To my knowledge the authors are the first to publish a spatial climatology of low-level jets over... more To my knowledge the authors are the first to publish a spatial climatology of low-level jets over the North Sea. This feat alone is a welcome initiative given the importance of this area for the wind energy sector, which will only increase in the coming decades. Moreover, the authors combine the spatial and temporal coverage of state-of-the-art reanalysis data with an extensive set of lidar observations from ten offshore platforms. Carefully addressing the pros and cons of both the model data and the observations, as well as properly discussing the difficulties in issues like LLJ detection, they provide a highly relevant and balanced overview of LLJ characteristics in the area of interest. C1
We describe and analyze the results of the third Global Energy and Water cycle Experiment (GEWEX)... more We describe and analyze the results of the third Global Energy and Water cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS) intercomparison and evaluation study for singlecolumn models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning time transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL. The diagrams include the results of a number of sensitivity runs performed with one ...
Numerical weather prediction and climate models continue to have large errors for stable boundary... more Numerical weather prediction and climate models continue to have large errors for stable boundary layers (SBL). To understand and to improve on this, so far three atmospheric boundary layer model inter-comparison studies have been organised within the Global Energy and Water Cycle Experiment (GEWEX) of the World Climate Research Programme (WCRP). The previous GEWEX ABL Studies (GABLS) have joined about 20 research groups to model the SBL (GABLS1), the diurnal cycle (GABLS2, GABLS3) and the nocturnal low-level jet (GABLS3). With GABLS4 we aim to increase the further understanding of performance and challenges of numerical models in very stable conditions and contribute to the development of parameterization schemes. In this study we explore the set-up of GABLS4 over the Brunt Ice Shelf, Antarctica, where the British Antarctic Survey carries out measurements at the Halley station, including flux and profile measurements from a 32-m-high instrumented mast as well as tethersonde and rawinsonde soundings. The preparatory work towards GABLS4 includes mesoscale model experiments for selected periods (pre-GABLS4). One of these periods is May 2003, when a very stable boundary layer developed at Halley. On 18th of May, the 1-m wind speed calmed down to less than 1 m/s, the 1-m air temperature dropped to -35°C, and a 15°C inversion was generated in the 31 m layer observed by the mast instruments. The observed air temperature and wind included many oscillations with typical periods of 0.5 to 2 h which introduce further challenges. The 18th of May case was simulated applying four mesoscale models: The Polar WRF, HIRLAM, AROME, and the Unified Model (UM). All models took the initial and boundary conditions from ECMWF analyses and applied two or three nested domains, with a 2.5 to 4 km horizontal resolution in the finest domain covering the Brunt Ice Shelf, mostly ice-covered ocean, and parts of the sloping ice sheet. The vertical resolution and physical parameterization schemes for ABL turbulence, radiation, clouds, and heat conduction in the snow varied between the models. The model results were validated against the observations paying attention, among others, to the decoupling of the snow surface and the SBL, and the surface energy balance terms. The modelling challenges include the heat conduction in the snow and the decrease of the downward sensible heat flux with increasing air-surface temperature difference. The pre-GABLS4 mesoscale experiments provided information that is essential for the selection of the GABLS4 case, to be addressed by single column (SCM) and LES models. The pressure gradient was weak during the study period, with mostly ageostrophic winds at Halley. Also the lateral heat advection was weak, and the mesoscale model results can probably be applied to prescribe it for the SCM experiments. The conditions favour the set-up for a GABLS4 inter-comparison case for SCM's but this needs further discussion.
Many observations of artic boundary layers and nighttime boundary layers in general show low temp... more Many observations of artic boundary layers and nighttime boundary layers in general show low temperatures and weak winds near the surface. These weak wind conditions coincide with extremely low intensities of turbulence. As a result, the upper part of the boundary seems to be de-coupled from the surface. Despite the omnipresence and importance of this decoupling phenomenon its physical background
Quarterly Journal of the Royal Meteorological Society, 2010
Single-column models (SCMs) are widely employed to evaluate boundary-layer parametrizations under... more Single-column models (SCMs) are widely employed to evaluate boundary-layer parametrizations under well-controlled conditions. To compare SCM results to observations, these models must be driven by realistic forcings of the three-dimensional (3D) atmospheric state. However, these forcings are inherently uncertain. The central research question is therefore: can observations be used to distinguish between different parametrization schemes in SCM simulations or is
The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk m... more The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5–7 m s−1, depending on ra...
The geophysical fluid dynamics laboratory of the French meteorological service research center ( ... more The geophysical fluid dynamics laboratory of the French meteorological service research center ( CNRM-GAME, URA1357 Météo-France and CNRS ) provides facilities for fundamental and applied study of homogeneous, stratified and/or rotating flows. The research activities of ...
