Planetary boundary layer | EPFL Graph Search

In meteorology, the planetary boundary layer (PBL), also known as the atmospheric boundary layer (ABL) or peplosphere, is the lowest part of the atmosphere and its behaviour is directly influenced by its contact with a planetary surface. On Earth it usually responds to changes in surface radiative forcing in an hour or less. In this layer physical quantities such as flow velocity, temperature, and moisture display rapid fluctuations (turbulence) and vertical mixing is strong. Above the PBL is the "free atmosphere", where the wind is approximately geostrophic (parallel to the isobars), while within the PBL the wind is affected by surface drag and turns across the isobars (see Ekman layer for more detail). Cause of surface wind gradient Wind shearWind gradientWind engineering and Ekman layer Typically, due to aerodynamic drag, there is a wind gradient in the wind flow ~100 meters above the Earth's surface—the surface layer of the planetary boundary layer. Wind speed increa

Improving the Turbulence Coupling between High Resolution Numerical Weather Prediction Models and Lagrangian Particle Dispersion Models

Balazs Szintai

For the modelling of the transport and diffusion of atmospheric pollutants during accidental releases, sophisticated emergency response systems are used. These modelling systems usually consist of three main parts. The atmospheric flow conditions can be simulated with a numerical weather prediction (NWP) model. The evolution of the pollutant cloud is described with a dispersion model of variable complexity. The NWP and the dispersion models have to be coupled with a so-called meteorological pre-processor. This means that all the necessary – in most cases turbulence related – variables which are not available from the standard output of the NWP model have to be diagnosed. The main difficulty of the turbulence coupling is that these subgrid scale variables of NWP models are not routinely verified and thus little is known concerning their quality and impact on dispersion processes. The general aim of the present work is to better understand and improve this coupling mechanism. For this purpose all the three main components of the emergency response system of MeteoSwiss are carefully evaluated and possible improvement strategies are suggested. In the first part, the NWP component of the system, namely the COSMO model, is investigated focusing on the model performance in the Planetary Boundary Layer (PBL). Three case studies, representing both unstable and stable situations, are analyzed and the COSMO simulations are validated with turbulence measurements and Large Eddy Simulation (LES) data. It is shown that the COSMO model is able to reproduce the main evolution of the boundary layer in dry convective situations with the operational parameter setting. However, it is found that the COSMO model tends to simulate a too moist and too cold PBL with shallower PBL heights than observed. During stable conditions the operational parameter setting has to be significantly modified (e.g., the minimum diffusion coefficient) to obtain a good model performance. The turbulence scheme of COSMO, which carries a prognostic equation for Turbulent Kinetic Energy (TKE), is studied in detail to understand the shortcomings of the simulations. The turbulent transport term (third order moment) in the TKE equation is found to be significantly underestimated by the COSMO model during unstable situations. This results in inaccurate TKE profiles and hence missing entrainment fluxes at the top of the PBL. A solution to increase the TKE transport in the PBL is proposed in the present work and evaluated during a three-month continuous period. While improving the TKE profile substantially, the modification is demonstrated to not impair other model output characteristics. The second component of the emergency response system, namely the meteorological pre-processor, is also validated on case studies and a continuous period. The main objective of this analysis is to compare the currently operational coupling approach, which is based on the direct usage of the prognostic TKE from the COSMO model, to a classical approach based on similarity theory considerations, thereby using turbulence measurements on the one hand and LES data on the other hand. To be able to use similarity theory approaches for the determination of turbulence characteristics, the PBL height has first to be diagnosed from the NWP model. In the present study, several approaches for the determination of PBL height have been implemented and validated with radio sounding measurements. Based on the verification results and the operational convenience, the method based on the bulk Richardson number method has been chosen for the diagnosis of the PBL height. Validation results of post-diagnosed turbulence characteristics show that during convective situations, the similarity approach tends to overestimate the turbulence intensity, while the approach based on the direct usage of TKE yields more accurate results. For stable conditions the different approaches are closer to each other and both give reasonable predictions. It is found that the main drawback of the direct approach is the isotropic assumption in the horizontal direction. A new hybrid method is proposed which uses similarity considerations for the partitioning of horizontal TKE between along-wind and cross-wind directions. In the last part, pollutant dispersion in complex terrain is studied using a new scaling approach for TKE that is suited for steep and narrow Alpine valleys. This scaling approach is introduced in the interface between COSMO and a Lagrangian particle dispersion model (LPDM), and its results are compared to those of a classical similarity theory approach and to the operational coupling type, which uses the TKE from the COSMO model directly. For the validation of the modelling system, the TRANSALP-89 tracer experiment is used, which was conducted in highly complex terrain in southern Switzerland. The ability of the COSMO model to simulate the valley-wind system is assessed with several meteorological surface stations. The dispersion simulation is evaluated with the measurements from 25 surface samplers. The sensitivity of the modelling system towards the soil moisture, horizontal grid resolution, and boundary layer height determination is investigated. It is shown that if the flow field is correctly reproduced, the new scaling approach improves the tracer concentration simulation compared to the classical coupling methods.

