From: Saji Njarackalazhikam Hameed <saji_at_nyahnyahspammersnyahnyah>

Date: Mon, 06 Feb 2006 15:31:59 -1000

Date: Mon, 06 Feb 2006 15:31:59 -1000

Hi Ben,

NCL has useful tools to make composites. I have used two approaches as

outlined below. In the following I assume that all the data have the same

time co-ordinates (say time is written as "days since 1900-01-05").

Approach 1. Sort a time series and take a portion of the values as

significant events. Let's use Nino3 time series to

make a composite of El Nino events.

..................................

nin=feof->nin ; read in DJF values of Nino3 time series from a file

years=nin&time ; time co-ordinate

ip = dim_pqsort(nin, 2) ; sort DJF Nino3 time series

nnin=dimsizes(nin) ; length of Nino3 time series

p=0.10 ; Assume upper/lower tenths

; of sorted Nino3 to be

; El Nino/La Nina

xlen=p*nnin ;

ist=0+floattoint(xlen) ;

ien=nnin-ist ;

lanina=years(ip(:ist-1)) ;

elnino=years(ip(ien:)) ;

; Read Zonal surface wind data

uas=fuas->uas_anm ;Zonalwind anomalies at surface

uas_3m=runave_Wrap(uas(lat|:,lon|:,time|:),3,0) ;

; a 3-point running mean to

; to get running seasonal

; values

utime=uas&time ; Time co-ordinate of this data

; has same time units as Nino3

uind=get1Dindex(utime,elnino) ; Convert time-coordinates

; of ElNino years into

; utime indices(see above)

unin=uas(time|uind,lat|:,lon:) ; collect data for

; ElNino years

unino=dim_avg_Wrap(unin(lat|:,lon|:,time|:)) ; since we took DJF

; nino3 these are

; DJF wind composites

; Changing uind=uind-3 and repeating the 2 steps above

; should give SON composites and so on, assuming

; data are monthly values

------------------------------------------------

Approach 2 - In the approach above, different phases of the

composite are assumed to have same time lags. In the case there are ENSO

events of different duration, you want to consider compositing by phase.

Let's say phase=0 when the Nino3 time series has maximum positive

amplitude. Then phase=180 degree is when Nino3 time series has maximum

negative amplitude. Transitional phases are -90 and 90 degrees. The code

for that is a bit more complicated. I can sent it to you if you want,

but the essential idea is as follows.

**** Warning *****

In practise the following idea may not work out as smoothly as

you like it to. So be careful.

*********************************

* Read in monthly Nino3 time series and find all local maxima (using the

new function local_minmax_1d).

* Select a few out of them that satisfy a specified criteria

(say having amplitude greater than 1 standard deviation).

* Determine time of maximum negative amplitude before and after

this local peak. These are phases -180 and 180. Similarly take zero

crossings as -90 and 90.

* Composite according to phase with the above information.

Let me know if you need an example code for this.

saji

* Benjamin Lamptey <lamptey_at_essc.psu.edu> [2006-02-06 19:16:22 -0500]:

*> Hello,
*

*> I should be grateful if someone could point me to an example in which
*

*> composite maps (or anomaly plots) are made in NCL. For example, making a
*

*> composite of El Nino events. The events are of different duration.
*

*> Thanks
*

*> Ben
*

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Received on Mon Feb 06 2006 - 18:31:59 MST

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