[ncl-talk] Rossby wave filtering

[Carl Schreck]

The 2-d Fourier transform used in kf_filter assumes that the data are
period in the zonal direction. That's pretty easy in the atmosphere, but
not in the ocean. I think you would have to do some tapering in the x
direction towards the coastlines, similar to what I had to do in time for
kf_filter.

If you want to try adapting it, you can find the underlying code on your
machine at

$NCARG_ROOT/lib/ncarg/nclscripts/contrib/kf_filter.ncl

or

https://k3.cicsnc.org/carl/carl-ncl-library/blob/master/kf/kf_filter.ncl

Good luck!

Carl


On Fri, Mar 2, 2018 at 5:17 AM, Wickramage Chathurika <
wickramagechathurika at rocketmail.com> wrote:

> *Hello,*
>
> *I want to filter Rossby waves from zonal ocean velocities. can I use the
> function "kf_filter" and can I use zonal velocities to filter Rossby waves
> in ocean? I have tried to do and the script is below;*
>
>
> fi=addfile("/mnt/e/data/HYCOM/u/2004_june.nc","r")
> vname=getfilevarnames(fi)
> print(vname)
>
> oc=fi->WATER_U(:,{0:200},{5.5},{76:86})
> printVarSummary(oc)
>
> t=fi->TIME
> printMinMax(t, False)
>
> obsPerDay=8
> tMin=731734
> tMax=731763.875
> kMin = 1
> kMax = 14
> hMin=5
> hMax=60
> waveName="Kelvin"
>
> x=oc(:,{40},:)
> x!0="time"
> x!1="lon"
>
> kf=kf_filter(x(:,:), obsPerDay, tMin, tMax, kMin, kMax, hMin, hMax,
> waveName)
>
> printMinMax(kf, False)
>
> *and the output information is below;*
>
> Variable: vname
> Type: string
> Total Size: 64 bytes
>             8 values
> Number of Dimensions: 1
> Dimensions and sizes:   [8]
> Coordinates:
> (0)     LON2876_3501
> (1)     LON2876_3501_bnds
> (2)     LAT876_1251
> (3)     LAT876_1251_bnds
> (4)     LEV1_28
> (5)     LEV1_28_bnds
> (6)     TIME
> (7)     WATER_U
>
> Variable: oc
> Type: float
> Total Size: 2782080 bytes
>             695520 values
> Number of Dimensions: 3
> Dimensions and sizes:   [TIME | 240] x [LEV1_28 | 23] x [LON2876_3501 |
> 126]
> Coordinates:
>             TIME: [731734..731763.875]
>             LEV1_28: [   0.. 200]
>             LON2876_3501: [  76..  86]
> Number Of Attributes: 5
>   LAT876_1251 : 5.51999998093
>   missing_value :       -30000
>   _FillValue :  -30000
>   long_name :   eastward water velocity [m/s]
>   history :     From 3hourly
> (0)     time (days since 0001-01-01 00:00:00) : min=731734   max=731763.875
> (0)     eastward water velocity [m/s] : min=-0.00192383   max=0.00160925
>
> P.S; When I try to filter "Mixed Rossby-Gravity" waves, all output results
> were "zero". In above script, I have filtered "Kelvin" waves. I really
> appreciate if anyone could help me to figure out this and any suggestion
> about how to filter "Rossby waves" is appreciated in advance.
>
> Kind Regard;
> Chathu
>
>
>
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>


-- 

<https://ncics.org/> *Carl J. Schreck III, PhD*
*Research Scholar*
North Carolina State University <http://ncsu.edu/>
North Carolina Institute for Climate Studies (NCICS) <https://ncics.org/>
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