COSMO General Meeting 7 September 200 9 , Offenbach, Germany

Federal Department of Home Affairs FDHAFederal Office of Meteorology and Climatology MeteoSwiss

Analyzing the TKE budget of the COSMO model for the LITFASS-2003 convective case: comparison with

turbulence measurements and LES data

COSMO General Meeting

7 September 2009, Offenbach, Germany

Balázs SzintaiMeteoSwiss

Daniel Nadeau EPFL, Switzerland

2 Analyzing the TKE budget of the COSMO model: comparison with turbulence measurements and LES data

Balázs Szintai (balazs.szintai@meteoswiss.ch)

Outline

• Goal of the work

• LITFASS-2003 campaign

• Large Eddy Simulation

• COSMO-3D simulations• Mean variables (temperature, wind)• TKE• TKE budget terms

• Summary and Outlook

Goal of the work

• Evaluate the current one-equation turbulence scheme of the COSMO model

• Investigation of both unstable and stable cases

• Comparison with measurements and LES data

• Eventually further tuning

LITFASS-2003

• Measurement campaign conducted in the area of Lindenberg

• Main goal: measurement of turbulent fluxes in an inhomogeneous terrain

• Case study: 2003-05-30 : convective day, calm winds, no clouds

• Simulation of the whole diurnal cycle

• Measurements used:• Radiosoundings• Micrometeorological stations

all land cover types• Tower turbulence measurements

(at 50 and 90 m)

Large Eddy Simulation

• Large Eddy Simulation runs were performed at the Environmental Fluid Mechanics Laboratory (EFLUM) at EPFL, Switzerland

• Model was developed at the Johs Hopkins University and was used several times previously to simulate the atmospheric boundary layer

• LES details:• Solves the filtered incompressible N-S equations• Lagrangian scale-dependent dynamic subgrid scale model • Spectral representation in the horizontal directions• Finite differences in the vertical direction• Periodic lateral boundary conditions• Stress-free lid as a top BC for unstable conditions• Surface temperature is imposed • Heat exchange using MOS

LES – ideal cases

Neutral Stable Unstable

Daniel Nadeau, EPFL

LES - LITFASS-2003 case

• Domain size• 6 x 6 km horizontal (Δx = Δy = 94 m)• 3 km vertical (Δz = 47 m)

• Time step: 0.2 s• 3 hours of simulation from 9 to 12 UTC

• Geostrophic forcing calculated from COSMO-7 analysis (constant in time, vertically changing)

• Changing lower BC from measurements for each land cover type

• 10 minute averaging in output • No space averaging

• Work in progress, preliminary results

COSMO-3D at 1 km

COSMO-7 COSMO-1

• Horizontal resolution: 0.0089° ~ 1 km

• Timestep : 10 s

• Operational external parameters (soil type) are not accurate in the area improved dataset from Felix Ament is used

• Soil analysis was produced with a one-month TERRA standalone run forced with COSMO-7 analyses

• Investigated:• Numerical horiz. diffusion• Implicit TKE diffusion• Finer vertical resolution

COSMO-3D runs

In the following the control run is presented:• no horizontal diffusion• explicit TKE diffusion• oper. 60 levels

COSMO LES

Wind speed[m/s]

Wind dir.[deg]

Wind profiles at 12 UTC

• Wind direction is simulated accurately

• Wind speed is overestimated by LES

• Problem in LES above the PBL – wave pattern

Wind speed Wind direction

Potential temperature

• COSMO simulates a too warm and too shallow PBL

• LES background stratification is too stable

COSMO LES

12 UTC

• COSMO gives higher values than LES in the lower half of the PBL

• Height of TKE maximum is too high in LES

• Double peak in the profiles of horizontal velocity variances in LES (not shown)

COSMO LES

12 UTC

Component testing

• Budget terms:• I. : Local tendency• II. : Buoyancy production term• III. : Shear production term• IV. : Turbulent transport of TKE and pressure correlation term• V. : Dissipation

Prognostic equation for TKE:

I. II. III. IV. V.

TKE budget terms I.

COSMO LES

Buoyancy

TKE budget terms II.

COSMO LES

Transport

Dissipation

Budget imbalance

TKE budget - measurements

• Average values computed from two sonic anemometers at 50 and 90 m• Turbulent transport: triple correlations (M. Mauder, Research Centre Karlsruhe)• Dissipation: turbulence spectrum (D. Michel, University Basel)

• Positive values of turbulent transport in COSMO• Large negative imbalance in measurements

Transport

Dissipation Budget imbalance

Buoyancy Shear

Summary and Outlook

• The diurnal cycle of a dry convective case has been simulated with the COSMO model at 1 km resolution

• TKE budget terms have been compared to preliminary LES data and measurements

• The shear production term shows vertical oscillations (in stable regimes)

• The transport term near the surface have different sign as compared to measurements and LES

• The TKE budget from turbulence measurements shows a large negative imbalance

• Outlook:

• More reliable LES data

• Stable case: GABLS-3

Federal Department of Home Affairs FDHAFederal Office of Meteorology and Climatology MeteoSwiss

Thank you for your attention!

Reserve slides

TERRA standalone run

• Upper levels of the produced soil analysis are colder and moister than measurements

COSMO-3D: Mean Vertical Velocity 2003-05-30 12 UTC, 800 m AGL

Without horiz. DiffusionCTRL-60

With horiz. DiffusionHDIFF-60

COSMO-3D: Turbulent Kinetic Energy2003-05-30 12 UTC, 800 m AGL

• Without horizontal diffusion the results are not very realistic

wavelength is on the order of the mesh size

• Horizontal diffusion reduces the amplitude of waves and increases the wavelength

• Cellular updrafts on areas with low winds vanish with horizontal diffusion

Without horiz. DiffusionCTRL-60

With horiz. DiffusionHDIFF-60

Validation of COSMO-3D – 10 m wind

• Too strong wind in analysis – vanishes after 2 hours• Small maximum in the morning hours not present in measurement

CTRL-60 IMPL-60

HDIFF-60 IMPL-74

-3D -3D

Validation of COSMO-3D – Surface sens. heat flux

• No significant differences between the experiments• Surface sensible heat flux is overestimated during daytime• Surface latent heat flux is underestimated, large horizontal variability (not

shown)

CTRL-60 IMPL-60

HDIFF-60 IMPL-74

-3D -3D

Validation of COSMO-3D – Temperature and humidity profiles 2003-05-30 12 UTC

• 60 level runs are very similar to diffusive in the entrainment zone• 74 levels bring improvement in the PBL

CTRL-60 IMPL-60

HDIFF-60 IMPL-74

Potential temp. [K] Potential temp. [K]Specific hum. [kg/kg] Specific hum. [kg/kg]

Validation of COSMO-3D – TKE timeseries at 90 m

• Experiments give similar results

• Maximum of TKE is earlier in the model

• Night time TKE is overestimated by the model

• Fairly accurate results for TKE with „wrong” surface heat flux?

CTRL-60 IMPL-60

HDIFF-60 IMPL-74

COSMO-3D: TKE budget terms

• Similar results to the idealized convective case

• Turbulent transport of TKE is too weak

• Negative buoyancy flux at PBL top is missing

• Higher vertical resolution in the entrainment zone and implicit formulation are improving the results

CTRL-60 IMPL-60

HDIFF-60 IMPL-74

LES – TKE budget profile

COSMO General Meeting 7 September 200 9 , Offenbach, Germany

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