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- Author or Editor: A. C. M. Beljaars x
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Abstract
The theory by Rice for extreme value statistics is used to study the influence of filters in a wind measuring system on the measured gusts. An extension is made to discretely sampled data. Model results are compared with strong wind data from the Cabauw tower. On the basis of this theory a definition is proposed for the duration of gusts. Also a data reduction scheme for standard synoptic and climatological stations is proposed. Such a standardization would enhance the applicability of wind gust climatology.
Abstract
The theory by Rice for extreme value statistics is used to study the influence of filters in a wind measuring system on the measured gusts. An extension is made to discretely sampled data. Model results are compared with strong wind data from the Cabauw tower. On the basis of this theory a definition is proposed for the duration of gusts. Also a data reduction scheme for standard synoptic and climatological stations is proposed. Such a standardization would enhance the applicability of wind gust climatology.
Abstract
In this paper a summary is given of observations and modeling efforts on surface fluxes, carried out at Cabauw in The Netherlands and during MESOGERS-84 in the south of France. Emphasis is put on those aspects that are important from a modeling point of view, e.g., surface roughness lengths for momentum and heat, stomatal resistance for evaporation, and related quantities. Special attention is paid to the problem of subgrid surface inhomogeneities up to horizontal scales of a few kilometers. A qualitative explanation is given for the apparent low values of the roughness length for heat. Simple flux parameterizations are compared with observations, and empirical closure functions are proposed to model the transfer coefficients between the surface and the first model layer.
Abstract
In this paper a summary is given of observations and modeling efforts on surface fluxes, carried out at Cabauw in The Netherlands and during MESOGERS-84 in the south of France. Emphasis is put on those aspects that are important from a modeling point of view, e.g., surface roughness lengths for momentum and heat, stomatal resistance for evaporation, and related quantities. Special attention is paid to the problem of subgrid surface inhomogeneities up to horizontal scales of a few kilometers. A qualitative explanation is given for the apparent low values of the roughness length for heat. Simple flux parameterizations are compared with observations, and empirical closure functions are proposed to model the transfer coefficients between the surface and the first model layer.
Abstract
Stimulated by the results of a simple SST anomaly experiment with the ECMWF forecast model, a study was carried out to examine the model parameterization of evaporation from the tropica] oceans. In earlier versions of the model, these fluxes were parameterized with neutral transfer coefficients in accordance with the Charnock relation with equal coefficients for momentum, heat, and moisture. Stability correction was applied using Monin-Obukhov theory. This parameterization resulted in an extremely weak coupling between atmosphere and ocean at wind speeds below 5 m s−1. The transfer coefficients for heat and moisture have now been modified for low wind speeds to bring them in accordance with the empirical scaling law for free convedion. It is shown that these revisions to the transfer coefficients at very low wind speeds (<5 m s1) have a dramatic positive impact on almost all aspects of the model's simulation of the tropics. These include much improved seasonal rainfall distributions (with the virtual elimination of a tendency to generate a double ITCZ in both winter and summer), a much improved Indian monsoon circulation, and substantially reduced tropical systematic errors. The model previously had an eagerly bias in the zonal-mean upper tropical tropospheric flow with a corresponding cold bias in the deep tropics; it is shown that the flux revision substantially reduces this. Furthermore, the revision to the fluxes greatly enhances the model's ability to represent interannual and intraseasonal variability (see also the companion paper by Palmer et al.).
Abstract
Stimulated by the results of a simple SST anomaly experiment with the ECMWF forecast model, a study was carried out to examine the model parameterization of evaporation from the tropica] oceans. In earlier versions of the model, these fluxes were parameterized with neutral transfer coefficients in accordance with the Charnock relation with equal coefficients for momentum, heat, and moisture. Stability correction was applied using Monin-Obukhov theory. This parameterization resulted in an extremely weak coupling between atmosphere and ocean at wind speeds below 5 m s−1. The transfer coefficients for heat and moisture have now been modified for low wind speeds to bring them in accordance with the empirical scaling law for free convedion. It is shown that these revisions to the transfer coefficients at very low wind speeds (<5 m s1) have a dramatic positive impact on almost all aspects of the model's simulation of the tropics. These include much improved seasonal rainfall distributions (with the virtual elimination of a tendency to generate a double ITCZ in both winter and summer), a much improved Indian monsoon circulation, and substantially reduced tropical systematic errors. The model previously had an eagerly bias in the zonal-mean upper tropical tropospheric flow with a corresponding cold bias in the deep tropics; it is shown that the flux revision substantially reduces this. Furthermore, the revision to the fluxes greatly enhances the model's ability to represent interannual and intraseasonal variability (see also the companion paper by Palmer et al.).
