, 2 m temperatures were strongly overestimated, though a better simulation was seen at higher tower levels. For Halley a better representation of the 2 m temperature was found, though aloft potential temperatures were underestimated. Hence, the three cases had an underestimated modelled temperature gradient in common.
This study also investigated how the forcing fields for the SCM should be prescribed. Model results for the three study cases all showed a significant deviation from the observed wind field without lateral forcings and time-invariant geostrophic wind speed. Including only a time-varying geostrophic wind speed did not improve the results. Prescribing additional momentum advection did have a positive impact on the modelled wind speed. The results regarding temperature, specific humidity and their stratification improved when temperature and humidity advection was also taken into account. Forcing the SCM field towards a prescribed 3D atmospheric state is not recommended, since unrealistic profiles were found below the threshold forcing height.
Having established the optimal model set-up, the SCM can be used as a tool to further study the small-scale processes for the three study cases, addressing the following questions:
Question 6: How do the model results with various process intensities compare with observations?
Question 7: Are any differences in relative process impacts found for the three contrasting sites?
Question 8: Does the model sensitivity vary between two different BL schemes?
The sensitivity analysis was performed with the WRF-SCM and repeated for two BL schemes. In general, the temperature and humidity stratifications intensified by decreasing the process strengths and hence were in better agreement with observations than the reference cases. The wind field was most sensitive to turbulent mixing, with a weaker low-level jet at a higher altitude for enhanced mixing and the opposite for less mixing, while the impact of the other processes was small. Contrary to the temperature profiles, a better agreement with wind observations was found with amplified mixing, except for Halley where results improved with reduced mixing.
Regarding the surface energy budget, the conductive heat flux was greatly overestimated at Cabauw due to an overestimated snow conductivity, while better agreements were found for the other sites. A revision of the definition for snow conductivity in the model is recommended, because rather large values were assumed for fresh snow, and indeed results improved when the coupling strength was reduced for Cabauw and Sodankylä. For Halley almost the same snow conductivity was modelled as was used to determine the observed conductive heat flux, however, then the temperature gradient through the first soil/snow layer was underestimated leaving the flux too small.
The net radiation was strongly too negative for the Cabauw and Halley case-studies. This is likely due to an underestimation of the incoming long wave radiation as part of a deficiency in the long wave radiation scheme. For all sites the sensible heat flux was overestimated, and decreased mixing improved the results. However, the eddy covariance measurements may have been made outside the constant flux layer, which hampers the model evaluation.
Though Question 6 aims to obtain understanding in which processes are most responsible for simulating model results that are in closer agreement with observations, measuring in these cold and dry circumstances is especially challenging. Furthermore, the measurements are mostly point measurements while the model grid represents a larger area, such that the measurements may be influenced by local features which are not captured in the model. These issues hinders a clear comparison of the model results with observations, and the observation uncertainties may be greater than what was represented in the process diagrams.
When comparing the process sensitivity for the different sites (Question 7), we found some distinct variations in relative process significance. The radiation impact was relatively large at Cabauw and Sodankylä where the specific humidity was higher such that a larger impact on the incoming long wave radiation can be obtained. The snow-surface coupling is more important at Halley. This is related to the higher snow cover at Halley compared to the other sites. Additionally, the conductivity of the underlying medium at Halley is set equal to that of snow. These two factors ensure that the impact of an altered snow conductivity is greater.
From the comparison of the sensitivity analyses for the two BL schemes (MYJ and YSU, Question 8), it followed that the overall direction of the sensitivity orientation is similar. However, stronger BL temperature stratifications were found with YSU, though between the surface and the first model level stronger stratifications were simulated with MYJ. This is related to the relatively high ratio of mixing in the boundary layer versus the surface layer with MYJ. Therefore the mixing in the BL is relatively more efficient and the surface layer cannot keep up the mixing to keep a smooth profile at the surface-layer / boundary-layer interface. This indicates the importance of a consistent transition between the BL and surface layer, as also pointed out by Svensson and Holtslag (2009). Furthermore, the non-linearity concerning the 2 m temperature behaviour discussed earlier is most profound with YSU, and not as obvious with MYJ due to a stronger implemented minimum diffusivity.
The results point towards the direction of focus for future research. This could be achieved by e.g. re-evaluating the snow representation, as well as investigating the long-standing problem of the underestimated long wave radiation. Additionally, the mixing seems to be too high in some of the simulations. As such, care should be taken in choosing the BL scheme and its constraints on the mixing, as these may hamper the development of the observed behaviour on non-linear near-surface temperature evolution for example.
