# vinth2p

Interpolates CAM (Community Atmosphere Model) hybrid coordinates to pressure coordinates.

## Prototype

function vinth2p ( datai : numeric, hbcofa [*] : numeric, hbcofb [*] : numeric, plevo [*] : numeric, psfc : numeric, intyp [1] : integer, p0 [1] : numeric, ii [1] : integer, kxtrp [1] : logical ) return_val : numeric

## Arguments

*datai*

An array of 3, 4, or 5 dimensions. Needs to contain a level dimension in hybrid coordinates. The order of the dimensions is specific. The three rightmost dimensions must be level x lat x lon [e.g. T(time,lev,lat,lon)]. The order of the level dimension must be top-to-bottom.

*hbcofa*

A one-dimensional array containing the hybrid A coefficients. Must
have the same dimension size as the level dimension of *datai*. The
order must be top-to-bottom. (These are expected to be normalized to
one (1.0). If not, divide by *P0*.)

*hbcofb*

A one-dimensional array containing the hybrid B coefficients. Must
have the same dimension size as the level dimension of *datai*. The
order must be top-to-bottom.

*plevo*

A one-dimensional array of output pressure levels in mb. These can be ordered top-to-bottom or bottom-to-top.

*psfc*

A multi-dimensional array of surface pressures in Pa. Must have the
same dimension sizes as the corresponding dimensions of
*datai* (minus the level dimension).

*intyp*

Scalar integer value equal to the interpolation type: 1 = LINEAR, 2 = LOG, 3 = LOG LOG

*p0*

Scalar numeric value equal to surface reference pressure in mb.

*ii*

Not used at this time. Set to 1.

*kxtrp*

Logical. False => no extrapolation when the pressure level is outside
of the range of *psfc*.

## Return value

A multi-dimensional array of the same size as *datai* except
that the level coordinate has been replaced by *plevo*. The
function automatically copies the metadata from *datai* to
*return_val*.

The type of the output data will be double only if *psfc* and
*datai* are of type double. Otherwise, the return type will be
float.

## Description

Interpolates CAM (Community Atmosphere Model) hybrid
coordinates to pressure coordinates. The type
of interpolation is currently a variant of transformed pressure
coordinates with the interpolation type specified by
*intyp*. All hybrid coordinate values are transformed to
pressure values. If *datai* is on midlevels (interfaces), then
*hyam*/*hybm* (*hyai*/*hybi*) coefficients
should be input.

This is the exact routine used within the old CCM (Community Climate Model)
Processor. The mixture of Pa for *psfc* and mb for *plevo*
and *p0* is specified by the source routine. Further, the
*datai* are required to be top-to-bottom BUT the *hyam*
and *hybm* are ordered bottom-to-top. Again, this is
the ordering expected by the original routine.

## See Also

Use the **vinth2p_ecmwf** for
the extrapolation of temperature and geopotential below ground.

## Examples

CESM1.0 output files in CCM History Tape (CCMHT) format. These files
may be read into NCL directly using **addfile** with a
".ccm" extension. Alternatively, these files may be converted to
netCDF using ccm2nc
The reason for mentioning this is that there are differences. The
netCDF files produced by ccm2nc
or output directly from later versions of the model:

- have the reference pressure (P0) contained within the file while earlier files do not. In the latter case the user must explicitly provide that information.
- produce variables with level, latitude and longitude being the three rightmost (fastest varying) dimensions [e.g. TS(time,lev,lat,lon)]. The variables in CCMHT format are ordered differently [e.g. (lat,lev,lon) or (time,lat,lev,lon)]. It is therefore necessary to reorder the variable before input.
- have a time dimension while the CCMHT files may or may not have a time dimension.

