Atmospheric Boundary Layer Model Research Articles

Upgrading the coupled vegetation boundary layer model HIRVAC by new soil water and interception modules

The coupled atmospheric boundary layer model HIRVAC (High Resolution Vegetation Atmosphere Coupler) combines a 1.5 dimensional planetary boundary layer model (developed at the Humboldt University Berlin) and the mechanistic photosynthesis module PSN6 (University of Bayreuth) as well as a soil water module. This multi layer model includes a vertically structured vegetation and covers a range from 0-2 km height. The model layers are coupled via the turbulent exchange using a K-approach. Recent work aimed to include several new modules to enable model calculations for realistic conditions. Therefore rainfall and a multi layer interception and a soil water module were included. Further the light extinction through the canopy was upgraded by including sunfleck dynamics for calculating a more realistic radiation distribution within and below canopy. The upgraded model version was run for the Anchor Station in Tharandt where various measurements for comparison with model results are available. The obtained results demonstrate that periods of several days can be modelled successfully. A good model performance was found for both, soil water and interception. The modelled energy fluxes finally confirm the reliability of the overall model performance.

Comparison of surface energy exchange models with eddy flux data in forest and grassland ecosystems of Germany

Latent heat, which plays a major role in the energy balance of ecosystems, is usually calculated in different types of models, e.g., water balance approaches, aggregated “big-leaf” models, multi-layer ecosystem process models, that can be generally catalogued as Soil–Vegetation–Atmosphere–Transport models (SVATs) in a broader sense, etc. In this study, four (multi-) layered models (PLant-ATmosphere INteraction model (PLATIN), MixFor-SVAT, SVAT-CN, PnET-N-DNDC) and an atmospheric boundary layer model including vegetation (HIgh Resolution Vegetation Atmosphere Coupler (HIRVAC)) were used to predict latent heat (LE) and sensible heat ( H) exchange in the target area of VERTIKO, a research project on vertical transports of energy and trace gases and their spatial/temporal extrapolation under complex natural conditions. Investigated vegetation types were a Norway spruce forest, and two grassland sites in Brandenburg and Saxony (Germany) during field campaigns in 2001 and 2002. Four models had half-hourly to hourly time steps, but PnET-N-DNDC had a daily time step. All models used meteorological conditions above the canopy as input, and computed LE and H independently, or H as the residual of the energy balance. Intercomparisons and discussions of model parameters focussed on vegetation type (or species), and LAI. Model predictions were compared to half-hourly and daily eddy covariance measurements of ecosystem LE and H during two periods (cold and wet fall 2001, warm and dry summer 2002) to include contrasting weather conditions in the model evaluation. The agreement between modeled and measured fluxes was reasonable for most models. Yet, as the models were principally based on energy balance, the lack of energy balance closure as found in the data made it difficult to evaluate models against data, indicating a need for more attention to measurement accuracy in future studies. Systematic differences between the models were obvious. In general, HIRVAC model predictions of LE were routinely the lowest, and PnET-N-DNDC the highest. The tendency of under- or overestimating LE fluxes was comparable in PLATIN, SVAT-CN and MixFor-SVAT. SVAT-CN especially underestimated LE and overestimated H of the forest site. Under- or overestimation by the models could not be attributed to vegetation type. Both, MixFor-SVAT and SVAT-CN underestimated LE of one of the grassland sites, and overestimated the other. When averaging all model outputs, daily fall fluxes reached between 75 and 115% of the data for the three sites (76–83% for the summer period), and between 65 and 90% for half-hourly fluxes (73–97% for summer). Sensitivity analysis for an important model parameter, LAI, revealed only small effects on modeled LE in most models, probably due to compensation in the relative amounts of evapotranspiration and soil evaporation, at least for the limited range of LAI we analyzed (mean ±1 m 2 m −2). The sensitivity to LAI was smaller for forest simulations than for grasslands. For a given site PLATIN and MixFor-SVAT showed lowest, SVAT-CN intermediate, and HIRVAC largest sensitivity to changes in LAI. For latent heat simulations of forests, the sensitivity of LE to changes in LAI was smaller than the differences in the LE predicted by the five models. Further, it is much more important to determine vegetation type and accurate leaf physiology than the LAI of the system. The models differed widely in their approach for canopy radiation, leaf physiology, energy balance, atmospheric transport or soil water modeling, yet the treatment of evaporative losses from soil and interception pools seemed to cause the larger differences between the models after rain events.

