GDPVSF vertically interpolates grid data to an arbitrary surface.
INPUT PARAMETERS
GDFILE Grid file
GDOUTF Output grid file
GFUNC Grid function
GDATTIM Grid date/time
GVCORD Grid vertical coordinate
STARTL Level to start search
STOPL Level to stop search
DESIRE Desired interpolation surface
GDOUTL Output grid level
GVOUTC Output vertical coordinate
GPACK Number of bits / packing type
GLIST List of grids to interpolate
PMAX Maximum pressure to search
PROGRAM DESCRIPTION
GDPVSF interpolates grid data vertically to an arbitrary
functional surface. In the following discussion, it will
be assumed that the functional surface is a surface of
constant potential vorticity.
GDFILE is the input grid to use for interpolation, and it
must contain the necessary grids at the specified times and
levels. For example, for GFUNC = MUL(AVOR(WND),STAP), GDFILE
must contain the grids UREL (UWND), VREL (VWND) and STAP at
the vertical coordinate given by GVCORD.
GDOUTF must be an already-existing grid file. GDPVSF will
not create a new grid file if GDOUTF does not exist. You
can use GDDIAG to create the necessary grids for GDPVSF.
The grid function (potential vorticity) is specified in
GFUNC. DESIRE specifies the value of the potential
vorticity surface to which the grids should be interpolated.
The vertical coordinate of the input grids is given by
GVCORD. The output vertical coordinate and grid level are
specified separately and arbitrarily by the user through
GVOUTC and GDOUTL, respectively. GVOUTC must be defined
either as PVBL (ascending) or PVAB (descending).
GDPVSF will not produce an interpolated grid of the input
vertical coordinate unless it is also specified in GLIST.
For example, if GVCORD is THTA but GLIST contains only UWND,
VWND and PRES, no output grid for the parameter THTA will
be written. It is good practice to always include GVCORD
in GLIST. Grid functions can not be interpolated.
The program begins the search for the desired pv surface
at STARTL. If the surface is not encountered by the time
STOPL is reached, the values at the grid point are set to
missing. The first time GFUNC=DESIRE is encountered, with
GFUNC increasing upward, the interpolation is performed and
the search is started at the next grid point. To prevent
searching the lower troposphere while in THTA coordinates,
PMAX may be set to a non-zero value.
Switching STARTL and STOPL causes the search to be made in
the opposite direction. Locations where the output is
different for a different search direction imply that
the surface is present at more than one level, i.e. that
the tropopause is folded.
EXAMPLES
1. Use an input grid file in THTA coordinates which
contains PRES, UWND, VWND, STAP!10, and RELH between 260 K
and 350 K, interpolate THTA, UWND, VWND, and PRES to the
1.5 PVU surface. Assign the output grids to the nominal
15 m grid level in the preexisting file PV.GRD. Look for
the lowest occurrence of the 1.5 PVU surface, assuming that it
occurs above 700 mb. Call the output vertical coordinate pvbl,
which represents the pv surface found from below.
GDFILE = input.grd
GDOUTF = pv.grd
GFUNC = mul(avor(obs),stap)
GDATTIM = last
GVCORD = thta
STARTL = 260
STOPL = 350
DESIRE = 0.00000015
GDOUTL = 15
GVOUTC = pvbl
GPACK =
GLIST = thta;uwnd;vwnd;pres
PMAX = 700
2. Repeat the search in the opposite direction. Change
the vertical coordinate to distinguish between this
set of grids and the previous set of grids.
GDFILE = input.grd
GDOUTF = pv.grd
GFUNC = mul(avor(obs),stap)
GDATTIM = last
GVCORD = thta
STARTL = 350
STOPL = 260
DESIRE = 0.00000015
GDOUTL = 15
GVOUTC = pvab
GPACK =
GLIST = thta;uwnd;vwnd;pres
PMAX = 700