(a) Map of MODIS-derived annual mean cloud droplet concentration, N0, for stratiform marine warm clouds. To be included in the annual mean, the daily warm cloud fraction in 1×1° boxes must exceed 50% to capture primarily marine stratocumulus clouds. (b) Cumulative distribution of daily 1×1° droplet number, N0, from MODIS for all ocean points. (c) A comparison of MODIS and C-130 aircraft-measured cloud droplet concentration estimates from the VOCALS regional experiment during October/November 2008 off the Chilean coast , for longitudes 70–77.5° W (more polluted) and 77.5–85° W (more pristine). There is good agreement between in situ and satellite-derived values that lends weight to the use of these data over the global oceans.
A comparison between model and observed precipitation, and investigation into the impacts of MCB on model precipitation (mm per day). (a) Comparison of the CMAP precipitation dataset with a current CO2 level simulation in HadGEM1. (b) The effect of increasing carbon dioxide from 440 to 560 ppm within the model. (c) The difference between a geoengineered simulation, 2CO2 and a control simulation. (Online version in colour.)
A comparison of the north and south polar sea-ice fraction averaged over the summer minimum for the final 20 years of the 70 year simulations. Sea-ice fraction can be interpreted as the fraction of time that ice is present at that location. The northern minimum is taken as September, and the southern minimum is taken as March. (a,b) The difference in north and south polar sea-ice fraction between 2CO2 and CON. (c,d) The difference in north and south polar sea-ice fraction between MCB and CON. The black contour shows the ice limit in CON. (Online version in colour.)
A comparison of the north and south polar sea-ice thickness (m) averaged over the summer minimum for the final 20 years of 70 year simulations. The northern minimum is taken as September, and the southern minimum is taken as March. (a,b) The difference in the north and south polar sea-ice thickness between 2CO2 and CON. (c,d) The difference in the north and south polar sea-ice thickness between MCB and CON. The black contour shows the ice limit in CON. (Online version in colour.)
Snapshots of the cloud albedo field when ships pass through the domain once from x=0 to 180 km, about 7 hours after the start of the simulations. The background aerosol number concentration varies linearly from a lower bound at x=0 to an upper bound at x=180 km; (a) clean case 60–150 mg−1 and (b) the polluted case 210–300 mg−1. Arrows indicate the direction of movement of the ships and the band of ship plumes emitted near the surface. Details on the model and experimental set-up can be found in Wang & Feingold [33,34].
Summary plots for the clean air mass. The number of activated drops without the addition of NaCl were 8.8 and 9.8 cm−3 for w=0.2 and 0.5 m s−1, respectively. (a) A contour of the number of activated cloud drops when a distribution of NaCl aerosols of different total number and median mass are added to a rising parcel moving at 0.2 m s−1. The masses added are on the x-axis, whereas the corresponding number added is on the y-axis. Plus signs denote the different runs used to calculate the contour plot; (b) same as (a) but for an updraught of 0.5 m s−1; (c) the difference in the albedo between the control run and the run with the indicated aerosol added (nadd, madd), in units of per cent, of the clouds resulting from seeding; (d) same as (c) but for 0.5 m s−1. Please refer to initial conditions in text for dry diameters corresponding to added dry particle masses.
Summary plots of the albedo change for the medium and dirty air-mass cases. For the medium case, the number of activated drops without the addition of NaCl were 142 and 179 cm−3 for w=0.2 and 0.5 m s−1, respectively, whereas for the dirty case these were 358 and 639 cm−3 for w=0.2 and 0.5 m s−1, respectively. (a) The difference in the albedo between the control and the run with the indicated aerosol (nadd,madd) for the medium case with 0.2 m s−1 updraught; (b) the same but for 0.5 m s−1; (c,d) the corresponding contours of albedo change for the dirty case.
Schematic of the proposed phase 2 and 3 field testing to evaluate the cloud responses to (a) a single-seeded plume; (b,c) multiple-seeded plumes. Examination of ship tracks from commercial ships  tells us that the plumes spread quasi-linearly with time at a rate of approximately 2 km h−1 , which for typical wind speeds of 5–10 m s−1 is a width of approximately 6–12 km at a distance of 100 km downwind of the source (a). For phase 3 testing, 5–10 ships (six shown in the example here) would be spaced approximately 10 km apart to generate a single plume 50–100 km wide at a distance of 100 km downwind (b). This broad plume and its surrounding unperturbed cloud would be sampled in the crosswind direction by stacked aircraft as discussed in the text (c). (Online version in colour.)