The Impact of Horizontal Resolution on the Tropical Heat Budget in an Atlantic Ocean Model

Abstract An ocean general circulation model (OGCM) of the tropical Atlantic is coupled to an advective atmospheric boundary layer model. This configuration is used to investigate the hypothesis that resolving tropical instability waves (TIWs) in OGCMs will remove the equatorial cold bias that is a feature common to coarse-resolution OGCMs. It is shown that current eddy parameterizations cannot capture the TIW heat flux because diffusion in coarse-resolution OGCMs removes heat from the warm pool to heat the equatorial cold tongue, whereas TIWs draw their heat mostly from the atmosphere. Thus, they can bring more heat to the equatorial cold tongue without cooling the warm pool, and the SST in the warm pool is higher and more realistic. Contrary to expectations, the SST in the equatorial cold tongue is not significantly improved. The equatorial warming due to TIWs is slightly greater than the warming due to diffusion, but this increased equatorial heat flux in the high-resolution experiment is compensated by increased equatorial entrainment there. This is attributed to the Equatorial Undercurrent being stronger, thereby increasing the entrainment rate through shear instability. Thus, higher resolution does not significantly increase the total oceanic heat flux convergence in the equatorial mixed layer.

Physical Modelling of Point Source Dispersion Inside Atmospheric Boundary Layer

AbstractOne of the most important problems in the turbulence theory is the process of turbulent mixing. Although many theoretical and experimental studies on the structure functions of a passive scalar as well as the probability density functions of its increment and gradient in a turbulent fluid have been published so far, the instantaneous feature of an advected scalar has not been explored very well. There are only a few experimental studies on a scalar concentration fluctuation analysis in turbulent flows. In this study the time‐series of both longitudinal velocity component and concentration analysis are to be presented. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Model Simulations of the Arctic Atmospheric Boundary Layer from the SHEBA Year

We present Arctic atmospheric boundary-layer modeling with a regional model COAMPS, for the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment. Model results are compared to soundings, near-surface measurements and forecasts from the ECMWF model. The near-surface temperature is often too high in winter, except in shorter periods when the boundary layer was cloud-capped and well-mixed due to cloud-top cooling. Temperatures are slightly too high also during the summer melt season. Effects are too high boundary-layer moisture and formation of too dense stratocumulus, generating a too deep well-mixed boundary layer with a cold bias at the simulated boundary-layer top. Errors in temperature and therefore moisture are responsible for large errors in heat flux, in particular in solar radiation, by forming these clouds. We conclude that the main problems lie in the surface energy balance and the treatment of the heat conduction through the ice and snow and in how low-level clouds are treated.

Arctic Ocean sea ice response to Northern Annular Mode‐like wind forcing

The response of the Arctic Ocean sea ice system to Northern Annular Mode‐like wind forcing has been investigated using an ocean/sea ice general circulation model coupled to an atmospheric boundary layer model. A series of idealized experiments was performed to investigate the Arctic Ocean's response to idealized winter wind anomalies on interannual to multi‐decadal time scales. The sea ice response of the model consists of a rapid change of ice movements leading to widespread variation in sea ice thickness and concentration. In most areas the response is largely independent of the forcing frequency with only a slight increase towards longer periods. Only the Greenland Sea exhibited a change in sign of sea ice concentration anomalies at about 20 years period which appears to be caused by slow adjustment of the oceanic circulation.

Variational Assimilation of Radiometric Surface Temperature and Reference-Level Micrometeorology into a Model of the Atmospheric Boundary Layer and Land Surface

Abstract Data assimilation provides a useful framework that allows us to combine measurements and models, by appropriately weighting the sources of error in both, to produce a statistically optimal and dynamically consistent estimate of the evolving state of the system. In this paper a variational approach is used to estimate regional land and atmospheric boundary layer states and fluxes via the assimilation of standard reference-level temperature and humidity and radiometric surface temperature measurements into a coupled land surface–atmospheric boundary layer model. Results from an application to a field experiment site show that using both surface temperature and reference-level micrometeorology measurements allows for the accurate and robust estimation of land surface fluxes even during nonideal conditions, where the evaporation rate is atmospherically controlled and processes that are not parameterized in the model (i.e., advection) are important. The assimilation scheme is able to provide estimates...

Indian Ocean Intraseasonal Variability in an Ocean General Circulation Model

Abstract The impact of atmospheric intraseasonal variability on the tropical Indian Ocean is examined with an ocean general circulation model (OGCM). The model is forced by observation-based wind stresses and surface heat fluxes from an atmospheric boundary layer model. Composites of 26 well-defined boreal spring and summer intraseasonal events from 1985 to 1994 are used to explore surface and subsurface impacts of intraseasonal oscillations in the ocean. The phase and amplitude of simulated intraseasonal sea surface temperature (SST) variations agree well with observations. The net surface heat flux dominates the composite mixed layer heat budget on intraseasonal timescales, while entrainment through the base of the mixed layer contributes locally. Horizontal advection is of secondary importance in the composite heat balance. However, inspection of individual events suggests that in individual intraseasonal events different processes may control their dynamics. A characteristic feature of equatorial intr...

