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The impact of snow DA changes has been investigated for 49r1. Snow depth on the Tibetan Plateau and the Rocky Mountains is reduced by assimilating IMS except on complex orography. RMSE of 2m temperature is reduced by 0.5% for winter and 1% for spring in the Northern Hemisphere. In addition, computational time for the 'snow' task is reduced about 30 seconds at 0 UTC by reducing a radius for observation scanning and number of IMS input in 2D-OI.
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In the current operational system, a simple data thinning is applied to IMS by "bufr_filter" (select 1 from every 36 raw observations). However, it leads to inhomogeneous coverage of IMS, especially in some areas. In order to improve it, new data thinning has been tested by "bufr_grid_screen_parallel" as same as other satellite observations. The thinning selects nearest observations on a reduced gaussian grid of 31km. As a result, the coverage becomes homogeneous and number of observations is reduced from 251926 to 109223.
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Snow water equivalent (SWE) in ERA5, ERA5-Snow and ERA5-Land has been validated against in situ observations (snow course in Canada) by Chris Derksen (ECCC). He pointed out that ERA5 and ERA5-Snow don't represent SWE more than 500mm (personal communication). We found that it is caused by a cap value of 1.4m in snow depth analysis. It has been changed to 3.0m in the latest experiment.
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The usage of IMS is a bit tricky in the current operational system. IMS is not used assimilated on model grids of IMS mask in 2D-OI instead of rejecting IMS directly. So large departures can be calculated on IMS mask and lead to large increments the impact propagates outside IMS mask. I found that suspicious increments can be accumulated on IMS mask by the following processes:
- Positive increments by in situ observation outside IMS mask at 6, 12, 18 UTC and 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 ...
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The following figures show snow depth averaged in Feb 2021. Snow depth on the Tibetan Plateau and the Rocky Mountains is reduced by assimilating IMS. Snow depth on the other mountains are increased by modifying a the cap value for snow depthchange.
| control (hut8) | test (hybs) | hybs - hut8 |
|---|---|---|
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RMSE of 2m temperature is reduced by about 0.5% for winter in the Northern Hemisphere , (especially in North America).
| RMSE for 2m temperature against analysis in the NH | RMSE for 2m temperature against observations in the NH |
|---|---|
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