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. 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 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 input in 2D-OI is reduced from 251926 to 169726.
  • The spatial distribution of observations can be more consistent between IMS and ESA CCI Snow.