traj_back_control.txt |    load control file |
83 09 28 13 |    back start time |
38.665 -71.369 216.4 |    back start location |
tdump_data |    backward file name |
tdump_isen |    constant potential temperature | ||||
tdump_dens |    constant density
tdump_sigma   constant vertical coordinate (zero)
tdump_divg   computed from velocity divergence |