1. USER'S GUIDE TO THE FORECAST MODEL

1.1. Introduction

The NMC Spectral Model became operational in l980. The original code, designed by J. Sela, was developed for the IBM computer and has been documented in detail (J. Sela, l980, 1982). Major revision of the code was made in the period between l984 and l986 to adopt the model to the Cyber 205 vector machine, and to incorporate the GFDL physics package.

Recently, the code has been modified to incorporate various diagnostics on dependent variables, surface parameters and physical processes for the Dynamical Extended Range Forecast (DERF) experiments. This documentation explains Version 3 of the DERF. Future (planned) modification to the code is presented in Appendix 8A.

1.2. Programs

In order to perform forecasts, several programs are needed to pre-process the input analysis field, prepare the surface field and post-process the predicted results. These programs also require some documentation in this section.

1.2.1. Pre-processor (SMI)

This program is needed only when initial states are available on pressure levels. In the operational system, analysis increments on the standard pressure levels are interpolated and incremented directly on the model sigma level initial guess field so that this program step is not required. The major function of the program is to convert the initial pressure level spherical coefficient data to grid point values, interpolate to sigma levels and convert back to the spherical coefficients.

1.2.2. Surface Merge (ASTC)

The program creates and/or merges surface data fields from climatology, analyzed sea surface temperature, and predicted surface fields. It also interpolates the monthly climatology to the date and time specified, and perfor ms the SST correction for spectrally truncated topography.

1.2.3. Radiation (HEAT)

The program computes long wave and short wave radiative heating rates and short and long wave flux at the surface and passes them to the forecast program. It also computes land-surface temperature from an energy balance at the surface for dead start. Inputs to the program are the sigma file generated either by the analysis or by the pre-processor program, and the surface fields created by the surface merge program. Outputs of the program are the short wave and long wave heating rates on the model Gaussian grids, modified surface fields (surface temperature and deep soil temperatures) for the dead start and two types of diagnostic files. This program is normally executed every l2 forecast hours but this interval can be modified by a change in the job control structure.

1.2.4. Forecast (SMF)

The program performs time integration and initialization. Input files to this program are the three output files from the radiation, i.e. sigma level dependent variables, surface fields and the radiative heating. The program also requires normal modes for the initialization. The outputs of the program are the predicted dependent variables, predicted surface parameters and four types of diagnostic fields.

1.2.5. Post Processor (SMP)

The predicted fields on the model sigma levels are interpolated to the standard pressure levels in this program. The input file consists of the sigma level dependent variables and the output file consists of the pressure level variables, which can be used as input to the preprocessor program (see 8.2.l). The file also contains several processed arrays (e.g. boundary layer parameters and tropopause parameters).

1.3. Input and Output

The files used in the forecast system are classified into the following types:

- Forecast/diagnostic files

- Constant files

- Namelist files

1.3.1. Forecast/Diagnostic Files

The Forecast/Diagnostic files contain the fields produced by the analysis/forecast system and are listed as follows:

l. Pressure file

Contains atmospheric variables on standard pressure surfaces; input to pre-processor and output of the post processor; created from sigma files (require model sigma levels and orography). (See Table 8.1 for the detailed structures.)

2. Sigma file

Contains atmospheric variables on sigma surface and model sigma levels as well as topography; input to the radiation and forecast program; output from forecast program; created by the model integration. (See Table 8.2 for the file structure.)

3. Surface file

Contains surface parameters; output from the surface merge program and from the forecast program; created from surface climatology and sea surface temperature analysis at the initial time and modified by the forecast pr ogram. (See Table 8.3 for the file structure.)

1.3.2. Diagnostic Files

There are six kinds of diagnostic files; two (H2D and H3D) are outputs from the radiation program and the rest are from the forecast program. The structure of the files is presented in Appendix 8B.

l. H2D: Various radiative fluxes, i.e. short wave and long wave at the surface and at the top of the model.

2. H3D: Three dimensional radiative cooling rates for short and long waves.

3. F2D: Surface parameters, precipitation, surface fluxes and stresses.

4. F3D: Three dimensional distribution of latent heating and moistening due to various physical processes

5. Zonal: Zonally averaged sigma level variables and physical diagnostics.

6. Grid point file: Time series of various and physical diagnostics for given grid points.

1.3.3. Constant Files

The major components of the constant files are the surface climatologies, the sea surface temperature analysis, and the model normal modes. Topography is also included in one of these files although it is also included in t he sigma file. The model sigma levels comprise another set of important constants but are specified in the program using a DATA statement. The list of the constant files and their structures are given in Table 8.4.

1.3.4. Namelist Files

The execution of the programs can be controlled by the namelist input parameters. Under normal conditions, it is not necessary to specify any namelist parameters since default values are set for standard runs. Details of t he namelist parameters for the major programs are listed in Table 8.5 to 8.9.

Table 8.1 Structure of a Pressure File

Pressure File (record number)	Contents	Length (bytes)	Type
1	see NMC Office Note 85	32	binary
2	forecast hour	4	real
	initial hour , month , day , year	4(each)	integer
3	Spherical Coefficients of Geopotential , Z , at 1000 mb	MDIM^(1) * 4	real
4	U-wind component at 1000 mb	MDIMV^(2) * 4	real
5	V-wind component at 1000 mb	MDIMV^(2) * 4	real
...	at standard p-levels	"	"
36,37,38	Same at 50 mb	"	"
39,...,	Spherical Coefficients of Relative Humidity at 1000 mb^(4) , then next five standard levels ending at...	MDIM * 4	real
44	300 mb	"	"
45	Temperature at 1000 mb	"	"
...	at standard p-levels	"	"
56	... at 50 mb	"	"
57	w at 1000 mb^(5)	"	"
...	at standard p-levels	"	"
66	... at 100 mb	"	"
67	Surface pressure (mb)	"	"
68	Mean RH between surface and 500 mb	"	"
69	Mean RH between model layers 2,3	"	"
70	Mean RH between model layers 4,5	MDIM * 4	real
71	Total precipitable water (up to 300 mb)	"	"
72	Potential temperature in lowest model layer