Re: [ferret_users] Averaging Values are Mismatching

To: ferret_users@xxxxxxxx

Subject: Re: [ferret_users] Averaging Values are Mismatching

From: "Ansley C. Manke" <ansley.b.manke@xxxxxxxx>

Date: Thu, 9 Mar 2017 09:55:53 -0800

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Hi,

Ferret computes averages as a weighted average, so the result will not in general be the same as a computing it as a sum divided by count.

In this case the weights are a 2-dimensional area of each grid cell. If the region you have specified cuts across grid cells, the weighting will take the area of those partial grid cells into account. When the computation is on the surface of the earth, Ferret also accounts for different size of longitude/latitude grid cells at different latitudes. That won't affect this calculation much since you are working near the equator. Please see this discussion about averaging in the Ferret documentation, and follow the links in that section to the topics of "General Information on transformations" and the @WGT transformation which will let you look at the weights used in the calculation.

-Ansley

On 3/9/2017 5:24 AM, Abhishek Savita wrote:

Dear Ferreters,

                  I am using Pyferret and getting some error when averaging the values. it is mismatching with the ferret calculation. Suppose I have average the values like

list/precision=7 D20[x=60.25:61.25@ave,y=-6.25:-5.25@ave,l=1]

its value has come 60.39426

when I have listed the values as

list/precision=7/x=60.25:61.25/y=-6.25:-5.25 D20[l=1]
             VARIABLE : TEMP[Z=@LOC:20]
                        Z=-0.02:5500
             FILENAME : soda_d20_1958_2010_indpac.nc
             SUBSET   : 3 by 3 points (LONGITUDE-LATITUDE)
             TIME     : 15-JAN-1958 00:00
                60.25E    60.75E    61.25E
                 61        62        63
5.25S / 70: 64.47244 65.52505 67.76488
5.75S / 69: 59.62996 59.87644 61.27785
6.25S / 68: 55.80148 55.17461 55.52123

I calculated analytical and It is giving 60.56349

When I have taken corresponding I and J values, I got as

list/precision=7 D20[i=61:63@ave,j=68:70@ave,l=1]

Its values has come   60.56349

Could you please suggest me, how to solve this problem

With regards

Abhishek

--

Regards
Abhishek Savita

Research Scholar (Earth System Science Technology)

Center For Oceans, Rivers, Atmosphere & Land Science Technology
Indian Institute of Technology, Kharagpur
+91-8609704619