Modelling activities

Modelling activities

The BLLAST project has a strong modeling component to complete the investigation of the dynamics of the afternoon transition and support the observations taken during the observational period.

The modelling tools:

Up to now the modeling tools available in BLLAST community are:

  • Three-dimensional models (in spatial scale order):
    Large-Eddy Simulations models: Meso-NH, NCAR-LES, DALES
    Mesoscale models: Meso-NH, WRF
    Operational models: ECMWF, Arpege, AROME
  • One-dimensional and conceptual models:
    One-dimensional: MESO-NH, WRF-1D
    Mixed layer model

Research strategy

We plan before, during and after the BLLAST experimental campaign a number of numerical experiments at different spatial and time scales. These are the following:

  • Idealized LES simulations (D. Pino, Z. Sorbjan, H. Jonker, F. Lohou)
  • Realistic LES case based on the observations gathered during the field campaign: (F. Couvreux, F. Guichard, F. Lohou, B. Beare)
  • Evaluation of the real-time outputs of forecasting models (AROME and ARPEGE) in order to determine the behavior of different physical parameterization during this transition phase
  • Evaluation and improvement of boundary-layer parameterizations in models: (E. Bazile, Y. Seity, F. Couvreux, W. Angevine)
  • Case study based on meso-scale simulations: (J. Cuxart, M.-A. Jimenez, M. Jonassen)

Scientific questions:

This section lists the different scientific questions the modeling group will try to answer.

A/ The common questions we should address before a LES study:

1/ How LES models simulate the transition period?

  • Intercomparison of different models (DALES/ Meso-NH/ NCAR) with imposed surface fluxes: same answer for the transition ?
  • Establishment of a real case study constrained as much as possible with the observations of the field campaign in order to complete and support the observational evidence during the afternoon transition
  • Do we have the same answers when intercomparing LES models coupled with a surface scheme?
  • Sensitivity study analysis on impact of large-scale forcing, upper conditions,…
  • Provide a first list of diagnostics to characterize the transition

2/ Can we provide some guidance for the field campaign what to look (which variables)? Where (which levels)? When?

3/ At what time the LES should start to correctly simulate the transition period from the atmospheric turbulence and surface point of view?

4/ Is the response very sensitive to resolution, numerical schemes and sub-grid scale parameterizations?

5/ How do operational models perform in describing the transition? This can help focusing on the major defaults of those models. Focusing in the physical parameterizations: boundary layer schemes, surface representations and radiation.

B/ The atmospheric boundary layer specific questions

1/vertical structure:

  • Determine the different layering and characterize the turbulence characteristics in each layer (scales, anisotropy, role of thermal turbulence versus mechanical turbulence, presence of aerosols)
  • What are the main processes in each layer (radiation, shear, buoyancy, waves, …)?

2/ timing:

  • determine relevant diagnostics for the different layers that enable to characterize the transition (how to define the end of the convective boundary layer? the beginning of the residual layer and the beginning of the stable layer?)? Does the LES agree with observations for the definition of this layering?
  • link between the evolution of the turbulence characteristics and the radiative budget at the surface?

3/ role of different processes:

  • entrainment processes: how does it evolve?
  • Organised spatial structures: how do they evolve? Can they persist? Through which mechanisms? Interaction of boundary layer and mesoscale scales.
  • Wind shear: what is its impact on the transition?
  • Gravity waves?
  • Clouds: Does their existence modify the transition (scales, timing, …) ? Through which processes :radiative impact, venting? Do they play a role in the formation of the residual/stable boundary layer?

C/ Surface processes

1/ What is the impact of the surface characteristics on the transition period?

  • How much do they influence the timing of the transition?
  • Is the Monin-Obukhov/mixed-layer scaling relevant and how long during the transition?

2/ What are the relevant surface parameters for the scaling: heat storage in the cover/ground, Bowen ratio, roughness , …

3/ Over heterogeneous surface

  • Are the differences between each surface patch strong enough to induce some secondary circulations during this transition period?
  • Does the surface patch length scale influence the turbulence length scale?

4/ Impact on in situ measurements:

  • How and what do we measure in this late afternoon transition period with EC systems?
  • Help the BLLAST campaign implementation and organization: measuring strategy with tethered balloon and higher up in the ABL by means of aircraft.

5/ Over topography:

What is the impact of the topography on the transition?

6/ Can we suggest an appropriate scaling which would cover this transition from the well-mixed BL to the stable BL and overlying residual layer ?