The Origin of the Ice Phase in Arctic Boundary Layer CloudsS

A paucity of observations in the Arctic means that neither the aerosol processes nor cloud properties are well understood or represented within models, with the result that aerosol and cloud-forcing of Arctic climate is poorly constrained.

Research investigating the lower atmosphere of the arctic has found that low cloud dominates the variability in arctic cloud cover. Temperature and humidity profiles show a high frequency of one or more temperature inversions and below these inversions stratus or stratocumulus clouds form. During the arctic summer these low clouds often consist of multiple layers, with a number of theories describing the separation of these clouds. These clouds have been observed during different seasons but the association between temperature and the formation of ice in these clouds is not well understood. Jayaweera and Ohtake (1973) found that the cloud microphysics consisted of very little ice above -20° but Curry et al., (1996) observed ice clouds at temperatures as high as -14°. It is possible that the variation in temperatures at which point glaciated clouds are observed is caused by variation in the number concentration and composition of aerosol.

Methodology

ACCACIA measurement campaigns took place during March-April 2012 and July 2012, in the region between Greenland and Norway. Measurements were made from both ship and aircraft platforms. Results presented here will be confined to those from aircraft which made most of their measurements in the regions around and to the south of the Svalbard archipelago. Two aircraft (the FAAM BAe 146 based in Kiruna Sweden, and the British Antarctic Survey (BAS) Twin Otter, based at Lonyearbyan Svalbard) flew during the late winter campaign while only one (the BAS Twin other) flew during the summer period. The overarching theme of the project is to reduce the large uncertainty in the effects of aerosols and clouds on the Arctic surface energy balance and climate. Key to the work presented here is the understanding the microphysical properties of Arctic clouds and their dependence on aerosol properties. During the late winter period (March and April 2012) the FAAM BAe-146 aircraft was flown within, below and above clouds in order to make high-resolution measurements of the cloud microphysical structures and of the properties the aerosol above and below cloud. Cloud microphysics was measured by a full suite of cloud probes on the 146, which included measurements of cloud drop size distributions using a DMT Cloud Droplet probe (CDP) and measurements of the size distribution and shape of ice particles using a SPEC-Inc Two-Dimensional Stereoscopic (2D-S) Probe. Aerosol measurements included measurements of size distribution and size resolved chemical composition. Volatile aerosol composition was measured using an Aerodyne compact Time of Flight (c-ToF) aerosol mass spectrometer (AMS) and non volatile components were measured by ESEM from filter samples. During the summer campaign the British Antarctic Survey's Twin Otter aircraft also carried aerosol sampling equipment, alongside CDP and 2D-S probes for the measurement of the cloud microphysics.

Examples of Results

Two examples from the winter campaign are FAAM flights B761 and B768. The main difference is that B768 was influenced by pollution aerosol with higher sulphate loadings advected from Europe, whereas B761 was carried out in much less polluted conditions. B768 contained many more cloud droplets attributable to the higher concentration of Cloud Condensation Nuclei (400 cm-3 compared to 70 cm-3 for B761), however, the numbers of ice crystals observed was very similar (about 0.5 l-1 and close to the number that would be expected from the number of ice nuclei detected. In both cases, cloud base temperatures were about -10¢ªC and cloud top temperatures about -20¢ªC. One example is also considered from the summer campaign. In this case, the droplet number concentration was similar to the clean winter case, being about 90 cm-3. However, the cloud base and cloud top temperatures, around -2¢ªC and -10¢ªC respectively, were much higher. In this case the number of ice crystals was also higher with a median value of around 3 L-1, which is much higher than observed during the winter campaign examples.. The observed ice crystal concentration enhancement, at temperatures between -2¢ª and -10¢ªC in the summer case, is consistent with a process of secondary ice particle production by the Hallett-Mossop process in the temperature zone around -6¢ªC. Further evidence for this was observed by the presence of riming ice particles and many small columnar crystals.

Conclusions

Generally it was found that in winter there were frequent incursions of polluted aerosol layers mostly advected from continental Europe or Russia and these were frequently lofted above the arctic boundary layer. These were often depleted of ice nuclei which are often found in the larger particles. The boundary layer stratocumulus clouds contained substantial amounts of supercooled water at all times. In winter the ice crystal concentrations were controlled by the number of ice nuclei. In the summer periods there were fewer incursions of polluted air and the often multi-layered cloud again contained supercooled water. However, at this time of year there were pockets of glaciation produced by secondary ice particle production where ice crystal concentrations exceeded those in the colder winter clouds.