Abstract
This paper discusses the results of a surface-layer experiment near the Cabauw meteorological mast. We measured momentum, heat and moisture fluxes at two heights, namely, 3.5 and 22.5 m. The measurements also include the mean wind speed and mean temperature profiles. The purpose was to investigate surface-layer similarity laws under nonideal fetch conditions. We found that under such conditions, the shell stress increases with height because of obstacles upstream. As a consequence flux-profile relationships differ from those over uniform terrain. It is shown that these deviations imply a slow relaxation in the exchange coefficient for heat and momentum over a terrain with changing surface roughness. Furthermore, we found that horizontal velocity fluctuations scale on a friction velocity representative of a large area. On the other hand, vertical velocity fluctuations scale on the local friction velocity.
Abstract
This paper discusses the results of a surface-layer experiment near the Cabauw meteorological mast. We measured momentum, heat and moisture fluxes at two heights, namely, 3.5 and 22.5 m. The measurements also include the mean wind speed and mean temperature profiles. The purpose was to investigate surface-layer similarity laws under nonideal fetch conditions. We found that under such conditions, the shell stress increases with height because of obstacles upstream. As a consequence flux-profile relationships differ from those over uniform terrain. It is shown that these deviations imply a slow relaxation in the exchange coefficient for heat and momentum over a terrain with changing surface roughness. Furthermore, we found that horizontal velocity fluctuations scale on a friction velocity representative of a large area. On the other hand, vertical velocity fluctuations scale on the local friction velocity.
Abstract
This study considers the question of what is the least complex bulk mass flux framework that can still conceptually reproduce the smoothly varying coupling between the shallow convective cloud layer and the subcloud mixed layer. To this end, the model complexity of the classic single bulk mass flux scheme is enhanced. Inspired by recent large-eddy simulation results, the authors argue that two relatively minor but key conceptual modifications are already sufficient to achieve this goal: (i) retaining a dry transporting updraft in the moist limit and (ii) applying continuous updraft area partitioning to this dual mass flux (DualM) framework. The dry updraft represents all internal mixed layer updrafts that terminate near the mixed layer top, whereas the moist updraft represents all updrafts that condense and rise out of the mixed layer as buoyant cumulus clouds. The continuous area partitioning between the dry and moist updraft is a function of moist convective inhibition above the mixed layer top. Updraft initialization is a function of the updraft area fraction and is therefore consistent with the updraft definition. It is argued that the model complexity thus enhanced is sufficient to allow reproduction of various phenomena involved in the cloud–subcloud coupling, namely (i) dry countergradient transport within the mixed layer that is independent of the moist updraft, (ii) soft triggering of moist convective flux throughout the boundary layer, and (iii) a smooth response to smoothly varying forcings, including the reproduction of gradual transitions to and from shallow cumulus convection.
The DualM framework is evaluated by implementing in the Eddy Diffusivity Mass Flux (EDMF) boundary layer scheme of the ECMWF’s Integrated Forecasting System. Single column model experiments are evaluated against large-eddy simulation results for a range of different cases that span a broad parameter space of cloud–subcloud coupling intensities. The results illustrate that also in numerical practice the DualM framework can reproduce gradual transitions to and from shallow cumulus convection. Model behavior is further explored through experiments in which model complexity is purposely reduced, thus mimicking a single bulk updraft setup. This gives more insight into the new model-internal interactions and explains the obtained case results.
Abstract
This study considers the question of what is the least complex bulk mass flux framework that can still conceptually reproduce the smoothly varying coupling between the shallow convective cloud layer and the subcloud mixed layer. To this end, the model complexity of the classic single bulk mass flux scheme is enhanced. Inspired by recent large-eddy simulation results, the authors argue that two relatively minor but key conceptual modifications are already sufficient to achieve this goal: (i) retaining a dry transporting updraft in the moist limit and (ii) applying continuous updraft area partitioning to this dual mass flux (DualM) framework. The dry updraft represents all internal mixed layer updrafts that terminate near the mixed layer top, whereas the moist updraft represents all updrafts that condense and rise out of the mixed layer as buoyant cumulus clouds. The continuous area partitioning between the dry and moist updraft is a function of moist convective inhibition above the mixed layer top. Updraft initialization is a function of the updraft area fraction and is therefore consistent with the updraft definition. It is argued that the model complexity thus enhanced is sufficient to allow reproduction of various phenomena involved in the cloud–subcloud coupling, namely (i) dry countergradient transport within the mixed layer that is independent of the moist updraft, (ii) soft triggering of moist convective flux throughout the boundary layer, and (iii) a smooth response to smoothly varying forcings, including the reproduction of gradual transitions to and from shallow cumulus convection.
