Date: Wed Apr 21 2010 - 07:45:36 MDT
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
>
>
> ------------------------------
> *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)
> 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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