5 day precipitation

[Arindam Chakraborty]

hi Ben,
        I'm glad to know that both the versions are giving same
results. However they could have been different if there were missing
values at certain times. From the algorithm given in the 1st mail I
somehow felt that outcome of sum(prec,t+0,t+4) should be used for
counting the number of days
and not prec. I'd have done exactly the same if I'd have written it in
fortran. Anyways, if the results are not satisfactory, can you please
check the expression for tenpanave? Your first mail shows that it is
'tenpanave = 0.1*((sum(prec,t=1,t=2922))/8)'
This is the 10% of total annual precipitation, averaged over 8 years,
but not 10% of annual average precipitation in the same unit of 'prec'
the way you may want it to be.

 Land-ocean masking can be done after the count was calculated. If you
have a land-ocean mask data file, assuming 1 over land and 0 over
ocean, in the same resolution of prec, then that masking is
'maskout(count,land(t=1)-1)'
where 'land' is the variable name for that file.

Hope this helps,

arindam
----------------------------
 ARINDAM CHAKRABORTY
 Department of Meteorology
 Florida State University
 Tallahassee, FL-32306, USA
 Tel: +001-850-6443524 (Off)
      +001-850-5758550 (Res)
 Fax: +001-850-6449642
____________________________

On Fri, 10 Feb 2006, Ben Burford wrote:

