gad levy northwest research associates and jordan c. alpert ncep/environmental modeling center

E1 Levy & Alpert: Impact of Scat-based sub-grid flux parameterization on NCEP GPS- Seattle OVWST

The Impact of Scatterometer-based Sub-grid Flux Parameterization on Medium Range Global

Forecasting in the Operational NCEP GFS

Gad LevyNorthWest Research Associates

andJordan C. Alpert

NCEP/Environmental Modeling Center

http://www.nwra.com/resumes/levy/papers/

March 24,2005: OVWST Meeting


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.


Objective and Method

o Test the impact of a subgrid parameterization (Levy and Vickers, 1999 and Levy, 2000) on the NCEP GPS (operational forecasting model).

Sensitivity tests:o Controlo Parameterization applied

with land mask (ASF1)o Parameterization applied

globally (ASF2)


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;

Foster et al. poster

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Fφ = ρ ⋅Cφ ⋅V(Φ sfc − Φ)

U = (V2 + Vsg2)1/2

Vsg =(X/D-1)


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)


Effect of Parameterization on Fluxes

Estimated based on climatology, Quickscat, and assumed 250 km grid

Latent Heat Sensible Heat Momentum


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 initially 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


The Synoptic Scenario (1000 mb)


Latent Heat Flux Evolution (ASF2)


Precipitation Systems Evolution


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)


Impact on precipitation


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.)


Current & Future Work

o Recover variability at smaller scales and in coastal areas

o Extend parameterization over lando Formulate as a function of region and regimeo Study interaction between subgrid and Cumulus

parameterizationso Compare impact to observationso Test impact on model ‘climatology’

gad levy northwest research associates and jordan c. alpert ncep/environmental modeling center

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