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Mathematics Programs

Raw Data File Mathematics

The program to perform mathematics on raw data file is called rawmath. Raw data can be either entered by hand or created by a program such as sfcwx or upairwx. Once a raw data file has been created, math can be performed on it using this program. The output of these computations can either be sent to mapplt for plotting or grid for gridding and contouring. The operations available with rawmath are:

rawmath rawa add rawb = rawc
rawmath rawa sub rawb = rawc
rawmath rawa mul rawb = rawc
rawmath rawa div rawb = rawc
rawmath rawa mod rawb = rawc
Standard functions add, subtract, multiply, divide and modulus
rawa, rawb, rawc are raw data files
rawmath rawa add #number = rawb Add a constant number to a station.
rawmath rawa mul #number = rawc Scale a value by number
rawmath sum rawa rawb ... = rawd Sum station values
rawmath ext rawa = rawd Extract a field from a raw file
rawmath rawa merge rawb = rawd Merge two like raw files
rawmath series rawa rawb ...= rawd

Also "-" can be used for a filename to read from standard input or write to standard output. This can be used to change units or to scale output. For operations between files, stations are matched before the operation is performed. This can be used to get a plot of 24 hour temperature change:

( sfcwx -re=us -pr=3 -va=st -of=std -dr=none -cu ; \
  sfcwx -re=us -pr=3 -va=st -of=std -dr=none -cu=24 ) | \
  rawmath - sub - = - "24 hour Temp change (F)" | \
  mapplt -re=us -if=raw -de=d -

Grid File Mathematics

A program to perform mathematics on grid files is called grdmath. Grid data files are generally not entered by hand but are generated from a gridding program such as sfccalc, upcalc, and grid or by directly referencing a GRIB file. Once the grid file exists, grdmath can be called and the output sent to programs such as contour or vector. The operations available with grdmath are:

grdmath grida add gridb = gridc
grdmath grida sub gridb = gridc
grdmath grida mul gridb = gridc
grdmath grida div gridb = gridc
Standard functions add, subtract, multiply and divide
grida, gridb, gridc, gridu, gridv are grid files
grdmath grida add #number = gridb Add a constant number to a grid
grdmath grida mul #number = gridc Scale a grid by number
grdmath sum grida gridb ... = gridd Sum grids
grdmath avg grida gridb ... = gridd Average grids
grdmath dx grida = gridb Finite difference in x
grdmath dy grida = gridb Finite difference in y
grdmath cvg gridu gridv = gridb Convergence
grdmath vort gridu gridv = gridb Vorticity
grdmath adv grida gridu gridv = gridb Advection of grida
grdmath spd gridu gridv = gridb Speed given u and v wind components
grdmath interp grida factor gridb = gridc Interpolates between two grids using factor
grdmath grida add #cor = gridb Add coriolis parameter to grida

Also "-" can be used for a filename to read from standard input or write to standard output. For example, to convert the temps to Celsius:

sfccalc -re=us -va=st -pl=none -of=std -cu -pf=calc | \
  grdmath - sub #32 = - | grdmath - mul #.555555 = - "Temperature (C)" |
  contour -in=5 -pl=cf -de=d -

Multiple grids can be moved around by using ():

( grdmath uwind.grd sub ugeos.grd = - ;
  grdmath vwind.grd sub vgeos.grd = - ) |
  vector -de=p -la "500 mb Ageos Wind vectors" - -

This will plot ageostrophic wind vectors. Notice that both the U and V component is read in through standard input.


For further information about WXP, email technical-support@weather.unisys.com
Last updated by Dan Vietor on June 8, 1998