Atmospheric turbulence over crops : confronting theories with observations
Boer, A. van de - \ 2015
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): Arnold Moene; A. Graf. - Wageningen : Wageningen University - ISBN 9789462572416 - 143
turbulentie - meteorologie - atmosfeer - gewassen - watergebruiksrendement - transpiratie - modellen - eddy-covariantie - turbulente stroming - turbulence - meteorology - atmosphere - crops - water use efficiency - transpiration - models - eddy covariance - turbulent flow
Atmospheric turbulence plays a key role in hydrological and carbon cycles, and in weather and climate. Understanding and forecasting turbulence is thereby relevant for human life and environment.
We deal with some major challenges for studying atmospheric turbulence over crops. Land-atmosphere interactions are specifically complex because of surface heterogeneity. Also, boundary-layer entrainment complicates measuring and studying surface fluxes. Furthermore, the absence of high-frequency observations and of measurements of underlying soil and vegetation processes impedes studying land-atmosphere interactions.
We show the applicability of analytical footprint models over a heterogeneous land surface, and the validity of Monin-Obukhov similarity theory for a strongly-convective boundary-layer. Moreover, we present improvements on a scheme that can be used to estimate the amount of atmospheric turbulence from single-level weather data. We furthermore suggest to improve the partitioning theory that is used to distinguish soil processes from plant processes in eddy-covariance flux observations.
Detection of Entrainment Influences on Surface-Layer Measurements and Extension of Monin–Obukhov Similarity Theory
Boer, A. van de; Moene, A.F. ; Graf, A. ; Schüttemeyer, D. ; Simmer, C. - \ 2014
Boundary-Layer Meteorology 152 (2014)1. - ISSN 0006-8314 - p. 19 - 44.
convective boundary-layer - temperature-humidity correlation - sonic anemometer - analytical-model - mixed-layer - turbulence - fluxes - transport - water - heat
We present a method to detect influences of boundary-layer processes on surface-layer measurements, using statistics and spectra of surface-layer variables only. We validated our detection method with boundary-layer measurements. Furthermore, we confirm that Monin–Obukhov similarity functions fit well to temperature-variance data obtained at two different homogeneous surfaces. However, we found that humidity variance measurements deviate from the universal functions above one of the two studied surfaces for days on which entrained air reached the surface layer. These results confirm that Monin–Obukhov similarity theory should be used with care in the analysis of surface-layer data. Finally, we propose the use of an extra term in flux-variance relations that depends on the entrainment ratio for humidity and on the boundary-layer height. If boundary-layer measurements are not available, we show how the entrainment ratio for humidity can be approximated from the skewness of the humidity distribution
Aspects of atmospheric turbulence related to scintillometry
Braam, M. - \ 2014
Wageningen University. Promotor(en): Bert Holtslag, co-promotor(en): F. Beyrich; Arnold Moene. - Wageningen : Wageningen University - ISBN 9789461739384 - 151
turbulentie - meteorologie - atmosfeer - scintillometrie - meteorologische waarnemingen - methodologie - turbulence - meteorology - atmosphere - scintillometry - meteorological observations - methodology
Aspects of atmospheric turbulence related to scintillometry
Atmospheric turbulence is the main vertical transport mechanism in the atmospheric boundary layer. The surface fluxes related to this turbulent transport are the sensible () and latent heat fluxes (). The area-averaged values of and are of interest to evaluate mesoscale numerical weather models and in water budget studies. Natural landscapes are often heterogeneous, i.e. and differ among fields.
The fluxes can be obtained with a scintillometer system, which consists of an electromagnetic beam transmitter at one end of a propagation path and a receiver at the other end. The intensity of the electromagnetic signal at the receiver varies due to fluctuations in the refractive index of air () caused by turbulence along the path. From the magnitude of these fluctuations the structure parameter of temperature () and of humidity () can be derived. Finally, and are used to determine path-averaged and via Monin-Obukhov similarity theory (MOST). The advantage of scintillometry is that the obtained fluxes are path-averaged, which makes scintillometry a more suitable method for obtaining area-averaged fluxes over natural landscapes than traditional point measurements. However, the disadvantage is that the fluxes are not directly measured. Therefore in this thesis four questions are answered related to the applicability of MOST and to the behaviour of structure parameters over heterogeneous surfaces. The latter is important because MOST assumes homogeneous surface conditions. For our studies we used meteorological data measured at three sites (Cabauw; the Netherlands, CASES-99 experiment; Leon; Kansas; USA, LITFASS-2009 and LITFASS-2010 experiments; Lindenberg; Germany) under unstable conditions (day-time).
