# month_to_annual_weighted

Converts monthly values to annual values weighted by the number of days in a month.

## Prototype

load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl" ; This library isautomatically loaded; from NCL V6.2.0 onward. ; No need for user to explicitly load. function month_to_annual_weighted ( yyyymm [*] : numeric, x : numeric, opt [1] : integer )

## Arguments

*yyyymm*

Year-month dates [*e.g.*, 198401,198402,...]. This must be
the same size as the "time" dimension of *x*.

*x*

An array containing monthly values. The 'temporal' dimension must be the leftmost dimension if there are multiple dimensions. The size of the 'temporal' dimension must be evenly divisible by 12.

*opt*

option:

opt=0 returns the weighted sum opt=1 return the weighted annual mean opt=2 return the weighted sum divided by 12 to get monthly mean value

## Return value

Returns an array where the 'temporal' dimension is decimated by a factor of 12.
Note: Upon return the 'temporal' dimension will be named * year*
to avoid confusing dimension names.

## Description

Typical usage is that an array of monthly precipitation rate (*e.g.*, mm/day)
or monthly mean temperatures are input. For precipitation, generally opt=0,
and the weighted average is returned. For temperature, generally opt=1,
and the weighted annual average is returned.

The weights are the appropriate number days for a month. February of leap years uses 29 days.

This function does not check for January (mm=1) to be the first month
of the *yyyymm*. Thus, if *yyyymm*(0)=198103 [say],
then the 12-month year will be March 1983 to April 1984.

If the time span is not divisible by 12 then the 'extra' months
are ignored. For example, if *yyyymm* spans 199101 to
199603, weighted annual means will be calculated for 1991 to 1995.
There will be no value returned for 1996.

Caveat: This function works as expected for one-dimensional arrays.
Also, it works fine when all grid points have 12 non-missing
monthly values for any year. However, if *any* grid point
does not have 12 valid values, then the annual values will
be returned as missing for all grid points for that year.

## See Also

## Examples

**Example 1**

A one-dimensional array of monthly precipitation [mm/day] is
to be converted to an annual total. If prc(time), and the size of time
is 240 monthly values, than 20 (=240/12) values (years) will be returned.
The 'temporal' dimension will be named * year*.

prc_annual_total =If the array contains, say, monthly mean temperatures (tmp), thenmonth_to_annual_weighted(yyyymm, prc, 0) ; prc_annual_total(20)

tmp_annual_mean =month_to_annual_weighted(yyyymm, tmp, 1) ; tmp_annual_mean(20)

**Example 2**

A multi-dimensional array of monthly precipitation totals [mm/day] is
to be converted to an annual total. If prc(time,lat,lon), and the size of time
is 1200 monthly values, than 100 (=120/12) values (years) will be returned.
The 'temporal' dimension will be named * year*.

PRC =If the array contains, say, monthly mean temperatures [tmp(time,lev,lat,lon)], thenmonth_to_annual_weighted(yyyymm, prc, 0) ; PRC(100,nlat,mlon)

TMP =month_to_annual_weighted(yyyymm, tmp, 1) ; TMP(100,klev,nlat,mlon)