# [ncl-talk] Computing winds in an annulus

Prashanth Bhalachandran prashanth.bhalachandran at gmail.com
Tue Jan 30 12:51:57 MST 2018

```Thanks, Alan. That is the right way to go about this!

Prashanth

> On Jan 30, 2018, at 2:40 PM, Alan Brammer <abrammer at albany.edu> wrote:
>
> to follow up with an important detail here.
>
> Average:
> (a) 0-6    and multiply by the area
> (b) 0-4,   and multiply by the area
> (c) subtract: (a) -(b)  then divide by ( area a - area b )
> e.g. ignoring the fact that the earth is a sphere
>
> outer =  dim_avg_n( masked0_6_array, (/0,1/) ) * (pi*6^2)      ; some array where we've already masked the data we want, or subset the area we want
> inner =  dim_avg_n( masked0_4_array, (/0,1/) ) * (pi*4^2)
> annulus = ( outer - inner  ) /  (  (pi*6^2) - (pi*4^2) )
>
> You can cancel all the pi's above for simplicity if desired. .
>
> ==============================
>
> To do this with circles rather than squares however you could also use the layout from the Katrina circle script on the below page and add an extra gc_inout() to mask the inner 0 to x km circle.
> Then you don't need to worry about areas so much.  (Should probably still weight by the latitude though)
> https://www.ncl.ucar.edu/Applications/latlon_subset.shtml <https://www.ncl.ucar.edu/Applications/latlon_subset.shtml>
>
>
>
>
>
>
> On Mon, Jan 29, 2018 at 10:31 PM, Dennis Shea via ncl-talk <ncl-talk at ucar.edu <mailto:ncl-talk at ucar.edu>> wrote:
> Average:
> (a) 0-6
> (b) 0-4,
> (c) subtract: (a)-(b)
>
> On Mon, Jan 29, 2018 at 11:23 AM, Prashanth Bhalachandran via ncl-talk <ncl-talk at ucar.edu <mailto:ncl-talk at ucar.edu>> wrote:
> Dear NCL’ers,
>
> I have a question regarding the computation of winds in an annulus.
>
>
>
> That is, say I have a center lat and lon : cenlat, cenlon
>
> limit    = 2 ; In degrees
>
>   b1 = cenlat-limit
>
>   b2 = cenlat+limit
>
>   b3 = cenlon-limit
>
>   b4 = cenlon+limit
>
>
>
> This is what I use if want to average the winds in a box that is 2 degrees on either side of the center (4 degree x 4degree n total).
>
> uav = dim_avg_n(dim_avg_n(uu(tcount,:,{b1:b2},{b3:b4}),2),1)
>
>
>
> However, what I am now interested in is averaging around an annulus (say between 4 degrees to 6 degrees from the center of the hurricane). What is the best way to do that?
>
>
>
> Thank you,
>
> Prashanth
>
>
>
>
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