Unrealistic retrievals in IT-1DVAR-04/6

[Ian Culverwell]

ROPP5 DRI highlighted an issue with these tests: delta T > 10K at 52km for the 5th profile in IT-1DVAR-04.nc (= COSMIC obs, UKMO bg) - see orange profiles here:

There was nothing apparently odd with the data or the bg for this profile, although it (like the 1st) is sited in the S Hemisphere (~50S), which has higher bg err covs - see red line here:

So I just reduced the SH errors to be the same as in the tropics (white line above), which implies a reduction in the sigmas by about 2/3. (This requires setting bg_covar_method= FSFC in the metoffice_refrac_1dvar.cf config file, and editing a local file.)

This had little effect. So I artificially further reduced all the bgerr covs by a factor of 10 above 35km. The result is shown here:

Clearly, much more reasonable temp changes: < 0.3K.

This is not to say that these are the correct bg err covs to use in this case; merely that a reasonable change in them can produce drastic change in the retrievals. In other words, this is a data issue, not a ROPP issue.

Interestingly, the original temperature increment is much reduced (to ~3K) by using the non-default LevMarq minimiser, rather than the default MinRopp - see here:

Again, it's resolved by reducing the bgerr covs at height:

For bangle, we have to reduce the bgerr covs by a factor of 10 at all heights to change this:

(temp incs of 1-2K)

to this:

(temp incs of < 0.1K).

But it can still be done.

So I suggest the root of the problem has been clarified.

[Ian Culverwell]

Test2_minropp.gif

[Ian Culverwell]

Test2_reduced_bgerrcov.gif

[Ian Culverwell]

Test2_levmarq.gif

[Ian Culverwell]

Test2_levmarq_reduced_bgerrcov.gif

[Ian Culverwell]

Test2_bangle.gif

[Ian Culverwell]

Test2_bangle_reduced_bgerrcov.gif