WRF Model Version 2: Experimental Nudging Options

General description

Features and advantages of observational nudging are discussed in (*) below. The method uses relaxation terms based on the model error at observational stations, and the relaxation is such as to reduce this error.Each observation has a radius of influence, a time window, and a relaxation time scale determined by user-specified input. These determine where, when, and how much it affects the model solution. Typical model grid points may be within the radius of influence of several observations, and their contributions are weighted according to the distance from the observation(s). Before performing obs-nudging, you will need to generate an observation input file for each WRF domain. The observation file(s) contain chronological lists of the 3D positions and values of each observation, in a specific format. It is critical that your observations be listed in chronological time order!

* Liu, Y., A. Bourgeois, T. Warner, S. Swerdlin and J. Hacker, 2005: An implementation of obs-nudging-based FDDA into WRF for supporting ATEC test operations. 2005 WRF user workshop. Paper 10.7.

How to use the obs-data converter

A utility program for converting observation data to the format required by WRF has been provided (RT_fdda_reformat_obsnud.pl). The converter assumes that your observation data is in standard LITTLE_R format.

To convert your data that is in LITTLE_R format:

RT_fdda_reformat_obsnud.pl yourfilename

where "yourfilename" is the obs-data in LITTLE_R format. The converter will produce a file named yourfilename.obsnud, in the format required by the WRF model.

Note that during the conversion process:

1). P,T,U,V and RH fields are extracted.

2). U and V are assumed to be the wind components rotated to the model map-projection (see 3DVAR and MM5 Little_R).

3). SPD, DIR and Td fields are ignored.

4). For upper-air data, currently WRF nudging only takes those data with valid pressure records. For obs with height levels (e.g. wind profilers data), users need to calculate or estimate the pressure value. Inaccurate estimate of pressure will lead to bad data assimilation.

Naming your obs-nudge input files

After you have converted your obs data file to the proper format for WRF, you will need to rename it according to the naming convention for the WRF domain on which the obs-nudging is to be performed. For example, for observations to be used in Domain 1, use the naming convention OBS_DOMAIN101, for Domain 2, OBS_DOMAIN201, etc.

These files must be present in your WRF run directory, along with the usual WRF input and boundary files.

Observation Input Format

The observation input file contains multiple records of the following:

read (nvol,101) date_char
read (nvol,102) latitude, longitude
read (nvol,1021) id, namef
read (nvol,103) platform, source, elevation, is_sound, bogus, meas_count

For single level data:

read (nvol, 105) slp_data, slp_qc, ref_pres_data, ref_pres_qc, height_data, height_qc, temperature_data, temperature_qc, u_met_data, u_met_qc, v_met_data, v_met_qc, rh_data, rh_qc, psfc_data, psfc_qc, precip_data, precip_qc

For sounding type data:

read (nvol, 104) pressure_data, pressure_qc, height_data, height_qc, temperature_data, temperature_qc, u_met_data, u_met_qc, v_met_data, v_met_qc, rh_data, rh_qc

where

date_char: 14 character date string. e.g. 20070114120000
latitude, longitude: south latitudes are negative, west longitudes are negative
id: station id
namef: data types
platform: (to come)
source: (to come)
elevation: station height
is_sound: logical, whether the data is a sounding
bogus: logical, whether the data is a bogus
meas_count: number of levels for this sounding
data_qc: qc stands for quality control, and it follows the definition for MM5/little_r data. If your data are all good, set it to 10.

The formats of these read statements are provided in this program, read_obs.f. A sample data file, called OBS_DOMAIN, is available for reference. Also see WRFV2/share/wrf_fddaobs_in.F for details.

How to activate obs-nudging

To activate the observational nudging option in WRF, you will need to set the obs_nudge_opt flag(s) in the WRF "fdda" namelist. Note that there is a unique flag for each WRF domain in which you want to activate obs-nudging. To activate the print statements within the obs-nudging subroutines ERROB, NUDOB, and IN4DOB, set the respective print flags obs_ipf_errob, obs_ipf_nudob, and obs_ipf_in4dob to ".true." You can then easily verify that you have activated observational nudging by observing text in your WRF "standard out" that tell you how many obs stations are being processed at given model timesteps. This information will look something like:

0****** CALL IN4DOB AT KTAU = 8 AND XTIME = 24.00: NSTA = 11040 ******
++++++CALL ERROB AT KTAU = 8 AND INEST = 1: NSTA = 11040 ++++++

These lines will print out for each nest in which you have activated nudging, while nudging is active on that domain.

Below is an example of a namelist set up to activate obs-nudging on domains 1, 2, and 3:

&fdda
obs_nudge_opt = 1,1,1,
max_obs = 150000,
nobs_ndg_vars = 5,
nobs_err_flds = 9,
fdda_start = 0., 0., 0.,
fdda_end = 99999., 99999., 99999.,
obs_nudge_wind = 1,1,1,1,1
obs_coef_wind = 6.E-4,6.E-4,6.E-4,
obs_nudge_temp = 1,1,1,
obs_coef_temp = 6.E-4,6.E-4,6.E-4,
obs_nudge_mois = 1,1,1,
obs_coef_mois = 6.E-4,6.E-4,6.E-4,
obs_rinxy = 240.,240.,180.,
obs_rinsig = 0.1,
obs_twindo = 0.666667 ! (in hours)
obs_npfi = 10,
obs_ionf = 2,
obs_idynin = 0,
obs_dtramp = 40.,
obs_ipf_errob = .true.
obs_ipf_nudob = .true.
obs_ipf_in4dob = .true.

And in namelist record &time_control, set (auxinput_11_interval_s should be set as frequently as obs_ionf):

auxinput11_interval_s = 120, 120, 120,
auxinput11_end_h = 6, 6, 6,