Energy exchange processes in the marginal ice zone of the Barents Sea, Arctic Ocean, during spring 1999

AbstractWe present some new results describing energy exchange processes of drifting sea ice in the marginal ice zone (MIZ) in the Barents Sea, Arctic Ocean. All measurements and observations of meteorological parameters and ice conditions were taken on board the Norwegian research vessel Lance from 3 to 22 May 1999. Components of surface heat balance were measured and correlated with ice conditions and synoptic observations. These results can be used in atmospheric boundary layer modelling as lower boundary conditions. A relationship was found between modelled turbulent heat fluxes and observed sea-ice concentrations.

Lagrangian Stochastic Modeling in Coastal Oceanography

Lagrangian stochastic (LS) modeling is a common technique in atmospheric boundary layer modeling but is relatively new in coastal oceanography. This paper presents some fundamental aspects of LS modeling as they pertain to oceanography. The theory behind LS modeling is reviewed and an introduction to the substantial atmospheric literature on the subject is provided. One of the most important properties of an LS model is that it maintains an initially uniform distribution of particles uniform for all time—the well-mixed condition (WMC). Turbulent data for use in an oceanic LS model (LSM) are typically output at discrete positions by a general circulation model. Tests for the WMC are devised, and it is shown that for inhomogeneous turbulence the data output by an oceanic general circulation model is such that the WMC cannot be demonstrated. It is hypothesized that this is due to data resolution problems. To test this hypothesis analytical turbulence data are constructed and output at various resolutions to show that the WMC can only be demonstrated if the resolution is high enough (the required resolution depending on the inhomogeneity of the turbulence data). The output of an LSM represents one trial of possible ensemble and this paper seeks to learn the ensemble average properties of the dispersion. This relates to the number of particles or trials that are performed. Methods for determining the number of particles required to have statistical certainty in one’s results are demonstrated, and two possible errors that can occur when using too few particles are shown.

Feedback between the Land Surface Energy Balance and Atmospheric Boundary Layer Diagnosed through a Model and Its Adjoint

Abstract In this paper, the differences in (daytime) sensitivity of surface turbulent fluxes to states and parameters for a coupled land–atmospheric boundary layer model in coupled and uncoupled modes are investigated using an adjoint framework. The adjoint approach yields diagnostic insight into sensitivity pathways that is not readily obtained when using a traditional forward sensitivity framework. It is shown that the corresponding uncoupled adjoint model is a special case of the coupled adjoint model in which the atmospheric feedback pathways are “closed.” These closed sensitivity pathways are directly responsible for the differences in sensitivity between the uncoupled and coupled models. Integrating the adjoint models about a consistent and common trajectory provided by the forward model yields the sensitivities of the surface turbulent fluxes to all of the model states and parameters, which are shown to be significantly different because of the boundary layer feedbacks. Furthermore, the adjoint sen...

Effects of river inputs into the Bay of Bengal

The effect of river runoff in the Bay of Bengal is examined using a reduced gravity primitive equation ocean model coupled to an atmospheric boundary layer model. Model simulations are carried out by including river discharges as surface freshwater forcing at the mouths of the rivers. To assess the effect of river inputs on the dynamics and thermodynamics of the tropical Indian Ocean, parallel simulations are carried out by neglecting the river inputs. Additionally, another set of parallel runs without penetrative radiation loss through the mixed layer is carried out. The freshwater flux due to rivers results in lower salinities and shallower mixed layers, as expected. However, the influence of this additional freshwater flux into the bay is rather counterintuitive. With the inclusion of river discharges more heat is absorbed by the ocean, but sea surface temperatures are slightly cooler in the bay because of enhanced entrainment cooling of the shallower mixed layer, enhanced penetrative radiation, and an enhanced effect of latent heat loss on the temperature tendency. This is despite the greater latent heat loss when river input is neglected. Conversely, neglect of penetrative radiation results in a shallower but slightly warmer mixed layer with river input. River input and penetrative radiation each affect the mixed layer depths, the salinity and temperature structure, and currents in the Bay of Bengal, but they have a small effect on SST. Annual SST, averaged over the Bay of Bengal, is only 0.1°C colder with river input. Neglecting penetrative radiation in the river run results in an increase of only 0.2°C for the annual SST. The lack of persistence of a barrier layer in the bay helps regulate SST even in the presence of enhanced buoyancy forcing due to river input. Averaged over the bay, a barrier layer forms as mixed layer detrainment occurs, and the thermocline deepens just after the southwest monsoon and the northeast monsoon. The barrier layer is short‐lived in each case it is eroded by mixing. The effect of riverine input in the bay is not confined to the surface waters. A pool of cold anomaly (−1°C) and fresher waters is centered near 100 m depth in the bay with riverine input. This cold pool beneath the mixed layer allows entrainment cooling of the mixed layer to be more effective, even though mass entrainment is lower relative to the case neglecting river input. The more diffuse thermocline in the bay is consistent with enhanced vertical mixing despite the large positive buoyancy forcing.

