Dear Rob, This is an extremely relevant discussion for any attempt to cool the Arctic in order to halt sea ice retreat. (There is strong evidence that the retreat is already having an effect on N Hemisphere climate due to jet stream disruption, so a strong argument to try cooling the Arctic ASAP.)
The two main approaches being considered are (i) to produce a reflecting stratospheric haze at mid to high latitude and (ii) to brighten marine clouds (MCB) in the troposphere over the North Atlantic and North Pacific. In both cases the aim is to cool surface water flowing into the Arctic and thereby slow sea ice melt and allow it to reform more easily. Much of the surface water at higher latitudes (between about 50N and 70N) finds its way into the Arctic. About 10% of the world's freshwater flows in the Arctic Ocean. Most of your discussion has involved consideration of MCB, creating the cloud condensation nuclei (CCN) from ships. Since there are so many unknowns about the effectiveness, couldn't we have some useful experiments from aircraft, or has this been done already? For example, you point out that turbulence changes could reduce or enhance the initial enhancement - and if it is a reduction this could be showstopper. Two further approaches for cooling the Arctic involve clouds: (iii) cloud removal to increase outgoing thermal radiation, and (iv) cloud seeding to produce fresh snow and thereby increase surface albedo on a regional scale. In both these cases, we need to test the production of CCN from aircraft and monitor effectiveness. Could you envisage a crash programme for testing of these various approaches, to see which is most appropriate and effective in different locations, at different times of year and under different circumstances (existing cloud conditions, etc)? Has anything like this been done already? Cheers, John On Thu, Jan 1, 2015 at 4:47 PM, Rob Wood <bobbywood2...@gmail.com> wrote: > A straightforward way to prevent plume sinking (if indeed it turns out to be > undesirable for particle dispersion), is to heat the stack. This happens > already on all cargo ships. > > I don't believe that coagulation will be a showstopper although experiments > will be necessary to confirm this because coagulation depends on the exact > size distribution and charging and this cannot be predicted from modeling > alone. Some degree of charging may well occur (not my expertise) but this > will likely depend on the spray method. Effervescent spray atomization (see > Cooper et al. article in Phil Trans 2014 special issue), does not seem to > make a lot of charged particles. > > Observations show that shiptracks are rarely observed in boundary layers > deeper than 1km. Globally, most stratocumulus occurs in PBLs deeper than > this. But shiptracks themselves (although highly visible) are not necessary > for MCB to work. Greater dispersion in the subcloud layer prior to ascent > into the stratocumulus deck in the intermittently coupled layer above, might > increase efficacy by producing a more evenly distributed droplet > concentration enhancement (Stephen alluded to this). > > That said, I disagree that the albedo enhancement required (e.g., to offset > CO2 doubling globally, i.e., about 4 W/m2) is small. Only 20% of the planet > has clouds that may be seedable, so the solar reflection of seeded clouds > would need to be enhanced by >20 W/m2 (this number being generous because it > is highly unlikely that uniform seeding is possible). This is about one fifth > to one quarter of typical cloud albedo. > > More important than whether the human eye can detect the brightening, is that > spatial albedo enhancement gradients, and changes to the condensate amounts > due to e.g. drizzle suppression, will drive turbulence changes and also > regional scale circulation changes that produce cloud adjustments that could > reduce or enhance the initial enhancement. For example, it is known that on > average, condensate amounts in shiptracks are lower than in surrounding > clouds (e.g. Coakley and Walsh 2002, Chen et al. 2014) because reduced > precipitation in the track leads to stronger turbulence which drives greater > entrainment of dry free tropospheric air that thins the cloud layer. Ackerman > et al. (2004, Nature) first noticed this in large eddy models, and I wrote a > paper that attempted to explain this behavior with a simple model (Wood, J. > Atmos. Sci. 2007). These responses are difficult to capture in climate models > as they depend upon subgrid scale processes that are poorly represented in > models with re > solutions greater than a few hundred meters horizontal and a few meters in > the vertical. A big challenge. This *might* be the biggest showstopper of all > for MCB. > > Regards > > Rob > > > > On 1/1/2015 4:36 AM, Alan Gadian wrote: > > > Stephen, > > I am afraid I cannot comment on the electrification, but I would like to > emphasise the dynamics again. WRF (and WRF Chem ) can be driven either by > an observed real data, or in an idealised WRF - LEM form ( with no BL > parameterisation scheme) driven from an atmospheric profile > > In all LEM modelling of Sc, an important feature is always entrainment and > mixing. The horizontal and vertical velocities