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netlw_fao56

Compute longwave radiation 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 netlw_fao56 (
		tmin       : numeric,  
		tmax       : numeric,  
		actvpr     : numeric,  
		radext     : numeric,  
		radsol     : numeric,  
		iounit [4] : integer   
	)

Arguments

tmin
tmax

Scalars or arrays of the same size, shape and units containing minimum and maximum temperatures. The units may be derees Celcius, Kelvin or Farenheit. See the iounit argument.

actvpr

A variable containing actual vapor pressure. It must have the same dimension sizes as the tmin and tmax variables. See satvpr_fao56 [tmin], actvpr_mnmx_fao56, actvpr_rhmean_fao56

radext

A variable containing extraterrestrial radiation. It must have the same dimension sizes as the tmin and tmax variables. See radext_fao56

radsol

A variable containing solar radiation. It must have the same dimension sizes as the tmin and tmax variables. It must have the same units as radext. See radsol_fao56, radsol_hargreaves_fao56

iounit

An integer array of length 4 indicating the units of input arguments tmin, tmax,

iounit(0)=0 ; input tmin and tmax are in degrees C (degC).
iounit(0)=1 ; input tmin and tmax are in degrees K (degK).
iounit(0)=2 ; input tmin and tmax are in degrees F.

iounit(1)=0 ; input actvpr are in mm/day
iounit(1)=1 ; input actvpr Pa
iounit(1)=2 ; input actvpr kPa

iounit(2)=0 ; input radext and radsol are in mm/day
iounit(2)=1 ; input radext and radsol are in MJ/(M2-day)
iounit(2)=2 ; input radext and radsol are in W/m2

iounit(3)=0 ; output units => mm/day
iounit(3)=1 ; output units => MJ/(m2-day)
iounit(3)=2 ; output units => W/m2

Return value

An array the same size as tmin containing the estimates net longwave radiation.

Description

Estimates longwave radiation equation 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 39 of Chapter 3.

Examples

Example 1: Replicate example 11 in Chapter 3. Here tunit=0 (degC). Use values in document.

  jday   = 135
  lat    = -22.9
  tmin   = 19.1     ; C; iounit(0)=0
  tmax   = 25.1     ; C
 
                    ; ---- FAO 56 ------
  ea     = 2.1      ; kPa; "recorded" (ie, observed);  iounit(1)=2
  rs     = 14.5     ; MJ/(m2-day); Example 10; eqn 35; iounit(2)=1
  ra     = 25.1     ; MJ/(m2-day); Example 10; eqn 21

  netlw  = netlw_fao56(tmin, tmax, ea, ra, rs, (/0,2,1,1/), False)  ; 3.508 MJ/(m2-day)

  print(netlw)

The output for 'netlw' with iounit=1 is

     Variable: netlw
     Type: float
     Total Size: 4 bytes
                 1 values
     Number of Dimensions: 1
     Dimensions and sizes:   [1]
     Coordinates: 
     Number Of Attributes: 4
       long_name :   net long wave radiation: FAO_39
       units :       MJ/(m2-day)
       url : http://www.fao.org/docrep/X0490E/x0490e07.htm
       info :        FAO 56; EQN 39
     (0)     3.508542

Example 2: Similar to Example 1 except the quantities are calculated via functions. The es is estimated using EQN 48 (same as EQN 14 but with Tmin<==>Tdew)..