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mixhum_convert

Convert mixing ratio to specific humidity or vice-versa.

Available in version 6.5.0 and later.

Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"  ; This library is automatically loaded
                                                             ; from NCL V6.2.0 onward.
                                                             ; No need for user to explicitly load.

	function mixhum_convert (
		wq         : numeric,  ; float, double, integer only
		wqType [1] : string,   
		iounit [2] : integer   
	)

	return_val [dimsizes(wq)] :  float or double

Arguments

wq

A scalar or array containing mixing ratio or specific humidity (units: kg/kg, g/kg).

wqType

  • wqType="w" or "W" means that wq is mixing ratio and a conversion to specific humidity is desired.
  • wqType="q" or "Q" means that wq is specific humidity and a conversion to mixing ratio is desired.

iounit

An integer array of length 2 which specifies the units of the input wq and returned variable

  • iounit(0)=0 input wq are kg/kg
  • iounit(0)=1 input wq are g/kg

  • iounit(1)=0 returned units are kg/kg
  • iounit(1)=1 returned units are g/kg

Return value

A variable of the same size and shape as wq.

Description

The mixing ratio and specific humidity are commonly denoted as w and q, respectively. As noted in the Glossary of Meteorology, these quantities are nearly identical. Hence, w and q are often used interchangeably. Still, for certain applications, it may be appropriate to make the conversion. Specifically:

         q = w/(1+w) [switch=True]  <===>   w = q/(1-q) [switch=False]

See Also

mixhum_ptd, mixhum_ptrh, relhum

Examples

Example 1: Convert mixing ratio to specific humidity and vice-versa.

   W   = 15.2       ; g/kg      iounit(0)=1
   q   = mixhum_convert(W, "w", (/1,1/)  ; q = 14.97; return g/kg, iounit(1)=1
   printVarSummary(q)

   w   = mixhum_convert(q, "q", (/1,1/)  ; w = 15.2 ; return g/kg, iounit(1)=1 
   printVarSummary(w)

Example 2: Read specific humidity (Q; kg/kg). Convert to mixing ratio and vice-versa.

   a  = addfile("cam35.h0.0008-07.nc","r") 
   Q  = a->Q       ; kg/kg     iounit(0)=0
   w   = mixhum_convert(Q, "q", (/0,0/); return kg/kg, iounit(1)=0  
   printVarSummary(w)   ; w(time,lat,lon)

   q   = mixhum_convert(w, "w" , (/0,1/); return kg/kg, iounit(1)=1  
   printVarSummary(q)   ; q(time,lat,lon)
The edited output:

   Variable: Q
   Type: float
   Total Size: 1437696 bytes
               359424 values
   Number of Dimensions: 4
   Dimensions and sizes:	[time | 1] x [lev | 26] x [lat | 96] x [lon | 144]
   Coordinates: 
               time: [3132..3132]
               lev: [3.54463800000001..992.5560999999998]
               lat: [ -90..89.99999999999999]
               lon: [   0..357.5]
   Number Of Attributes: 3
     units :	kg/kg
     long_name :	Specific humidity
     cell_method :	time: mean

   (0)	Specific humidity (kg/kg) : min=4.95837e-07   max=0.0220418
   
   Variable: w
   Type: float
   Total Size: 1437696 bytes
               359424 values
   Number of Dimensions: 4
   Dimensions and sizes:	[time | 1] x [lev | 26] x [lat | 96] x [lon | 144]
   Coordinates: 
               time: [3132..3132]
               lev: [3.54463800000001..992.5560999999998]
               lat: [ -90..89.99999999999999]
               lon: [   0..357.5]
   Number Of Attributes: 4
     long_name :	mixing ratio
     equation :	w=q/(1-q)
     units :	kg/kg
     NCL :	mixhum_convert

   (0)	mixing ratio (kg/kg) : min=4.95838e-07   max=0.0225386
   
   
   Variable: q 
   Type: float
   Total Size: 1437696 bytes
               359424 values
   Number of Dimensions: 4
   Dimensions and sizes:	[time | 1] x [lev | 26] x [lat | 96] x [lon | 144]
   Coordinates: 
               time: [3132..3132]
               lev: [3.54463800000001..992.5560999999998]
               lat: [ -90..89.99999999999999]
               lon: [   0..357.5]
   Number Of Attributes: 4
     long_name :	specific humidity
     equation :	q=w/(1+w)
     units :	 g/kg
     NCL :	mixhum_convert
   (0)	specific humidity (kg/kg) : min=4.95837e-04   max=2.20418