Water balance versus land surface model in the simulation of Rhine river discharges
Hurkmans, R.T.W.L. ; Moel, H. de; Aerts, J.C.J.H. ; Troch, P.A. - \ 2008
Water Resources Research 44 (2008). - ISSN 0043-1397 - 14
hydrologie van stroomgebieden - afvoer - rijn - klimaatverandering - modellen - catchment hydrology - discharge - river rhine - climatic change - models - climate-change - parametrization schemes - hydrological model - basin - impact - parameterization - variability - management - streamflow - resources
Accurate streamflow simulations in large river basins are crucial to predict timing and magnitude of floods and droughts and to assess the hydrological impacts of climate change. Water balance models have been used frequently for these purposes. Compared to water balance models, however, land surface models carry the potential to more accurately estimate hydrological partitioning and thus streamflow, because they solve the coupled water and energy balance and are able to exploit a larger part of the information provided by regional climate model output than water balance models. Owing to increased model complexity, however, they are also more difficult to parameterize. The purpose of this study is to investigate and compare the accuracy of streamflow simulations of a water balance approach (Spatial Tools for River basins and Environment and Analysis of Management (STREAM)) and a land surface model (Variable Infiltration Capacity (VIC)) approach. Both models are applied to the Rhine river basin using regional climate model output as atmospheric forcing, and are evaluated using observed streamflow and lysimeter data. We find that VIC is more robust and less dependent on model calibration. Although STREAM performs better during the calibration period (Nash-Sutcliffe efficiency (E) of 0.47 versus E = 0.29 for VIC), VIC more accurately simulates discharge during the validation period, including peak flows (E = 0.31 versus E = 0.21 for STREAM). This is the case for most locations throughout the basin, except for the Alpine part where both models have difficulties due to the complex terrain and surface reservoirs. In addition, the annual evaporation cycle at the lysimeters is more realistically simulated by VIC.
Global reactive nitrogen deposition from lightning NOx
Shepon, A. ; Gildor, H. ; Labrador, L.J. ; Butler, T. ; Ganzeveld, L.N. ; Lawrence, M.G. - \ 2007
Journal of Geophysical Research: Atmospheres 112 (2007). - ISSN 2169-897X - 14 p.
general-circulation model - dry deposition - atmospheric chemistry - organic nitrogen - climate-change - trace gases - distributions - parameterization - cycle - emissions
We present results of the deposition of nitrogen compounds formed from lightning (LNO x ) using the global chemical transport Model of Atmospheric Transport and Chemistry¿Max Planck Institute for Chemistry version. The model indicates an approximately equal deposition of LNO x in both terrestrial and oceanic ecosystems, primarily in the tropics and midlatitudes open ocean, despite much higher intensities of lightning flashes above landmasses. The highest values of deposition are due to wet convective deposition, with highest values concentrated in the tropical continents. Nonconvective wet deposition, associated with large-scale weather patterns, occurs over large areas of the ocean amid lower values per square meter, manifesting the long-range transport of NO y , including long-lived species such as HNO3 at high altitudes and PAN. Dry deposition is concentrated primarily above landmasses, yet oceanic deposition over wide areas is still observed. Combined together, the total LNO x deposition exhibits maximal influx values over land, whereas oceanic deposition over wider areas renders the integrated deposition over both ecosystems almost identical. Peaks of terrestrial deposition values (located in Africa, South America, and Asia) show seasonal variability by meridionally penetrating the northern or southern midlatitude following the corresponding summer hemisphere, in accordance with the migration of LNO x production sites. On land, wet and dry deposition rates are more or less equal with a small bias toward wet deposition, whereas above the ocean, wet deposition is markedly higher because of a small water uptake efficiency and relatively small surface roughness. Further work of modeling additional species and obtaining more information on different compounds is required
Water-side turbulence enhancement of ozone deposition to the ocean
Fairall, C.W. ; Helmig, D. ; Ganzeveld, L.N. ; Hare, J. - \ 2007
Atmospheric Chemistry and Physics 2007 (2007)7. - ISSN 1680-7316 - p. 443 - 451.
dry deposition - boundary-layer - gas-exchange - wind-speed - parameterization - surface - fluxes - trends
A parameterization for the deposition velocity of an ocean-reactive atmospheric gas (such as ozone) is developed. The parameterization is based on integration of the turbulent-molecular transport equation (with a chemical source term) in the ocean. It extends previous work that only considered reactions within the oceanic molecular sublayer. The sensitivity of the ocean-side transport to reaction rate and wind forcing is examined. A more complicated case with a much more reactive thin surfactant layer is also considered. The full atmosphere-ocean deposition velocity is obtained by matching boundary conditions at the interface. For an assumed ocean reaction rate of 103 s¿1, the enhancement for ozone deposition by oceanic turbulence is found to be up to a factor of three for meteorological data obtained in a recent cruise off the East Coast of the U.S.
