Re: Question about the OLR space-time Spectra

Hi, Carl

Thanks for your reply. It help me a lot.

I used the input data starting from 1980. There are no missing values. The abnormal large zonal mean appears only if the input data is longer than 1998 days. For example, there is no problem when the input data is from 1jan1980 to 31dec1985 or from 1jan1986 to 31dec1990. However, if the input data is from 1jan1980 to 31dec1990, the abnormal large zonal mean appears.

After checking the function wkSpaceTime detailedly, I found that the error is from the function "dtrend_msg". When the dimension sizes is larger than 1998, the dtrend_msg output a missing value at the first location. And wkSpaceTime delete the missing value attribution to adapt the FFT function. Thus, abnormal large power at wavenumber zero are created.

The missing value output may be a bug of the function dtrend_msg.

For example:

begin
 ntime = 1999
 temp = random_normal(10, 5, ntime+1)
 print(num(ismissing(temp)))
 b = dtrend_msg(ispan(0,ntime,1)*1.0,temp , False, False)
 print(num(ismissing(b)))
 print(ind(ismissing(b)))
end

This simple script will get the result as:

 NCAR Command Language Version 5.1.1
 The use of this software is governed by a License Agreement.
 See http://www.ncl.ucar.edu/ for more details.
(0) 0 ; there is no missing value before detrending
(0) 1 ; there is a missing value after detrending!
(0) 0 ; the missing value place in the first location.



Regards,

Huang Ping



2010-04-22



HUANG, Ping



·¢¼þÈË£º Carl J. Schreck, III
·¢ËÍʱ¼ä£º 2010-04-21 21:45:40
ÊÕ¼þÈË£º HUANG, Ping; ncl forum
³­ËÍ£º
Ö÷Ì⣺ Re: [ncl-talk] Question about the OLR space-time Spectra[SEC=UNCLASSIFIED]
 
Dear Huang Ping,


In your longer (> 2000 days) plots, is it possible that you are including times from before 1 January 1979? The OLR data contain a lot of missing data before this date, but none after it. For the missing periods, the entire globe would have missing values of 32766, which is a substantial zonal mean! I do not know if/how wkSpaceTime handles these missing points, but treating them as regular data could cause that issue.


Hope this helps.


     Cheers,
     Carl


2010/4/21 HUANG, Ping <ph0007@ustc.edu>

Dear Dr. Wheeler,

Thanks for your reply.

I found that the problem is from the contributed function wkSpaceTime in NCL. My input data don't have a large zonal mean component. And only if the time range of input data are larger than 2000 days, there are the abnormal powers in wavenumber zero. In wkSpaceTime, all the steps operate the 96 and 65 overlapping days segment except removing the long term linear trend and annual cycle. Actually, the annual cycle of input data has been removed and the linear trend of each segment has been removed too. Thus, I deleted this two processes and the problem was figured out. I don't know why these two steps create so strong zonal mean when the input data have long time range.

Regards,

Huang Ping





·¢¼þÈË£º Matthew Wheeler
·¢ËÍʱ¼ä£º 2010-04-21 09:15:35
ÊÕ¼þÈË£º 'HUANG, Ping'
³­ËÍ£º
Ö÷Ì⣺ RE: Question about the OLR space-time Spectra [SEC=UNCLASSIFIED]
Dear Huang Ping,

Wavenumber zero is the zonal mean. Hence, your spectrum appears to be indicating that you have a large zonal mean component, and this component has strong variability at a range of frequencies. Can you think of a reason why your input data may have an abnormally large zonal mean component? You could try removing the zonal mean at each time step, but this would then give zero power at wavenumber 0. It would be a good test to try.

Regards,

-----------------------------------------------------------
Matthew C. Wheeler
Centre for Australian Weather and Climate Research (CAWCR)
A partnership between the Bureau of Meteorology and CSIRO
Post: GPO Box 1289, Melbourne, VIC 3001
Street: 700 Collins Street, Melbourne, VIC 3008
Phone: +61 (0)3 9669 4068 Fax: +61 (0)3 9669 4660
m.wheeler@bom.gov.au
http://cawcr.gov.au/staff/mwheeler (new)
http://cawcr.gov.au/bmrc/clfor/cfstaff/matw.htm (old)
-----------------------------------------------------------





From: HUANG, Ping [mailto:huangping@mail.iap.ac.cn]
Sent: Wednesday, 21 April 2010 00:34
To: Matthew Wheeler
Subject: Question about the OLR space-time Spectra


Dear Dr. wheeler,

I am a research from China. Recently, I analyzed the Wheeler-Kiladis space-time spectra using the NCL function wkSpacetime and OLR daily data. However, the symmetric power has an abnormal value at wavenumber zero. Please see the following figure.

The annual cycle of input daily data has been removed.

Could you give some suggestions? Thanks.



Regards,



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--
Carl J. Schreck, III
PhD Student
Department of Atmospheric and Environmental Sciences
University at Albany, ES 330
carl@atmos.albany.edu
http://www.atmos.albany.edu/student/carl/

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