Function : mvl_flexpart_read_hl
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# Syntax : fieldset mvl_flexpart_read_hl (inFile: string,
# param: string, level: number,
# step: number, ageclass: number)
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# Category : FLEXPART
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# OneLineDesc : Extract fields on height levels from FLEXPART output GRIB files
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# Description :
#
# Parameters : file - the FLEXPART ouput GRIB file
# param - the shortName of the parameter to extract
# level - the height level (m) to extract. -1 means all levels.
# step - the forecast step to extract. -1 means all steps.
# ageclass - the ageclass to extract. -1 mean all ageclasses.
#
#
# Return Value : mtext
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# Dependencies : none
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# Example Usage :