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Boundary Layer Parametrization in Numerical Weather Prediction Models
Stockholm University, Faculty of Science, Department of Meteorology .
2015 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Numerical weather prediction (NWP) and climate models have shown to have a challenge to correctly simulate stable boundary layers and diurnal cycles. This aim of this study is to evaluate, describe and give suggestions for improvements of the descriptions of stable boundary layers in operational NWP models. Two papers are included. Paper I focuses on the description of the surface and the interactions between the surface and the boundary layer in COAMPSR, a regional NWP model. The soil parametrization showed to be of great importance to the structure of the boundary layer. Moreover, it showed also that a low frequency of radiation calculations caused a bias in received solar energy at the surface.

In paper II, the focus is on the formulation of the turbulent transport in stable boundary layers. There, an implementation of a diffusion parametrization based on the amount of turbulent kinetic energy (TKE) is tested in a single column model (SCM) version of the global NWP model Integrated Forecast System (IFS). The TKE parametrization turned out to behave similarly as the currently operational diffusion parametrization in convective regimes and neutral regimes, but showed to be less diffusive in weakly stable and stable conditions. The formulations of diffusion also turned out to be very dependent on the length scale formulation. If the turbulence and the gradients of wind temperature and wind are weak, the magnitude of turbulence can enter an oscillating mode. This oscillation can be avoided with the use of a lower limit of the length scale.

Abstract [sv]

Det har visat sig att det är en stor utmaning för numeriska väderprognosmodeller (NWP-modeller) att simulera stabilt skiktade atmosfäriska gränsskikt och gränsskiktets dygnscykel på ett korrekt sätt. Syftet med denna studien är att utvärdera, beskriva och ge förslag på förbättringar av beskrivningen av gränsskiktet i NWP-modeller. Studien innehåller två artiklar. Den första fokuserar på beskrivningen av markytan och interaktionen mellan marken och gränsskiktet i den regionala NWP-modellen COAMPS R . Det visade sig att beskrivningen av markytan har en signifikant inverkan på gränsskiktets struktur. Det framkom också att strålningsberäkningarna endast görs en gång i timmen vilket bland annat orsakar en bias i inkommande solinstrålning vid markytan.

Den andra artikeln fokuserar på beskrivningen av den turbulenta transporten i stabila skiktade gränsskikt. En implemenering av en diffusionsparametrisering som bygger på turbulent kinetisk energy (TKE) testas i en endimensionell version av NWP-modellen Integrated Forecast System (IFS), utvecklat vid European Center for Medium Range Weather Forecasts (ECMWF). Den TKE-baserade diffussionsparametriseringen är likvärdigt med den nuvaran de operationella parametriseringen i neutrala och konvektiva gränsskikt, menär mindre diffusivt i stabila gränsskikt. Diffusionens intensitet är beroende påden turbulenta längdskalan. Vidare kan turbulensen i TKE-formuleringen hamna i ett oscillerande läge om turbulensen är svag samtidigt som temperatur- och vindgradienten är kraftig. Denna oscillation kan förhindras om längdskalans minsta tillåtna värde begränsas.

Place, publisher, year, edition, pages
Stockholm: Department of Meteorology, Stockholm University , 2015. , 27 p.
Keyword [en]
Boundary-Layer Meteorology, Numerical Weather Prediction, Turbulence parametrization, COAMPS, IFS
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
URN: urn:nbn:se:su:diva-117134ISBN: 978-91-7649-194-2 (print)OAI: oai:DiVA.org:su-117134DiVA: diva2:810688
Presentation
2015-05-29, C 609, Svante Arrhenius väg 16 C, Stockholm, 09:19 (English)
Opponent
Supervisors
Available from: 2015-05-18 Created: 2015-05-08 Last updated: 2015-05-18Bibliographically approved
List of papers
1. Sensitivity tests with COAMPS in stable boundary layers over the CASES 99 area
Open this publication in new window or tab >>Sensitivity tests with COAMPS in stable boundary layers over the CASES 99 area
(English)Manuscript (preprint) (Other academic)
Keyword
COAMPS, Numerical Weather Prediction, Boundary-Layer Meteorology, Surface parametrization
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-117131 (URN)
Available from: 2015-05-08 Created: 2015-05-08 Last updated: 2016-01-29Bibliographically approved
2. Evaluation of a TKE diffusion parametrization in IFS Single Column Model
Open this publication in new window or tab >>Evaluation of a TKE diffusion parametrization in IFS Single Column Model
(English)Manuscript (preprint) (Other academic)
Abstract [en]

An implementation of a diffusion parametrization based on Turbulent KineticEnergy (TKE) in a single column model of the weather forecast model Integrated Forecast System (IFS) is evaluated. Simulations are performed for four test cases GABLS1,GABLS3, DICE and GABLS4. The test cases have different idealization and gives together a comprehensive framework for evaluating the performance for a NWP in representing stably stratified atmospheric boundary layers, diurnal cycles and transition between stable andunstable stratification. The sensitivity to the mixing length scale is evaluated by simulations with alternative formulations and by diagnostic evaluation. An estimation of ageostrophicmass flux is made for idealized test cases.The TKE parametrization is less diffusive than the currently used operational parametrization and shows more realistic stable boundary layers. The magnitude of the diffusivity shows to be very sensitive to length scale formulation. The more diffusive formulations yield weaker gradients of temperature and wind at the surface and smaller angle between surface stress and geostrophic wind. The lower limit of the length scale showed to be ofimportance to avoid unrealistic vertical gradients of wind and temperature close to the surface. Moreover, the diagnosed length scales formulations showed to have very simila rbehaviour when turbulence ceases in stable conditions. The ageostrophic mass flux was higher in simulations with stronger diffusion, except that the new TKE scheme in one test case showed similar values as the operational parametrization.

Keyword
Boundary-layer meteorology, Integrated Forecasting System, Length scale, Single Column Model, Stable boundary layer
National Category
Meteorology and Atmospheric Sciences
Research subject
Meteorology
Identifiers
urn:nbn:se:su:diva-117130 (URN)
Available from: 2015-05-08 Created: 2015-05-08 Last updated: 2016-01-29Bibliographically approved

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