IPCC results indicate that the main bulk of uncertainties on global warming is within aerosol-cloud interactions. Based on observations this thesis aims to measure how anthropogenic aerosol from mid-latitudes increase emissivity of clouds in the Arctic, thus increasing Arctic surface temperatures.
Until recently this effect have been thought insignificant, but recent studies indicate that in the Arctic, many clouds may be susceptible to changes in emissivity. This is due to the few CCN and low liquid water paths in the Arctic, making clouds more sensitive and thus this is a cliamtologically significant effect in Arctic regions (Lubin and Vogelmann 2006)
With a long time series of measurements from Ny Aalesund (Svalbard), long term trends in clouds, climatology and aerosols were examined. A statistical approach was then used to investigate differences in longwave surface forcing from clean and polluted instances.
When investigating low clouds with small liquid water paths over Ny Aalesund a significant warming effect of 3.54 W/m^2 [3.2-4.3] was found in the presence of high accumulation mode aerosols compared to low accumulation aerosol concentrations. This is linked to cloud droplets having a smaller effective radius, giving the clouds higher emissivity, and is comparable to recent observations based findings by (Garrett and Zhao (2006); Lubin and Vogelmann (2006)).
Annual results show a potential significant surface forcing by what isrecognized as a cloud emissivity effect, the actual annual surface heating of this effect is established to have a potential of (0.24-0.29 W/m^2) which is the same order of magnitude as the modelled results Alteskjær et. al. (2010), showing that the frequency of affectable clouds probably is as low as model results indicate. A decreasing trend in accumulation mode aerosol number concentrations were uncovered and the origin of the aerosols were investigated.
This study show that more specific instruments for observations of clouds are needed at Ny Aalesund, but that the cloud emissivity effect can be measured with relatively simple low-maintenance instruments.