Minutes of the aerosol working group meeting 22/11/2013

From UKCA

Notes from UKCA aerosol teleconference held Friday 22nd November between Leeds & MO

Present: Ken Carslaw, Graham Mann, Colin Johnson, Jane Mulcahy, Ben Johnson, Steve Rumbold, Alastair Sellar.

Summary of discussion:

There were 2 main topics of discussion for the teleconference aimed as resolving the two remaining issues re: the current v8.4 GA4 CheT+GLOMAP job:

1. AOD and sea-spray too high in Southern Ocean (and also Arctic)

2. AOD and BC, OC (also sulphate?) column-mass too low in biomass burning regions

Actions:

1. JM/AJ to try no-DMS run also to see how much the DMS is contributing to the AOD there.

2. GM to try sensitivity study where multiply sedimentation velocity by 2 (modal underpredict sedi?)

3. CJ to investigate precipitation rates in Southern Ocean region and whether wet dep may be inefficient there.

4. JM to report how bias looks at GA6 CheT+GLOMAP job when all bug-fixes have been applied.

5. CJ/BJ to try sensitivity run with convective scavenging coeffs reduced from 1.0 to 0.5

6. Next UKCA aerosol teleconf 2pm Thurs 12th Dec

Notes:

1. S. Ocean AOD high bias

At Sept 12th telecon, discussion was held about the very high AODs in the model in the S. Ocean. At that time job amtjn had large region with AOD>0.5 during SH summer (DJF) and max value of ~0.45 in JJA (winter). Had identified either sea-spray or DMS as potential causes. Had discovered S. Ocean DMS was very high-biased (b=0.29, 3.04, 3.31 at 3 S. Ocean sites). Had resolved to try changing Wanninkhof  Liss Merlivat sea-air transfer (known to generate much lower DMS emission). Equivalent job ajkzi (using LM sea-air) reduced DMS high bias substantially (b=-0.11,1.66,1.22 for same sites). AOD from ajkzi job (using LM sea-air) reduced AOD slightly in S. Ocean – peak now just below 0.5 in S. Ocean during DJF. Comment from marine biogeochemistry group was why use Liss Merlivat since it was so old. After a discussion MO folks resolved to try runs with Nightingale instead. But Leeds commented this falls within Wann & LM so this not expected to reduce DMS high-bias.

Action: JM/AJ to try no-DMS run also to see how much the DMS is contributing to the AOD there.

2nd progress since last time was that SW had found factor-0.5 was being applied in fall velocity due to code-merge error.

JM had circulated plots (5th Nov) showing impact of sedi bug-fix – DJF AOD in SO then peaks at 0.4 rather than 0.5. Bigger impact on AOD during SH winter when sea-salt is dominant contribution (SO peak at ~0.35 rather than ~0.45). But although AOD reduced, still much too high against collection-6 MODIS and AERONET which show AOD ~0.1 there). BJ explained he had checked AODs in each mode in the region and found coarse mode AOD contributed ~0.3 AOD on the annual mean so main source of high-AOD likely to be sea-salt not DMS. CJ explained budget analysis shows removal of sea-salt showed 45% by dry-dep-sedi and 55% by nucscav/imscav. This was surprising as coarse mode expected to show sedimentation as dominant removal process.

Action: GM to try sensitivity study where multiply sedimentation velocity by 2 (modal underpredict sedi?).

Action: CJ to investigate precipitation rates in Southern Ocean region and whether wet dep may be inefficient there.

Note: CLASSIC sea-spray AOD was found to much higher at GA4 than GA3 or GA5 so expect reduction anyway GA4GA5/6.

Action: JM to report how bias looks at GA6 CheT+GLOMAP job when all bug-fixes have been applied.

2. AOD too low in biomass burning regions

GM had suggested at UKESM-UKCA meeting on Wed that BC/OC seemed reasonable in biomass regions so why AOD low? BJ had checked and although BC and OC mass burdens do show expected plumes in biomass regions, they are lower than at v7.3 which suggests AOD low-bias is consistent with BC/OC burdens so not an issue re: RADAER. BJ explained that spatial patterns in column-integrated SO4, BC and POM suggested too much scavenging in tropical regions --- likely that convective (plume) scavenging implementation was too aggressively scavenging. Omission of re-evaporation also likely to mean scavenging needs turning down.

Action: CJ/BJ to try sensitivity run with convective scavenging coeffs reduced from 1.0 to 0.5.

Next UKCA aerosol teleconf meeting will be held at 2pm on Thursday 12th December. This is scheduled for 1 week ahead of UKESM-UKCA meeting on 19th Dec (target date for GA6 CheT+GLOMAP evaluation).