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prsatm_z_fao56

Compute atmospheric pressure using the approximation described in FAO 56: Chapter 3; equation 7.

Available in version 6.4.0 and later.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/crop.ncl"  ; This library is automatically loaded
                                                      ; from NCL V6.5.0 onward.
                                                      ; No need for user to explicitly load.

	function prsatm_z_fao56 (
		z          : numeric,  
		P0     [1] : numeric,  
		Z0     [1] : numeric,  
		iounit [2] : integer   
	)

Arguments

A scalar or array of any dimensionality containing elevation (m).

A scalar specifying the atmospheric pressure at sea level. Most commonly: P0= 1013.0 hPa, 101300 Pa, 101.3 kPa See iounit(0) argument.

A scalar specifying the reference level elevation (m) corresponding to P0. Most commonly: Z0=0.0 (m).

iounit

An integer array of length 2 indicating the units P0 and the returned pressure value(s).

iounit(0)=0 ; hPa
iounit(0)=1 ; Pa
iounit(0)=2 ; kPa

iounit(1)=0 ; hPa
iounit(1)=1 ; Pa
iounit(1)=2 ; kPa

Return value

An array with the same dimensionality and type as z containing estimated atmospheric pressure.

Description

Compute atmospheric pressure described in the Food and Agriculture Organization (FAO) Irrigation and Drainage Paper 56 entitled: Crop evapotranspiration - Guidelines for computing crop water requirement . Specifically, see equation 7 of Chapter 3. The is a simplification of equation 3-2 Annex 3.

Examples

Example 1: Replicate Example 2 deriving pressure using equation 7, Chapter 3.

  P0  = 101.3         ; kPa
  z0  =  0.0          ; m
  z   =  0.0          ; m

  prs = prsatm_z_fao56( z, P0, z0, (/2,2/))  ; 81.778 kPa
  print(prs)

The output for 'prs' is

     Variable: prs
     Type: float
     Total Size: 4 bytes
                 1 values
     Number of Dimensions: 1
     Dimensions and sizes:	[1]
     Coordinates: 
     Number Of Attributes: 4
       units :	kPa
       long_name :	atmospheric pressure
       url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
       info :	FAO 56; Annex: EQN 3-2

Example 2: Match the 50-1000 elevation values in Annex 2: Table 2.1

            ; match Table 2.1
      zz    = ispan(50,1000,50)
      pres  = prsatm_z_fao56(zz, P0, z0, (/2,2/) )

      printVarSummary(pres)
      print("")
      print("Annex 2: Table 2.1")
      print(zz+"  "+pres)

The (edited) output is: