Variables

The ability to specify a specific parameter to plot is generally the last major item to determine. Variables can be those contained in the input files or a hybrid variable based on the simple variables.  WXP provides a large range of functions to generate new variable types.

The simple variables are determined by input file type.  The hybrid variables are determined through use of a var file.  This file can be used to create hybrid variables as well as specify plot information such as color, maps, and multi-panel plots.  Variables can be aliased to other variables if needed.  Each plotting program has an associated var file.

Menu

The var file specifies which variables are to appear in this menu and it is dependent on the program.

        Variable Menu

  1: All Data                        13: Cloud Cover                   
  2: Temperature                     14: Pressure Tendency             
  3: Dewpoint                        15: 24 Hr Precip                  
  4: Wind Direction                  16: 24 Hr Maximum Temp            
  5: Wind Speed                      17: 24 Hr Minimum Temp            
  6: Wind Gust                       18: Snow Depth                    
  7: Wind Barb                       19: Wind Chill Temp               
  8: Sea Level Pressure              20: Heat Index                    
  9: Altimeter Setting               21: Relative Humidity             
 10: Present Weather                 22: Equiv Potential Temp          
 11: Visibility                      23: Convergence                   
 12: Cloud Ceiling                   24: Rel Vorticity                 
  0: Return to previous menu         -1: Exit program                  

Enter the variable:

This menu only lists a small subset of what WXP can calculate.  Again, you can list the resource values with the "?" but this will only list those values enabled in the menu.  If you want all the available values, you can use the "list" command. The items in the variable menu can be tailored on and off within the ".var" variable menu file. Special variable definitions are specified in that file as well.

Specifying Variables

There are several methods for specifying variables.  The simplest is to provide the alias as listed in the var file.  

   temp    Temperature         1  +temp    [F]

In this case, you would specify "temp".  This will plot the simple variable "temp".  Simple variables are specified by a "+" prefix.  From the var file, the default plot parameters are to plot the temperature in F (Fahrenheit).

To modify the units of a particular variable, just put the desired units after the variable name separated by a colon ":":

Enter the variable: temp:C

The plot parameter information can be more extensive:

Enter the variable: temp:C:cf:in=5

In addition to changing the units, the plot type is set to color fill with a contour interval of 5.  

Hybrid variables can also by specified for the variable resource:

Enter the variable: conv(wind)

which will compute the convergence of the real winds.  The value of wind is aliased in the var file to the U and V wind components specified as a vector quantity.  You can generate vector quantities with some functions:

Enter the variable: grad(temp)

This will compute the gradient of temperature.  Then by specifying a plot type of vectors, the vector field will be plotted. 

Variables in GRIB files

The variables in a GRIB file are specified using a single number that is cross-referenced to a simple variable in the variable.lup lookup file.  In this file, the numbers from 1-255 are reserved for GRIB parameters.  Numbers above 255 are user configurable.

   8  hght  Height
  11  temp  Temperature

This file can be used to access GRIB products not normally in the Family of Services feed.  In some cases, the simple variable can mean many different GRIB variables.  A good example of this is pressure where there are 4 possible pressure values.  In this case, the lookup file contains the possibilities separated by commas:

   1,2,128,129,130  pres  Pressure

This way, the value "pres" can be used to capture all of these possible GRIB pressure products.

To get a feel for what is needed to correctly specify a variable in a GRIB file, use the griblook program. By setting the message level to out3 a listing of the parameters is printed out:

448: 12 hour ETA valid 12Z MON 29 SEP 97-500 mb Abs vorticity (/s)
     Model: ETA  Time: 12hr         Level: 500mb        Variable: avort   Units: /s

The specific time, level and variable name are listed. You can use these directly in programs like grbcalc.

For more information on the variables available, see variable.lup