Estimation of the regional evaporative fraction over the Tibetan Plateau area by using Landsat-7 ETM data and the field observations
Ma, Y. ; Song, M. ; Ishikawa, H. ; Yang, K. ; Koike, T. ; Jia, L. ; Meneti, M. ; Su, Z. - \ 2007
Journal of the Meteorological Society of Japan 85A (2007). - ISSN 0026-1165 - p. 295 - 309.
soil heat-flux - surface-energy balance - heterogeneous landscape - net-radiation - model - parameterization - game/tibet - profiles - length - budget
This study proposed a parameterization methodology based on Landsat-7 ETM data and field observations and tested it for deriving an evaporative fraction (EF) over a heterogeneous landscape. As a case study, the methodology was applied to the experimental area of CAMP/Tibet located on the central Tibetan Plateau. Four scenes of Landsat-7 ETM data were used in the study. Scenes of 9 June 2002 and 28 August 2002 were selected as case examples of summer and autumn, respectively; the scene of 2 December 2002 was selected as a winter case; and 24 March 2003 was selected as a spring case (or pre-monsoon period). To validate the proposed methodology, the Landsat-7 ETM derived EFs were compared to ground-measured values in four different months that spanned a wide range of surface conditions and surface features. This comparison revealed that the predictions were in good accordance with the ground measurements with absolute percent differences of less than 9.5%. It was concluded that the proposed methodology successfully facilitates the retrieval of EF using Landsat-7 ETM data and field observations over the study area.
Wind profiles, momentum fluxes and roughness lengths at Cabauw revisited
Verkaik, J.W. ; Holtslag, A.A.M. - \ 2007
Boundary-Layer Meteorology 122 (2007)3. - ISSN 0006-8314 - p. 701 - 719.
grenslaagmeteorologie - windsnelheid - fluctuaties - experimenten - boundary-layer meteorology - wind speed - fluctuations - experiments - atmospheric boundary-layer - surface-layer - complex terrain - land-surface - parameterization - similarity - turbulence - footprint - location - exchange
We describe the results of an experiment focusing on wind speed and momentum fluxes in the atmospheric boundary layer up to 200 m. The measurements were conducted in 1996 at the Cabauw site in the Netherlands. Momentum fluxes are measured using the K-Gill Propeller Vane. Estimates of the roughness length are derived using various techniques from the wind speed and flux measurements, and the observed differences are explained by considering the source area of the meteorological parameters. A clear rough-to-smooth transition is found in the wind speed profiles at Cabauw. The internal boundary layer reaches the lowest k-vane (20 m) only in the south-west direction where the obstacle-free fetch is about 2 km. The internal boundary layer is also reflected in the roughness lengths derived from the wind speed profiles. The lower part of the profile (<40 m) is not in equilibrium and no reliable roughness analysis can be given. The upper part of the profile can be linked to a large-scale roughness length. Roughness lengths derived from the horizontal wind speed variance and gustiness have large footprints and therefore represent a large-scale average roughness. The drag coefficient is more locally determined but still represents a large-scale roughness length when it is measured above the local internal boundary layer. The roughness length at inhomogeneous sites can therefore be determined best from drag coefficient measurements just above the local internal boundary layers directly, or indirectly from horizontal wind speed variance or gustiness. In addition, the momentum and heat fluxes along the tower are analysed and these show significant variation with height related to stability and possibly surface heterogeneity. It appears that the dimensionless wind speed gradients scale well with local fluxes for the variety of conditions considered, including the unstable cases.
