The addition of the -ntasks argument to build-namelist is a convenience feature. It is used only to provide a default setting of the namelist variable npr_yz, which describes the decomposition used by the FV dycore only. It is only necessary to set npr_yz if the number of tasks being used is more than can be used by the default 1D decomposition. For example, on the 1.9x2.5 grid the maximum 1D decomposition can only use 32 tasks because there must be at least 3 latitude slices in each latitude subdomain. So to use 64 tasks with that grid it is necessary to specify a 2D decomposition using the npr_yz variable. You could add this to the namelist by, for example, setting -namelist "&atm npr_yz=32,2,2,32 /" on the commandline. An alternative method is to use the -ntasks argument to build-namelist.
