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The usage of IMS is a bit tricky in the current operational system. IMS is not used on IMS mask in 2D-OI instead of rejecting IMS directly. So large departures can be calculated on IMS mask and they lead to large increments outside IMS mask. I found that suspicious increments can be accumulated on IMS mask by the following processes:
- Positive increments by in situ observation at 6, 12, 18 UTC and they propagate to grids on IMS mask
- Negative increments around in situ observation by assimilating IMS at 0 UTC
- Positive increments again by in situ observation at 6, 12, 18 UTC ...
In order to reduce such suspicious increments, the following changes have been applied:
- a
- a
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- Reject IMS on IMS mask
- IMS thinning at a resolution of 31km (a bit larger than previous setting)
- Reduce vertical correlation (from green line to red line)
Suspicious flip-flop increments are accumulated on IMS mask before these modifications but they are reduced by them.
| control | before modifications | after modifications |
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Radius for observation scanning in 2D-OI
SCAN_RAD has been changed from 3000km to 300km. This change reduces computational cost for sorting observations.
Experiment list
Results
Results for winter 2020/21 and summer 2020 (against Andrew's control)
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