[ncl-talk] Question about Spectral Analysis(computation of variance)

[Lyndon Mark Olaguera]

Hi Sir Dennis,

Thanks for the help. I was looking for peaks with periods of 30 to 60 days.
The data is not good for this analysis. I also tried in the raw daily
values (attached in this email) but there is still a very large
variance/frequency.



On Sat, Jan 7, 2017 at 1:42 AM, Dennis Shea <shea at ucar.edu> wrote:

> [1]
> As noted in the documentation:
>    "The units are variance/(unit frequency interval)."
>     **not**
>    "variance"
>
> [2]
> Look at the data ... literally
>    plot = gsn_csm_y(wks,rain,False)                      ; see attachment
>
> Basically, the annual cycle of rain.
>
> What is it you expect from a time series like this? The lag-1 day auto
> correlation is 0.96. A ***very red** process.
>
> Assuming this is a climatology, I did a [four)info] on the data, The 1st
> harmonic has amplitude 7.3 with the phase max at day 220 and it explains
> 82% of the variance. The 2nd harmonic has amplitude 2.3; phase day 32; 8%
> of the variance.
>
>
>
> On Fri, Jan 6, 2017 at 1:57 AM, Lyndon Mark Olaguera <
> olagueralyndonmark429 at gmail.com> wrote:
>
>> Hi All,
>>
>> I'm trying to perform spectral analysis using the specx_anal function in
>> ncl.
>> I'm getting a variance around 1000 near the zero mark.I dont know if I'm
>> missing a step prior to performing the spectral analysis. I would like to
>> ask for any suggestions on how can I improve this.
>>
>>
>> Attached are the data set and script.
>>
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_code.ncl"
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/gsn_csm.ncl"
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/contributed.ncl"
>> load "$NCARG_ROOT/lib/ncarg/nclscripts/csm/shea_util.ncl"
>>
>> begin
>> filename = "type1_clim.csv"
>> lines    = asciiread(filename,-1,"string")
>> data     = lines(1:)
>> rain    = tofloat(str_get_field(data,10,","))
>>
>> d = 1
>> sm = 12
>> pct = 0.10
>>
>> spec= specx_anal(rain,d,sm,pct)
>> printVarSummary(spec)
>>
>> wks  = gsn_open_wks("png","test_fldmean")             ; Opens a ps file
>> res = True
>> res at tiMainString = "Type1_Spectrum"                     ; title
>> f = spec at frq
>> res at tiXAxisString = "Frequency (cycles/day)"  ; xaxis
>> res at tiYAxisString = "Variance"                       ; yaxis
>>
>> splt = specx_ci(spec, 0.05, 0.95)
>>
>> res at xyLineColors        = (/"foreground","green","blue","red"/)
>> plot = gsn_csm_xy(wks,f,splt,res)     ; create plot
>> end
>>
>> I'll appreciate any help.
>>
>> Many thanks,
>>
>> Lyndz
>>
>>
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