MOST is restricted to the part of the atmosphere close to the surface: the atmospheric surface layer (ASL). The depth of the ASL is not constant during the day, and is relatively shallow during the morning. At those moments, the scintillometer observation level can be located outside the ASL, which can question the validity of MOST. Therefore, we proposed and compared two variations in MOST (MOST using local fluxes and MOST using surface fluxes). We found that during the afternoon when both concepts have to be valid, the values of are comparable. During the morning, the data do not unequivocally support one of the two concepts: MOSTl shows the correct temporal behaviour in, but underestimates by a factor of ten.
The universal function that links the surface fluxes with the structure parameter in MOST needs to be determined empirically. In literature a great variety of these function can be found. Therefore, we investigate to what extent the expression for this function depends on the specific regression approaches, stability range and observation level. First, we found that applying various regression approaches has an impact on the expression. This means that studies always should specify their regression approach, when presenting new functions. We advise to use an orthogonal distance regression method, applied to the logarithmic transformation of both dimensionless groups, and weighted such that unreliable data points have a smaller influence on the fit. Second, we found that the observation height and the stability range have an impact on the coefficients too. This implies that variations found in literature may result from variations in the height and stability ranges among the datasets. Furthermore, application of a given expression on a dataset measured at a different height or within a different stability range has to be done with care.
In order to investigate whether variations in and along a scintillometer path or aircraft flight leg are within the range of local variability, or could be attributed to surface heterogeneity, we analysed the amount of local variability in the structure parameters at different heights and under different stability regimes. We found that the variability is determined by stability and by the size of the averaging window over which the structure parameters are calculated. If instability increases, differences in structure parameters between upward motions and downward motions increase. If the averaging window size increases, the variance of the logarithmic structure parameters decreases. A rough estimation of this decrease is made by fitting a simple linear regression between this variances and the averaging window size. From this we found that for various stability classes both the offset and slope (in absolute sense) decrease with increasing instability. The offset and slope can be used to quantify the local variability, which in turn can give an indication if variations in the structure parameters along a scintillometer or flight path might be attributed to surface heterogeneity.
Finally, our last study is an elaboration of the study of Beyrich et al. (2012). They compared obtained with the unmanned meteorological mini aerial vehicle (M2AV) with obtained with the large-aperture scintillometer (LAS) for five flights on one single day during LITFASS-2009 experiment. We investigated if the systematically larger values of the M2AV as observed by them, can be found for other days, and if these differences could be reduced or explained through a more elaborate processing of the data of both instruments. We concluded that the difference can be found for other days during LITFASS-2009 and LITFASS-2010 as well. obtained from the M2AV data is larger, which is not improved by the more elaborate data analysis. Moreover, an exact synchronization of the LAS data with the time intervals of the M2AV data does not eliminate the discrepancy between both datasets.
All in all, this thesis defines better the borders of MOST and shows the behaviour of the structure parameters in the atmospheric surface layer.
Beyrich F, Bange J, Hartogensis OK, Raasch S, Braam M, van Dinther D, Gräf D, van Kesteren B, van den Kroonenberg AC, Maronga B, Martin S, Moene AF (2012) Towards a validation of scintillometer measurements: The LITFASS-2009 experiment. Boundary-Layer Meteorol 144:83-112
Role of the residual layer and large-scale subsidence on the development and evolution of the convective boundary layer
Blay-Carreras, E. ; Pino, D. ; Vilà-Guerau de Arellano, J. ; Boer, A. van de; Coster, O. de; Darbieu, C. ; Hartogensis, O.K. ; Lohou, F. ; Lothon, M. ; Pietersen, H.P. - \ 2014
Atmospheric Chemistry and Physics 14 (2014). - ISSN 1680-7316 - p. 4515 - 4530.
large-eddy-simulation - morning transition - carbon-dioxide - mixed-layer - water-vapor - order-jump - part i - turbulence - entrainment - inversion
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.
