In the current operational system, a data thinning is applied to IMS by using "bufr_filter" to pick up part of IMS (select 1 every 36 IMS raw observations). However, it looks inhomogeneous especially in some areas. The In order to improve it, the new data thinning has been tested by using "bufr_grid_screen_parallel" as same as satellite observations. The following configuration have has been used:
"bufr_filter" can be switched to "bufr_grid_screen_parallel" by inc_obs.py.
The impacts are as below.
- The new thinning leads to more homogeneous spatial distribution at the horizontal resolution of 25 km.
- The number of IMS observations used input in 2D-OI is reduced from 251926 to 169726.
- The spatial distributions distribution of observations can be more consistent between IMS and ESA CCI Snow.
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