Simulations of atmospheric dispersion in an urban stable boundary layer

City centers have to cope with an increasing amount of air pollution. The supply of fresh air is crucial yet difficult to ensure, especially under stable conditions of the atmospheric boundary layer. This case study used the PArallelized Large eddy simulation (LES) Model PALM to investigate the wind field over an urban lake that had once been built as a designated fresh air corridor for the city center of Münster, northwest, Germany. The model initialization was performed using the main wind direction and stable boundary layer conditions as input. The initial wind and temperature profiles included a weak nocturnal low-level jet. By emitting a passive scalar at one point on top of a bridge, the dispersion of fresh air could be traced over the lake’s surface, within street canyons leading to the city center and within the urban boundary layer above. The concept of city ventilation was confirmed in principle, but the air took a direct route from the shore of the lake to the city center above a former river bed and its adjoining streets rather than through the street canyons. According to the dispersion of the passive scalar, half of the city center was supplied with fresh air originating from the lake. PALM proved to be a useful tool to study fresh air corridors under stable boundary layer conditions.

Download Full-text

Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

Boundary-Layer Meteorology ◽

10.1007/s10546-021-00619-z ◽

2021 ◽

Author(s):

Lena Pfister ◽

Karl Lapo ◽

Larry Mahrt ◽

Christoph K. Thomas

Keyword(s):

Boundary Layer ◽

Stable Boundary Layer ◽

Side Wall ◽

Cold Pool ◽

Continuous Data ◽

Valley Bottom ◽

Near Surface ◽

Cold Air ◽

Stable Boundary ◽

Surface Temperatures

AbstractIn the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena.

Download Full-text

The Height Correction of Similarity Functions in the Stable Boundary Layer

Boundary-Layer Meteorology ◽

10.1007/s10546-011-9653-x ◽

2011 ◽

Vol 142 (1) ◽

pp. 21-31 ◽

Cited By ~ 18

Author(s):

Zbigniew Sorbjan

Keyword(s):

Boundary Layer ◽

Stable Boundary Layer ◽

Similarity Functions ◽

Stable Boundary

Download Full-text

Tethered Balloon Observations Of The Nocturnal Stable Boundary Layer In A Valley

Boundary-Layer Meteorology ◽

10.1023/a:1002628924673 ◽

2000 ◽

Vol 97 (1) ◽

pp. 1-24 ◽

Cited By ~ 22

Author(s):

J. J. Holden ◽

S. H. Derbyshire ◽

S. E. Belcher

Keyword(s):

Boundary Layer ◽

Stable Boundary Layer ◽

Tethered Balloon ◽

Stable Boundary

Download Full-text

On stable boundary-layer height estimation using backscatter lidar data and variance processing

10.5194/egusphere-egu21-8765 ◽

2021 ◽

Author(s):

Marcos Paulo Araujo da Silva ◽

Constantino Muñoz-Porcar ◽

Umar Saeed ◽

Francesc Rey ◽

Maria Teresa Pay ◽

...

Keyword(s):

Boundary Layer ◽

Stable Boundary Layer ◽

Adaptive Estimation ◽

Microwave Radiometer ◽

Minimum Variance ◽

Initial Guess ◽

Layer Height ◽

Stable Boundary ◽

Boundary Layer Height ◽

Temporal Variance

This study describes a method to estimate the nocturnal stable boundary layer height (SBLH) by means of lidar observations. The method permits two approaches which yield independent retrievals through either spatial or temporal variance vertical profiles of the attenuated backscatter. Then, the minimum variance region (MVR) on this profile is identified. Eventually, when multiple MVRs are detected, a temperature-based SBLH estimation derived from radiosonde, launched within the searching time, is used to disambiguate the initial guess. In order to test the method, two study cases employing lidar-ceilometer (Jenoptik CHM 15k Nimbus) measurements are investigated. Temperature-based estimates from a collocated microwave radiometer permitted validation, using either temporal or spatial backscatter variances. The dataset was collected during the HD(CP)2 Observational Prototype Experiment (HOPE) [1]. &#160;&#160;[1] U. Saeed, F. Rocadenbosch, and S. Crewell, &#8220;Adaptive Estimation of the Stable Boundary Layer Height Using Combined Lidar and Microwave Radiometer Observations,&#8221; IEEE Trans. Geosci. Remote Sens., 54(12), 6895&#8211;6906 (2016), DOI: 10.1109/TGRS.2016.2586298.[2] U. L&#246;hnert, J. H. Schween, C. Acquistapace, K. Ebell, M. Maahn, M. Barrera-Verdejo, A. Hirsikko, B. Bohn, A. Knaps, E. O&#8217;Connor, C. Simmer, A. Wahner, and S. Crewell, &#8220;JOYCE: J&#252;lich Observatory for Cloud Evolution,&#8221; Bulletin of the American Meteorological Society, 96(7), 1157-1174 (2015). DOI: 10.1175/BAMS-D-14-00105.1

Download Full-text

Local Characteristics of the Nocturnal Boundary Layer in Response to External Pressure Forcing

Journal of Applied Meteorology and Climatology ◽

10.1175/jamc-d-17-0011.1 ◽

2017 ◽

Vol 56 (11) ◽

pp. 3035-3047 ◽

Cited By ~ 17

Author(s):

Steven J. A. van der Linden ◽

Peter Baas ◽

J. Antoon van Hooft ◽

Ivo G. S. van Hooijdonk ◽

Fred C. Bosveld ◽

...

