Hello Sudheer,
A note in the documentation for the LSL_LOWPASS function,
http://ferret.pmel.noaa.gov/Ferret/documentation/users-guide/appendix-a-external-functions/LSL-LOWPASS
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?
Ansley
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
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Sudheer Joseph
Scientist
Indian National Centre for Ocean Information Services (INCOIS)
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