Nudging meteorology

This process is WRF-specific but may be useful for your WRF-GC runs. The namelist options described here are also described in the WRF User’s Guide

Grid nudging using input FNL data

  1. Add the following lines to your namelist.input, into a new section &fdda as shown below:

&fdda
 grid_fdda                           =   1,    1,    1,
 gfdda_inname                        = "wrffdda_d<domain>",
 gfdda_interval_m                    = 360,  360,  360,
 gfdda_end_h                         = 9999, 9999, 9999,
 io_form_gfdda                       =   2,
 fgdt                                =   0,    0,   0,
 if_no_pbl_nudging_uv                =   1,    1,   1,
 if_no_pbl_nudging_t                 =   1,    1,   1,
 if_no_pbl_nudging_q                 =   1,    1,   1,
 if_zfac_uv                          =   1,    1,   1,
 k_zfac_uv                           =  10,   10,  10,
 if_zfac_t                           =   0,    0,  10,
 k_zfac_t                            =  10,   10,  10,
 if_zfac_q                           =   0,    0,   0,
 k_zfac_q                            =  10,   10,  10,
 guv                                 = 0.0003, 0.0003, 0.0003,
 gt                                  = 0.0003, 0.0003, 0.0003,
 gq                                  = 0.0003, 0.0003, 0.0003,
 if_ramping                          = 0,
 dtramp_min                          = 60.0
 /

The configuration options you need to change with a brief description are listed below.

  • grid_fdda (per-domain): 0 for grid analysis nudging off; 1 for grid analysis nudging on.

  • gfdda_inname: name of fdda input file (wrffdda_d<domain>) that will be generated when running real.exe.

  • gfdda_interval_m (per-domain): time interval (in mins) for NCEP FNL analysis times.

  • gfdda_end_h (per-domain): time (in hours) to stop nudging after the start of the simulation.

  • io_form_gfdda: analysis data format (2 for netCDF format).

  • fgdt (per-domain): calculation frequency (in mins) for analysis nudging; 0 = every time step (recommended).

  • if_no_pbl_nudging_uv/if_no_pbl_nudging_t/if_no_pbl_nudging_q (per-domain): 0 for nudging in the PBL; 1 for no nudging of uv-winds/air temperature/water vapor in the PBL.

  • if_zfac_uv/if_zfac_t/if_zfac_q (per-domain): 0 for nudge uv-winds/air temperature/water vapor in all layers; 1 for limit nudging to levels above k_zfac_uv\k_zfac_t\k_zfac_q.

  • k_zfac_uv/k_zfac_t/k_zfac_q (per-domain): model level below which nudging is switched off for uv-winds/air temperature/water vapor.

  • guv/gt/gq (per-domain): nudging coefficient for uv-winds/air temperature/water vapor (s-1).

  • if_ramping: 0 for nudging ends as a step function; 1 for ramping nudging down at the end of the period.

  • dtramp_min: time (in mins) for ramping function.

To change whether winds, air temperature, and/or water vapor are nudged you can change if_no_pbl_nudging_uv (t or q.)

  1. Re-run real.exe. This will generate wrffdda_d<domain> files in your run directory.

  2. Run wrf.exe.

Analysis nudging using surface and upper-air observations

  1. Downloading surface and upper air observation data in rda.ucar.edu (ds461.0 and ds351.0).

  1. Upper air observational data

  2. Surface observational data

  1. Compiling obsgrid.exe and get_rda_data.exe.

  1. To compile obsgrid.exe, please refer to OBSGRID Github. If successful, this will generate the executable obsgrid.exe.

git clone https://github.com/wrf-model/OBSGRID.git OBSGRID
./configure
./compile

  1. To compile get_rda_data.exe, please refer to OBSGRID/util/get_rda_data.f.

cd OBSGRID/util
ifort -FR get_rda_data.f -o get_rda_data.exe

  1. Using combineSurfaceToObs.sh to combine the downloaded files SURFACE_OBS:YYYYMMDDHH and OBS:YYYYMMDDHH into new files (e.g. C_OBS:YYYYMMDDHH). This script is available in Github WRF_Nudging.

  2. Using combineCobsToRdaobs.sh to combine those new files (C_OBS:YYYYMMDDHH) into rda_obs. This script is available in Github WRF_Nudging.

  3. Using get_rda_data.exe to generate files (OBS:YYYY-MM-DD-HH) for obsgrid.exe.

  4. Setting up the options in configuration file (namelist.oa). Here is an example of OBSGRID Namelist for running obsgrid.exe.

  5. Running obsgrid.exe. This will generate files metoa_em* and wrfsfdda_d01. Please note that obsgrid.exe needs to be run after metgrid.exe. The files met_em* must be available in OBSGRID/ directory.

./obsgrid.exe >& obsgrid.out