Large Off-Shore Windfarms: Linking Wake Models with Atmospheric Boundary Layer Models

Gerard Schepers1, Rebecca Barthelmie2, Kostas Rados3, Bernhard Lange4 and Wolfgang Schlez5 1Energy Research Centre,Wind Energy, 1755 ZG Petten,The Netherlands. E-mail 2Wind Energy Dept., Riso National Laboratory, 40 0 0 Roskilde, Denmark 3The Robert Gordon University, School of Mechanical and Offshore Engineering,Aberdeen AB10 1FR, Scotland 4Dept. of Energy and Semiconductor Research EHF,University of Oldenburg,D-26111 Oldenburg,Germany 5Wolfgang Schlez, Garrad Hassan and Partners Ltd, Bristol BS2 0 QD, UK

Quantifying the coupling degree between land surface and the atmospheric boundary layer with the coupled vegetation-atmosphere model HIRVAC

Abstract. In the present study, the ability of different indices to quantify the coupling degree between a vegetated surface and the atmospheric boundary layer is tested. For this purpose, a one-and-a-half dimensional atmospheric boundary layer model, including a high resolved vegetation canopy, was applied (HIRVAC) and indices, such as the decoupling factor Ω, as well as other measures derived from model out-put were used. The aim of the study was to show that the quite complex coupling and feedback mechanisms can be described with these relatively simple measures. Model results illustrate that the vegetation and the atmosphere are well coupled (expressed by a lower Ω) under conditions of a tall and dense canopy, as well as under strong dynamic forcing. This better aerodynamic coupling leads to an increase in evapotranspiration, as well as an increase in the evaporative fraction. This fact was also shown by the second coupling measure: the relative changes in daily model evapotranspiration. This measure was inspired by the assumption that these changes are primarily dependent on the coupling degree between the surface and the atmosphere, if the other boundary conditions in the model are fixed. A third sensitivity measure was used according to Jacobs and de Bruin (1992). It shows that the sensitivity of evaporative fraction to stomata resistance is much higher with a better aerodynamic coupling. The results of the factor Ω; are in a good agreement with the findings of Jacobs and de Bruin: they stress that it is a valuable strategy to group vegetation into two simple categories (smooth and rough) for the understanding of vegetation-atmosphere coupling.Key words. Atmospheric composition and structure (biosphere- atmosphere interactions) – Hydrology (evapotranspiration; hydroclimatology)

An objective approach for selecting ice or wet–snow design loads on transmission lines

Ice and wet–snow design loads affect the investment costs and the potential maintenance costs for overhead lines more than any other single loading case in most countries exposed to snow and freezing temperatures during winter. The dependence of ice loads on investment costs for both steel towers and wood poles as well as the climatic variations of icing are shown. The paper summarizes the need for such data and refers to IEC recommendations for procedures and measurements of loads, especially as probabilistic methods for design require more comprehensive information on ice accretions on overhead lines, including variations due to conductor configurations. Icing information from icing models based on general meteorological data is summarized, including the potential for future application of atmospheric boundary–layer models as used by weather forecasting centres. A methodology for handling some topographic influences is introduced. The paper concludes with two case studies, one for glaze ice and one for wet–snow loadings.