Technical Note: An implementation of the dry removal processesDRY DEPosition and SEDImentation in the Modular EarthSubmodel System (MESSy)
Kerkweg, A. ; Buchholz, J. ; Ganzeveld, L.N. ; Pozzer, A. ; Tost, H. ; Jöckel, P. - \ 2006
Atmospheric Chemistry and Physics 6 (2006)12. - ISSN 1680-7316 - p. 4617 - 4632.
general-circulation model - parameterization - chemistry
We present the submodels DRYDEP and SEDI for the Modular Earth Submodel System (MESSy). Dry deposition of gases and aerosols is calculated within DRYDEP, whereas SEDI deals with aerosol particle sedimentation. Dry deposition velocities depend on the near-surface turbulence and the physical and chemical properties of the surface cover (e.g. the roughness length, soil pH or leaf stomatal exchange). The dry deposition algorithm used in DRYDEP is based on the big leaf approach and is described in detail within this Technical Note. The sedimentation submodel SEDI contains two sedimentation schemes: a simple upwind zeroth order scheme and a first order approach
Evaluation of Peroxide Exchanges over a Coniferous Forest in a Single-Column Chemistry-Climate Model
Ganzeveld, L.N. ; Valverde-Canossa, J. ; Moortgat, G. ; Steinbrecher, R. - \ 2006
Atmospheric Environment 40 (2006)Supp.1. - ISSN 1352-2310 - p. S68 - S80.
reactive trace gases - general-circulation model - red spruce needles - dry deposition - hydrogen-peroxide - flux experiments - ozone - nox - parameterization - emissions
A single-column chemistry-climate model has been applied to evaluate peroxide exchanges measured over a coniferous forest during the BEWA2000 field campaign, July¿August 2001. Simulations indicate that for suppressed nocturnal turbulent mixing, the H2O2 mixing ratios are sensitive to the representation of sources and sinks, e.g., non-stomatal uptake and chemical transformations, the latter tightly linked to atmosphere¿biosphere NOx exchanges through its control on HO2 production. Comparison of observed and simulated H2O2 fluxes suggests that the commonly applied method to estimate uptake resistances results in a significant underestimation of the dry deposition flux. By using a very small surface uptake resistance, as observed, the modeled surface fluxes are still too low due to an underestimation of the simulated turbulent transport. Further, a reasonable agreement between simulated and observed methylhydroperoxide and hydroxymethylhydroperoxide mixing ratios in and above the canopy air is observed. Our analysis indicates the important role of daytime as well as nocturnal turbulent exchanges, which control the efficiency of dry deposition and downward transport of peroxides that are chemically produced higher up in the boundary layer. In turn, this chemical production depends on the upward transport of emitted precursor gases and their oxidization products. This demonstrates that improved simulations of atmosphere¿biosphere peroxide exchanges rely heavily on improved model representations of boundary layer and canopy turbulent exchanges.
The sensitivity of aerosol in Europe to two different emission inventories and temporal distribution of emissions
Meij, A. de; Krol, M.C. ; Dentener, F. ; Vignati, E. ; Cuvelier, C. ; Thunis, P. - \ 2006
Atmospheric Chemistry and Physics 6 (2006)12. - ISSN 1680-7316 - p. 4287 - 4309.
anthropogenic sulfate aerosols - sea-salt aerosol - zoom model tm5 - optical-properties - transport - chemistry - simulation - ozone - parameterization - climate
The sensitivity to two different emission inventories, injection altitude and temporal variations of anthropogenic emissions in aerosol modelling is studied, using the two way nested global transport chemistry model TM5 focussing on Europe in June and December 2000. The simulations of gas and aerosol concentrations and aerosol optical depth (AOD) with the EMEP and AEROCOM emission inventories are compared with EMEP gas and aerosol surface based measurements, AERONET sun photometers retrievals and MODIS satellite data. For the aerosol precursor gases SO2 and NOx in both months the model results calculated with the EMEP inventory agree better ( overestimated by a factor 1.3 for both SO2 and NOx) with the EMEP measurements than the simulation with the AEROCOM inventory ( overestimated by a factor 2.4 and 1.9, respectively). Besides the differences in total emissions between the two inventories, an important role is also played by the vertical distribution of SO2 and NOx emissions in understanding the differences between the EMEP and AEROCOM inventories. In December NOx and SO2 from both simulations agree within 50% with observations. In June SO4= evaluated with the EMEP emission inventory agrees slightly better with surface observations than the AEROCOM simulation, whereas in December the use of both inventories results in an underestimate of SO4 with a factor 2. Nitrate aerosol measured in summer is not reliable, however in December nitrate aerosol calculations with the EMEP and AEROCOM emissions agree with 30%, and 60%, respectively with the filter measurements. Differences are caused by the total emissions and the temporal distribution of the aerosol precursor gases NOx and NH3. Despite these differences, we show that the column integrated AOD is less sensitive to the underlying emission inventories. Calculated AOD values with both emission inventories underestimate the observed AERONET AOD values by 20 - 30%, whereas a case study using MODIS data shows a high spatial agreement. Our evaluation of the role of temporal distribution of anthropogenic emissions on aerosol calculations shows that the daily and weekly temporal distributions of the emissions are only important for NOx, NH3 and aerosol nitrate. However, for all aerosol species SO4=, NH4+, POM, BC, as well as for AOD, the seasonal temporal variations used in the emission inventory are important. Our study shows the value of including at least seasonal information on anthropogenic emissions, although from a comparison with a range of measurements it is often difficult to firmly identify the superiority of specific emission inventories, since other modelling uncertainties, e. g. related to transport, aerosol removal, water uptake, and model resolution, play a dominant role.
