Dust

AeroCom wiki discussion entry

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AeroCom working group DUST

Participants for analysis

Ginoux, Balkanski, Mahowald, Schulz, Winker, Mann, Takemura, Miller?, Tegen?, Zender?,

NEXT Telephone conference: 10th of January 17 UTC (= 18 Paris time, 12 ET, 10 Boulder Time?)

Goals

Investigate the possible impact of anthropogenic dust sources
Compare dust simulations to multiple observational datasets
Recommend properties for dust size and refractive index

Processes and possible Diagnostics

Dust erosion - wind frequency and speed ; threshold velocities; effective source fluxes; source size distribution

Dust settling - dry removal velocities per size bin;

Wet Dust removal - wet scavenging efficients; vertical distribution; wet deposition of Calcium, dust;

Scattering and absorption - refractive index; AOD; size; absorption

Short-term actions/experiments

Organise regular teleconferences

Prepare a natural and an anthropogenic dust source for a joint experiment
Follow up on ideas from earlier Mahowald publications.

Assemble observations
(Visibility, IDDI, MISR, MODIS, MODIS Deepblue, Deposition, Aeronet, Aeroce, Calipso, Asian dust observations, Ca concentrations in rain)
Retrieve from AeroCom database dust variables and wind speed fields

Eventually participate in dust conference in Italy

Data to look at

The AeroCom database has unanalysed data on dust deposition and wind speed statistics in some models.

Aeronet dust dominated sites

Refractive index and size in literature

Dust, Fe and Ca concentrations in air and precipitation

Satellite AOD in dust dominated regions (MISR, MODIS, TOMS, DEEPBLUE-MODIS, CALIPSO)

Fine mode AOD in dust regions?

Enter the DISCUSSION HERE

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Michael Schulz: Dec. 14th 2006

Dear dust colleagues,

Having met Nathalie and Paul in Paris during the GEIA conference I am motivated again for suggesting joint dust work. I think there is a good chance to get a better 'dust analysis' going on the basis of the AeroCom data. But we probably need to do/propose some additional experiments.

During the AeroCom workshop we have said that a working group should be installed and Paul kindly agreed to lead it (for now). As suggested by the meeting we intend to maintain wiki pages for joint discussion and I have set up one with basic initial thoughts:

http://wiki.esipfed.org/index.php/Dust

However - Nathalie and me thought that some few telephone conferences would be really helpful to get things going. I have no real experience but can only propose skype. Any better suggestion would be perfect!! I just tested: we could also download images on the wiki page prior/during a telecon. Not so complicated. I suggest you use the wiki page to enter propositions for a first teleconference.

Right now I am not sure we will manage to start before Christmas. But some reaction to this proposal would be nice,

have nice winter days! Michael

Ina Tegen: Dec. 15th 2006,

Dear colleagues, a dedicated AEROCOM-DUST analysis is a good idea and is quite timely considering the ever-increasing interest in dust aerosol, and I would be happy to contribute. I agree with Michael that some new experiments would be needed, which obviously have to be carefully designed. To have Paul lead a working group on this is great. I suggest to also include Stephanie Woodward from the Hadley Center in this exercise. (stephanie.woodward@metoffice.gov.uk).

After having looked at the WIKI page, here are some additional comments:

- What is the best choice for an anthropogenic dust source? While the estimates for total anthropogenic dust contribution to total dust are now considerably lower compared to the IPCC 2001 number, this emission and the geographical distribution is still very uncertain. In addition to ‘direct’ anthropogenic dust sources as from agricultural soil disturbance it would be nice to have a ‘second order’ anthropogenic dust source that is caused by anthropogenic climate change, ie present-preindustrial sources taking into account any changes in surface wind and precipitation as well as vegetation, such as estimated recently by Natalie. In addition to her results it would be useful to have additional estimates from other models to minimize a possible model dependence of these results. Maybe putting together a map of vegetation changes from different vegetation model results for the preindustrial to present can be helpful.

