MODEL SENSITIVITY TO SPACE- AND IN SITU- BASED SUB-GRID FLUX PARAMETERIZATION
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10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium MODEL SENSITIVITY TO SPACE- AND IN SITU- BASED SUB-GRID FLUX PARAMETERIZATION Gad Levy NorthWest Research Associates and Jordan C. Alpert NCEP/Environmental Modeling Center http://www.nwra.com/resumes/levy/ papers/ January 13,2005: 85thAMS Meeting
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MODEL SENSITIVITY TO SPACE- AND IN SITU- BASED SUB-GRID FLUX PARAMETERIZATION. Gad Levy NorthWest Research Associates and Jordan C. Alpert NCEP/Environmental Modeling Center http://www.nwra.com/resumes/levy/papers/ January 13,2005: 85thAMS Meeting. Motivation & Background. - PowerPoint PPT Presentation
Transcript of MODEL SENSITIVITY TO SPACE- AND IN SITU- BASED SUB-GRID FLUX PARAMETERIZATION
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
MODEL SENSITIVITY TO SPACE- AND IN SITU- BASED SUB-GRID FLUX PARAMETERIZATION
Gad LevyNorthWest Research Associates
andJordan C. Alpert
NCEP/Environmental Modeling Center
http://www.nwra.com/resumes/levy/papers/
January 13,2005: 85thAMS Meeting
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Motivation & Background
o Significant uncertainty in model fluxes due to unresolved subgrid variability.
o Some variability is consistent and can be parameterized based on observations.
o Large-scale atmospheric models surface fluxes are sensitive to unresolved (subgrid) variability in the wind (bulk formulas: function of the surface winds).
o Advances in observing systems allow parameterizing subgrid flux due to wind directional variability.
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Objective and Method
o Test the impact of a subgrid parameterization (Levy and Vickers, 1999 and Levy, 2000) on the NCEP operational forecasting model.
Sensitivity tests:o Controlo Parameterization applied
with land mask (ASF1)o Parameterization applied
globally (ASF2)
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
The Parameterization1
o Accounts for subgrid variability due to directional variability.
o Formulated as a velocity scale term added to current model bulk formulation.
o Formulated as a function of model grid scale.
o Based on a best fit of collocated scatterometers (2) and buoy data.
1Levy & Vickers, 1999; Levy, 2000
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Fφ = ρ ⋅Cφ ⋅V(Φ sfc − Φ)
U = (V2 + Vsg2)1/2
Vsg =(X/D-1)
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
The Parameterization (cont’d)
o Some seasonal and regional variability is evident in data used
o Flux enhancement) is largest for low winds & for larger grids.
o Satellite (ERS, NSCAT, Qscat) /buoy results consistent with aircraft results (including over land and equatorial)
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Effect of Parameterization on Fluxes
Estimated based on climatology, Quickscat, and assumed 250 km grid
Latent Heat Sensible Heat Momentum
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Model; Experimental Design; Limitations
o NCEP global forecasts system (GFS) model used
o Spectral model using a 64 level sigma vertical coordinate system
o Triangular truncation of T254 with a physical Gaussian grid of 768x384
o Existing model flux parameterization already include stratification effects; low-wind minimum
o Model run globally
o Mean values for Vsg used (regional variation neglected)
o Vsg likely underestimated over land
o Only one case (5 days forecast with initial conditions from 8 February 2004 ) run
o Impact of momentum flux enhancement hard to assess in non-coupled model
o Focus on W. Hemisphere
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
The Synoptic Scenario (1000 mb)
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Latent Heat Flux Evolution (ASF2)
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Precipitation Systems Evolution
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Impact on Heat Fluxes
o All significant impacts limited to just a few locationso Significant (20%-80%) impact in LHF in convective (tropical)
systems shows early; reflected in convective precipitationo Impact in LHF at mid/high latitudes shows later; limited to frontal
systems; reflected in geopotential and (total) precipitation redistribution
o Significant impact in sensible heat flux limited (frontal & land)
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Impact on precipitation
10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization- 9th IOAS-AOLS Symposium
Summary and Conclusionso Parameterization has significant direct (flux
enhancement) and indirect (convective and total precipitation, synoptic wave amplification) impacts on GFS forecasts
o Significant impacts are not evenly distributedo Impacts on middle and higher latitude systems:
• amplify the synoptic waves and change their propagation• redistribute stratiform and convective precipitation• impacts develop (noted) later (at 96 hrs. or 120 hrs.)
o Impacts on equatorial and tropical systems:• enhance latent heat fluxes • enhance tropical convective precipitation• Impacts develop early (at 24 hrs.)
