[gradsusr] If Statements for Gridded Model Data Open GrADS

Baker, Matt (Quebec - Atlantic) matt.baker at amecfw.com
Thu Jun 23 11:51:07 EDT 2016 

Hi Stephen,

I’m attempting your route with the if statements; however, I want to make sure I’m executing them correctly.

Here’s the new code with the Ifs.

        'define NAMHail = if(NAMcape,>,1500,if(NAMcape,<,2200,NAMHail+1,NAMHail),NAMHail)'
        'define NAMHail = if(NAMcape,>,2199,if(NAMcape,<,3100,NAMHail+3,NAMHail),NAMHail)'
        'define NAMHail = if(NAMcape,>,3099,NAMHail+4,NAMHail)'

That code currently only displays values of 1 when the SBCAPE is greater than 2200. I’m wondering why the expression that’s bolded doesn’t return that value to NAMHail even though its being defined.

Cheers,

Matt

From: gradsusr-bounces at gradsusr.org [mailto:gradsusr-bounces at gradsusr.org] On Behalf Of Stephen McMillan
Sent: June-22-16 2:09 PM
To: GrADS Users Forum <gradsusr at gradsusr.org>
Subject: Re: [gradsusr] If Statements for Gridded Model Data Open GrADS

Matt,

The "if" construct you are using will not work on a gridded field, and it appears that's what you are trying to do.  The extension "if" function was designed to work on such fields.

Here's a partial example using the first set of conditions you provided:

'define NAMHail=if(NAMcape,>,1500,if(NAMcape,<,2200,NAMHail+1,NAMHail),NAMHail)'

That's, of course, assuming you have defined a field called "NAMHail" as zero prior to executing the "if" function.  Alternatively, you could have 0 or -u (for undefined) as your false expression, without first defining NAMHail.  After that, you could execute the "if" again by adding an additional set of conditions, until you have applied all conditions.  Another option is to nest several conditions, but it gets a little complicated that way.

Stephen McMillan


On Wed, Jun 22, 2016 at 12:11 PM, Baker, Matt (Quebec - Atlantic) <matt.baker at amecfw.com<mailto:matt.baker at amecfw.com>> wrote:
Thanks Bernd and Stephen,

Essentially if the CAPE, LI, and 500 mb relative humidity are within a certain range, I want to add to NAMHail variable (NAMHail = NAMHail + 1) and then display the variable with contour lines.

Stephen, in my code there are already if statements for this. Is there something wrong with my syntax and could that be the reason GrADS isn’t executing in the if statement?

As an example, is this statement written correctly?
if (NAMcape > 1500 & NAMcape < 2200)
    'NAMHail = NAMHail + 1'
endif
Again, NAMHail is defined by:  ‘NAMHail = 0’ before the if the statements.

Thanks for the help!

Matt

From: gradsusr-bounces at gradsusr.org<mailto:gradsusr-bounces at gradsusr.org> [mailto:gradsusr-bounces at gradsusr.org<mailto:gradsusr-bounces at gradsusr.org>] On Behalf Of Stephen McMillan
Sent: June-22-16 12:39 PM
To: GrADS Users Forum <gradsusr at gradsusr.org<mailto:gradsusr at gradsusr.org>>
Subject: Re: [gradsusr] If Statements for Gridded Model Data Open GrADS

Hi Matt,

In addition to Bernd's suggestion, also check out the "if" function, which should have come with your opengrads package as part of the following extension function set:

http://opengrads.org/doc/udxt/libbjt/

The "if" function syntax:
if(EXPR1,OP,EXPR2,TRUE_EXPR,FALSE_EXPR)

This function compares EXPR1 and EXPR2, where 5 arguments are expected. if their relation is true, TRUE_EXP is returned, otherwise expression FALSE_EXPR is returned.
OP

One of the following: ==, =, >, <, >=, <=, !=
EXPR2, TRUE_EXPR, FALSE_EXPR

These can be a expression, constant or -u.



I have found this function to be very useful and much easier to handle than using maskout and const, although there are probably cases where the latter is more appropriate or advisable.

Stephen McMillan

On Wed, Jun 22, 2016 at 9:24 AM, Baker, Matt (Quebec - Atlantic) <matt.baker at amecfw.com<mailto:matt.baker at amecfw.com>> wrote:
Hi all,

I have a question regarding Open GrADS.

I’m trying to use if statements for scalar model data (eg. GFS and NAM) over a grid and I’m wondering if it’s possible to do this. As an example, I’m using if statements for these parameters: surface based CAPE, LI, and 500 mb relative humidity to create an index to plot on a map. So far GrADS isn’t showing that this is working, however, I can plot the model data individually so I don’t think it’s a case of missing data.

Here’s some of the code for the NAM model. Note that I define the variable NAMHail to equal 0 at the start of the function.

   if (modelNumberNAM>0)
        set_maps(_map,_hasNegativeNAM)
        'set grads off'
        'set grid off'
        'set gxout contour'
        'set ccolor 0'
        'set clopts -1 1 0.15'
        'set cint 1'
        'set cthick 6'
        'set clab on'
        'set csmooth on'

        'NAMcape = CAPEsfc.'modelNumberNAM
        if (NAMcape > 1500 & NAMcape < 2200)
            'NAMHail = NAMHail + 1'
        endif
        if (NAMcape > 2199 & NAMcape < 3100)
            'NAMHail = NAMHail + 2'
        endif
        if (NAMcape > 3099)
            'NAMHail = NAMHail + 3'
        endif

        'set lev 500'
        'NAMrh500=RHprs.'modelNumberNAM
        if (NAMrh500 < 35)
            'NAMHail = NAMHail + 3'
        endif
        if (NAMrh500 > 35 & NAMrh500 < 44)
            'NAMHail = NAMHail + 1'
        endif

    endif

    'set gxout contour'
    'set ccolor 0'
    'set clopts -1 1 0.15'
    'set cint 200'
    'set cthick 6'
    'set clab on'
    'set csmooth on'

    'd NAMcape'

Thanks!

Matt Baker
Meteorologist, Met-Ocean Services, Amec Foster Wheeler Environment & Infrastructure
133 Crosbie Road, PO Box 13216, St. John’s, NL A1B 4A5, Canada
matt.baker at amecfw.com<mailto:matt.baker at amecfw.com>­­      amecfw.com<http://amecfw.com>
ISO 9001:2008 Quality Management System (St. John's)

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