Representing Sheared Convective Boundary Layer by Zeroth- and First-Order-Jump Mixed-Layer Models: Large-Eddy Simulation Verification
Pino, D. ; Vilà-Guerau de Arellano, J. ; Kim, S.W. - \ 2006
Journal of Applied Meteorology and Climatology 45 (2006)9. - ISSN 1558-8424 - p. 1224 - 1243.
entrainment zone - wind shear - cumulus convection - kinetic-energy - part ii - inversion - growth - dynamics - parameterization - budget
Dry convective boundary layers characterized by a significant wind shear on the surface and at the inversion are studied by means of the mixed-layer theory. Two different representations of the entrainment zone, each of which has a different closure of the entrainment heat flux, are considered. The simpler of the two is based on a sharp discontinuity at the inversion (zeroth-order jump), whereas the second one prescribes a finite depth of the inversion zone (first-order jump). Large-eddy simulation data are used to provide the initial conditions for the mixed-layer models, and to verify their results. Two different atmospheric boundary layers with different stratification in the free atmosphere are analyzed. It is shown that, despite the simplicity of the zeroth-order-jump model, it provides similar results to the first-order-jump model and can reproduce the evolution of the mixed-layer variables obtained by the large-eddy simulations in sheared convective boundary layers. The mixed-layer model with both closures compares better with the large-eddy simulation results in the atmospheric boundary layer characterized by a moderate wind shear and a weak temperature inversion. These results can be used to represent the flux of momentum, heat, and other scalars at the entrainment zone in general circulation or chemistry transport models.
Modelling the Arctic Stable boundary layer and its coupling to the surface
Steeneveld, G.J. ; Wiel, B.J.H. van de; Holtslag, A.A.M. - \ 2006
Boundary-Layer Meteorology 118 (2006)2. - ISSN 0006-8314 - p. 357 - 378.
large-eddy simulation - intermittent turbulence - land-surface - atmospheric models - energy-balance - parameterization - fluxes - oscillations - temperature - validation
The impact of coupling the atmosphere to the surface energy balance is examined for the stable boundary layer, as an extension of the first GABLS (GEWEX Atmospheric Boundary-Layer Study) one-dimensional model intercomparison. This coupling is of major importance for the stable boundary-layer structure and its development because coupling enables a realistic physical description of the interdependence of the surface temperature and the surface sensible heat flux. In the present case, the incorporation of a surface energy budget results in stronger cooling (surface decoupling), and a more stable and less deep boundary layer. The proper representation of this is a problematic feature in large-scale numerical weather prediction and climate models. To account for the upward heat flux from the ice surface beneath, we solve the diffusion equation for heat in the underlying ice as a first alternative. In that case, we find a clear impact of the vertical resolution in the underlying ice on boundary-layer development: coarse vertical resolution in the ice results in stronger surface cooling than for fine resolution. Therefore, because of this impact on stable boundary-layer development, the discretization in the underlying medium needs special attention in numerical modelling studies of the nighttime boundary layer. As a second alternative, a bulk conductance layer with stagnant air near the surface is added. The stable boundary-layer development appears to depend heavily on the bulk conductance of the stagnant air layer. This result re-emphasizes the fact that the interaction with the surface needs special attention in stable boundary-layer studies. Furthermore, we perform sensitivity studies to atmospheric resolution, the length-scale formulation and the impact of radiation divergence on stable boundary-layer structure for weak windy conditions
Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer
Cuxart, J. ; Holtslag, A.A.M. ; Beare, R.J. ; Bazile, E. ; Beljaars, A. ; Cheng, A. ; Conangla, L. ; Ek, M.B. ; Freedman, F. ; Hamdi, R. ; Kerstein, A. ; Kitagawa, H. ; Lenderink, G. ; Lewellen, D. ; Mailhot, J. ; Mauritsen, T. ; Perov, V. ; Schayes, G. ; Steeneveld, G.J. ; Svensson, G. ; Taylor, P. ; Weng, W. ; Wunsch, S. ; Xu, K.M. - \ 2006
Boundary-Layer Meteorology 118 (2006)2. - ISSN 0006-8314 - p. 273 - 303.