EPFL2010

A physics-based model for wind turbine wake expansion in the atmospheric boundary layer

Fernando Porté Agel, Dara Vahidi

Analytical wind turbine wake models are widely used to predict the wake velocity deficit. In these models, the wake growth rate is a key parameter specified mainly with empirical formulations. In this study, a new physics-based model is proposed and validated to predict the wake expansion downstream of a turbine based on the incoming ambient turbulence and turbine operating conditions. The new model utilises Taylor diffusion theory, the Gaussian wake model, turbulent mixing layer theory and the analogy between wind turbine wake expansion and scalar diffusion. These components ensure that the model conserves mass and momentum in the far wake and accounts for the ambient turbulence and turbine-induced turbulence effects on the wake expansion. To account for the turbulence relevant scales that contribute to the wake expansion, the model uses the root-mean-square of the low-pass filtered radial velocity component. A simplified version that only requires the unfiltered velocity standard deviation and turbulence integral scale is also proposed. In addition, a new relation for the near-wake length is derived. The model performance is validated using large-eddy simulation data of a wind turbine wake under neutral atmospheric conditions with a wide range of incoming turbulence levels. The results show that the proposed model yields reasonable predictions of the wake width, maximum velocity deficit and near-wake length. In the case with a relatively low incoming streamwise turbulence intensity of 0.05, the ambient and turbine-induced terms in the model contribute almost equally to the wake width, rendering them both crucial for reasonable wake predictions.

2022

Impact of transatlantic transport episodes on summertime ozone in Europe

Isabelle Bey

This paper reports on the transport of ozone

{O}_{3}

and related species over the North Atlantic ocean and its impact on Europe. Measurements of nitrogen dioxide (

{NO}_{2}

)and carbon monoxide (CO) columns from the GOME and MOPITT satellite instruments, respectively, are used in conjunction with the GEOS-CHEM global model of transport and tropospheric chemistry to identify the major events of long range transport that reach Europe over the course of summer 2000. Sensitivity model simulations are used to analyse observed

{O}_{3}

distributions with respect to the impact of long range transport events. For that purpose, we used in-situ

{O}_{3}

observations taken at the mountain site of Jungfraujoch as well as O3 vertical profiles taken in the vicinity of central European cities. Over the course of summer 2000, we identified 9 major episodes of transatlantic pollution transport; 7 events are associated with transient cyclones while 2 events occur through zonal transport (e.g. by advection in the strong low-level westerly winds established in summer between the Azores anticyclone and transient cyclones). We find that on average three episodes occur per month with the strongest ones being in June. The number and frequency of long range transport events that reach Europe are driven by the position and strength of the Azores anticyclone. Model sensitivity simulations indicate that the summer mean North American

{O}_{3}

contribution ranges from 3 to 5 ppb (7–11%) in the planetary boundary layer and 10 to 13 ppb (18–23%) in the middle and upper troposphere. During particular episodes, North American sources can result in

{O}_{3}

enhancements up to 25–28 ppb in the layer between 800–600 hPa and 10–12 ppb in the boundary layer. The impact of the zonal transport events on

{O}_{3}

distribution over Europe is more clearly seen below 700 hPa as they tend to transport pollution at lower levels while the events associated with transient cyclones are more likely to have an impact on the middle and upper troposphere (i.e. above 600 hPa). The air mass origins found in the GEOS-CHEM model are clearly confirmed by back trajectory analyses. During most of the 9 events, a strong contribution in North American

{O}_{3}

is in general associated with only little European

{O}_{3}

and viceversa (in particular at the Jungfraujoch). A substantial North American contribution (e.g., 30% or higher) to

{O}_{3}

over Europe does not always result in pronounced

{O}_{3}

enhancements in the observations during our period of study.

2006