| 1. |
|
|
| 2. |
- Esau, Igor, 1969-
(författare)
-
Large Eddy Simulation of Non-Local Turbulence and Integral Measures of Atmospheric Boundary Layers
- 2003
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A new large eddy simulation (LES) code is developed and used to investigate non-local features of turbulent planetary boundary layers (PBLs). The LES code is based on filtered Navier-Stokes equations, which describe motions of incompressible, Boussinesq fluid at high Reynolds numbers. The code computes directly large-scale, non-universal turbulence in the PBL whereas small-scale, universal turbulence is parameterized by a dynamic mixed subgrid closure. The LES code is thoroughly tested against high quality laboratory and field data. This study addresses non-local properties of turbulence which emphasis on the stable stratification. Its basic results are as follows. The flow stability in PBLs is generally caused by two mechanisms: the negative buoyancy force (in the stable density stratification) and the Coriolis force (in the rotating system). The latter stabilizes the flow if the earth’s vorticity and the turbulent vorticity are anti-parallel. The Coriolis force stability suppresses large-scale turbulence and makes large eddies asymmetric. The density stratification suppresses vertical scales of turbulence. Joint actions of the Coriolis and the buoyancy forces result in a more complex behavior of turbulence. Particularly, the layers of vigorous turbulence may appear in the course of development of low-level jets in baroclinic atmosphere. Non-local effects determine integral measures of PBLs, first of all the PBL depth. This study clearly demonstrates its pronounced dependences on the Coriolis parameter, the Kazanski-Monin internal stability parameter, and newly introduced imposed-stability and baroclinicity parameters. An LES database is created and used to validate an advanced PBL-depth formulation. LES support the idea that PBLs interact with the stably stratified free flow through the radiation of gravity waves, excited by large turbulent eddies at the interface.
|
|
| 3. |
|
|
| 4. |
- Esau, Igor
(författare)
-
The Coriolis effect on coherent structures in planetary boundary layers
- 2003
-
Ingår i: Journal of turbulence. - : Informa UK Limited. - 1468-5248 .- 1468-5248. ; 4, s. 1-10
-
Tidskriftsartikel (refereegranskat)abstract
- Coherent structures are often visible in atmospheric boundary layers as convective clouds and irregular fog. Large eddy simulations (LES) provide data to study the coherent structures by means of multivariate methods of statistical analysis. One of such methods is a proper orthogonal decomposition (POD). A POD can isolate most energetic three-dimensional structures in turbulent boundary layers. Coherent structures in planetary boundary layers (PBLs) were found to be inherently different from those in laboratory boundary layers without background rotation. This study attributes the differences to an interaction between the vorticity of coherent structures and the constant background vorticity of the planet rotation. LES of neutrally stratified PBLs clearly show the Coriolis force effect. The Coriolis effect supports vortices with vorticity parallel to the planetary vorticity. Simultaneously, the Coriolis effect destroys vortices with vorticity counter-parallel to the planetary vorticity. Convective PBLs show an interplay of vorticities of shear- and buoyancy-induced coherent structures. The Coriolis effect is less important in convective PBLs in comparison with neutral PBLs. The horizontal and vertical components of the Coriolis force act in essentially different ways. The vertical component mainly damps the coherent structures. It also turns the structures relative to the geostrophic wind. The horizontal component mainly makes the structures asymmetrical.
|
|
| 5. |
- Lappalainen, Hanna K., et al.
(författare)
-
Pan-Eurasian Experiment (PEEX) : towards a holistic understanding of the feedbacks and interactions in the land-atmosphere-ocean-society continuum in the northern Eurasian region
- 2016
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 16:22, s. 14421-14461
-
Tidskriftsartikel (refereegranskat)abstract
- The northern Eurasian regions and Arctic Ocean will very likely undergo substantial changes during the next decades. The Arctic-boreal natural environments play a crucial role in the global climate via albedo change, carbon sources and sinks as well as atmospheric aerosol production from biogenic volatile organic compounds. Furthermore, it is expected that global trade activities, demographic movement, and use of natural resources will be increasing in the Arctic regions. There is a need for a novel research approach, which not only identifies and tackles the relevant multi-disciplinary research questions, but also is able to make a holistic system analysis of the expected feedbacks. In this paper, we introduce the research agenda of the Pan-Eurasian Experiment (PEEX), a multi-scale, multi-disciplinary and international program started in 2012 (https://www.atm.helsinki.fi/peex/). PEEX sets a research approach by which large-scale research topics are investigated from a system perspective and which aims to fill the key gaps in our understanding of the feedbacks and interactions between the land-atmosphereaquatic-society continuum in the northern Eurasian region. We introduce here the state of the art for the key topics in the PEEX research agenda and present the future prospects of the research, which we see relevant in this context.
|
|
| 6. |
- Mauritsen, Thorsten, et al.
(författare)
-
A total turbulent energy closure model for neutrally and stably stratified atmospheric boundary layers
- 2007
-
Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 64:11, s. 4113-4136
-
Tidskriftsartikel (refereegranskat)abstract
- This paper presents a turbulence closure for neutral and stratified atmospheric conditions. The closure is based on the concept of the total turbulent energy. The total turbulent energy is the sum of the turbulent kinetic energy and turbulent potential energy, which is proportional to the potential temperature variance. The closure uses recent observational findings to take into account the mean flow stability. These observations indicate that turbulent transfer of heat and momentum behaves differently under very stable stratification. Whereas the turbulent heat flux tends toward zero beyond a certain stability limit, the turbulent stress stays finite. The suggested scheme avoids the problem of self-correlation. The latter is an improvement over the widely used Monin–Obukhov-based closures. Numerous large-eddy simulations, including a wide range of neutral and stably stratified cases, are used to estimate likely values of two free constants. In a benchmark case the new turbulence closure performs indistinguishably from independent large-eddy simulations.
|
|
| 7. |
|
|
| 8. |
|
|
| 9. |
- Zilitinkevich, Sergej, et al.
(författare)
-
The effect of baroclinicity on the equilibrium depth of neutral and stable planetary boundary layers
- 2003
-
Ingår i: Quarterly Journal of the Royal Meteorological Society. - : Wiley. - 0035-9009 .- 1477-870X. ; 129:595, s. 3339-3356
-
Tidskriftsartikel (refereegranskat)abstract
- Earlier studies of the depths of the geophysical planetary boundary layers (PBLs) basically focused on the effects of the earth's rotation and density stratification. One more inherent feature of geophysical flows, namely their baroclinicity, has not been considered in that context. The present paper makes up for this deficiency. Diagnostic equations for the depths of the equilibrium, baroclinic, neutral or stable PBLs are derived and verified against data from large-eddy simulations (LES). These latter represents the most convenient empirical-validation tool, which allows considering separately different types of PBL: barotropic and baroclinic; truly neutral and conventionally neutral; nocturnal stable and long-lived stable. The LES are performed using a new code specifically designed for modelling the neutral and stable PBL. Results from the comparison show an excellent correspondence between the proposed equations and LES data. Copyright © 2003 Royal Meteorological Society
|
|
