Minutes of the aerosol working group meeting 12/09/2013


Notes from UKCA aerosol teleconference held Thursday 12th November between Leeds & MO

Present: Ken Carslaw, Graham Mann, Colin Johnson, Jane Mulcahy, Mohit Dalvi.


From a general discussion over plots circulated in advance of the meeting the following points were discussed in detail:

1. AOD in Southern Ocean too high in v8.4 and v8.2 runs

2. DMS too high during SH summer in Southern Ocean

3. sea-spray too high in Southern Ocean

4. Arctic AOD too high.

5. Wildfire signal in AOD seems very low

6. General Arctic seasonal cycle against observations

Notes of discussion:

1. AOD in Southern Ocean seems too high in the v8.4 CheT+GLOMAP run (amtjn).

In fact in all v8.4 runs (and v8.2 too?). 2 suggested causes here -- too-high sea-salt and/or too-high DMS. The AOD was higher in the SH summer (Dec) than in SH winter (Jun) which suggested the dominant cause could be DMS.

2. DMS:

Comparing to the Dumont D’Urville (DD) DMS seasonal cycle showed the DMS was much too high during SH summer whereas we generally seem to get the DMS about right at DD in TOMCAT-GLOMAP (e.g. see Mann et al., 2010 and Spracklen et al. 2005). Too high DMS could be caused by the use of the Wanninkhof sea-air transfer relation which tends to give a high DMS flux and may have been used for historical reasons with the CLASSIC scheme (which has lower chemical sink). Could try switching from Wanninkhof to Liss & Merlivat which is also available in the UMUI as a different option. I recall us making this change in the v7.3 CheT+GLOMAP runs to address a Southern Ocean SO2/DMS/SO4 high-bias

3. sea-spray:

The annual-mean comparison to the U. Miami surface NaCl observations suggests the S. Ocean sea-salt is a bit high – and for some reason there was very little drop-off of the sea-salt from the ocean to the Antarctic continen (particularly in SH winter, June). This was also the case in the Arctic winter. There seems in general to be very little removal of aerosol during the cold periods of the year. Could this be related to the way the scavenging shuts off if the cloud liquid fraction is zero? But then it was mentioned that actually this was a key control for transport to the Arctic for example. An alterative issue could be to do with Possible issue with dry deposition or sedimentation that is leading to too high sea-salt over the continents. Roughness issues?

4. Arctic AOD too high too?

Could be similar issue to the high sea-salt over the – need to check use of roughness length and sub-optimal implementation of aerosol dry deposition (still using old approach inherited from TOMCAT-GLOMAP whereby roughness length is used to infer land-surface types for coefficients in aerosol-drydep routine). Or could there be an issue with the BL mixing taking too much aerosol out of the layer that is subject to dry-dep? We do the emissions simultaneously with the BL mixing but then the ddep is after that (could this lead to too inefficient dry-dep?)

5. Wildfire signal in AOD seems very low

Don’t see any indications of biomass-burning /wildfire in AOD maps – for example SON AOD seems very low in biomass burning regions – could there be an issue with the emissions ancils being used? Suggest to look into this with biomass burning on/off experiments (could Ben Johnson look into this?).

6. General Arctic seasonal cycle against observations for v8.4 GA4 and GA5 CheST+GLOMAP runs.

One thing seen in the GA5 CheST+GLOMAP run is that there seems to be more transport to the Arctic in GA5 than GA4. Jo Browse had done analysis of these 2 v8.4 runs against several sets of Arctic aerosol observations from her PhD. There was very encouraging comparisons of SO2 seasonal cycle against the observations at several sites and also the BC seasonal cycle looked good with both runs a little low against the measurements – but much improved compared to v7.3 runs. There was a general question-mark about the rather high AOD seen in the model during the winter and spring – this was in agreement with the seasonal cycle in the measurements but the AOD seemed too high at 0.3-ish? Perhaps could be related to an issue with the sea-salt transport being much too efficient over the continents in cold seasons? (as discussed in items 3 and 4).