and the "rolls" or > "eddies" are critical in this. If there is a decoupled layer near the > surface, for example, as is sometimes / often observed then this will > critically affect the dispersion. I am still uncertain what was run in the > WRF chem simulation, what BL scheme was used in the IGAP runs, but the > argument I am proposing was that the velocity structure is unlikely to > be correct, unless actually verified with observations. > > I am trying the think of examples. Yamaguchi & Feingold , 2014, show the > changes in turbulence patterns, Wang and Fiengold (2009) and other work > including that of Wood ( not mentioned as he is part of this discussion) > show examples of this importance of the turbulence and eddies. > > I know that volcanoes are completely different, and this work is not at > the required resolution for SC clouds, but the attached poster, probably > without video, is some work that we did. We had to run WRF in LEM mode to > get anywhere near the correct eddy structures for the near volcanic plume > eddies. Again looking at the high resolution modelling work of AndrejczuK > (some of which Rob was again involved with), the role of the interaction > between the dynamical eddy structure and the microphysics and latent heat > exchange is crucial. > > Thus again, I feel that there are a lot of uncertainties in the modelling > work, and the only way to see if MCB works is to do an experiment > > Alan Gadian > > > > > > On Thu, 1 Jan 2015, Stephen Salter wrote: > > Hi All > > The words 'charge' and 'electrostatic' do not appear in Stuart et 2013. > People cleaning oil tanks in the 1960's found the painful way that its is > difficult NOT to generate charge, see > http://www.infostatic.co.uk/Papers/TankWashingRisks.pdf . There are at > least two ways by which we can control charge. > > The Stuart paper used a size distribution of 100 size bins, spaced > logarithmically between 10 nm and 10 μm in wet diameter rather than > mono-disperse spray. This is a range of 1000:1. I hope to keep within > 20%. Coagulation requires a relative velocity between drops. Viscous > forces are very large at sub-micron dimension. Particles will behave like > sand in honey. Small scale turbulence will tend to vary the velocity of > particles but while the Stokes drag force goes with the first power of > diameter the mass resisting acceleration goes with the cube. If there is > a > wide range of drop diameters, local turbulence will produce much larger > range of relative velocities. It would be useful to know coagulation > rates > for narrow ranges of drop diameter. > > In my paper on the detection of small contrast changes I assumed a loss of > 50% which would not be a show stopper. > > In figure 2 of of the Stuart paper there is no sign of any initial drop > due > to evaporative cooling. > > Stephen > > > Emeritus Professor of Engineering Design. School of Engineering. > University > of Edinburgh. Mayfield Road. Edinburgh EH9 3JL. Scotland s.sal...@ed.ac.uk > Tel +44 (0)131 650 5704 Cell 07795 203 195 WWW.see.ed.ac.uk/~shs YouTube > Jamie Taylor Power for Change > > On 01/01/2015 02:48, Alan Gadian wrote: > Rob, > I agree here with you. With LEM modelling with WRF Chem, the bdy > layer schemes can be very diffusive. Ignoring the electrostatics > charge element, I am concerned that the PDFs of the vertical > velocities are critical. From experience 20m is not good enough > resolution in the vertical. How does the model cope with changes in > cloud droplet number, as seen in andrejczuk (2012 aNd 2014) . The > vocals profiles provide data on the BL dynamical profiles, and I fear > with the wef chem LEM results, the dynamics and hence the dispersion > are inadequately represented. WRF Chem is about 20 times slower than > WRF without the chemistry package, and thus the representation of the > dynamics has to be compromised for the inclusion of the chemistry. I > would like it clarified about how these results compare with > observations. > > The papers of Andrejcuck provide a surprisingly efficient and rapid > dispersion, and compare reasonably well with observations. > > Alan > > > T --- Alan Gadian, NCAS, UK, ( sent from a mobile device ) Email: > a...@env.leeds.ac.uk or alan...@gmail.com > Tel: +44 / 0 775 451 9009 or +44 / 0 113 343 7246 > T --- > > On 31 Dec 2014, at 23:46, Rob Wood <bobbywood2...@gmail.com> > <bobbywood2...@gmail.com> wrote: > > Dear All, > > I think that some degree of coagulation given such > localized point sources of large numbers of particles is > inevitable, as shown in the paper by Stuart et al. (2013). > This will also be the case with charged particles. > Nevertheless, I don't think that this is necessarily a > fundamental limitation. After all, shiptrack formation, > where even larger numbers of particles are produced, still > occurs. Coagulation must be considered in the > calculations. That said, in our recent paper (Connolly et > al. 2014), we found significant albedo enhancement in a > parcel model even with quite broad size distributions. The > optimal median particle size becomes smaller as the size > distribution spread broadens (e.g. from coagulation). For > broader distributions typical of those produced in lab > tests, the