Vtables
The Vtables are the means by which the standard pregrid programs
are told which fields to extract from the analysis files. Tables like these
are expedient, because the archives tend to label fields by code numbers,
and different archives frequently use different code numbers. The
Vtables also allow users to request that additional fields be extracted
from the archives, without having to modify the pregrid programs
themselves. The Vtable files have a fairly strictly defined format:
-
Any lines before the first line beginning with "-" are ignored.
-
All lines after the first line beginning with "-", and up to the next line
beginning with "-", are interpreted.
-
Any subsequent lines after the next line beginning with '-' are ignored,
until the next line beginning with '-' is hit, and then the following lines
are interpreted. This pattern continues until the end of the file.
-
Any lines beginning with "#" are ignored. This allows for comments.
-
A corollary to the above points is that multiple Vtables, each with its
own header lines, can be combined via the UNIX "cat" command and the result
is still a valid Vtable format.
For each line to be interpreted, there are the following fields,
separated by "|" (vertical bar, not an exclamation point):
-
A code number associated with the variable to extract
-
A level code number associated with the type of level to extract. This
generally refers to pressure levels, surface, or ground.
-
A "Level 1" value, denoting which level to extract (* indicates all levels
of the type of level described by the level code).
-
A "Level 2" value, needed for those fields which represent averages or
sums over more than one level.
-
REGRID Name, up to nine characters, which is what the field will be called
in the regridder output, and in subsequent programs of the MM5 system.
-
REGRID Units, a string of up to 25 characters which describes the units
of the field. This is a description of the units as used in the archives.
Do not try to change the units of a field by changing the REGRID Units
string.
-
REGRID Description, up to 46 characters describing the field.
Example:
GRIB | Level| Level| Level| REGRID |
REGRID | REGRID
|
Code | Code | 1 | 2 | Name
| Units | Description
|
-----+------+------+------+----------+----------+------------------------------+
11 | 100 | * |
| T | K
| Temperature
|
33 | 100 | * |
| U | m s{-1} | U
|
34 | 100 | * |
| V | m s{-1} | V
|
52 | 100 | * |
| RH | %
| Relative Humidity
|
7 | 100 | * |
| HGT | m
| Height
|
11 | 105 | 2 |
| T | K
| Temperature at 2 m
|
33 | 105 | 10 |
| U | m s{-1} | U at 10
m
|
34 | 105 | 10 |
| V | m s{-1} | V at 10
m
|
52 | 105 | 2 |
| RH | %
| Relative Humidity at 2 m |
2 | 102 | 0 |
| PMSL | Pa
| Sea-level Pressure
|
-----+------+------+------+----------+----------+------------------------------+
There are a few subtleties built into the Vtable usage:
-
If the REGRID Description field is left blank, pregrid will read
the named field from the archives, but will not write it to the final output
(for input to regridder).
-
Certain fields may be computed "on the fly", with fields that have already
been read.
A certain situation often occurs with humidity variables. The
MM5 preprocessors use relative humidity, yet the archives may specify specific
humidity, mixing ratio, dewpoint, or other humidity variables. So
the pregrid program has to compute relative humidity given the fields
available to it. The Vtable must include an entry for relative humidity,
or the pregrid program will not write out a relative humidity field.
But this Vtable entry should not have a "GRIB code" field. The Vtable
must also include an entry for the alternate humidity variable, or the
pregrid program will not read that variable, and so will not be able to
compute relative humidity. So this entry would not include a "REGRID
Description" field. The Vtable might then include the lines:
52 | 100 | * |
| SPECHUMD | kg kg{-1}|
|
| 100 | * |
| RH | %
| Relative Humidity
|
Fields which may be computed with pregrid are:
-
Relative humidity, from specific humidity, pressure, and temperature.
-
Relative humidity, from dewpoint, pressure, and temperature
-
Relative humidity,
-
Height (m), from geopotential (gpm)
This list may grow as users encounter various situations in
which a computed field is necessary.
If you need to make your own Vtable, there are plenty of
examples in the grib.misc directory.
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Last modified: Tue Jun 6 13:54:27 MDT 2000