Evaluation of the Weather Research and Forecasting mesoscale model for GABLS3: Impact of boundary-layer schemes, boundary conditions and spin-up
Kleczek, M.A. ; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2014
Boundary-Layer Meteorology 152 (2014)2. - ISSN 0006-8314 - p. 213 - 243.
nonlocal closure-model - low-level jets - wrf model - part i - vertical diffusion - diurnal cycles - sea-ice - turbulence - sensitivity - parameterization
We evaluated the performance of the three-dimensional Weather Research and Forecasting (WRF) mesoscale model, specifically the performance of the planetary boundary-layer (PBL) parametrizations. For this purpose, Cabauw tower observations were used, with the study extending beyond the third GEWEX Atmospheric Boundary-Layer Study (GABLS3) one-dimensional model intercomparison. The WRF model (version 3.4.1) contains 12 different PBL parametrizations, most of which have been only partially evaluated. The GABLS3 case offers a clear opportunity to evaluate model performance, focusing on time series of near-surface weather variables, radiation and surface flux budgets, vertical structure and the nighttime inertial oscillation. The model results revealed substantial differences between the PBL schemes. Generally, non-local schemes tend to produce higher temperatures and higher wind speeds than local schemes, in particular, for nighttime. The WRF model underestimates the 2-m temperature during daytime (about TeX K) and substantially underestimates it at night (about TeX K), in contrast to the previous studies where modelled 2-m temperature was overestimated. Considering the 10-m wind speed, during the night turbulent kinetic energy based schemes tend to produce lower wind speeds than other schemes. In all simulations the sensible and latent heat fluxes were well reproduced. For the net radiation and the soil heat flux we found good agreement with daytime observations but underestimations at night. Concerning the vertical profiles, the selected non-local PBL schemes underestimate the PBL depth and the low-level jet altitude at night by about 50 m, although with the correct wind speed. The latter contradicts most previous studies and can be attributed to the revised stability function in the Yonsei University PBL scheme. The local, turbulent kinetic energy based PBL schemes estimated the low-level jet altitude and strength more accurately. Compared to the observations, all model simulations show a similar structure for the potential temperature, with a consistent cold bias (TeX2 K) in the upper PBL. In addition to the sensitivity to the PBL schemes, we studied the sensitivity to technical features such as horizontal resolution and domain size. We found a substantial difference in the model performance for a range of 12, 18 and 24 h spin-up times, longer spin-up time decreased the modelled wind speed bias, but it strengthened the negative temperature bias. The sensitivity of the model to the vertical resolution of the input and boundary conditions on the model performance is confirmed, and its influence appeared most significant for the non-local PBL parametrizations
Variability of the Structure Parameters of Temperature and Humidity Observed in the Atmospheric Surface Layer Under Unstable Conditions
Braam, M. ; Moene, A.F. ; Beyrich, F. - \ 2014
Boundary-Layer Meteorology 150 (2014)3. - ISSN 0006-8314 - p. 399 - 422.
sonic anemometer - water-vapor - aperture scintillometer - heterogeneous surface - local-structure - boundary-layer - sensible heat - fluxes - turbulence - momentum
The structure parameters of temperature and humidity are important in scintillometry as they determine the structure parameter of the refractive index of air, the primary atmospheric variable obtained with scintillometers. In this study, we investigate the variability of the logarithm of the Monin-Obukhov-scaled structure parameters (denoted as log(2s )) of temperature and humidity. We use observations from eddy-covariance systems operated at three heights (2.5, 50, and 90 m) within the atmospheric surface layer under unstable conditions. The variability of log(C2 s ) depends on instability and on the size of the averaging window over which log(C2 s ) is calculated. If instability increases, differences in log(C2s ) between upward motions (large C2 s ) and downward motions (small C2 s ) increase. The differences are, however, not sufficiently large to result in a bimodal probability density function. If the averaging window size increases, the variances of log(C2 s ) decrease. A linear regression of the variances of log(C2 s ) versus the averaging window size for various stability classes shows an increase of both the offset and slope (in absolute sense) with increasing instability. For temperature, data from the three heights show comparable results. For humidity, in contrast, the offset and slope are larger at 50 and 90 m than at 2.5 m. In the end we discuss how these findings could be used to assess whether observed differences in C2 s along a scintillometer path or aircraft flight leg are just within the range of local variability in C2 s or could be attributed to surface heterogeneity. This is important for the interpretation of data measured above a heterogeneous surface.
