draw isentropic potential vorticity by NCL using WRF data
IPVsingle reads only one WRFout file
IPVset reads a set of WRFout files
verticalIPV shows the vertical IPV on a given longitude
zonalmeanIPV shows how the zonal mean IPV changes with time
latvstimeIPV shows the change of IPV on a given longitude and isentropic level
Author: (HC)Leonidas, aka Haochang Luo