turbulence closure-model - large-eddy simulation - part i - scheme - parameterization - formulation - diffusion - dynamics - surfaces - system
The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercomparison made for the same case by eleven different models are used as a guiding reference. The single-column parameterizations include research and operational schemes from major forecast and climate research centres. Results from first-order schemes, a large number of turbulence kinetic energy closures, and other models were used. There is a large spread in the results; in general, the operational schemes mix over a deeper layer than the research schemes, and the turbulence kinetic energy and other higher-order closures give results closer to the statistics obtained from the large-eddy simulations. The sensitivities of the schemes to the parameters of their turbulence closures are partially explored
Simulating of leaf wetness duration within a potato canopy
Jacobs, A.F.G. ; Heusinkveld, B.G. ; Kessel, G.J.T. - \ 2005
NJAS Wageningen Journal of Life Sciences 53 (2005)2. - ISSN 1573-5214 - p. 151 - 166.
gewasteelt - aardappelen - gewasbescherming - fungiciden - meteorologie - micrometeorologie - meteorologische factoren - neerslag - weer - dauw - relatieve vochtigheid - pesticiden - toepassing - toepassingsdatum - simulatie - modellen - simulatiemodellen - phytophthora - phytophthora infestans - crop management - potatoes - plant protection - fungicides - meteorology - micrometeorology - meteorological factors - precipitation - weather - dew - relative humidity - pesticides - application - application date - simulation - models - simulation models - phytophthora - phytophthora infestans - estimating dew duration - meteorological models - parameterization - field
A leaf wetness duration experiment was carried out in a potato field in the centre of the Netherlands during the growing season of 2003. A within-canopy dew simulation model was applied to simulate leaf wetness distribution in the canopy caused by dew and rainfall. The dew model is an extension of an earlier-developed energy budget model, distinguishing three layers within the potato canopy. To run the dew model successfully, information on the above-canopy wind speed, air temperature, humidity and net radiation as well as the within-canopy temperature and humidity must be available. In most cases leaf wetting starts in the top layer followed by the centre and the bottom layer, in that order. Leaf drying shortly after sunrise takes place in the same order. Leaf wetness lasted longest in the bottom layer. Rainfall was accounted for by applying an interception model. The results of the dew model agreed well with leaf wetness recorded with a resistance grid
Transport and chemical transformations influenced by shallow cumulus over land
Vilà-Guerau de Arellano, J. ; Kim, S.W. ; Barth, M.C. ; Patton, E.G. - \ 2005
Atmospheric Chemistry and Physics 5 (2005). - ISSN 1680-7316 - p. 3219 - 3231.
large-eddy-simulation - convective boundary-layer - cloud - turbulence - ozone - model - flux - photodissociation - parameterization - atmosphere
The distribution and evolution of reactive species in a boundary layer characterized by the presence of shallow cumulus over land is studied by means of two large-eddy simulation models: the NCAR and WUR codes. The study focuses on two physical processes that can influence the chemistry: the enhancement of the vertical transport by the buoyant convection associated with cloud formation and the perturbation of the photolysis rates below, in and above the clouds. It is shown that the dilution of the reactant mixing ratio caused by the deepening of the atmospheric boundary layer is an important process and that it can decrease reactant mixing ratios by 10 to 50 percent compared to very similar conditions but with no cloud formation. Additionally, clouds transport chemical species to higher elevations in the boundary layer compared to the case with no clouds which influences the reactant mixing ratios of the nocturnal residual layers following the collapse of the daytime boundary layer. Estimates of the rate of reactant transport based on the calculation of the integrated flux divergence range from to ¿0.2 ppb hr-1 to ¿1 ppb hr-1, indicating a net loss of sub-cloud layer air transported into the cloud layer. A comparison of this flux to a parameterized mass flux shows good agreement in mid-cloud, but at cloud base the parameterization underestimates the mass flux. Scattering of radiation by cloud drops perturbs photolysis rates. It is found that these perturbed photolysis rates substantially (10¿40%) affect mixing ratios locally (spatially and temporally), but have little effect on mixing ratios averaged over space and time. We find that the ultraviolet radiance perturbation becomes more important for chemical transformations that react with a similar order time scale as the turbulent transport in clouds. Finally, the detailed intercomparison of the LES results shows very good agreement between the two codes when considering the evolution of the reactant mean, flux and (co-)variance vertical profiles.
