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satvpr_mean_fao56

Compute mean saturation vapor pressure using minimum and maximum temperature temperature as described in FAO 56.

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 satvpr_mean_fao56 (
		tmin       : numeric,  
		tmax       : numeric,  
		iounit [2] : integer   
	)

Arguments

tmin

A scalar or array of any dimensionality containing minimum temperatures.

tmax

A scalar or array of containing maximum temperatures. The units and dimsionality must match tmin.

iounit

An integer array of length 2 which indicates the units of the temp and returned saturation vapor pressure.

iounit(0)=0 means the input are in degrees C (degC).
iounit(0)=1 means the input are in degrees K (degK).
iounit(0)=2 means the input are in degrees F (degF)

If iounit(0)=1 or 2, the tmin and tmax will be converted to degC internally prior to the computation. The input values are unaltered.

iounit(1) refers to the units of the returned saturation vapor pressure

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

Return value

An array with the same dimensionality and type as tmin containing mean saturation vapor pressure.

Description

Compute 'mean' saturation vapor pressure (kPa) as 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 12 of Chapter 3.

Tetens' Formula is uised. This is an empirical expression for saturation vapour pressure with respect to liquid water that includes the variation of latent heat with temperature.

Tetens, O. 1930. Uber einige meteorologische Begriffe. Z. Geophys.. 6. 297–309

Examples

A table which displays saturation vapor pressures for different temperatures is here.

Example 1: Replicate example 3 in Chapter 3. Use different temperature units to illustrate that the answers are identical.


  tminC   = 15.0                         ; degC
  tmaxC   = 24.5
                                         ; iounit=(/0,2/) , degC, kPa
  esmeanC = satvpr_mean_fao56(tminC, tmaxC, (/0,2/))   ; 2.39 kPa 
;---

  tminK   = tminC+273.16                 ; degK
  tmaxK   = tmaxC+273.16
                                         ; iounit=(/1,2/) , degK, kPa
  esmeanK = satvpr_mean_fao56(tminK, tmaxK, (/1,2/))   ; 2.39 kPa 
;---

  tminF   = 1.8*tminC + 32               ; deg Farenheit
  tmaxF   = 1.8*tmaxC + 32
                                         ; iounit=(/2,2/) , degF, kPa
  esmeanF = satvpr_mean_fao56(tminK, tmaxK, (/1,2/))   ; 2.39 kPa 
;---
  printVarSummary(esmeanF)

The output for 'esmeanF' is


   Variable: esmeanF
   Type: float
   Total Size: 4 bytes
               1 values
   Number of Dimensions: 1
   Dimensions and sizes:	[1]
   Coordinates: 
   Number Of Attributes: 4
     long_name :	mean saturation vapor pressure
     units :	kPa
     url :	http://www.fao.org/docrep/X0490E/x0490e07.htm
     info :	FAO 56; EQN 11 and 12

Example 2: Consider TMIN(time,lat,lon) and TMAX(time,lat,lon) with sizes (3,73,144) and the temperatures have units of degK.


  SATVPR  = satvpr_mean_fao56(TMIN, TMAX, (/1,2/))  
  printVarSummary(SATVPR)

The output for 'SATVPR' is