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Re: [ferret_users] LSL_LOWPASS

Hello Sudheer,
A note in the documentation for the LSL_LOWPASS function,

says the following:
Note on tidal filtering: For hourly time series containing tidal signals, some investigators use this filter with a 35-hour cutoff period and a filter span of xxx hours to remove at least 99.5 % of the energy for periods less than 25 hours.
This would indicate that to remove annual signal from a monthly series you would need a longer cutoff period than 11 months.

I believe that the reference given in that same documentation, Bloomfield, is  "Fourier Analysis of Time Series: An Introduction (Wiley Series in Probability and Statistics)", by Peter Bloomfield.

Does anybody have further guidance to pass on to Sudheer?


Sudheer Joseph wrote:
Dear Users,
                                Can any one educate me on the use of the below function. What should be the unit of the t axis if I want to use this function. I tried to use 11 as the cut of period for a monthly time series. My intention was to remove all sub-annual signals but the resulted time series had reduced amplitude for annual cycle.
use "http://apdrc.soest.hawaii.edu:80/dods/public_data/satellite_product/TRMM/TRMM_TMI/trmm_monthly"
let box1=sst[x=60:70@ave,y=5:10@aveo,d=1]
go grd_T sst ! getting sst to regular monthly time axis( 30.42 days)
let box1_rg=box1[gt=t_eqm,d=1]
plot box1_rg

LSL_LOWPASS(box1_rg,11, 11)

with best regards


Sudheer Joseph                                                                  
Indian National Centre for Ocean Information Services (INCOIS)    
Ocean Valley, Post Box No# 21,
IDA Jeedimetla P.O.                  
Hyderabad, Ranga Reddy District - 500 055
Andhra Pradesh, India.
E-mail:sjo@xxxxxxxxxxxxxx; sudheer.joseph@xxxxxxxxx;  sjo@xxxxxxxxxxxxx.
Web- http://oppamthadathil.tripod.com
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