Quantification of the dry aeolian deposition of dust on horizontal surfaces: an experimental comparison of theory and measurements
Goossens, D. - \ 2005
Sedimentology 52 (2005)4. - ISSN 0037-0746 - p. 859 - 873.
wind-tunnel - particles - accumulation - velocity - gauge - flux - parameterization - sedimentation - resuspension - performance
Eight techniques to quantify the deposition of aeolian dust on horizontal surfaces were tested in a wind tunnel. The tests included three theoretical techniques and five measurement techniques. The theoretical techniques investigated were: the gradient technique, the inferential technique without grain-shape correction, and the inferential technique corrected for grain shape. The measuring techniques included the following surrogate surfaces: a water surface, a glass surface, a metal surface, a vertical array of metal plates, and an inverted frisbee filled with glass marbles. The efficiency of the techniques was investigated for the sediment as a whole (all grain sizes together) as well as for a large number of grain sizes extending from 1 to 104 m. The surrogate surfaces showed more or less comparable catch efficiencies, although the water surface nearly always caught the highest quantities of dust and the marble-filled frisbee and the vertical array of metal plates the lowest quantities of dust. The dust fluxes calculated by theoretical methods were markedly different from those obtained by direct measurements. The fluxes calculated by the inferential technique approximated those of the direct measurements only for grain sizes between 30 and 40 m. For smaller and coarser grains, deviations from the measured fluxes were high. The gradient method, in its turn, provided extremely low calculated fluxes for grains in all size classes investigated. The latter technique was not considered very reliable for the dust used in the tests
Impact of Amazonian deforestation on atmospheric chemistry
Ganzeveld, L.N. ; Lelieveld, J. - \ 2004
Geophysical Research Letters 31 (2004). - ISSN 0094-8276 - 5 p.
general-circulation model - dry deposition - emissions - parameterization - ozone
A single-column chemistry and climate model has been used to study the impact of deforestation in the Amazon Basin on atmospheric chemistry. Over deforested areas, daytime ozone deposition generally decreases strongly except when surface wetness decreases through reduced precipitation, whereas nocturnal soil deposition increases. The isoprene and soil nitric oxide emissions decrease although nitrogen oxide release to the atmosphere increases due to reduced canopy deposition. Deforestation also affects vertical transport causing substantial ozone and hydroxyl changes, also depending on soil moisture. The analysis shows that assessment of the impact of land cover and land use changes on atmospheric chemistry requires the development of explicitly coupled chemistry and meteorological models including surface trace gas exchanges, micro-meteorology and the hydrological cycle
An updated length-scale formulation for turbulent mixing in clear and cloudy boundary layers
Lenderink, G. ; Holtslag, A.A.M. - \ 2004
Quarterly Journal of the Royal Meteorological Society 130 (2004)604. - ISSN 0035-9009 - p. 3405 - 3427.
grenslaagmeteorologie - wolken - turbulentie - kinetische energie - boundary-layer meteorology - clouds - turbulence - kinetic energy - shallow cumulus convection - large-eddy simulation - atmospheric models - parameterization - scheme - stratocumulus - fluxes - transport - impacts - wind
A new mixing-length scale is presented for turbulence-closure schemes, with special emphasis on neutral-to-convective conditions in clear and cloudy boundary layers. The length scale is intended for a prognostic turbulent-kinetic-energy closure. It is argued that present-day length-scale formulations may easily fail in one of two limiting situations. Schemes based on a local stability measure (e.g.the Richardson number) display unrealistic behaviour and instabilities in the convective limit. This strongly limits the representation of mixing in cloudy boundary layers. On the other hand, it is shown that non-local parcel methods may misrepresent mixing near the surface. The new length-scale formulation combines local and non-local stability in a new way; it uses vertical integrals over the stability (the Richardson number) in a simple 'parcel' framework. The length scale matches with surface-layer similarity for near-neutral conditions and displays a realistic convective limit. The use of the length-scale formulation can be extended easily to cloudy boundary layers. The scheme is numerically stable and computationally cheap. The behaviour of the length scale is evaluated in a single-column model (SCM) and in a high-resolution limited-area model (LAM). The SCM shows good behaviour in three cases with and without boundary-layer clouds. The prediction of the near-surface wind and temperature in the LAM compares favourably with tower measurements at Cabauw (the Netherlands).