The DualM framework is evaluated by implementing in the Eddy Diffusivity Mass Flux (EDMF) boundary layer scheme of the ECMWF’s Integrated Forecasting System. Single column model experiments are evaluated against large-eddy simulation results for a range of different cases that span a broad parameter space of cloud–subcloud coupling intensities. The results illustrate that also in numerical practice the DualM framework can reproduce gradual transitions to and from shallow cumulus convection. Model behavior is further explored through experiments in which model complexity is purposely reduced, thus mimicking a single bulk updraft setup. This gives more insight into the new model-internal interactions and explains the obtained case results.
Abstract
The Agulhas Current is the major western boundary current of the Southern Hemisphere. South of Africa it retroflects back into the southwest Indian Ocean, transporting relatively warm water into the midlatitudes. Large sensible and latent heat transfers from the Agulhas Current and its retroflection to the atmosphere occur throughout the year, but particularly during winter. This study suggests that the NCEP and ECMWF models tend to underestimate these fluxes because they are unable to adequately represent the air–sea fluxes over the warmest waters in the core of the current. This core is only 80–100 km wide and it is suggested that the SST data used by these models do not have fine enough spatial resolution to properly represent the Agulhas Current and its mesoscale variability.
Abstract
The Agulhas Current is the major western boundary current of the Southern Hemisphere. South of Africa it retroflects back into the southwest Indian Ocean, transporting relatively warm water into the midlatitudes. Large sensible and latent heat transfers from the Agulhas Current and its retroflection to the atmosphere occur throughout the year, but particularly during winter. This study suggests that the NCEP and ECMWF models tend to underestimate these fluxes because they are unable to adequately represent the air–sea fluxes over the warmest waters in the core of the current. This core is only 80–100 km wide and it is suggested that the SST data used by these models do not have fine enough spatial resolution to properly represent the Agulhas Current and its mesoscale variability.
Abstract
Two simplifying assumptions adopted in the current ECMWF surface scheme are explored: a uniform skin temperature for all grid-box fractions with variable latent heat release and a fixed value of an effective heat conductivity defining the soil heat flux density. This paper proposes relatively simple modifications of the ECMWF scheme with a better physical basis, without large input or computer infrastructure requirements.
A uniform skin temperature overestimates evaporation from relatively wet surface fractions when the other surface components are dry and warm. This is shown to be the case for an evaporating soil after rain and vegetation evaporation in a sparse Mediterranean vineyard canopy. Allowing different temperatures for each surface fraction significantly reduces the overestimations and introduces only little additional computation.
The default effective conductivity value (7 W m−2K−1) employed by the current ECMWF scheme is shown to be too low for the sparse vineyard canopy. By raising the conductivity to 17 W m−2 K−1 for the bare-soil part of the surface, the daytime simulated soil heat flux was improved considerably.
Abstract
Two simplifying assumptions adopted in the current ECMWF surface scheme are explored: a uniform skin temperature for all grid-box fractions with variable latent heat release and a fixed value of an effective heat conductivity defining the soil heat flux density. This paper proposes relatively simple modifications of the ECMWF scheme with a better physical basis, without large input or computer infrastructure requirements.
A uniform skin temperature overestimates evaporation from relatively wet surface fractions when the other surface components are dry and warm. This is shown to be the case for an evaporating soil after rain and vegetation evaporation in a sparse Mediterranean vineyard canopy. Allowing different temperatures for each surface fraction significantly reduces the overestimations and introduces only little additional computation.
The default effective conductivity value (7 W m−2K−1) employed by the current ECMWF scheme is shown to be too low for the sparse vineyard canopy. By raising the conductivity to 17 W m−2 K−1 for the bare-soil part of the surface, the daytime simulated soil heat flux was improved considerably.
Abstract
By forcing a third-generation wave-prediction model with surface stresses from the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric model, it was discovered that lower wave heights were generated than by forcing with the ECMWF surface winds. The apparent inconsistency between surface stresses and surface winds in the atmospheric model turns out to be time-step dependent. A similar conclusion may be inferred from results of the WAMDI group.
