# Vertical Conversion of Observations

 Jump to DART Documentation Main Index version information for this file: \$Id: vertical_conversion.html 11403 2017-03-27 23:26:22Z hendric@ucar.edu \$
In Lanai vertical conversion of observations occurs in get_close_obs. The Lanai code in filter_assim is as follows:

```SEQUENTIAL_OBS do i = 1, obs_ens_handle%num_vars
...
broadcast increments for observation i
call get_close_obs()
call convert_vertical_location(observation(i))
enddo```

If this algorithm was followed in RMA DART, all processors would have to communicate to calculate the location of observation i. This is a huge amount of contention since all processors are doing exactly the same calculation so need to access exactly the same state elements. This causes the code to run very slowly, for example 1 minute for 1000 observations, versus 5 seconds for 1000 observations for Lanai.

However, there is no need to calculate the vertical conversion inside the SEQUENTIAL_OBS do loop, since the mean state vector used is not updated during the loop. (In Lanai it is the array passed to the model_mod by ens_mean_for_model in filter_main). Also this calculation does not scale, because all processors do the same calculation.

In DART RMA the owner of an observation converts the vertical location of an observation and broacasts it to all other processors as part of the broadcast in the SEQUENTIAL_OBS do loop.

The DART RMA code calculates the vertical of all observations before the loop. This potentially scales better because processors only calculate their own observation conversions, but does require model_mod interfaces for vertical conversion.

The DART RMA code in filter_assim is as follows:

```do i =, obs_ens_handle%my_num_vars
call convert_vertical_location(my_obs_loc(i))
end do
SEQUENTIAL_OBS do i = 1, obs_ens_handle%num_vars
...
broadcast increments and vertical location for observation i
...
enddo
```

### Bitwise Problem

Moving the `convert_vertical_location` changes the number of `get/set location` calls. There is a bitwise creep of the location when you do this. This is in the conversion from degrees to radians and back again. If you want to do the exact number of `get/set location` you can change the line lanai_bitwise = .false. to lanai_bitwise = .true. in assim_tools_mod.f90. Note this is not a namelist option because production code should not be run with lanai_bitwise = .true. For more detail on running bitwise with Lanai see bitwise considerations.

[top]