An evaluation of mass flux closures for diurnal cycles of shallow cumulus
Neggers, R.A.J. ; Siebesma, A.P. ; Lenderink, G. ; Holtslag, A.A.M. - \ 2004
Monthly Weather Review 132 (2004)11. - ISSN 0027-0644 - p. 2525 - 2538.
large-eddy simulation - convective adjustment scheme - planetary boundary-layer - atmospheric radiation - cloud - parameterization - subcloud - model - heat - validation
Three closure methods for the mass flux at cloud base in shallow cumulus convection are critically examined for the difficult case of a diurnal cycle over land. The closure methods are first evaluated against large-eddy simulations (LESs) by diagnosing all parameters appearing in the closure equations during simulations of two different observed diurnal cycles of shallow cumulus. This reveals the characteristic behavior of each closure mechanism purely as a result of its core structure. With these results in hand the impact of each closure on the development of the cloudy boundary layer is then studied by its implementation in an offline single-column model of a regional atmospheric climate model. The LES results show that the boundary layer quasi-equilibrium closure typically overestimates the cloud-base mass flux after cloud onset, due to the neglect of significant moisture and temperature tendencies in the subcloud layer. The convective available potential energy (CAPE) adjustment closure is compromised by its limitation to compensating subsidence as the only CAPE breakdown mechanism and the use of a constant adjustment time scale. The closure method using the subcloud convective vertical velocity scale gives the best results, as it catches the time development of the cloud-base mass flux as diagnosed in LES.
Analyis of the role of the planetary boundary layer schemes during a severe convective storm
Wisse, J.S.P. ; Vilà-Guerau de Arellano, J. - \ 2004
Annales Geophysicae-Atmospheres Hydrospheres and Space Sciences 22 (2004)6. - ISSN 0992-7689 - p. 1861 - 1874.
numerical-simulation - model - sensitivity - parameterization - precipitation - evaporation - catalonia - flood - event - tests
The role played by planetary boundary layer (PBL) in the development and evolution of a severe convective storm is studied by means of meso-scale modeling and surface and upper air observations. The severe convective precipitation event that occurred on 14 September 1999 in the northeast of the Iberian Peninsula was simulated by means of the mesoscale model MM5 (version 3) using three different PBL schemes. The numerical results show a large impact of the PBL schemes on the precipitation fields associated to the convective storm. The schemes are based on different physical assumptions: the nonlocal first order Medium-Range Forecast (MRF) and Blackadar (BLA) scheme and the local, one-and-a-half order ETA scheme. Surface and radar observations are used to validate the model results. The comparison focuses on three aspects: the evolution, the spatial distribution and the 24-h accumulated precipitation. The comparison with rain gauge observations shows that the MRF, BLA and ETA schemes predicted most of the precipitation during the morning, whereas the rain gauge stations recorded rainfall during the evening. The evaluation performed with the radar data shows that all three numerical simulations produced a realistic spatial accumulated precipitation distribution. According to the quantity distribution, all three numerical simulations were able to predict precipitation quantities comparable to the rain gauge measurements. The MRF scheme predicted the largest average accumulated precipitation and the largest average precipitation rate, whereas the ETA scheme predicted the smallest accumulated precipitation and average precipitation rate. However, the ETA scheme yielded the highest extreme precipitation rates. The performance of the three schemes is analyzed in terms of the vertical distribution of potential temperature, specific humidity and conserved variables, like equivalent potential temperature and total water content. The MRF scheme showed more evidence of enhanced mixing than did the other schemes. Due to this process, more moisture was more efficiently transported to the free atmosphere. Consequently, the MRF scheme predicts more widespread precipitation. Furthermore, the enhanced mixing led to a less sharp capping inversion. However, the stronger inversion resulting from suppressed mixing processes in the case of the ETA scheme yielded higher values of convective available potential energy (CAPE) than did the other two schemes. Consequently, the more extreme precipitation rates are simulated by MM5 when the ETA scheme is used.
