For the Lesson: Eta Characteristics, Biases, and Usage

December 1998

ETA-32 MODEL CHARACTERISTICS

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

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

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?

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

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

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?

Sigma CoordinateVersus 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

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

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

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

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?

Eta TerrainRepresentation

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

Eta TerrainRepresentation Continued

Eta model uses step-mountain topography– The step-

mountain is raised or lowered to closest vertical interface after interpolation to eta native grid

Eta TerrainRepresentation Continued

– Mountains represented as discrete steps whose tops coincide exactly with model layer interfaces

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

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

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

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

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

Precipitation Verification

• Observed precipitation greater than Eta forecast

• Heaviest amounts concentrated near higher terrain

• Much lesser amounts in valleys

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?

DiscussionQuestions