[gradsusr] what is the limit of function(aave) ?

[lz007700]

Dear GrADS Users,

I'm trying to calculate the area average wind ,when I use the function (aave) in gs,as follows:
'define u200=aave(UGRDprs,lon=lon1-10,lon=lon1+10,lat=lat1-10,lat=lat1+10)'
it prompt such warnings "Syntax Error:  Invalid dimension expression
  Dimension value missing or invalid
Error from AAVE:  Invalid dimension expression"
lon1,lat1 has been transformed to real number,not character.
IS it must be specific numeral when we use function(aave)?
when I define u200 as follows:
'define u200=aave(UGRDprs,lon=111.5,lon=131.5,lat=17.9,lat=37.9)' 
it works.why?
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