- A while ago I looked at the AEROCOM results for dust– it seemed that the differences between the results for Experiment B (fixed sources) were larger than to be expected from differences in deposition schemes alone (although these may be considerable too)- and indeed it seems that while everyone used the same emission fluxes they were distributed quite differently on the different particle sizes, due to the different treatments of size distribution in the models (mode vs bin schemes) and of course if the same mass it put into larger size bins, the atmospheric load depletes more quickly due to higher gravitational settling velocities, and the importance of the different deposition processes (as seen in the Figure included in the Wiki page)) partly differs simply due to this. It is crucial to pay attention to this for any new runs comparing results with identical dust source fluxes– if the effect of the different transport and deposition schemes are to be compared it has to be ensured that the sources are in fact identical, including identical initial size distribution.

- The choice of the dust refractive index is of course important for computing radiative flux changes, but to my knowledge no model uses yet refractive indices which vary with the source region to account eg for hematite/goethite content (my apologies if I have overlooked something here). Also, I see that you intend to compare Ca contents of the dust – this should also vary for the different source regions. So I am not sure if we can make any real progress by model intercomparison, apart from compiling appropriate literature data. BTW, the German SAMUM dust experiment that took place in Morocco past summer should have some very good data on dust optical properties by next year (the measurement results are currently still being analysed).

- Which should be the focus year(s) for those models which are driven by reanalysis fields or nudged meteorology? For the AEROCOM years I suggest to concentrate on 2001, which had Ace Asia, and some nice Saharan dust events. If additional years will be simulated, I suggest 2006 of course, if we want to make use of Calipso data. Also, for this year eventually some results from the SAMUM and AMMA field campaigns results will become available.

- If this is possible it would be useful to perform an emissions only experiment in addition to the full atmospheric dust run to systematically compare the performance of the different schemes, e.g. computing dust emissions for the different schemes with identical surface wind speeds in addition to compute the fluxes with their own wind fields. Of course it will difficult to decide on the ‘best’ emission scheme by this as there are no global scale dust emission flux data, but a test of the performance could possibly be done be in terms of modelled ’emission events’ compared to meteorological dust storm data, IDDI, and TOMS AI > 0.7 (?)events (any other ideas?).

Regarding a telephone conference, sometime January would be fine. Having never set up one myself, I cannot offer any other suggestions on the best way to do this, sorry.

A happy holiday season to all of you, Best wishes, Ina

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Dr. Ina Tegen Leibniz Institute for Tropospheric Research Permoserstr. 15 04318 Leipzig Germany

Tel.: +49 (0)341 235 2146 Fax: +49 (0)341 235 2139 Email: itegen@tropos.de

Stefanie Woodward: Dec. 18th 2006,

Dear Ina et al,

I think a dedicated AEROCOM dust intercomparison would definitely be a good idea, and I'd be interested in being involved, if possible.

I agree with the points that Ina makes. The idea of a comparison of emissions schemes is particularly interesting, as they must account for a large part of the differences between models as they are actually run. It could be a challenge to set up a level comparison, though, as emissions schemes tend to be tuned to the GCM they're run in (intentionally or otherwise), so if the prescribed wind fields etc vary significantly from those in the "parent" GCM, dust emissions could be very different from what the scheme normally produces. As to looking at "second order anthropogenic dust", I have some data which might be useful: fields from HadAM3 simulations run with vegetation fractions and SSTs taken at 2000 and 2100 from a coupled climate - carbon-cycle simulation, which allowed the vegetation changes to feed back on the climate.

I don't know what the projected timescale for an AEROCOM dust analysis is, but we'd be in a much better position to take part in something later in the year. Our latest climate model is still in the process of being tuned, though this should, hopefully, be completed fairly early next year. Also, we don't at the moment have the facility for running the model for specific time periods: this should be available later in the year.

Best wishes, Steph