Apparently, a number of atmospheric models have inaccuracies in the boundary-layer scheme near the surface. In this paper it is argued that the reason for the inaccuracies is related to the numerical integration scheme that is used in these models. It is shown that a numerical scheme that treats physics and dynamics separately has an equilibrium that is time-step dependent. An alternative scheme—namely, simultaneous, implicit treatment of both physics and dynamics—removes this deficiency. Possible consequences for atmospheric-, wave-, and ocean-circulation models are briefly discussed.
Abstract
By forcing a third-generation wave-prediction model with surface stresses from the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric model, it was discovered that lower wave heights were generated than by forcing with the ECMWF surface winds. The apparent inconsistency between surface stresses and surface winds in the atmospheric model turns out to be time-step dependent. A similar conclusion may be inferred from results of the WAMDI group.
Apparently, a number of atmospheric models have inaccuracies in the boundary-layer scheme near the surface. In this paper it is argued that the reason for the inaccuracies is related to the numerical integration scheme that is used in these models. It is shown that a numerical scheme that treats physics and dynamics separately has an equilibrium that is time-step dependent. An alternative scheme—namely, simultaneous, implicit treatment of both physics and dynamics—removes this deficiency. Possible consequences for atmospheric-, wave-, and ocean-circulation models are briefly discussed.
The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.
The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2-m temperature and 10-m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, the authors review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art as well as findings and recommendations from three intercomparison studies held within the Global Energy and Water Exchanges (GEWEX) Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear-sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models, and large-eddy simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas, and Cabauw in the Netherlands. From these studies, we find that even for the noncloudy boundary layer important parameterization challenges remain.
Abstract
In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m−2 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (±10 W m−2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of30 W m−2 and 25 W m−2, respectively. Annual totals of evapotranspiration and runoff, into which the precipitation is partitioned, both have ranges of 315 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resistance under non-water-stressed conditions. Many schemes tend to underestimate latent heat flux and overestimate sensible heat flux in summer, with a reverse tendency in winter. For six schemes, root-mean-square deviations of predictions from monthly observations are less than the estimated upper bounds on observation errors (5 W m−2 for sensible heat flux and 10 W m−2 for latent heat flux). Actual runoff at the site is believed to be dominated by vertical drainage to groundwater, but several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of total pore volume) in the simulated annual mean root-zone soil moisture. Unfortunately, no measurements of soil moisture were available for model evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to explain the large variance in soil moisture. However, many of the extreme values of soil moisture could be explained in terms of the particulars of experimental setup or excessive evapotranspiration.
Abstract
In the Project for Intercomparison of Land-Surface Parameterization Schemes phase 2a experiment, meteorological data for the year 1987 from Cabauw, the Netherlands, were used as inputs to 23 land-surface flux schemes designed for use in climate and weather models. Schemes were evaluated by comparing their outputs with long-term measurements of surface sensible heat fluxes into the atmosphere and the ground, and of upward longwave radiation and total net radiative fluxes, and also comparing them with latent heat fluxes derived from a surface energy balance. Tuning of schemes by use of the observed flux data was not permitted. On an annual basis, the predicted surface radiative temperature exhibits a range of 2 K across schemes, consistent with the range of about 10 W m−2 in predicted surface net radiation. Most modeled values of monthly net radiation differ from the observations by less than the estimated maximum monthly observational error (±10 W m−2). However, modeled radiative surface temperature appears to have a systematic positive bias in most schemes; this might be explained by an error in assumed emissivity and by models’ neglect of canopy thermal heterogeneity. Annual means of sensible and latent heat fluxes, into which net radiation is partitioned, have ranges across schemes of30 W m−2 and 25 W m−2, respectively. Annual totals of evapotranspiration and runoff, into which the precipitation is partitioned, both have ranges of 315 mm. These ranges in annual heat and water fluxes were approximately halved upon exclusion of the three schemes that have no stomatal resistance under non-water-stressed conditions. Many schemes tend to underestimate latent heat flux and overestimate sensible heat flux in summer, with a reverse tendency in winter. For six schemes, root-mean-square deviations of predictions from monthly observations are less than the estimated upper bounds on observation errors (5 W m−2 for sensible heat flux and 10 W m−2 for latent heat flux). Actual runoff at the site is believed to be dominated by vertical drainage to groundwater, but several schemes produced significant amounts of runoff as overland flow or interflow. There is a range across schemes of 184 mm (40% of total pore volume) in the simulated annual mean root-zone soil moisture. Unfortunately, no measurements of soil moisture were available for model evaluation. A theoretical analysis suggested that differences in boundary conditions used in various schemes are not sufficient to explain the large variance in soil moisture. However, many of the extreme values of soil moisture could be explained in terms of the particulars of experimental setup or excessive evapotranspiration.