Keyword(s):

Boundary Layer ◽

Wind Speed ◽

Stable Boundary Layer ◽

Dynamical Behavior ◽

Geostrophic Wind ◽

External Pressure ◽

Near Surface ◽

Stable Boundary ◽

Wind Speeds ◽

Turbulent Characteristics

AbstractGeostrophic wind speed data, derived from pressure observations, are used in combination with tower measurements to investigate the nocturnal stable boundary layer at Cabauw, the Netherlands. Since the geostrophic wind speed is not directly influenced by local nocturnal stability, it may be regarded as an external forcing parameter of the nocturnal stable boundary layer. This is in contrast to local parameters such as in situ wind speed, the Monin–Obukhov stability parameter (z/L), or the local Richardson number. To characterize the stable boundary layer, ensemble averages of clear-sky nights with similar geostrophic wind speeds are formed. In this manner, the mean dynamical behavior of near-surface turbulent characteristics and composite profiles of wind and temperature are systematically investigated. The classification is found to result in a gradual ordering of the diagnosed variables in terms of the geostrophic wind speed. In an ensemble sense the transition from the weakly stable to very stable boundary layer is more gradual than expected. Interestingly, for very weak geostrophic winds, turbulent activity is found to be negligibly small while the resulting boundary cooling stays finite. Realistic numerical simulations for those cases should therefore have a comprehensive description of other thermodynamic processes such as soil heat conduction and radiative transfer.

Download Full-text

Energy cascade for highly anisotropic turbulence in the Stable Boundary Layer

10.5194/ems2021-167 ◽

2021 ◽

Author(s):

Federica Gucci ◽

Lorenzo Giovannini ◽

Dino Zardi ◽

Nikki Vercauteren

Keyword(s):

Boundary Layer ◽

Kinetic Energy ◽

Stable Boundary Layer ◽

Energy Cascade ◽

Isotropic Case ◽

Limiting State ◽

Stable Boundary ◽

Turbulence Anisotropy ◽

Small Scales ◽

Energy Exchanges

The broad variety of phenomena occurring on multiple scales under stably stratified conditions and their complex interactions make it difficult to get a full description of the Stable Boundary Layer (SBL). Near-surface turbulence may be intermittent and highly anisotropic even at small scales. By studying the invariants of the anisotropy Reynolds stress tensor, it is possible to analyse the eddy kinetic energy distribution over the three components of the flow. Recent analyses of SBL turbulence data highlighted a prevalence of one-component limiting state of anisotropy. The causes of this particular limiting state are not fully understood, but there is evidence that submeso activity influences turbulence topology.&#160;This open question motivated the present work, that addresses the issue from the point of view of space dimensionality. In large-scale atmospheric and oceanic dynamics it is well known that turbulent motions may transfer energy both to the large and to the small scales, according to density stratification and rotation. These two properties act as constraints on the flow, giving it a 2D structure, and leading turbulence to be more complex than the homogeneous and isotropic case. For a SBL in low-wind speed conditions, atmospheric stratification might be very strong and we investigate if some of the peculiar characteristics of this regime might be related to a quasi-2D dynamics, with the occurrence of an inverse energy cascade, typical of 2D-like turbulence.Energy exchanges across larger and smaller scales are studied by analysing the direction of the momentum flux with different methods, including a coarse-graining approach based on Large Eddy Simulation (LES) theory. The SnoHATS dataset was used to this purpose, where two vertically-separated horizontal arrays of sonic anemometers over the Plaine Morte Glacier (Switzerland) allowed the computation of the full three-dimensional velocity gradient. In order to fully characterize the energy exchanges according to different states of turbulence anisotropy, energy conversion processes between eddy kinetic and potential energy have also been considered and analysed at different heights. To this purpose, the dataset FLOSSII was used, providing turbulence measurements up to 30 m above a flat grass surface, often covered by snow.&#160;Results seem to suggest that turbulent kinetic energy in the SBL is distributed mainly in one component more as a consequence of wave-turbulence interactions than of development of 2D-like turbulence. This gives insights on mechanisms driving turbulence anisotropy that might be used to improve turbulence parameterizations in the SBL.

Download Full-text