zonal mean scavenging rate intergrated between 0-2km, 2-5km and 10+km for control and ice-phase simulations EFFECT OF SCAVENGING PROCESSES (ICE-PHASE CLOUDS)
School of Earth and Environment INSTITUTE FOR CLIMATE AND ATMOSPHERIC SCIENCE The scavenging processes controlling the seasonal cycle in Arctic CCN: Implications in a sea-ice free Arctic Jo Browse¹, Ken Carslaw¹, Steve Arnold¹ ,Kirsty Pringle¹, Olivier Boucher² 1.University of Leeds 2. UK met office Acknowledgements: Cathryn Birch (ASCOS group)
Background
Nordenskiöld, Nordenskiöld on the inland ice of Greenland. Science, 1883 “The sky was covered with a thin veil of clouds…but it was not wet, but dry—yes, so dry that our wet clothes absolutely dried in it.” “Everywhere a fine dust, gray in color, and, when wet, black or dark brown, is distributed over the inland ice” By absorbing sunlight, the “cryoconite” created narrow cavities… up to a meter deep. Nansen, 1883 “…the surface of the larger sheets … had a dirty, grayish, or even brownish hue… There is dust in the air… taken down onto the ice with every snowfall” George Simpson, 1906: “All who have traveled in Arctic regions know the peculiar haze which fills the air… On the other hand, at the end of the summer the air reached a degree of transparency which I have never seen equaled” J. Murray Mitchell, Ptarmigan reconnaissance flights in 1950s “…haze at flight altitudes which restricts visibility to 3–8 km” Rahn, Borys, and Shaw, The Asian source of Arctic haze bands. Nature, 1977 “Arctic haze is the end product of massive transport of air pollution from various midlatitude sources … on a scale that could have never been imagined, even by the most pessimistic observer.” BACKGROUND Zeppelin mountain (Svalbard) aerosol size distribution (Ström et al. 2003)
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BACKGROUND Stohl (2006) Kahl et al (1996)
BACKGROUND Modified from Quinn et al 2007
Why do we care at all how well we model Arctic aerosol concentrations? -stress seasonality of direct and indirect aerosol effects in the arctic -stress values are estimates Finish with – Shindell and Faluvegi 2008 study which attributed 2/3 of current Arctic warming to changing aerosol concentrations (this includes the global reduction in sulphate emissions from the 1980s onwards)
GLOMAP- mode GLOMAP-mode: two-moment modal global model which calculates both aerosol mass and number in 7 modes (4 soluble, 3 insoluble) for five aerosol species (Sulphate, Black Carbon, Organic carbon, Sea-Salt and dust) Wet deposition scheme Calculates the fraction of CCN removed by convective and frontal rainfall only (Mann et al. 2010) New wet deposition scheme includes Drizzle scavenging in low clouds Ice-phase cloud scavenging GLOMAP-mode Global model of aerosol processes (GLOMAP) Research pages: http://researchpages.net/glomap Boundary layer CCN lifetime with respect to drizzle scavenging Ratio of modelled snow to total precipitation per m²
Drizzle+ice scavenging Ice scavenging Drizzle scavenging control
School of something FACULTY OF OTHER
Model evaluation (BC) MODEL VALIDATION
Green –model with ice-cloud scavenging Red- control model Yellow- model with both parameters Purple- model with drizzle scavenging Figures show the inter-annual mean from 2000 to 2002 of equivalent black carbon observations and model output, model-observation agreement quantitively better at both stations seasonal cycle is now in phase with observations. High summertime concentrations at Barrow are the consequence of fresh insoluble BC wildfire emissions in the region which are unaffected by low cloud scavenging. ‹#›
Effect of drizzle on surface SO4 concentrations Model Evaluation (Sulphate) EFFECT OF SCAVENGING PROCESSES (DRIZZLE) Percentage change in deposition flux
zonal mean scavenging rate intergrated between 0-2km, 2-5km and 10+km for control and ice-phase simulations EFFECT OF SCAVENGING PROCESSES (ICE-PHASE CLOUDS)
Scavenging distribution Ice cloud scavenging
CONCLUSIONS
Figure shows mass scavenging flux in milligrams per m2 per day between the surface and two kilometers (red), 2 and 5 km (blue) and 5-10km (green) for the control run (dashed lines) and a model run with the ice-cloud scavenging scheme ‹#›
Conclusions
CCN in an sea-ice free Arctic (negative feedback?) sea-spray emissions Wind speed Surface water temperature Sea-ice cover Surface albedo Increased Arctic warming Decreased Arctic warming CO2 radiative forcing Direct aerosol radiative forcing CCN indirect forcing Fossil fuel combustion Anthro Aerosol emission Anthro CO2 emission Ice-albedo-aerosol (-) Ice-sea-spray (-) Ice-albedo (+) Ice-sea-spray (+) Anthropogenic forcing Combined Arctic ocean radiation climate forcing feedback loops DMS flux increase (CLAW) Sea-spray emission feedback 23% increase in natural AOD -0.2 - -0.4W/m² (negative feedback) (Struthers et al. 2010) 75% increase in Arctic sea-salt aerosol burden (Jones et al. 2007) DMS-emission Feedback ? Modified from Struthers et al. (2010)
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Model evaluation (Sulphate) MODEL VALIDATION (ASCOS) Observations courtesy of Cathryn Birch Mauritsen et al. (2001)
Scavenging Distribution low cloud scavenging
Zero sea-ice experiments -sea ice fraction reduced to zero from July-September -DMS climatology remains the same (Kettle and Andreae, 2000) -ECMWF meteorological forcing remains the same -Drizzle scavenging suppressed in second simulation
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SUMMARY The seasonal cycle in Arctic aerosol characteristics is controlled by the onset of ice-phase cloud scavenging in the Arctic and amplified by the occurrence of drizzle in the Arctic boundary layer Future increases in spring and autumn temperatures in the marginal Arctic are likely to decrease CCN concentrations in this region Future changes to Arctic stratus are likely to have only a small effect on CCN as Arctic drizzle scavenging is already highly effective In a ice-free Arctic CCN concentrations (particularly in the central Arctic) are sensitive to boundary layer deposition processes However, the effect of sea-ice reductions on DMS emission and wind speed remain highly uncertain Current work: assessing the sensitivity of CCN concentrations in a ice-free Arctic to DMS climatology and wind speed
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