The Effect of a New Calibration Procedure on the Measurement Accuracy of Scintec's Displaced-Beam Laser Scintillometer
Kesteren, A.J.H. van; Hartogensis, O.K. ; Kroonenberg, A.C. van den - \ 2014
Boundary-Layer Meteorology 151 (2014)2. - ISSN 0006-8314 - p. 257 - 271.
atmospheric surface-layer - stable boundary-layer - optical scintillation - inner scale - structure parameter - flux measurements - refractive-index - sensible heat - turbulence - spectrum
We describe a new calibration procedure included in the production process of Scintec’s displaced-beam laser scintillometers (SLS-20/40) and its effect on their measurement accuracy. The calibration procedure determines the factual displacement distances of the laser beams at the receiver and transmitter units, instead of assuming a prescribed displacement distance of 2.70 mm. For this study, four scintillometers operated by Wageningen University and the German Meteorological Service were calibrated by Scintec and their data re-analyzed. The results show that significant discrepancies may exist between the factual and the prescribed displacement distances. Generally, the factual displacement is about 0.1 mm smaller than 2.70 mm, but extremes varied between 0.04 and 0.24 mm. Correspondingly, using non-calibrated scintillometers may result in biases as large as 20 % in the estimates of the inner-scale length, l0, the structure parameter of the refractive index, Cn2, and the friction velocity, u*. The bias in the sensible heat flux was negligible, because biases in Cn2 and u* cancel. Hence, the discrepancies explain much of the long observed underestimations of u * determined by these scintillometers. Furthermore, the calibration improves the mutual agreement between the scintillometers for l 0 , but especially for Cn2. Finally, it is noted that the measurement specifications of the scintillometer do not expire and hence the results of the calibration can be applied retroactively
Calibration and Quality Assurance of Flux Observations from a Small Research Aircraft
Vellinga, O.S. ; Dobosy, R.J. ; Dumas, E.J. ; Beniamino, G. ; Elbers, J.A. ; Hutjes, R.W.A. - \ 2013
Journal of Atmospheric and Oceanic Technology 30 (2013). - ISSN 0739-0572 - p. 161 - 181.
boundary-layer development - lift-induced upwash - carbon-dioxide - wind-tunnel - co2 fluxes - airborne - turbulence - surface - variability - landscape
Small environmental research aircraft (ERA) are becoming more common for detailed studies of air–surface interactions. The Sky Arrow 650 ERA, used by multiple groups, is designed to minimize the complexity of high-precision airborne turbulent wind measurement. Its relative wind probe, of a nine-port design, is furthermore used with several other airplanes. This paper gives an overview of 1) calibration of the model that converts the probe’s raw measurements to meteorological quantities; 2) quality control and assurance (QC–QA) in postprocessing of these quantities to compute fluxes; and 3) sensitivity of fluxes to errors in calibration parameters. The model, an adapted version of standard models of potential flow and aerodynamic upwash, is calibrated using an integrated method to derive a globally optimum set of parameters from in-flight maneuvers. Methods of QC–QA from the tower flux community are adopted for use with airborne flux data to provide more objective selection criteria for large datasets. Last, measurements taken from a standard operational flight are used to show fluxes to be most sensitive to calibration parameters that directly affect the vertical wind component. In another test with the same data, varying all calibration parameters simultaneously by ±10% of their optimum values, the model computes a response in the fluxes smaller than 10%, though a larger response may occur if only a subset of parameters is perturbed. A MATLAB toolbox has been developed that facilitates the procedures presented here
On the use of horizontal acoustic doppler profilers for continuous bed shear stress monitoring
Vermeulen, B. ; Hoitink, A.J.F. ; Sassi, M.G. - \ 2013
International Journal of Sediment Research 28 (2013)2. - ISSN 1001-6279 - p. 260 - 268.
broad-band adcp - tidal channel - turbulence - velocity - flow - layer
Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean. Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler (ADCP) to monitor bed shear stress, applying a prescribed boundary layer model, previously used for discharge estimation. The model parameters include the local roughness length and a dip correction factor to account for sidewall effects. Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data. We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP. To validate the results, co-located coupled ADCPs were used to infer bed shear stress, both from Reynolds stress profiles and from mean velocity profiles. From HADCP data collected over a period of 1.5 years, a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River, East Kalimantan, Indonesia. A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs. The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow, which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP. Bed shear stresses from the coupled ADCP are representative of a much more localized flow, while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales. Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods, and highlight the robustness of the method to uncertainty in the estimates of the roughness length.
Stable atmospheric boundary layers and diurnal Cycles-Challenges for Weather and Climate Models
Holtslag, A.A.M. ; Svensson, G. ; Baas, P. ; Basu, S. ; Beare, B. ; Beljaars, A.C.M. ; Bosveld, F.C. ; Cuxart, J. ; Lindvall, J. ; Steeneveld, G.J. ; Tjernstrom, M. ; Wiel, B.J.H. van de - \ 2013
Bulletin of the American Meteorological Society 94 (2013). - ISSN 0003-0007 - p. 1691 - 1706.
low-level jets - land-surface - contrasting nights - soil-moisture - ecmwf model - sea-ice - turbulence - cases-99 - parameterization - fluxes
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, we 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 inter-comparison 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 at Cabauw in the Netherlands. From these studies, we find that even for the non-cloudy boundary layer important parameterization challenges remain.
