Mask outside map

[Ranyére Nóbrega]

Dear All,

I would like their help to generate an image in GrADS with isolines just
in a certain area, as one cut out.

For example, I want to generate a temperature map for Brasilian Amazon,
but do not would like that the isothermal appeared outside Brasilian
Amazon, as if  put a mask outside Brasilian Amazon Map.

Does anybody know as to help me?

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Ranyére Silva Nóbrega
Analista Intelectual
Divisão de Meteorologia e Climatologia - SIPAM
Centro Técnico e Operacional de Porto Velho