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Cold bias
April ‘07
Previous work by Taro suggested that reducing ice albedo also killed off the MOC. Combining the deep bathymetry (with its initial MOC strengthening) and ideas about changing the ice albedo are proving fruitful however. Using the lower high albedo, as mentioned below, does start to reduce ice coverage with the deeper bathymetry. Standard albedoes, with a reduced “new ice” thickeness (0.5 → 0.25) doesn’t help much, but reduced thickness with a lower low ice albedo (0.5 → 0.2) produces much better looking ice coverage with a sensible strength MOC which reaches about as far north as
Had CM 3’s does. AABW’s still not great though. Whilst more physically justifiable than lowering the high-end ice albedo (e.g. meltponds can really lower the albedo of melting ice; the new ice thickness seems to be a bit of a tuned guess anyway) these are pretty big changes - they do at least prove that this is a potential way forward though. I haven’t looked at the seasonal cycle, or the southern hemisphere yet either. Hopefully, an optimal tuned set of ice parameters will turn up in the next release that will give much better results.
June ‘07
A 100yr “spinup” suggests that the above values (deep N.Atlantic bathymetry, new ice thickness(H0)=0.25, melting ice albedo(ALPHAM)=0.2) do produce significant, long term improvements (relative to HadCM3) to the surface temperature, radiation budgets and ice fraction, so that’s what I’m going to go with at the moment. The MOC spins up a little (~24Sv max), there’s not much in the way of AABW, and surface albedo north of 70N or so is now too low (it was far too high before), but the cold bias in zonal surface air temp. is pretty much removed. The 0 fraction ice line isn’t radically different in a lot of the N.Atlantic from before, but the fractions are down quite a bit. This is all just looking at annual average plots. A brief look at other times/regions suggests that there’s also a slight improvement in the southern hemisphere, and that the seasonal cycle isn’t much changed (it’s still too large), just shifted downwards in the mean. I’ll do some more checking up on this though. See below for practical details.
All low resolution versions of HadCM3 have had a cold bias and too much sea-ice at high latitudes. FAMOUS partially improves on the situation by the removal of Iceland to allow more northward ocean heat transport in the Atlantic, but the issue is still problematic - particularly if FAMOUS is to be used to look at the onset of glaciation.
- The solutions:
Use of an envelope orography (e.g. Slingo and Pearson, QJRMS 1987) in low resolution atmosphere models has been shown to improve variability and blocking in a similar manner to parameterisations of gravity-wave drag. Preliminary attempts at in FAMOUS this also seem to improve the low latitude warm bias (esp. with the new ozone), and possibly help the high latitude cold one. Applying the envelope “algorithm” over Antarctica clearly makes the temperature worse and makes the orography look worse too so it’s not applied here - to a certain extent this is true of high northern latitudes too, although how to shade between areas where it is applied and those where it isn’t is not yet clear. The effect on atmospheric variability does not seem to be as expected - EKE diagnostics show a mixed bag of increases and decreases at different locations, but explicit N.Atlantic stormtrack/blocking plots show decreases in both, which is the opposite from what was expected. Storm track activity is too low anyway (common in GCMs, but especially so here), so this is bad news. More work required before this gets anywhere near a release.
| qrparm.customX_orog | ancil | replaces standard atmospheric orog. ancil. - atmospheric reconfiguration needs to be on |
Even with the removal of Iceland by Jones et al. the Atlantic MOC does not penetrate as far north as that in HadCM3. The bathymetry between Greenland and Scotland is still clearly higher than that in HadCM3 - deepening this channel allows the MOC to push further north. However, it also initially strengthens to unrealistic values whilst doing little to melt the excessive ice or raise SSTs except very locally to the deeper channel cut for it.
| adtaao@dao41c1-DEEP | ocean restart | a new ocean restart needs to be created with a new bathymetry header and appropriate data. This has been done via a run with newrestart.mod and a bathymetry header specified on fort.102, but is really not nice. Xancil should be able to do this soon, Paul Valdes et al at Bristol (I assume) have a better solution (and much more experience at changing landmasks etc.) |
| adbip@dao41c1-DEEP | ocean restart | as above, but contains biogeochemsitry fields for HadOCC |
In an attempt to melt some of the over-enthusiastic sea-ice, the “cold ice” albedo of 0.8 was lowered to match the “melting ice” albedo of 0.5. Whilst this improves the surface air temperature and the surface fluxes, it does not substantially change the actual area covered by the ice, so cannot be counted a success. Not to mention not really being physically justifiable. Improved below.
| sice_albedo.mod | model mod | does simply what it says above |
Changing the melting ice albedo is rather more justified than changing the cold ice one, and combining this with tuning the new ice thickness and the deeper bathymetry can be effective - see note for June ‘07 above. My current sice_albedo.mod makes these changes to the ice, but they can be specified through the UMUI instead. The melting ice albedo is under Atmosphere → Scientific Parameters → Section by section → SW Radiation → Minimum Albedo of Sea Ice and the new ice thickness is at Ocean → Scientific Parameters → Sea Ice Model → Minimum Local Ice Depth. I haven’t tried these without the deep bathymetry restarts above, but I should think they’d help plenty on their own.