CHANGES TO THE ETA MODELS
On February 18, 1997, a series of changes was implemented in both the operational early-Eta (48km/38level) model and the operational meso-Eta (29km/50level). These affect model physics as well as processing and output. A brief description of the changes follows:
1. Model Changes
- Addition of a form-drag scheme.
- Implementation of a more efficient and accurate positive definite advection
scheme for moisture and cloud.
- Eccentric (rather than circular) orbit around the Sun.
- Corrected atmospheric ozone distribution.
- Effect of aerosols on net shortwave radiation added.
- Reduced albedo over snow.
- Model radiation impacted by cloud in each individual model layer, rather than through three deep layers.
- Improved computation of cloud fraction.
- Improved input fields of vegetation greenness fraction and initial soil moisture.
- Enhanced bare soil evaporation.
- Corrections to physical processes related to melting snow.
- Increased the background level of turbulent mixing.
2. Post-Processor Code and Output Changes
- Implementation of a more efficient version of the Eta model post-processing code.
- The following grids will be directly output by the Early Eta model post-processor (complementing the existing grids) at 6-h forecast intervals:
- AWIPS grid 211 (80 km Lambert Conformal over CONUS).
- AWIPS grid 214 (48 km polar stereographic over Alaska).
- The following grids will be directly output by the Early Eta model post-processor
at 3-h forecast intervals:
These additional fields will be accessible via anonymous ftp on the nic server (nic.fb4.noaa.gov or 18.104.22.168). The content and timeliness of the current Eta model products on the OSO server remains unchanged. The additional grids described above will be available on the OSO server in the near future.
- AWIPS grid 212 (40 km Lambert Conformal over CONUS)
- Eta model computational grid on pressure levels.
- Eta model computational grid on model levels.
Coming in the spring will be another set of model changes, and most significantly--the meso-Eta (29km/50level) will replace the early-Eta. The domain of the "new" early-meso Eta will be expanded over the current meso-domain (Figure 1 ), but it will remain smaller than the present early Eta. Here are highlights of other changes:
- The current early-Eta, based on older slow code, takes over 45 min clock time to do a 48 hr forecast. New FAST Eta code can do a 48 hr forecast on the expanded early-meso Eta domain in the same amount of time. Thus, there should be no impact on completion times.
- NCEP will continue to generate LFM look-alike, NGM look-alike, and AWIPS 211
grid products (for AFOS, FAX and FOS). Also AWIPS 212 and 215 (plus Puerto Rico
and Alaska grids).
- Stage 1: Put early-meso Eta in at 00Z and 12Z.
- Stage 2: Add early-meso runs at 06Z and 18Z.
- Data Assimilation System
- Replace OI with 3D-VAR.
- EDAS cycle independent of GDAS.
- Adjustment to observed precip.
- Cloud initialization scheme.
- Radiosonde balloon drift computation.
- 10m level over land (N + 1).
- High terrain convection refined.
- 4-layer soil package.
- New shortwave radiation package.