source: trunk/WRF.COMMON/WRFV2/test/em_real/README.obs_fdda @ 2756

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. Note that you may split your obs data into
hourly input for each domain. For example, observations with timestamps in
the first model hour can be listed in file OBS_DOMAIN101, those with
timestamps in the second model hour in file OBS_DOMAIN102, etc.

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



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                       = 1,1,1,0,0   
 max_obs                             = 150000,
 fdda_start                          =     0.,     0.,     0.,     0.,     0.
 fdda_end                            = 99999., 99999., 99999., 99999., 99999.
 obs_nudge_wind                      = 1,1,1,1,1
 obs_coef_wind                       = 6.E-4,6.E-4,6.E-4,6.E-4,6.E-4
 obs_nudge_temp                      = 1,1,1,1,1
 obs_coef_temp                       = 6.E-4,6.E-4,6.E-4,6.E-4,6.E-4
 obs_nudge_mois                      = 1,1,1,1,1
 obs_coef_mois                       = 6.E-4,6.E-4,6.E-4,6.E-4,6.E-4
 obs_rinxy                           = 240.,240.,180.,180,180
 obs_rinsig                          = 0.1,
 obs_twindo                          = 40.
 obs_npfi                            = 10,
 obs_ionf                            = 2,
 obs_idynin                          = 0,
 obs_dtramp                          = 40.,
 obs_ipf_errob                       = .true.
 obs_ipf_nudob                       = .true.
 obs_ipf_in4dob                      = .true.