Entrainment process of carbon dioxide in the atmospheric boundary layer
Vilà-Guerau de Arellano, J. ; Gioli, B. ; Miglietta, F. ; Jonker, H.J.J. ; Klein Baltink, H. ; Hutjes, R.W.A. ; Holtslag, A.A.M. - \ 2004
Journal of Geophysical Research: Atmospheres 109 (2004). - ISSN 2169-897X - p. D18110 - D18110.
large-eddy simulation - sensible heat - flux measurements - aircraft - turbulence - model - parameterization - moisture - budget - latent
Aircraft and surface measurements of turbulent thermodynamic variables and carbon dioxide (CO2) were taken above a grassland in a convective atmospheric boundary layer. The observations were analyzed to assess the importance of the entrainment process for the distribution and evolution of carbon dioxide in the boundary layer. From the observations we were able to estimate the vertical profiles of the fluxes, the correlation coefficients, and the skewness of the virtual potential temperature, the specific humidity, and the carbon dioxide. These profiles indicate that important entrainment events occurred during the observed period. The data were also used to estimate the budgets for heat, moisture, and carbon dioxide. By studying this observational data we find that the entrainment of air parcels containing lower concentrations of water vapor and carbon dioxide significantly dries and dilutes the concentration of these two constituents in the boundary layer. This process is particularly important in the morning hours which are characterized by a rapidly growing boundary layer. The observations show that the CO2 concentration in the boundary layer is reduced much more effectively by the ventilation with entrained air than by CO2 uptake by the vegetation. We quantify this effect by calculating the ratio of the entrainment flux of CO2 to the surface flux of CO2(beta(c)=-((wc) over bar)(e)/((wc) over bar)(o)). A value of beta(c) equal to 2.9 is estimated at around 1300 UTC from the vertical profile of the carbon dioxide flux. We corroborate this observational evidence by reproducing the observed situation using a mixed layer model. The mixed layer model also yields the variation in time of beta(c). During the morning the ventilation process is more important than the CO2 uptake by the vegetation (beta(c)>1), whereas in the afternoon the assimilation by grass at the surface becomes the dominant process (beta(c) <1). This research points out the relevance of the entrainment process on the budget of carbon dioxide in the lower troposphere and the relevance of boundary layer dynamics in controlling the diurnal variation of carbon dioxide.
The microphysical structure of extreme precipitation as inferred from ground-based raindrop spectra
Uijlenhoet, R. ; Smith, J.A. ; Steiner, M. - \ 2003
Journal of the Atmospheric Sciences 60 (2003). - ISSN 0022-4928 - p. 1220 - 1238.
drop-size distributions - radar reflectivity - catastrophic rainfall - convective clouds - conceptual-model - tropical rain - stratiform - equilibrium - disdrometer - parameterization
The controls on the variability of raindrop size distributions in extreme rainfall and the associated radar reflectivity-rain rate relationships are studied using a scaling-law formalism for the description of raindrop size distributions and their properties. This scaling-law formalism enables a separation of the effects of changes in the scale of the raindrop size distribution from those in its shape. Parameters controlling the scale and shape of the scaled raindrop size distribution may be related to the microphysical processes generating extreme rainfall. A global scaling analysis of raindrop size distributions corresponding to rain rates exceeding 100 mm h(-1), collected during the 1950s with the Illinois State Water Survey raindrop camera in Miami, Florida, reveals that extreme rain rates tend to be associated with conditions in which the variability of the raindrop size distribution is strongly number controlled (i.e., characteristic drop sizes are roughly constant). This means that changes in properties of raindrop size distributions in extreme rainfall are largely produced by varying raindrop concentrations. As a result, rainfall integral variables (such as radar reflectivity and rain rate) are roughly proportional to each other, which is consistent with the concept of the so-called equilibrium raindrop size distribution and has profound implications for radar measurement of extreme rainfall. A time series analysis for two contrasting extreme rainfall events supports the hypothesis that the variability of raindrop size distributions for extreme rain rates is strongly number controlled. However, this analysis also reveals that the actual shapes of the (measured and scaled) spectra may differ significantly from storm to storm. This implies that the exponents of power-law radar reflectivity-rain rate relationships may be similar, and close to unity, for different extreme rainfall events, but their prefactors may differ substantially. Consequently, there is no unique radar reflectivity-rain rate relationship for extreme rain rates, but the variability is essentially reduced to one free parameter (i.e., the prefactor). It is suggested that this free parameter may be estimated on the basis of differential reflectivity measurements in extreme rainfall.