Impacts of Aerosol Shortwave Radiation Absorption on the Dynamics of an Idealized Convective Atmospheric Boundary Layer
Wilde Barbaro, E. ; Vilà-Guerau de Arellano, J. ; Krol, M.C. ; Holtslag, A.A.M. - \ 2013
Boundary-Layer Meteorology 148 (2013)1. - ISSN 0006-8314 - p. 31 - 49.
actinic flux - energy-budget - heating rates - closure-model - turbulence - entrainment - simulation - photolysis - climate - surface
We investigated the impact of aerosol heat absorption on convective atmospheric boundary-layer (CBL) dynamics. Numerical experiments using a large-eddy simulation model enabled us to study the changes in the structure of a dry and shearless CBL in depthequilibrium for different vertical profiles of aerosol heating rates. Our results indicated that aerosol heat absorption decreased the depth of the CBL due to a combination of factors: (i) surface shadowing, reducing the sensible heat flux at the surface and, (ii) the development of a deeper inversion layer, stabilizing the upper CBL depending on the vertical aerosol distribution. Steady-state analytical solutions for CBL depth and potential temperature jump, derived using zero-order mixed-layer theory, agreed well with the large-eddy simulations. An analysis of the entrainment zone heat budget showed that, although the entrainment flux was controlled by the reduction in surface flux, the entrainment zone became deeper and less stably stratified. Therefore, the vertical profile of the aerosol heating rate promoted changes in both the structure and evolution of the CBL. More specifically, when absorbing aerosols were present only at the top of the CBL, we found that stratification at lower levels was the mechanism responsible for a reduction in the vertical velocity and a steeper decay of the turbulent kinetic energy throughout the CBL. The increase in the depth of the inversion layer also modified the potential temperature variance. When aerosols were present we observed that the potential temperature variance became significant already around 0.7zi (where zi is the CBL height) but less intense at the entrainment zone due to the smoother potential temperature vertical gradient.
Crosswind from a Single Aperture Scintillometer using Spectral Techniques
Dinther, D. van; Hartogensis, O.K. ; Moene, A.F. - \ 2013
Journal of Atmospheric and Oceanic Technology 30 (2013)1. - ISSN 0739-0572 - p. 3 - 21.
windsnelheid - vliegtuigen - scintillometrie - klimaatverandering - wind speed - airplanes - scintillometry - climatic change - flevoland field experiment - heterogeneous surface - wind measurements - turbulence - fluxes - scintillations - profiles - terrain - layer - beam
In this study, spectral techniques to obtain the crosswind from a single large aperture scintillometer (SLAS) time series are investigated. The crosswind is defined as the wind component perpendicular to a path. A scintillometer obtains a path-averaged estimate of the crosswind. For certain applications this can be advantageous, e.g. monitoring the crosswind along airport runways. The essence of the spectral techniques lies in the fact that the scintillation power spectrum shifts linearly along the frequency domain as a function of the crosswind. Three different algorithms are used, which we named the Corner Frequency (CF), Maximum Frequency (MF), and the Cumulative Spectrum (CS) technique. The algorithms track the frequency shift of a characteristic point in different representations of the scintillation power spectrum. The spectrally derived crosswinds compare well with sonic anemometer estimates. The CS algorithm obtained the best results for the crosswind when compared with the sonic anemometer. However, the MF algorithm was most robust in obtaining the crosswind. Over short time intervals (
Characterization of a boreal convective boundary layer and its impact on atmospheric chemistry during HUMPPA-COPEC-2010
Ouwersloot, H.G. ; Vilà-Guerau de Arellano, J. ; Nölscher, A.C. ; Krol, M.C. ; Ganzeveld, L.N. ; Breitenberger, C. ; Mammarella, I. ; Williams, J. ; Lelieveld, J. - \ 2012
Atmospheric Chemistry and Physics 12 (2012). - ISSN 1680-7316 - p. 9335 - 9353.
