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| current system | candidate for 49r1 | |
|---|---|---|
| IMS mask | based on altitude (>1500m) | based on SDFOR (>200) |
| IMS thinning | select 1 for every 36 | select nearest IMS on a gaussian grid of 31km (TL639) |
| IMS rejection on IMS mask | No | Yes |
| Cap value for snow depth | 1.4m | 3.0m |
| RSCALE_Z | 800m | 500m |
| SCAN_RAD | 3000km | 300km |
| ODB | Improve number of O-A |
IMS mask
In the current operational system, IMS is not assimilated on mountainous areas over 1500m (the same as ERA5). However, Orsolini et al.(2019) found:
- The high temporal correlation coefficient (0.78) between the IMS snow cover and the in situ observations
- ERA-Interim, which assimilates IMS on mountainous areas over 1500m, has smaller biases of snow depth on the Tibetan Plateau than ERA5.
In order to improve the biases, the condition to assimilate IMS has been changed. In the latest experiment, IMS is not assimilated if standard deviation of filtered sub-grid orography (SDFOR) is more than 200. This is based on the idea that IMS snow cover from satellite observations could be less accurate on complex orography. The following figures show IMS masks for the current system and the latest experiment.
| IMS mask based on altitude (>1500m) | IMS mask based on SDFOR (>200) |
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