(back up to Science Work Robin)
Sea Ice tuning
version XFHCY
Up to and including XFXWB FAMOUS, sea ice is simply advected with the surface currents. Amongst other things, this stops the ice from drifting north off the coast of Antarctica like it should, which we thought might be important in correcting the Southern Ocean warm bias and lack of AABW. UM4.5 does include a subroutine for advecting the ice independently of the surface currents that was intended for HadCM4 but never used(?). The code’s clearly a bit flakey, as it initially failed on the QUEST cluster until I fixed a memory allocation error, but it appears to run fine on other platforms. This is being tuned up for the MOSES2.2 release of FAMOUS
The earlier low-albedo fix for the cold bias isn’t really required once the excessive atmospheric filtering is reduced, so for XFHCY I’ve gone back to the higher sea-ice albedoes/thicknesses as for Had CM 3. These may change during the tuning exercise, however.
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versions XFXWB and earlier
Much of the northern high-latitude cold bias is linked to excessive ice production. Encouraging the melting of the ice thus improves the high latitude surface temperatures considerably. Sea-ice albedo is temperature dependent, between a low “melting” value and a high “frozen” value. For FAMOUS releases up to XDBUA, the GMD paper reference version, the melting value has been lowered from 0.5 to 0.2. This is a bit extreme, although individual meltponds may have albedoes this low. This value was found to help temperature and ice fraction the most, through a short-ish series of sea-ice parameter tests. The overall Northern Hemisphere ice albedo in summer is now a little unrealistically low, but it’s actually better than it was for the rest of the year, and temperatures, year-round, are much improved. There’s also a slight average improvement in the Southern Hemisphere ice coverage. There’s still too much ice cover for much of the year in the Northern Hemisphere, then too little after the summer melt. Interestingly, the excessive ice in the Pacific really isn’t impacted much by this - really, it’s good for the temperatures via the ice albedo change, rather than by the actual removal of ice fraction. In addition, the tunable parameter of the minimum ice thickness of 0.5m is also reduced, to 0.25m.
The melting ice albedo is under Atmosphere → Scientific Parameters → Section by section → SW Radiation → Minimum Albedo of Sea Ice
New ice thickness is at ‘’Ocean → Scientific Parameters → Sea Ice Model → Minimum Local Ice Depth’