= Bug on linear interpolation =

Normally, the wind stress is interpolated with the bilinear interpolation scheme of the [[http://oceans11.lanl.gov/trac/SCRIP/|Scrip]] library included in OASIS-MCT.

In IPSLCM6 at resolution ORCA1 x LMD 144x144, the bilinear interpolation of the wind stress is bugged. Two NEMO points around 0°N/180°W takes there wind stress values from LMDZ points around 0°N/0°E, and the weights, base on the distance between the source and destination points, are totally wrong..

The bilinear interpolation of OASIS-MCT compute weights and adresses that are written in a file :  `rmp_tlmd_to_uorc_BILINEAR.nc`,  `rmp_tlmd_to_torc_BILINEAR.nc` or `rmp_tlmd_to_torc_BILINEAR.nc`.

 == Resolutions which are know to have the bug : ==

 === IPSLCM6 LR : eORCA1.2 x LMD144x144 ===

 Wrong fields are wind stress on T and U grids : `COTAUXXU`, `COTAUYYU`, `COTAUZZU` and `COWINDSP`.

 Fields on V grids are OK.
 
=== IPSLCM6 MR : eORCA1.2 x LMD 280x180 ===

Wrong fields are wind stress on T and U grids : `COTAUXXU`, `COTAUYYU`, `COTAUZZU` and `COWINDSP`.

 Fields on V grids are OK.

=== IPSLCM6 MR : eORCA1.2 x LMD 256x256 ===

Wrong fields are wind stress on T and U grids : `COTAUXXU`, `COTAUYYU`, `COTAUZZU` and `COWINDSP`.

 Fields on V grids are OK.

== Resolutions which are know to be free of the bug  : ==
 * IPSLCM5A2 VLR : ORCA2.3 x LMD96x95

== How to detect the bug == 

[[Image(BugOasisBilinear.png, 900px )]]

Plot of the wind stress on the ocean grid, at U point. There is an obvious wrong point at 0°N/180°W.

== Correction ==

A python program has been developed. It  makes a few corrections of adresses and weights in a copy of the file.

The program is available at https://forge.ipsl.jussieu.fr/igcmg/browser/TOOLS/BUG_OASIS_BILINEAR

== To run the correction, the steps are: ==

1. Run the model with bilinear interpolation of the wind stress ;

2. Check the wind stress fields on the NEMO grid ;

3. If they are wrong, get the weigths files `rmp_tlmd_to_[tuv]orc_BILINEAR.nc`. They are produced by OASIS, but not saved  by libIGCM ;

4. Correct them with the program. You may need to adapt the program to the bug you observe :-(

5. Put them in the IGCM repository where the other weights file lies ;

6. Use MOZAIC interpolation for the wind stress, using these new weights files.

7. Check the result ;-)

== Namcouple content extract ==

=== for bilinear interpolation (1) ===
{{{
############################################################################
#
#                      ATMOSPHERE  --->>>  OCEAN
#                      -------------------------
############################################################################   
#
# Field 8 : wind stress along X axis (a->o 1)
#
COTAUXXU O_OTaux1 23 <freq_coupling>  1  flxat.nc   <output_mode>
256 257 362 332 tlmd    uorc  LAG=<lag_atm>
P  0 P  2
SCRIPR
BILINEAR LR SCALAR LATLON 10
}}}

=== for mozaic interpolation with corrected weights (6) ===
{{{
############################################################################
#
#                      ATMOSPHERE  --->>>  OCEAN
#                      -------------------------
############################################################################   
#
# Field 8 : wind stress along X axis (a->o 1)
#
COTAUXXU O_OTaux1 23 <freq_coupling>  1  flxat.nc   <output_mode>
256 257 362 332 tlmd    uorc  LAG=<lag_atm>
P  0 P  2
MAPPING
# Interpolation method ou parametres mozaic
rmp_tlmd_to_uorc_BILINEAR_Corrected.nc dst
}}}