1. 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

2. 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.

3. 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 Control o Parameterization applied with land mask (ASF1) o Parameterization applied globally (ASF2)

4. 10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization - 9th IOAS-AOLS Symposium The Parameterization 1 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. 1 Levy & Vickers, 1999; Levy, 2000 U = (V 2 + V sg 2 ) 1/2 V sg =  (  X/D-1) 

5. 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)

6. 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 HeatMomentum

7. 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

8. 10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization - 9th IOAS-AOLS Symposium The Synoptic Scenario (1000 mb)

9. 10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization - 9th IOAS-AOLS Symposium Latent Heat Flux Evolution (ASF2)

10. 10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization - 9th IOAS-AOLS Symposium Precipitation Systems Evolution

11. 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 locations o Significant (20%-80%) impact in LHF in convective (tropical) systems shows early; reflected in convective precipitation o 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)

12. 10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization - 9th IOAS-AOLS Symposium Impact on precipitation

13. 10.1 Levy & Alpert: Model sensitivity to sub-grid flux parameterization - 9th IOAS-AOLS Symposium Summary and Conclusions o 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 distributed o 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.)