R: Re: [ferret_users] Spatial average on irregular domain

["markus.mingel@xxxxxxxxx" <markus.mingel@xxxxxxxxx>]

Dear Ansley,
thanks for the suggestion, it works and I am able to create the mask. However 
I still have a question, since I am not sure about the averaging. What I did 
is:

! Create Mask
let Mask_Tropics = if lat gt -30 and lat lt 30 then 1

!  Mask the variable also multiplying it by grid area
New_SST = Mask_Tropics * Area * SST 

! Compute and Plot spatial mean
plot  New_SST[i=@ave,j=@ave]

I think last step is wrong since ferret attempts to compute a "fake" area of 
cells. So how can I avoid it accounts for the area? Maybe  could I use @sum 
instead of @ave and then divide by the number of points that fall inside the 
sub-region?! If so, how can I compute the number of points where Mask_Tropics = 
1?

Thanks for help
Regards 
Markus

>----Messaggio originale----
>Da: ansley.b.manke@xxxxxxxx
>Data: 03/11/2011 17.40
>A: "markus.mingel@xxxxxxxxx"<markus.mingel@xxxxxxxxx>
>Cc: <oar.pmel.ferret_users@xxxxxxxx>
>Ogg: Re: [ferret_users] Spatial average on irregular domain
>
>Hi-
>This is not a simple question, is it.  The size of the grid cells on a 
>curvilinear grid may vary from cell to cell, so to compute an accurate 
>average you would need to find a way to compute the area of the cells.  
>On a rectilinear grid, Ferret computes the actual size of the grid cells 
>using a simple latitude correction, but it does not have a means of 
>doing that on a curvilinear grid.  Does the dataset have variables that 
>describe the area of cells? Some datasets do include that information.  
>If so then we could find a way to create a mask using the curvilinear 
>coordinate variables, along the lines of
>
>let mask = if curv_lat gt -30 and curv_lat lt 30 then 1
>
>and compute the average of the data multiplied by that mask. Another 
>option would be to regrid the data to a rectilinear grid and take the 
>average of the regridded field over the region.
>
>Ansley
>
>On 11/3/2011 6:09 AM, markus.mingel@xxxxxxxxx wrote:
>> Dear All,
>> I am trying to perform a spatial average on irregular domain.
>>
>> I know how to do that on regular domains:
>>
>> let regional_ SST = SST[x=-180:180@ave,y=-30:30@ave]
>>
>> however this command does not work or curvilinear grids. Could you please
>> suggest me how to compute spatial averages in such case?
>>
>> Thanks for help
>>
>> Regards
>> Markus
>>
>>
>