A Simple Model for the Afternoon and Early Evening Decay of Convective Turbulence Over Different Land Surfaces

A simple model to study the decay of turbulent kinetic energy (TKE) in the convective surface layer is presented. In this model, the TKE is dependent upon two terms, the turbulent dissipation rate and the surface buoyancy fluctuations. The time evolution of the surface sensible heat flux is modelled based on fitting functions of actual measurements from the LITFASS-2003 field campaign. These fitting functions carry an amplitude and a time scale. With this approach, the sensible heat flux can be estimated without having to solve the entire surface energy balance. The period of interest covers two characteristic transition sub-periods involved in the decay of convective boundary-layer turbulence. The first sub-period is the afternoon transition, when the sensible heat flux starts to decrease in response to the reduction in solar radiation. It is typically associated with a decay rate of TKE of approximately t −2 (t is time following the start of the decay) after several convective eddy turnover times. The early evening transition is the second sub-period, typically just before sunset when the surface sensible heat flux becomes negative. This sub-period is characterized by an abrupt decay in TKE associated with the rapid collapse of turbulence. Overall, the results presented show a significant improvement of the modelled TKE decay when compared to the often applied assumption of a sensible heat flux decreasing instantaneously or with a very short forcing time scale. In addition, for atmospheric modelling studies, it is suggested that the afternoon and early evening decay of sensible heat flux be modelled as a complementary error function.

Atmospheric Boundary Layer Dynamics of Transitional Flows over Complex Terrain

Daniel Nadeau

Recent advances in boundary-layer meteorology are beginning to allow the study of atmospheric flow phenomena that have previously been poorly understood. In this dissertation, we study the effects of complex terrain and unsteady regimes on the atmospheric boundary layer (ABL) dynamics, with the help of field measurements and theoretical analyses. We consider transitions generated by sharp surface discontinuities and by the daily cycle of solar heating. When air flows over an inhomogeneous landscape, it encounters a series of parcels (vegetated areas, mountains, cities, etc.) with different mechanical and thermal properties. Each parcel's boundary triggers a transition layer in the atmosphere. Substantial changes in momentum and heat exchanges are found when air flows over an urban area, due to the complex arrangement of buildings and the various thermal properties of the surface materials. To study the daytime heat exchanges between the built environment and the atmosphere, we conducted a field campaign over the EPFL campus in 2006-07 with a wireless network of 92 weather stations and an atmospheric profiler. In this analysis, the heat exchanges are successfully estimated with Monin-Obukhov similarity and the thermal roughness length method. We also illustrate how one carefully-selected station inside the urban canopy can provide a satisfying estimate of the sensible heat flux over the campus. The diurnal cycle of solar heating also induces transitions in the ABL. During the day, the ABL is usually characterized by an unstable stratification and large turbulent exchanges of momentum, heat and moisture. At night, the ABL is stably stratified and weak turbulent exchanges with the surface are typically observed. This dissertation presents a simple model to track the decay of atmospheric turbulence during the evening transition period, when the ABL shifts from its daytime to its nighttime regime. First, we describe a function to model the sensible heat flux during the transition period. This function is then inserted into a simplified version of the turbulent kinetic energy (TKE) budget, which we validate with several eddy covariance datasets. This study shows that the decay of convective turbulence over flat and unobstructed terrain occurs in three different steps: (i) the TKE is relatively constant for several convective eddy turnover time scales; (ii) the TKE decays with a t-2 rate, where t is time after the start of the decay; (iii) an abrupt decay rate of TKE near the sunset is observed indicating a rapid collapse of turbulence. We also study the evening transition period for slope flows developing over alpine terrain under clear skies and weak synoptic conditions. Slope winds travel upslope during the day and downslope at night. Little is known about the transition between these two wind regimes over steep slopes, mostly because they are extremely challenging to monitor. For this reason, in summer 2010, we deployed a suite of meteorological stations on a 25 to 45 degrees slope of the Swiss Alps. The results show that a 'shading front' induced by surrounding topography triggers the evening transition. The impact on the surface temperatures is substantial and in some cases, drops of 10 °C in less than 10 min are found. This is usually followed by an early evening calm period with low turbulence levels and small wind speeds (< 0.5 ms-1). At night, a shallow layer of 'skin' downslope flow forms, with the maximum wind velocity just above the surface (< 1.5 m).

EPFL2011

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 Simple Model for the Afternoon and Early-Evening Decay of Turbulence over Different Land Surfaces

Chad Higgins, Daniel Nadeau, Eric Richard Pardyjak, Marc Parlange

Recent years have seen an increasing interest in the late-afternoon transition between the convective and stable regimes of the atmospheric boundary layer. There are several differences between the two regimes. On one hand, the convective boundary layer is characterized by an unstable stratification, turbulent mixing of mass, momentum and heat and buoyancy-driven eddies. On the other hand, the stable boundary layer is associated with a strong stable stratification that tends to suppress vertical motions generated by mechanical turbulence. One of the key processes of this complex transition period is the forcing time scale associated with the surface heat flux. Unfortunately, very few modeling studies have used realistic decaying time scales for the sensible heat flux. Therefore, in the first part of this study, we present a new function that better represents the afternoon and early-evening transitions and validate it with eddy covariance measurements over different land surfaces. The objectives are to capture the buoyancy forcing time scales observed in nature and the influence of surface properties. In the second part of the study, we show preliminary results of large-eddy simulation of atmospheric flow over heterogeneous cooling stripes. We focus our attention on the temperature advection between the different stripes as a result of their different cooling rates. Overall, this study is one of the first to model the convective decay of turbulence using realistic time scales over heterogeneous terrain.