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prsatm_tz_fao56

Compute atmospheric pressure as described in FAO 56.

Available in version 6.4.0 and later.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/crop.ncl"

	function prsatm_tz_fao56 (
		t          : numeric,  
		z          : numeric,  
		P0     [1] : numeric,  
		Z0     [1] : numeric,  
		iounit [3] : integer   
	)

Arguments

t

A scalar or array of any dimensionality containing temperature. See iounit(0) argument.

z

A scalar or array of any dimensionality containing elevation (m). If not a scalar, the size and shape must match t.

P0

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

Z0

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

iounit

An integer array of length 3 indicating the units t, P0 and the returned pressure value(s).

  • iounit(0)=0 ; degrees C (degC)
  • iounit(0)=1 ; degrees K (degK)
  • iounit(0)=2 ; degrees Farenheit

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

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

Return value

An array with the same dimensionality and type as t containing estimated atmospheric pressure (kPa).

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 or equation 3-2 in Annex 3.

See Also

Crop & Evapotranspiration functions

Examples

Example 1: Replicate example in Annex 3.

  P0  = 101.3         ; kPa
  z0  =  0.0          ; m

  t   = 20.0          ; degC
  z   =  0.0          ; m

  prs = prsatm_tz_fao56(t, z, P0, z0, (/0,2,2/)  ; 81.756  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)
      tt    = conform(zz, 20.0, -1)
      pres  = prsatm_tz_fao56(tt, zz, P0, z0, (/0,2,2/) )

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

The (edited) output is:

     Variable: pres
     Type: float
     Total Size: 80 bytes
                 20 values
     Number of Dimensions: 1
     Dimensions and sizes:	[20]
     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

             Annex 2: Table 2.1

                 z     prs
     (0) 	 50   100.711
     (1)	100   100.125
     (2)	150    99.542 
     (3)	200    98.961
     (4)	250    98.383
     (5)	300    97.808
     (6)	350    97.235
     (7)	400    96.665
     (8)	450    96.099
     (9)	500    95.534
     (10)	550    94.973
     (11)	600    94.414
     (12)	650    93.857
     (13)	700    93.304
     (14)	750    92.753
     (15)	800    92.205
     (16)	850    91.659
     (17)	900    91.116
     (18)	950    90.575
     (19)      1000    90.038