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time_reassign_cv2var

Reassign (replace) a CF-conforming "time" coordinate variable associated with a variable by calculating the mid-time values using the "bounds" attribute.

Available in version 6.4.0 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 time_reassign_cv2var (
		f            ,          
		timeName [1] : string,  
		varName  [1] : string   
	)

	return_val [dimsizes(varName)]

Arguments

A variable of type 'file' or 'list'. The file(s) must be CF-conforming.

timeName

A string specifying the name of the time variable on the file(s). Most commonly the name is "time". This must a CF-conforming variable with units such as "seconds/minutes/hours/days since ...". It must have a bounds attribute. If not the original time will be returned. A string specifying the name of the variable.

Return value

An array of the same size and type as the variable specified by varName.

Description

Some models output a 'time' (timeName) that is not consistent with the date(s) indicated within the file name. Consider a model file named (say) FOO.2012-01.nc. This contains monthly averages for January 2012. However, the 'time' variable points to February 2012. Specifically,


     f    = addfile("FOO.2012-01.nc","r")     ; 'f' is type 'file'
     time = f->time                           ; "days since 2011-01-01 00:00:00"
     date = cd_calendar(time, 0)             ; (/ 2012,2, 1, 0, 0, 0 /)

Note that the month is 'off-by-one'. It indicates February 2012 rather than January 2012. The reason is that the model accumulates statistics until the time switches months (here from January to February). Unfortunately, the statistic (eg, monthly mean) is 'tagged' with the current model time (February) and not the previous month (January). Actually, the time 'should be' the center-of-times of the previous month (eg, January 16).

To address this issue, the CF conforming bounds attribute associated with the specified timeName variable is used to access the temporal bounds used to calculate the mean. These boundary times are averaged.


     time = f->time                        ; timeName => "time"
     if (isatt(time,"bounds")) then        ; CF attribute
         tbnd = f->time@bounds             ; (ntim,2)
         time = (tbnd(:,0)+tbnd(:,1))*0.5  ; center of 'time' bounds
     end if
     date = cd_calendar(time, 0)          ; (/ 2012, 1,16,12, 0, 0 /)

Using the correct times can be particulary important when doing (say) compositing.

Examples

Example 1

This CAM file has monthly means from 1850-01 to 2005-12. Perform two tasks: (a) reassign (replace) the "time" values with the temporal monthly time means; (b) reassign (replace) the coordinate variable (cv) associated with the specified variable.

     dir    = "./"
     fil    = "b40.20th.track1.2deg.001.cam2.h0.PSL.1850-01_cat_2005-12.nc"
     f      = addfile(dir+fil, "r")
     
     time   = time_reassign(f, "time")                 ; 'new' time
     printVarSummary(time)
     
     psl    = time_reassign_cv2var(f, "time", "PSL")   ; 'new' time coordinate variable (cv)
     printVarSummary(psl)

The (edited) output from the above would be:

     Variable: time
     Type: double
     Dimensions and sizes:	[time | 1872]
     Coordinates: 
                 time: [15.5..56924.5]    <=== reassigned    original: time: [  31..56940]
     Number Of Attributes: 5
       bounds :	time_bnds
       calendar :	noleap
       units :	days since 1850-01-01 00:00:00
       long_name :	time
       NCL :	function time_reassign used to reassign time to mid-value of bounds
     
     
     Variable: psl
     Dimensions and sizes:	[time | 1872] x [lat | 96] x [lon | 144]
     Coordinates: 
                 time: [15.5..56924.5]      <=== reassigned    original: time: [  31..56940]
                 lat: [ -90..89.99999999999999]
                 lon: [   0..357.5]
     Number Of Attributes: 4
       cell_methods :	time: mean
       long_name :	Sea level pressure
       units :	Pa
       NCL :	function time_reassign used to reassign time coordinate variable to mid-value of bounds

If the original and reassigned yyyy/mm/dd/hh were printed, you would readily see the more appropriate dates.