large-eddy simulation - aerosol formation - forest - model - entrainment - turbulence - evolution - inversion - emissions - exchanges
We studied the atmospheric boundary layer (ABL) dynamics and the impact on atmospheric chemistry during the HUMPPA-COPEC-2010 campaign. We used vertical profiles of potential temperature and specific moisture, obtained from 132 radio soundings, to determine the main boundary layer characteristics during the campaign. We propose a classification according to several main ABL prototypes. Further, we performed a case study of a single day, focusing on the convective boundary layer, to analyse the influence of the dynamics on the chemical evolution of the ABL. We used a mixed layer model, initialized and constrained by observations. In particular, we investigated the role of large scale atmospheric dynamics (subsidence and advection) on the ABL development and the evolution of chemical species concentrations. We find that, if the large scale forcings are taken into account, the ABL dynamics are represented satisfactorily. Subsequently, we studied the impact of mixing with a residual layer aloft during the morning transition on atmospheric chemistry. The time evolution of NOx and O3 concentrations, including morning peaks, can be explained and accurately simulated by incorporating the transition of the ABL dynamics from night to day. We demonstrate the importance of the ABL height evolution for the representation of atmospheric chemistry. Our findings underscore the need to couple the dynamics and chemistry at different spatial scales (from turbulence to mesoscale) in chemistry-transport models and in the interpretation of observational data.
The effect of vegetation on wind-blown mass transport at the regional scale: A wind tunnel experiment
Youssef, I.F. ; Visser, S.M. ; Karssenberg, D. ; Erpul, G. ; Cornelis, W.M. ; Gabriels, D. ; Poortinga, A. - \ 2012
Geomorphology 159-160 (2012). - ISSN 0169-555X - p. 178 - 188.
northern chihuahuan desert - sand transport - erosion control - forest edge - flow - area - turbulence - threshold - cover - speed
Wind erosion is a global environmental problem. Re-vegetating land is a commonly used method to reduce the negative effects of wind erosion. However, there is limited knowledge on the effect of vegetation pattern on wind-blown mass transport. The objective of this study was to investigate the effect of vegetation pattern on this phenomenon within a land unit and at the border between land units. Wind tunnel experiments were conducted with artificial shrubs representing Atriplex halimus. Wind runs at a speed of 11 m s– 1 were conducted and sand translocation was measured after 200–230 s using a graph paper prepared for this purpose.This research showed that: 1) the transport within a land unit is affected by the neighboring land units and by the vegetation pattern within both the unit itself and the neighboring land units; 2) re-vegetation plans for degraded land can take into account the 'streets' effect (zones of erosion areas similar to streets); 3) the effect of neighboring land units includes sheltering effect and the regulation of sediment passing from one land unit to the neighboring land units and 4) in addition to investigation of the general effect of vegetation pattern on erosion and deposition within the region, it is important to investigate the redistribution of sediment at smaller scales depending on the scope of the project.
Case study of the diurnal variability of chemically active species with respect to boundary layer dynamics during DOMINO
Stratum, B.J.H. van; Vilà-Guerau de Arellano, J. ; Ouwersloot, H.G. ; Dries, K. van den; Laar, T.W. van; Martinez, M. ; Lelieveld, J. ; Diesch, J.M. ; Drewnick, F. ; Fischer, H. ; Hosaynali Beygi, Z. ; Harder, H. ; Regelin, E. ; Sinha, V. ; Adame, J.A. ; Sörgel, M. ; Sander, R. ; Bozem, H. ; Song, W. ; Williams, J. ; Yassaa, N. - \ 2012
Atmospheric Chemistry and Physics 12 (2012). - ISSN 1680-7316 - p. 5329 - 5341.
volatile organic-compounds - tropical forest - oh reactivity - isoprene - chemistry - campaign - air - segregation - turbulence - gabriel
We study the interactions between atmospheric boundary layer (ABL) dynamics and atmospheric chemistry using a mixed-layer model coupled to chemical reaction schemes. Guided by both atmospheric and chemical measurements obtained during the DOMINO (Diel Oxidant Mechanisms in relation to Nitrogen Oxides) campaign (2008), numerical experiments are performed to study the role of ABL dynamics and the accuracy of chemical schemes with different complexity: the Model for Ozone and Related chemical Tracers, version 4 (MOZART-4) and a reduced mechanism of this chemical system. Both schemes produce satisfactory results, indicating that the reduced scheme is capable of reproducing the O3-NOx-VOC-HOx diurnal cycle during conditions characterized by a low NOx regime and small O3 tendencies (less than 1 ppb per hour). By focusing on the budget equations of chemical species in the mixedlayer model, we show that for species like O3, NO and NO2, the influence of entrainment and boundary layer growth is of the same order as chemical production/loss. This indicates that an accurate representation of ABL processes is crucial in understanding the diel cycle of chemical species. By comparing the time scales of chemical reactive species with the mixing time scale of turbulence, we propose a classification based on the Damk¨ohler number to further determine the importance of dynamics on chemistry during field campaigns. Our findings advocate an integrated approach, simultaneously solving the ABL dynamics and chemical reactions, in order to obtain a better understanding of chemical pathways and processes and the interpretation of the results obtained during measurement campaigns.