> Hi Arindam,
>
> Again, thanks very much for your help.
>
> Sorry if my last email was not clear - my meaning was that I didn't think yo
> ur version of the algorithm was working correctly.  After receiving your ema
> il I studied the documentation to understand your original version.  My unde
> rstanding of the documentation was that, in the case of the simpler (my) ver
> sion:
> 'count = sum(  const(  const(  maskout(prec,sum(prec,t+0,t+4)-tenpanave)  ,1
> )  ,0,-u)  ,t=1,t=2918)'
> for:
> maskout(prec,sum(prec,t+0,t+4)-tenpanave)
>
> if sum(prec,t+0,t+4) is greater than tenpanave, then no change;
> if sum(prec,t+0,t+4) is less than tenpanave, then change the value of prec (
> at the current value of t) to "missing value";
>
> in your original verion of maskout(sum(prec,t+0,t+4),sum(prec,t+0,t+4)-tenpa
> nave)
> I don't know what its doing.  In the case of:
> if sum(prec,t+0,t+4) is less than tenpanave, then change the value of sum(pr
> ec,t+0,t+4) to "missing value";
> Where is "sum(prec,t+0,t+4)" such that it can be changed to missing value?
> Is it in the array prec, in an array representing "sum(prec,t+0,t+4)", in th
> e array "count"?  I don't understand what Grads is doing internally, so this
>  version didn't make any sense to me, and I thought there was an error.
>
> After I applied your original version I was pretty surprised at the results
> I got, so I doubted that your version was working correctly.  However, after
>  I went home and was able to apply both versions (yours and mine) I found th
> at the results were the same!  So now I'm both confused by how Grads execute
> s your version (and surprised with (confused about) the results from my data
> ). . . . .
>
>
> In any case, I'd really appreciate your help with one additional item.  I wo
> uld like to apply a mask, to mask out ocean and only apply the algorithm to
> land data.  Can you show me how to apply a land only mask (if possible) with
> in your complex nesting process?
>
> Thanks very much,
>
> Ben
>
>
>
>
>
> At 11:12 2006/02/09 -0500, you wrote:
> > hi Ben,
> >         your modified expression is correct if you want 'prec' to be
> > undefined rather than 'sum(prec,t+0,t+4)'. But both of these are going
> > to give the same result if you do not have undefined values in the
> > original data (prec) since at last we are not taking the value of the
> > variable to calculate anything but the number of time points it is
> > defined (i.e., greater than or equal to tenpanave).
> >         Why do you feel that the results are very close but not exact?
> >
> > regards,
> >
> > arindam
> > ----------------------------
> >  ARINDAM CHAKRABORTY
> >  Department of Meteorology
> >  Florida State University
> >  Tallahassee, FL-32306, USA
> >  Tel: +001-850-6443524 (Off)
> >       +001-850-5758550 (Res)
> >  Fax: +001-850-6449642
> > ____________________________
> >
> > On Thu, 9 Feb 2006, Ben Burford wrote:
> >
> > > Hello Arindam,
> > >
> > > This is amazing!  Thank you very much.
> > >
> > > I've been working on figuring out the functions and the logic, and I thi
> nk t
> > > here is a mistake in your answer (but its very close).  I'd appreciate i
> t if
> > >  you would check this.
> > >
> > > In the innermost part you have:
> > > maskout(sum(prec,t+0,t+4),sum(prec,t+0,t+4)-tenpanave)
> > >
> > > I think this means: "if the sum (of the 5 days of prec) is not greater t
> han
> > > tenpanave, then change 'sum(prec,t+0,t+4)' to missing value".
> > >
> > > I think the logic should be "if the sum (of the 5 days of prec) is not g
> reat
> > > er than tenpanave, then change 'prec' to missing value".
> > >
> > > So I think the innermost part should be:
> > > maskout(prec,sum(prec,t+0,t+4)-tenpanave)
> > >
> > >
> > > and the overall function would be:
> > > 'count = sum(  const(  const(  maskout(prec,sum(prec,t+0,t+4)-tenpanave)
>   ,1
> > > )  ,0,-u)  ,t=1,t=2918)'
> > >
> > >
> > > Is this right (or perhaps wrong)?
> > >
> > > Thank you very much,
> > >
> > > Ben
> > >
> > >
> > >
> > >
> > > At 15:21 2006/02/07 -0500, you wrote:
> > > > hi Ben,
> > > >         you have an interesting problem to solve in grads. Probably
> > > > it is possible to get the count and I'm giving the expression below,
> > > > although I've not checked the answer thoroughly for my own data.
> > > > You may try it out and see if it makes sense.
> > > >
> > > > If the threshold precipitation value is 'tenpanave' (you can
> > > > define it the way you want before the expression to calculate
> > > > the count, the way you showed the example in your mail).
> > > > Then, the count is nothing but
> > > > 'count = sum(const(const(maskout(sum(prec,t+0,t+4),sum(prec,t+0,t+4)-t
> enpa
> > > nave),1),0,-u),t=1,t=2918)'
> > > > ...
> > > >
> > > >
> > > > Hope this helps,
> > > > ----------------------------
> > > >  ARINDAM CHAKRABORTY
> > > >  Department of Meteorology
> > > >  Florida State University
> > > >  Tallahassee, FL-32306, USA
> > > >  Tel: +001-850-6443524 (Off)
> > > >       +001-850-5758550 (Res)
> > > >  Fax: +001-850-6449642
> > > > ____________________________
> > > >
> > > > On Tue, 7 Feb 2006, Ben Burford wrote:
> > > >
> > > > > Hello All,
> > > > >
> > > > > I'm trying to implement a script.  I studied functions (e.g. tloop)
> and
> > > look
> > > > > ed in the archives for help, but so far its over my head as a beginn
> er,
> > > I'm
> > > > > not even sure if Grads can do this.
> > > > >
> > > > > I have an 8 year (t=1,2922) global gridded daily precipitation data
> set.
> > >   Fo
> > > > > r each grid cell I want to go through the time series and check each
>  fiv
> > > e da
> > > > > y period (2918 possible 5 day periods) to see if the total of the pr
> ecip
> > > itat
> > > > > ion for the five days is greater than a certain level, if it is I wa
> nt t
> > > o in
> > > > > crement a counter.  The result will be a single value (total count)
> for
> > > each
> > > > >  grid cell (global grid of a single value per grid cell).
> > > > >
> > > > > The value that I want to check the five day total against is "10% of
>  the
> > >  ann
> > > > > ual average rainfall" (tenpanave) which I'm calculating as follows (
> add
> > > up a
> > > > > ll the daily values for 8 years, divide by 8 to get the annual avera
> ge,
> > > then
> > > > >  take 10% of that):
> > > > > tenpanave = 0.1*((sum(prec,t=1,t=2922))/8)
> > > > >
> > > > > A flow chart for the script might look like:
> > > > > * Calculate "10% of the annual average rainfall" (tenpanave)
> > > > > tenpanave = 0.1*((sum(prec,t=1,t=2922))/8)
> > > > >
> > > > > * calculate the precipitation over 5 days (5dayprec), if that 5 day
> tota
> > > l is
> > > > >  greater than or equal to ten percent of the annual average precipit
> atio
> > > n (t
> > > > > enpanave) for that grid cell, then increment the counter for that gr
> id c
> > > ell.
> > > > >   Perform this process for all grid cells, over the full time period
>  of
> > > 2922
> > > > >  days (x from 1 to 2918).
> > > > > x=1
> > > > > 5dayprec=sum(prec,t=x,t=x+4)
> > > > > if 5dayprec is greater than or equal to tenpanave then count=count+1
> > > > > x=x+1
> > > > > if x=2919 then quit, else go up three steps and calculate the next 5
>  day
> > >  win
> > > > > dow (5dayp)
> > > > >
> > > > > * Display results
> > > > > d count
> > > > >
> > > > >
> > > > > As I say, I don't know if this can be done with Grads.  Any help wou
> ld b
> > > e gr
> > > > > eatly appreciated.
> > > > >
> > > > > Thank you,
> > > > >
> > > > > Ben
> > > > >
> > > >
> > >
> > >
> >
>