The WELSONS experiment: overview and presentation of first results on the surface atmospheric boundary-layer in semiarid Spain

Abstract. This study presents the preliminary results of the local energy budget and dynamic characteristics of the surface atmospheric boundary-layer (SBL) during the WELSONS (wind erosion and losses of soil nutrients in semiarid Spain) experiment. Some Mediterranean regions suffer land degradation by wind erosion as a consequence of their particular soil and climate conditions and inappropriate agricultural practice. In Spain, where land degradation by water erosion is well known, the lack of field studies to quantify soils losses by wind erosion resulted in the European Community organizing a scientific program for this specific issue. The European programme known as WELSONS was devoted to study the wind erosion process in central Aragon (NE Spain). This multidisciplinary experiment, which began in 1996 and finished in 1998, was carried out over an agricultural soil which was left fallow. Within the experimental field, two plots were delimited where two tillage treatments were applied, a mould-board ploughing (or conventional tillage denoted CT) and chisel ploughing (reduced tillage denoted RT). This was to study on bare soil the influence of tillage method on surface conditions, saltation flux, vertical dust flux, erosion rates, dynamics characteristics such as friction velocity, roughness length, etc., and energy budget. The partitioning of the available energy, resulting from the dynamics of the SBL, are quite different over the two plots because of their own peculiar soil and surface properties. The first results show that the RT treatment seems to provide a wind erosion protection. Because of the long data recording time and particular phenomena (formation of a crust at the soil surface, very dry conditions, high wind speed for instance), these microclimatological data acquired during the WELSONS programmes may be helpful to test atmospheric boundary-layer models coupled with soil models.Key words: Hydrology (desertification) - Meterology and atmospheric dynamics (general)

Local scale PC-oriented Eulerian pollutant transport model PolTran-1-2

A one-dimensional non-stationary numerical Atmospheric Boundary Layer (ABL) model is combined with an Eulerian advective scheme, called TRAP. The input information includes meteorological measurements at the surface and high level information from weather forecast centres. The Boundary Layer model includes a comprehensive set of equations governing the meteorological processes (momentum, turbulence, heat, short and infrared radiation, humidity, soil heat transfer). The pollutant transport model is a single layer one, based on the mixing layer concept. The TRAP scheme was chosen after careful comparison of some of the most widespread advection schemes. Graphic facilities on a PC show the changes of the pollutant fields at specified time steps. Examples using real meteorological data demonstrate the complex configurations of pollutant concentration and deposition in time and space and also show how easily the model can handle them.

The WELSONS experiment: overview and presentation of first results on the surface atmospheric boundary-layer in semiarid Spain

<strong class="journal-contentHeaderColor">Abstract.</strong> This study presents the preliminary results of the local energy budget and dynamic characteristics of the surface atmospheric boundary-layer (SBL) during the WELSONS (wind erosion and losses of soil nutrients in semiarid Spain) experiment. Some Mediterranean regions suffer land degradation by wind erosion as a consequence of their particular soil and climate conditions and inappropriate agricultural practice. In Spain, where land degradation by water erosion is well known, the lack of field studies to quantify soils losses by wind erosion resulted in the European Community organizing a scientific program for this specific issue. The European programme known as WELSONS was devoted to study the wind erosion process in central Aragon (NE Spain). This multidisciplinary experiment, which began in 1996 and finished in 1998, was carried out over an agricultural soil which was left fallow. Within the experimental field, two plots were delimited where two tillage treatments were applied, a mould-board ploughing (or conventional tillage denoted CT) and chisel ploughing (reduced tillage denoted RT). This was to study on bare soil the influence of tillage method on surface conditions, saltation flux, vertical dust flux, erosion rates, dynamics characteristics such as friction velocity, roughness length, etc., and energy budget. The partitioning of the available energy, resulting from the dynamics of the SBL, are quite different over the two plots because of their own peculiar soil and surface properties. The first results show that the RT treatment seems to provide a wind erosion protection. Because of the long data recording time and particular phenomena (formation of a crust at the soil surface, very dry conditions, high wind speed for instance), these microclimatological data acquired during the WELSONS programmes may be helpful to test atmospheric boundary-layer models coupled with soil models.<br><br><b>Key words:</b> Hydrology (desertification) - Meterology and atmospheric dynamics (general)

Relationship between Urban Canopy Configration and Heat Island Intensity Derived from Two-Dimensional Atmospheric Boundary Layer Model

Relationship between Urban Canopy Configration and Heat Island Intensity Derived from Two-Dimensional Atmospheric Boundary Layer Model

Active control of turbulence for an atmospheric boundary layer model in a wind tunnel

Many types of meteorological and boundary layer wind tunnels have been used for a long time to simulate the atmospheric effects on large structures and the transport of air pollutants. In addition, a few actively controlled wind tunnels have been proposed to obtain more realistic effects on structures. New types of multiple-fan wind tunnels have been developed in our laboratory. To produce the required wind turbulence in the tunnels, fans and oscillating airfoils were controlled independently by a computer. Statistical parameters, such as mean wind velocity, turbulence intensity, turbulence scale and power spectral density profiles in a boundary layer, were satisfactorily reproduced in the tunnel. In addition to these parameters, a Reynolds stress profile was achieved and corresponding coherent motions were observed in the boundary-layer flow.