Composite hodographs and inertial oscillations in the nocturnal boundary layer
Baas, P. ; Wiel, B.J.H. van de; Brink, L. van den; Holtslag, A.A.M. - \ 2012
Quarterly Journal of the Royal Meteorological Society 138 (2012)663. - ISSN 0035-9009 - p. 528 - 535.
low-level jet - wind-structure variations - southern great-plains - climatology - cabauw - intermittent - turbulence - cases-99 - model - land
In this work the dynamic behaviour of the wind in the nocturnal boundary layer is studied, with a particular focus on systematic behaviour of the near-surface wind. Recently, an extension of the well-known Blackadar model for frictionless inertial oscillations above the nocturnal boundary layer was proposed by Van deWiel et al., which accounts for frictional effects within the nocturnal boundary layer. It appears that the nocturnal wind velocity profile tends to perform an inertial oscillation around an equilibrium wind profile, rather than around the geostrophic wind vector (as in the Blackadar model). In the present studywe propose the concept of ‘composite hodographs’ to evaluate the ideas and assumptions of the aforementioned analytical model. Composite hodographs are constructed based on a large observational dataset from the Cabauw observatory. For comparison and deeper analysis, this method is also applied to single-column model simulations that represent the same dataset. From this, it is shown that winds in the middle and upper part of the nocturnal boundary layer closely follow the dynamics predicted by the model by Van de Wiel et al. In contrast, the near-surface wind shows more complex behaviour that can be described by two different stages: (1) a decelerating phase where the wind decreases rapidly in magnitude due to enlarged stress divergence in the transition period near sunset (an aspect not included in the analytical model), and (2) a regular type of inertial oscillation, but with relatively small amplitude as compared to the oscillations in the middle and upper parts of the nocturnal boundary layer
On the segregation of chemical species in a clear boundary layer over heterogeneous land surfaces
Ouwersloot, H.G. ; Vilà-Guerau de Arellano, J. ; Heerwaarden, C.C. van; Ganzeveld, L.N. ; Krol, M.C. ; Lelieveld, J. - \ 2011
Atmospheric Chemistry and Physics 11 (2011). - ISSN 1680-7316 - p. 10681 - 10704.
large-eddy simulation - volatile organic-compounds - tropical rain-forest - atmospheric chemistry - deciduous forest - heat-flux - emission - campaign - scale - turbulence
Using a Large-Eddy Simulation model, we have systematically studied the inability of boundary layer turbulence to efficiently mix reactive species. This creates regions where the species are accumulated in a correlated or anti-correlated way, thereby modifying the mean reactivity. We quantify this modification by the intensity of segregation, IS, and analyse the driving mechanisms: heterogeneity of the surface moisture and heat fluxes, various background wind patterns and non-uniform isoprene emissions. The heterogeneous surface conditions are characterized by cool and wet forested patches with high isoprene emissions, alternated with warm and dry patches that represents pasture with relatively low isoprene emissions. For typical conditions in the Amazon rain forest, applying homogeneous surface forcings and in the absence of free tropospheric NOx, the isoprene- OH reaction rate is altered by less than 10 %. This is substantially smaller than the previously assumed IS of 50% in recent large-scale model analyses of tropical rain forest chemistry. Spatial heterogeneous surface emissions enhance the segregation of species, leading to alterations of the chemical reaction rates up to 20 %. The intensities of segregation are enhanced when the background wind direction is parallel to the borders between the patches and reduced in the case of a perpendicular wind direction. The effects of segregation on trace gas concentrations vary per species. For the highly reactive OH, the differences in concentration averaged over the boundary layer are less than 2% compared to homogeneous surface conditions, while the isoprene concentration is increased by as much as 12% due to the reduced chemical reaction rates. These processes take place at the sub-grid scale of chemistry transport models and therefore need to be parameterized.
Comments on “An extremum solution of the Monin-Obukhov similarity equations"
Wiel, B.J.H. van de; Basu, S. ; Moene, A.F. ; Jonker, H.J.J. ; Steeneveld, G.J. ; Holtslag, A.A.M. - \ 2011
Journal of the Atmospheric Sciences 68 (2011)6. - ISSN 0022-4928 - p. 1405 - 1408.
atmospheric boundary-layer - flux-profile relationships - temperature-fluctuation method - stable conditions - surface-layer - models - turbulence