Date: Wed Apr 21 2010 - 06:34:38 MDT
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
发件人： Matthew Wheeler
发送时间： 2010-04-21 09:15:35
收件人： 'HUANG, Ping'
抄送：
主题： 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)
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-----------------------------------------------------------
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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