To my knowledge the authors are the first to publish a spatial climatology of low-level jets over... more To my knowledge the authors are the first to publish a spatial climatology of low-level jets over the North Sea. This feat alone is a welcome initiative given the importance of this area for the wind energy sector, which will only increase in the coming decades. Moreover, the authors combine the spatial and temporal coverage of state-of-the-art reanalysis data with an extensive set of lidar observations from ten offshore platforms. Carefully addressing the pros and cons of both the model data and the observations, as well as properly discussing the difficulties in issues like LLJ detection, they provide a highly relevant and balanced overview of LLJ characteristics in the area of interest. C1
We describe and analyze the results of the third Global Energy and Water cycle Experiment (GEWEX)... more We describe and analyze the results of the third Global Energy and Water cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS) intercomparison and evaluation study for singlecolumn models. Each of the nineteen participating models was operated with its own physics package, including land-surface, radiation and turbulent mixing schemes, for a full diurnal cycle selected from the Cabauw observatory archive. By carefully prescribing the temporal evolution of the forcings on the vertical column, the models could be evaluated against observations. We focus on the gross features of the stable boundary layer (SBL), such as the onset of evening momentum decoupling, the 2-m minimum temperature, the evolution of the inertial oscillation and the morning time transition. New process diagrams are introduced to interpret the variety of model results and the relative importance of processes in the SBL. The diagrams include the results of a number of sensitivity runs performed with one ...
Numerical weather prediction and climate models continue to have large errors for stable boundary... more Numerical weather prediction and climate models continue to have large errors for stable boundary layers (SBL). To understand and to improve on this, so far three atmospheric boundary layer model inter-comparison studies have been organised within the Global Energy and Water Cycle Experiment (GEWEX) of the World Climate Research Programme (WCRP). The previous GEWEX ABL Studies (GABLS) have joined about 20 research groups to model the SBL (GABLS1), the diurnal cycle (GABLS2, GABLS3) and the nocturnal low-level jet (GABLS3). With GABLS4 we aim to increase the further understanding of performance and challenges of numerical models in very stable conditions and contribute to the development of parameterization schemes. In this study we explore the set-up of GABLS4 over the Brunt Ice Shelf, Antarctica, where the British Antarctic Survey carries out measurements at the Halley station, including flux and profile measurements from a 32-m-high instrumented mast as well as tethersonde and rawinsonde soundings. The preparatory work towards GABLS4 includes mesoscale model experiments for selected periods (pre-GABLS4). One of these periods is May 2003, when a very stable boundary layer developed at Halley. On 18th of May, the 1-m wind speed calmed down to less than 1 m/s, the 1-m air temperature dropped to -35°C, and a 15°C inversion was generated in the 31 m layer observed by the mast instruments. The observed air temperature and wind included many oscillations with typical periods of 0.5 to 2 h which introduce further challenges. The 18th of May case was simulated applying four mesoscale models: The Polar WRF, HIRLAM, AROME, and the Unified Model (UM). All models took the initial and boundary conditions from ECMWF analyses and applied two or three nested domains, with a 2.5 to 4 km horizontal resolution in the finest domain covering the Brunt Ice Shelf, mostly ice-covered ocean, and parts of the sloping ice sheet. The vertical resolution and physical parameterization schemes for ABL turbulence, radiation, clouds, and heat conduction in the snow varied between the models. The model results were validated against the observations paying attention, among others, to the decoupling of the snow surface and the SBL, and the surface energy balance terms. The modelling challenges include the heat conduction in the snow and the decrease of the downward sensible heat flux with increasing air-surface temperature difference. The pre-GABLS4 mesoscale experiments provided information that is essential for the selection of the GABLS4 case, to be addressed by single column (SCM) and LES models. The pressure gradient was weak during the study period, with mostly ageostrophic winds at Halley. Also the lateral heat advection was weak, and the mesoscale model results can probably be applied to prescribe it for the SCM experiments. The conditions favour the set-up for a GABLS4 inter-comparison case for SCM's but this needs further discussion.
Many observations of artic boundary layers and nighttime boundary layers in general show low temp... more Many observations of artic boundary layers and nighttime boundary layers in general show low temperatures and weak winds near the surface. These weak wind conditions coincide with extremely low intensities of turbulence. As a result, the upper part of the boundary seems to be de-coupled from the surface. Despite the omnipresence and importance of this decoupling phenomenon its physical background
Quarterly Journal of the Royal Meteorological Society, 2010
Single-column models (SCMs) are widely employed to evaluate boundary-layer parametrizations under... more Single-column models (SCMs) are widely employed to evaluate boundary-layer parametrizations under well-controlled conditions. To compare SCM results to observations, these models must be driven by realistic forcings of the three-dimensional (3D) atmospheric state. However, these forcings are inherently uncertain. The central research question is therefore: can observations be used to distinguish between different parametrization schemes in SCM simulations or is
The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk m... more The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5–7 m s−1, depending on ra...
The geophysical fluid dynamics laboratory of the French meteorological service research center ( ... more The geophysical fluid dynamics laboratory of the French meteorological service research center ( CNRM-GAME, URA1357 Météo-France and CNRS ) provides facilities for fundamental and applied study of homogeneous, stratified and/or rotating flows. The research activities of ...
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Papers by Bert Holtslag