Direct model output from the National Centers for Environmental Prediction's (NCEP's) Limited-area Fine-mesh Model (LFM) was produced in Forecast U. S. (FOUS) 60-78 messages (hereafter referred to as FRH)(National Weather Service (NWS) 1981) on AFOS beginning in 1981. The FRH contained analysis and forecast values for a subset of model fields which were interpolated to selected sites. The 80-km Eta model (Early Eta) replaced the LFM as the early look at the synoptic-scale forecast in June 1993 (Black et al. 1993). At that time, the Early Eta became the source of the direct model output given in the FRH. In addition to the new source model, the format of the FRH was changed (Treadon 1993) to correspond with the FOUS messages from the Nested Grid Model (NGM) (hereafter referred to as FRHT; NWS 1985a,b). The Early Eta was updated in October 1995, including an increase in resolution to 48 km (Rogers et al. 1995). The Early Eta-based FRH are available for the same stations as the NGM-based FRHT and obsolete LFM-based FRH.
The information in this Technical Procedures Bulletin (TPB)
is largely based on information provided by Russell Treadon,
formerly of NCEP's Environmental Modeling Center. This TPB
addresses how differences in the NGM and Early Eta models affect
the manner in which the model fields get interpolated to the FOUS
data points. For a more complete discussion on the FRHT, see
Technical Procedures Bulletin No. 3531 (NWS 1985b) and
2. EXPLANATION OF BULLETINS
Early Eta-based direct model output guidance is generated in a manner similar to that produced from the NGM. FRH and FRHT products list forecast values for corresponding model layers. However, since the Early Eta uses a vertical coordinate system that is much different from that of the NGM, some interpolation is needed in the post-processed output fields.
In the NGM, the direct model output guidance is obtained from the sigma layers used in the model. In fact, many of the FRHT variables are defined by sigma layers. In the Eta model, an eta coordinate system (Black et al. 1993; Mesinger 1988) is used for the vertical structure. Hence, to obtain model output values that represent comparable layers, variables from the appropriate Eta model layers are interpolated to the approximate mid-point pressure level of the corresponding NGM sigma layers. Table 1 lists the terrain height and normal surface pressure for each of the FRH stations so that the forecaster can determine where lower layers are at or below the surface. These were obtained by horizontal interpolation of the model terrain height from the forecast grid-points to the station location, followed by a calculation of the pressure at this height in the U. S. Standard Atmosphere. Note that Table 1 is only valid for this specific 48-km configuration of the Eta model.
A sample FRH bulletin and a summary explanation of the FOUS variables are shown in Figure 1. A series of post-processing interpolations are done to obtain the Early Eta forecast temperatures for the first (T1), third (T3), and fifth (T5) sigma layers, the wind components in the first sigma layer (DDFF), and the relative humidity in the first sigma layer (R1). Recall that the sigma layers correspond to those defined in the NGM. The Early Eta-based relative humidities R2 and R3 correspond to layer means from approximately 35 mb above the surface to 500 mb and from approximately 500 mb to the tropopause, respectively.
The 700-mb vertical velocity is an instantaneous value as it
is in the FRHT. The lifted index (LI) is based on the six lowest
Eta model layers (each 30 mb deep) above the Eta model surface.
The definitions of all other variables in the FRH (PTT, PS, and
HH) coincide with those used in the FRHT and are given in
Figure 1. For a more detailed discussion on the vertical interpolation
from eta to sigma levels, see Treadon (1993).
The Early Eta FOUS messages are available on AFOS and to the
Family of Services at approximately 0400 and 1600 UTC for the
groups (bulletins) of stations given in Table 2. Table 2 also
lists the AFOS product identifier (PIL) and the World
Meteorological Organization (WMO) bulletin headers. Map views of
the areas corresponding to each bulletin are shown in Figure 2.
TPB No. 294 (NWS 1981) provides latitudes and longitudes for some
of the oceanic sites.
I wish to recognize and thanks the following people for their
contributions to this TPB: the reviewers for their comments that
improved this paper, Paul Dallavalle for technical help on in
getting complete and accurate data, and Captain Fred Meyer,
U. S. Air Force for the graphics.
Black, T., D. Deaven, and G. DiMego, 1993: The step-mountain eta coordinate model: 80 km 'Early' version and objective verifications. NWS Technical Procedures Bulletin No. 412, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 31 pp.
Mesinger, F., Z. I. Janic, S. Nickovic, D. Gavrilov, and D. G. Deaven, 1988: The step-mountain coordinate: model description and performance for cases of Alpine lee cyclogenesis and for a case of an Appalachian redevelopment. Mon. Wea. Rev., 116, 1493-1518.
National Weather Service, 1981: FOUS60-78 Bulletins. NWS Technical Procedures Bulletin No. 294, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 11 pp.
_____, 1985a: Pre-implementation results from the Regional Analysis and Forecast System (RAFS). NWS Technical Procedures Bulletin No. 350, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 13 pp.
_____, 1985b: FOUS messages from the RAFS. NWS Technical Procedures Bulletin No. 351, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 7 pp.
Rogers, E., T. Black, D. Deaven, G. DiMego, Q. Zhao, Y. Lin, N. Junker, and M. Baldwin, 1995: Changes to the NMC oper-ational Eta model analysis/forecast system. NWS Technical Procedures Bulletin No. 423, National Oceanic and Atmos-pheric Administration, U.S. Department of Commerce, 51 pp.
Treadon, R., 1993: The NMC Eta model post processor: a documentation. NMC Office Note No. 394, NWS, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, 42 pp.