Running grid nudging
--------------------
Grid nudging is available in Version 2.2.
This nudges the WRF run towards a gridded analysis linearly interpolated in time between
specified analyses. It only requires multiple time periods of analyses for each domain
to be nudged, and these are input to real in the same format as the initial conditions.
To run an fdda job

- run real.exe with grid_fdda = 1
  This will generate an extra input file with the time-varying analyses called wrffdda_d0<n>
- note that each domain can have grid_fdda = 1 or 0 when running nesting
- run wrf.exe with the same grid_fdda setting to use the wrffdda files generated
  Also select wrf run-time options from the list described below to determine which fields to nudge,
    how strongly to nudge, whether to nudge in the boundary layer, or whether to truncate
    nudging below a certain grid level
- There is a ramping capability for dynamic initialization that turns nudging off over a
    specified period

 &fdda
 grid_fdda (max_dom)                 = 1        ; grid-nudging fdda on (=0 off) for each domain
 gfdda_inname                        = "wrffdda_d<domain>" ; defined name in real
 gfdda_interval_m (max_dom)          = 360      ; time interval (min) between analysis times
 gfdda_end_h (max_dom)               = 6        ; time (h) to stop nudging after start of forecast
 io_form_gfdda                       = 2        ; analysis data io format (2 = netCDF)
 fgdt (max_dom)                      = 0        ; calculation frequency (minutes) for grid-nudging (0=every step)
 if_no_pbl_nudging_uv (max_dom)      = 0        ; 1= no nudging of u and v in the pbl, 0=nudging in the pbl
 if_no_pbl_nudging_t (max_dom)       = 0        ; 1= no nudging of temp in the pbl, 0=nudging in the pbl
 if_no_pbl_nudging_q (max_dom)       = 0        ; 1= no nudging of qvapor in the pbl, 0=nudging in the pbl
 if_zfac_uv (max_dom)                = 0        ; 0= nudge u and v all layers, 1= limit nudging to levels above k_zfac_uv
  k_zfac_uv (max_dom)                = 10       ; 10=model level below which nudging is switched off for u and v
 if_zfac_t (max_dom)                 = 0        ; 0= nudge temp all layers, 1= limit nudging to levels above k_zfac_t
  k_zfac_t (max_dom)                 = 10       ; 10=model level below which nudging is switched off for temp
 if_zfac_q (max_dom)                 = 0        ; 0= nudge qvapor all layers, 1= limit nudging to levels above k_zfac_q
  k_zfac_q (max_dom)                 = 10       ; 10=model level below which nudging is switched off for qvapor
 guv (max_dom)                       = 0.0003   ; nudging coefficient for u and v (sec-1)
 gt (max_dom)                        = 0.0003   ; nudging coefficient for temp (sec-1)
 gq (max_dom)                        = 0.0003   ; nudging coefficient for qvapor (sec-1)
 if_ramping                          = 0        ; 0= nudging ends as a step function, 1= ramping nudging down at end of period
 dtramp_min                          = 60.0     ; time (min) for ramping function, 60.0=ramping starts at last analysis time, 
                                                                                  -60.0=ramping ends at last analysis time