Issue n. 1 in H2O MIPAS / ESA - Level 2 v8.0 products

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1 Issue n. 1 in H2O MIPAS / ESA - Level 2 v8.0 products (+ 5% v8 bias vs v7. Continued from webex teleconf 25 January 2017 ) Piera Raspollini (IFAC CNR) Luca Sgheri (IAC CNR) Marco Ridolfi (DIFA UB) Presentation at MIPAS QWG meeting 43 Karlsruhe, 5 6 July /17

2 Summary of previous investigations (see MR presentation at QWG webex telecon from 25/01/2017) On average, the H2O VMR in stratosphere in OR v8 will be 2 to 5% larger than in v7. This increase will worsen the agreement with correlative measurements (especially MLS and ACE). The difference was found to be caused by the difference in spectroscopic H2O line data in MW H2O_0332. Test showing the difference A) B) between H2O retrievals with: A) all LUTs from ADF2 v9 B) all LUTs from ADF2 v9 but the H2O LUT of MW H2O_0332 from v8.04 2/17

3 MWs used in OR v7 and v8 ESA H2O retrievals: MW# Start (cm-1) Stop (cm-1) ================================= H2O_ H2O_ <<< === MW causing + 5% bias in v8 H2O_ H2O_ H2O_ /17

4 Spectrum and residuals in MW H2O_0332 (from MR presentation at QWG 25). Tangent height = 27 km. (mainly NO2 line) 4/17

5 Spectrum and residuals in MW H2O_0332 (from MR presentation at QWG 25). Tangent height = 13.5 km. (mainly NO2 line) Fully saturated signal === >> large correlation between H2O VMR and CTNM parameter. 5/17

6 Remarks Differences due to the new H2O lines introduced in DB v4.45 were observed also in the tests presented at QWG#25 (21-23 March 2011), however at that time we were not concerned as validation results were not available. See plot from MR presentation at QWG#25: Note: 0.25 ppmv 30km corresponds to about +5% (H2O VMR ~ 5 ppmv) 6/17

7 A similar H2O VMR bias was also observed when comparing the new (4.45) H2O line data with HITRAN 12, see MR presentation at QWG# July 2015: However, during QWG#40 we were happy to see this increase in H2O VMR because this shift was expected to improve the agreement with correlative measurements with Frost Point Hygrometers (FPH, see Michael s Kiefer presentation at QWG#40). 7/17

8 Note that a +5% H2O MIPAS / ESA v7 / v8 bias is also perfectly compatible with the error budget evaluated by Anu at Oxford... Conclusion / way forward from webex teleconf 25 January 2017 The systematic differences between H2O VMR v7 and v8 (v8 = v7 + 5%) are caused by the changes in the H2O spectroscopic line data (changes from DB v3.2 to 4.45). The systematic difference increases the discrepancy between MIPAS and correlative measurements, however the new (v4.45) spectroscopic data are more accurate than the old ones (maybe we have a problem of undercompensation of another systematic error component?) We do not recommend to change again spectroscopic data. Maybe a revision of MW H2O_0332 could be useful (why was it chosen by MWMAKE if no H2O lines are visible?) 8/17

9 April 2017: new MWs were provided by Oxford Anu could not find a suitable single-microwindow replacement for H2O_0332 so instead he did a complete reselection based on a slightly different selection criterion. New microwindows are H2O_0341 H2O_0348, however the OM is limited to just the first five These MWs are in common with other species. New MWs: H2O_34* MW# start wn stop wn zmin zmax absorbers ================================================================================= 1 H2O_ H2O_ H2O_ H2O_ H2O_ Piera checked the differences between H2O VMR retrieved with 34* MWs and 33* MWs included in ADF2 v9.00 Average H2O VMR profile differences by latitude band, test retrieval from 1 OR orbit. 9/17

10 Average H2O differences by lat. band. Profile with new 34* MWs profile with 33* MWs of ADF2_V9.00 The sign and the amplitude of the H2O VMR changes are such that v8 v7 differences would be significantly reduced, however the noise error would be much larger.. (see error bars in this plot wrt the ones in the next plot) 10/17

11 Reminder. With MWs H2O_33* the global (i.e. due to both new ADF2 and new v8 code) v8 v7 H2O VMR differences are: 11/17

12 Suggestion for a further refinement of H2O MW selection (based on results of MA retrievals) Remark: MW H2O_0332 is used also in v8 OR MA retrievals Does v8 MA H2O has the same + 5 % bias identified in NOM retrievals? 12/17

13 Does v8 MA H2O has the same + 5 % bias identified in NOM retrievals? Mean h2o at 10 hpa See the results of this experiment: daily averages of H2O VMR at 10 hpa vs day. Days from 1st January 2008 The low values correspond to days with MA measurements ===> NOM retrievals show a + 5 % wrt MA retrievals. However, MW H2O_0332 has only little impact on MA retrievals, as demonstrated by this further test. 13/17

14 Impact of MW H2O_0332 on OR MA v8 retrievals For orbit (MA) we carried-out the v8 retrieval with a) ADF2 V9.00 and b) with ADF2 V9.00 but MW H2O_0332. Here is the plot of the relative differences a) b). ===> negligible impact on MA retrievals. 14/17

15 Next question: is the detrimental impact of H2O_0332 MW in NOM retrievals due to its usage at altitudes below the lower bound (18 km) of the MA scan? ===> we tried artificially to set to 18 km the lowest altitude of the OR NOM retrieval. H2O differences NOM ADFV9 NOM ADFV8, retrieval starts at 18 km === >> No, the negative impact of MW H2O_0332 in OR NOM retrievals is not limited to low altitudes. 15/17

16 Possible conclusion / lesson learned from this test Spectroscopic errors of H2O in MW H2O_0332 have a significant impact (+5%) in NOM retrievals, however their impact in MA retrievals is only marginal this could be due to the fact that in MA retrievals the effect of MW H2O_0332 is mitigated by the use of several other MWs such as H2O_42* and H2O_43* 16/17

17 Overall conclusions The new MWs H2O_034* are successful in reducing the v8 H2O bias around 10 hpa, but they seem to have small information content at low altitudes. Probably the comparison between MA and NOM MWs could help in identifying a proper set of MWs for NOM measurements. Any suggestion? Note: the H2O v8 issue is still open :-( 17/17