The content of some useful diagnostic files are as follows:
cost_fn and grad_fn: These files hold (in ASCII format) WRFDA cost and gradient function values, respectively, for the first and last iterations. If you run with PRINT_DETAIL_GRAD=true, however, these values will be listed for each iteration; this can be helpful for visualization purposes. The NCL script WRFDA/var/graphics/ncl/plot_cost_grad_fn.ncl may be used to plot the content of cost_fn and grad_fn, if these files are generated with PRINT_DETAIL_GRAD=true.