I'm not shure if I understand your commandline...
But, as far as i know Left does only take 2 parameters. the first one ist the string which should be left-cutted, and the second one is the length.
in your example you used 3...
Left(GetWind(Object.PersistStorage("wind_s"), Object.PersistStorage("wind_t"),15))
Hope I can help you
PhX