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The Version 1 header has been in use since 1994. During the period since, several ways to improve on it became apparent, and these are put into effect with Version 3. Firstly, Version 1 files are unnecessarily long because before every time period there are about 3.5 Mbytes of header record, only a small fraction of which is used. Version 1 had a first effort at a format that was self-describing, in that the header contains information about all the fields in the file. However it was made a little awkward to use because this information on each field is buried in the header. When adding a new field, the location and order in the header had to be consistent, as well as updating the total number of fields, so the files were difficult to manipulate.

Version 3 improves in both respects: the files are shorter, and it becomes easier to add or retrieve selected fields. The length of the header is reduced. The header still has 20 sections (second index which indicates program name), but now only 50 integers, and 20 reals, together with their 80-character descriptions, are in each section. This makes the header size a little over 100 kbytes. Moreover, there is only one header which is at the beginning of the file, but there is enough generality in the format to allow more headers at other times such as at the beginning of a restart run or when a nest moves. The header still contains information about the preprocessor options, and domain characteristics and location. However it no longer contains 1-dimensional fields, such as sigma or pressure levels, nor information about what is in the rest of the file. Version 3 introduces the concept of a sub-header, a 1-record description directly ahead of each field. This description includes information on the name, dimensionality, index order, index range, size and time of the following field. Flags in the file indicate whether to read a "big header'', or sub-header and field, or whether it is the end of a time period. It can be seen that it is easy to insert a field as long as it is accompanied by a relevant flag and sub-header. It is also easy to search for a given field by reading sub-headers until a match is found then reading the following field.

An MM5 Version 3 modeling system output file contains the following records:

 (first time period)
                 big header flag (integer value of 0)
                 big header
                 sub-header flag (integer value of 1)
                 sub-header
                 field
                 sub-header flag (integer value of 1)
                 sub-header
                 field
                 sub-header flag (integer value of 1)
                 sub-header
                 field
                 ....
                 ....
                 end-of-time-period flag (integer value of 2)
                 (second time period)
                 sub-header flag (integer value of 1)
                 sub-header
                 field
                 sub-header flag (integer value of 1)
                 sub-header
                 field
                 sub-header flag (integer value of 1)
                 sub-header
                 field
                 ....
                 ....
                 end-of-time-period flag (integer value of 2)
                 (and so on ....)

No particular order of fields is assumed, other than that they are chronologically grouped. When reading files in the modeling system, each field has to be read and matched to an expected 8-character name before being assigned to a variable in the program. Note that 1D, 2D, and 3D fields could be mixed, but that the sub-header gives enough information to assign an appropriate array to the read statement.

Thus, a simple read program would look like:

 10 continue
                 read (input_unit, end=900) flag
                 if (flag.eq.0) the
                 read (input_unit) big header
                 go to 10

                 else if (flag.eq.1) then
                 read (input_unit) sub header
                 read (input_unit) field
                 go to 10
                 else if (flag.eq.2) then
                 print *, `end of time period'
                 go to 10
                 end if
                 900 continue

In addition, the boundary condition file in V1/V2 system is separated into two files with one containing only the lateral boundary arrays, and the other the substrate temperature array (lower boundary condition file). Both files are brought to the same file structure as the rest of modeling system output files.

Sigma-level data in V3 are no longer coupled (e.g. multiplied by p*).

The units in V3 header are mostly MKS, e.g. pressure unit is Pascal, grid distance is in m, etc.

2.1 Big header

The big header has four 2-D arrays similar to that in the V1/V2 system, which we refer to in the V3 modeling system programs as

   BHI,BHR,BHIC,BHRC

and the dimensions of these arrays are

   BHI(50,20),BHR(20,20),BHIC(50,20),BHRC(20,20)

where BHI is an integer array, and BHIC is the companion array that contains the description of what is in BHI. Similarly BHR is a real array, and BHRC contains the description of what is in BHR.

The first value in the header, BHI(1,1), still represents data types. But there are some changes as shown below:

BHI(1,1)		Data Types

1		Terrain
2		Regrid
3		Rawins or little_r
4		Rawins' surface analysis
5		Model initial condition file
6		Model lower boundary condition file (substrate temp array)
7		Model lateral boundary condition file
8		Interpolated model output on pressure levels
11		Model output

MM5 model output actually occupies header locations 11 through 16.

2.2 Sub header

A sub-header contains the following information:

where

ndim:

integer

dimension of the field (integer)

start_index:

integer(4)

starting indices of the field array (generally 1's)

end_index:

integer(4)

ending indices of the field array (generally IX, JX, KX, and 1) (the fourth dimension is not yet used)

xtime:

real

the integration or forecast time for this field

staggering:

char*4

whether the field is at dot or cross point (character C or D)

ordering:

char*4

the order of the field array dimension (4-character string with the following values)
YXP:	3-D field, pressure data dimensioned by (IX,JX,KXP)
YXS:	3-D field, sigma data dimensioned by (IX,JX,KXS)
YXW:	3-D field, sigma data dimensioned by (IX,JX,KXS+1) (e.g. vertical motion in MM5)
YX:	2-D field, with array dimensioned by (IX,JX) with IX in Y direction)
CA:	2-D field, with array dimensioned by (landuse-categories,2). Arrays to store land property values, such as albedo, roughness length, etc.
XSB:	3-D field, containing north and south boundary arrays, dimensioned by (JX,KXS,5)
YSB:	3-D field, containing west and east boundary arrays, dimensioned by (IX,KXS,5)
XWB:	3-D field, containing north and south boundary arrays for vertical motion, dimensioned by (JX,KXS+1,5)
YWB:	3-D field, containing west and east boundary arrays for vertical motion, dimensioned by (IX,KXS+1,5)
P:	1-D field, pressure level array
S	: 1-D field, sigma level array

current_date:

char*24

24-character representation of date valid for this field

name:

char*9

8-character field name (kept the same as in Version 1/2 system)

unit:

char*25

25-character unit description

description:

char*46

field description (kept mostly the same as in Version 1/2 system)

Version 3 MM5 Modeling System At A Glance