**Example 1**

The models which produce files in CCM History Tape format write the variables in the following order ([time],lat,lev,lon). This example has no time dimension. vinth2p requires either the level dimension to be the slowest varying (leftmost) dimension or time as the slowest followed by level. Thus, the following example uses dimension reordering to put the dimensions into the required order. The surface pressure (PS) has two dimensions (lat,lon). This example explicitly reads all the required variables to memory and reorders the dimensions of the input at the time the function is invoked. Tnew is a 3D array returned by vinth2p with the same size as T except that the level dimension has been replaced by the size of pnew.

fccm =addfile("dummy.ccm" , "r") ; a CCM file pnew = (/ 850.0,200.0 /) ; desired output levels [hPa/mb] hyam = fccm->hyam ; read to memory [optional] hybm = fccm->hybm T = fccm->T ; say "T" is (lat,lev,lon) P0mb = 1000. ; reference pressure [mb] Tnew =vinth2p(T(lev|:,lat|:,lon|:),hyam,hybm,pnew,fccm->PS,1,P0mb,1,True)

**Example 2**

Let's assume that the above file had been converted to netCDF format via ccm2nc with the following dimension order (lev,lat,lon). In this case, the dimension order is correct. Also, the reference pressure P0 has been added to the file so it may be retrieved directly.

fncf =addfile("dummy.nc" , "r") ; a CCM file converted to netCDF via ccm2nc P0mb = 0.01*fncf->P0 ; change Pa to mb (as required) . . . Tnew =vinth2p(T,hyam,hybm,pnew,fncf->PS,1,P0mb,1,True)

**Example 3**

A variable with a time dimension is read in directly from a CCMHT formatted file. Each variable's dimension order is (time,lat,lev,lon). Only those variables with four dimensions are to be interpolated to the specified pressure levels. The output is dimension order is to be (time,lev_new,lat,lon). Also demonstrated is the technique to name a dimension and transfer appropriate coordinate variables to that named dimension.

fccm =addfile("dummy.ccm" , "r") ; a CCM file pnew = (/ 850., 500., 200. /) ; desired output levels hyam = fccm->hyam ; read to memory (optional) hybm = fccm->hybm time = fccm->time P0mb = 1000. ; reference pressure [mb] vname=getfilevarnames(fccm) ; get all variable names on input file lev_new = pnew ; create a new coordinate variable lev_new!0 = "lev_new" ; variable and dimension name the same do i=0,dimsizes(vnames)-1 dims =getfilevardims(fccm,vname(i)) nrank =dimsizes(dims) ; determine the number of dimensions if (nrank.eq.4) then ; only interp 4D variables Vold = fccm->$vname(i)$ ; read each variable to memory ; (faster dimension reordering) Vnew =vinth2p(Vold(time|:,lev|:,lat|:,lon|:),hyam,hybm,pnew,fccm->PS,\ 1,P0,1,True) end if ; do something with Vnew end do

**Example 4**

In this example, no dimension reordering is required.

fncf =oraddfile("dummy.nc" , "r") hyam = fccm->hyam ; read to memory (optional) hybm = fccm->hybm [snip] Vold = fncf->$vname(i)$ ; read to memory [could be large] Vnew =vinth2p(Vold,hyam, hybm, pnew ,fncf->PS, 1 \ ,P0mb, 1, True)

Vnew =vinth2p(fncf->$vname(i)$,hyam, hybm, plev ,fncf->PS, 1 \ ,P0mb, 1, True)

**Example 5**

Let x(time,lev,lat,lon) be a variable from a model in which the level coordinate is in sigma levels but is in the correct top-to-bottom order:

; vinth2p requires the lev_p to be expressed in mb [hPa] ; eother top-to-bottom or bottom-to-top ordering is allowed lev_p = (/ 10, 20, 30, 50, 70,100,150,200,250 \ , 300,400,500,600,700,850,925,1000 /) lev_p!0 = "lev_p" ; variable/dim name lev_p&lev_p = lev_p ; create coordinate variable lev_p@long_name = "pressure" ; attach some attributes lev_p@units = "hPa" lev_p@positive = "down" ; CAM outputs bottom-to-top hybm = f->sigma ; hybm is the 'sigma' coordinate hyam = hybm ; create a bogus hyam 1D array hyam = 0.0 ; set all values to zero P0mb = f->P0*0.01 ps = f->ps ; these are in Pa x = f->X xp =vinth2p(x, hyam, hybm, lev_p, ps, intyp, P0mb, 1, False )