optimal median droplet diameters need to be > somewhat smaller than 0.1 micron. > > I tend to agree with Stephen that near-surface spreading > due to initial negative buoyancy from evaporation of water > from the small seawater droplets may not necessarily be a > tremendous problem for the reasons he states. This has not > yet been considered in any model that I know of, but could > easily be done with large eddy models. > > Rob Wood > > > On 12/30/2014 8:35 AM, Stephen Salter wrote: > Hi All > > Piers Forster's concern in his video about spray > coagulation would be reduced if his model had used > mono-disperse drops with an electrostatic charge as > specified in our 2008 paper on sea-going hardware. > > His concern about detecting the effectiveness is > because the cloud contrast change needed to save > humanity is below the detection threshold of the > human eye. However contrast can be enhanced by the > superposition of satellite aligned images. I have > previously circulated some to this group and hope > that the idea will give quantitative results in a > few days. > > The picture of spray plumes shown in box 3 of his > IAGP practicalities note must have been using warm > air from a chimney. Depending on the temperature > and relative humidity of the surrounding ambient air > there will be several degrees of temperature drop > due to the latent heat of evaporation. The increase > of density will lead to a rapid fall of the cooled > air which will spread out over the sea surface like > a spilt liquid until it has been warmed by the large > area of contact with sea. You can show this fall and > dispersion very cheaply with a pond fogger, £19.99 > from Maplin. We want this dispersion because a low > dose over a large area is more effective than a high > point dose. > > Forster seems to be ignoring completely the idea of > coded modulation of CCN concentration in climate > models even though the satisfactory operation was > demonstrated by Ben Parkes doing a PhD in Forster's > own Department at Leeds in 2012. This might allow us > to get an everywhere-to-everywhere transfer function > of marine cloud brightening and win-win result with > more rain in dry places and less in wet. The high > frequency response means that we can give a tactical > spraying based local day-to-day observations. > > It is a puzzle that the Parkes thesis has, yet > again, vanished from the Leeds University website. > > Stephen > > > > Emeritus Professor of Engineering Design. School of > Engineering. University of Edinburgh. Mayfield Road. > Edinburgh EH9 3JL. Scotland s.sal...@ed.ac.uk Tel > +44 (0)131 650 5704 Cell 07795 203 195 > WWW.see.ed.ac.uk/~shs YouTube Jamie Taylor Power for > Change > On 28/12/2014 20:03, Andrew Lockley wrote: > > Integrated Assessment of Geoengineering > Proposals…: http://youtu.be/FFjzzfCLCqw > > Poster's note : I personally have found it > very difficult to access and appraise the > science behind the IAGP project. Despite this, > a vast amount of publicity has been obtained > for the project. I think the IAGP team could > do more to encourage early, in-depth access to > their material, particularly bearing in mind > the huge media interest. > > -- > You received this message because you are > subscribed to the Google Groups > "geoengineering" group. > To unsubscribe from this group and stop > receiving emails from it, send an email to > geoengineering+unsubscr...@googlegroups.com. > To post to this group, send email to > geoengineering@googlegroups.com. > Visit this group at > http://groups.google.com/group/geoengineering. > For more options, visit > https://groups.google.com/d/optout. > > > -- > You received this message because you are subscribed to > the Google Groups "geoengineering" group. > To unsubscribe from this group and stop receiving emails > from it, send an email to > geoengineering+unsubscr...@googlegroups.com. > To post to this group, send email to > geoengineering@googlegroups.com. > Visit this group at > http://groups.google.com/group/geoengineering. > For more options, visit > https://groups.google.com/d/optout. > > > The University of Edinburgh is a charitable body, registered in > Scotland, with registration number SC005336. > > > -- > You received this message because you are subscribed to the > Google Groups "geoengineering" group. > To unsubscribe from this group and stop receiving emails from > it, send an email to > geoengineering+unsubscr...@googlegroups.com. > To post to this group, send email to > geoengineering@googlegroups.com. > Visit this group at > http://groups.google.com/group/geoengineering. > For more options, visit https://groups.google.com/d/optout. > > > > > > > -- > You received this message because you are subscribed to the Google Groups > "geoengineering" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to geoengineering+unsubscr...@googlegroups.com. > To post to this group, send email to geoengineering@googlegroups.com. > Visit this group at http://groups.google.com/group/geoengineering. > For more options, visit https://groups.google.com/d/optout. > -- You received this message because you are subscribed to the Google Groups "geoengineering" group. 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