1. For the Lesson: Eta Characteristics, Biases, and Usage December 1998 ETA-32 MODEL CHARACTERISTICS

2. ETA-32 Job Stream  48-h forecasts twice a day at 0000 and 1200 UTC  33-h forecast at 0300 UTC  30-h forecast at 1800 UTC

3.  32-km replaced the 48-km configuration  Was a compromise among several factors –Increasing the resolution of the Early Eta system to be as close as possible to the Eta-29 –Keeping the model horizontal domain size nearly the same as the current 48-km grid  Eta-32 output is available on the same 80-km grids as the Eta-48 Horizontal Resolution

4. Discussion Questions  With 32-km horizontal resolution, what types of phenomena can the Eta be expected to resolve?  What effects does the remapping of model output to an 80-km grid have on the resolution of features?

5. Horizontal Domain  Eastern boundary of 32-km grid captures as much of tropical Atlantic as possible and keeps Puerto Rico inside domain  Northern boundary for Alaska virtually unchanged  Biggest difference - along the western boundary, Hawaii is much closer to the boundary than with 48- km grid

6. Vertical Resolution  45 vertical layers  Better distribution of layers over high terrain than Eta-48 (38 levels)  Not as good vertical definition as Eta-29 (50 levels)  Represents a compromise between 38 levels in Eta-48 and 50 levels in Eta-29

7. Discussion Questions  What considerations should be taken into account about the vertical resolution of the boundary layer when using Eta model guidance within your CWA?  Why is it important to have greater vertical resolution within the boundary layer?  Where else in the atmosphere would model forecasts benefit from greater vertical resolution?

8. Sigma Coordinate Versus Eta Characteristics of terrain representation result in computational differences in basic model equations –Compute temperature and pressure gradient terms differently –Can introduce large errors near steep or complex terrain

9. Sigma Coordinate  Near sloped terrain, temp. changes on a sigma surface are partially a result of hydrostatic temperature changes due to change in elevation –Vertical temperature gradient much larger than horizontal temp. gradient –Vertical gradients have dominating influence on pressure gradient calculation –Leads to large temperature errors, especially near steep terrain in the sigma terrain following coordinate system

10. Eta Coordinate  Eta coordinate reduces errors in computing PGF, advection, and diffusion near steep terrain –Result of surface terrain heights at discrete sets of values or steps

11. Eta Coordinate Continued  Values or steps dependent upon vertical resolution of model and mountain height –Terrain appears step-wise rather than smooth and continuous as in the sigma coordinate –For a given range of elevations, the eta coordinate allows the terrain to exist on more than one eta surface –In the sigma coordinate, the terrain can only exist on one sigma surface

12. Discussion Questions  Why is it important for a model to accurately solve the basic equations of motion and thermodynamics?  What effects can large errors in the temperature advection and gradient fields have on other model forecast fields such as winds, pressure, vertical motion, and precipitation?  What types of adjustments may be necessary to account for computational errors in these fields?

13. Eta Terrain Representation  Model terrain much smoother than in reality, even in the eta coordinate  Terrain smoothing can be large source of error in regions affected by small-scale terrain features –Terrain smoothing done partly because airflow over complex terrain can generate small-scale noise in the model –Small-scale noise can mask larger-scale signal

14. Eta Terrain Representation Continued  Eta model uses step-mountain topography –The step- mountain is raised or lowered to closest vertical interface after interpolation to eta native grid

15. Eta Terrain Representation Continued –Mountains represented as discrete steps whose tops coincide exactly with model layer interfaces

16. Eta Topography: West U.S.  Model resolution affects depiction of topography  Eta-29 and Eta-32 models show considerably more detail than Eta-48  Better definition of Sierra Nevada and Cascade ranges in Eta-29 and Eta-32

17. Eta Topography: West U.S. Continued  Exception - between Eta-29 and Eta-32 in the Great Basin in northern Nevada –Eta-29 terrain shows most of the region at one elevation –Eta-32 depicts this region on 3 different steps

18. Eta Topography: CONUS  Over the contiguous U.S., mountains spread over a slightly greater horizontal domain than in reality  Terrain averaging over each grid box causes model representation of terrain slope to be too shallow  Can affect model vertical motion and precipitation forecasts

19. Eta Topography Effects: Vertical Motion  Insufficient terrain slope in model results in vertical motion field’s being shifted away from mountains and steepest terrain  In example, inadequate definition of Sierra Nevada shifted maximum vertical motions westward away from the steepest topography

20. Eta Topography: Precipitation  Impact of terrain smoothing - misplacement of precipitation in vicinity of complex terrain  For this example, precipitation field shifted west of the highest/steepest terrain  Eta model often predicts precipitation too far west, away from mountain peaks

21. Precipitation Verification Observed precipitation greater than Eta forecast Heaviest amounts concentrated near higher terrain Much lesser amounts in valleys

22.  If terrain is a concern in your area of responsibility –How will the Eta’s terrain resolution and treatment of terrain influence its forecasts of precipitation? –What adjustments to the model forecast would be necessary within your forecast area based on known terrain features and Eta model characteristics? –Would the adjustments to model forecasts be regime dependent? If so, how might they vary? Discussion Questions