import pygrib as pg

grb = pg.open(file_name.grib)
temperature = grb.select(name='Temperature')
geop_height = grb.select(name='Geopotential')

import multiprocessing as mp

params = ['Temperature', 'Geopotential']
pool = mp.Pool(processes = len(params))

def readparam(grb_file, param):
    return grb_file.select(name=param)

pool.map(readparam, params)