id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
232	Unrealistic retrievals in IT-1DVAR-04/6	Ian Culverwell	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: [[Image(Test2_minropp.gif)]]

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: [[Image(cov.png)]]

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: [[Image(Test2_reduced_bgerrcov.gif)]]

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: [[Image(Test2_levmarq.gif)]]

Again, it's resolved by reducing the bgerr covs at height:
[[Image(Test2_levmarq_reduced_bgerrcov.gif)]]


For bangle, we have to reduce te bgerr covs by a factor of 10 everywhere to change this: [[Image(Test2_bangle.gif )]]

(temp incs of 1-2K) 

to this: [[Image(Test2_reduced_bgerrcov.gif)]]

(temp incs of 0.1K). 

But it can still be done. 

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

"	defect	new	normal	5.0	ropp_1dvar	4.1