Influences on formation and dissipation of high arctic fogs during summer and autumn and their interaction with aerosol

Abstract

Radiosondes established that the air in the near surface mixed layer was very frequently near saturation during the International Arctic Ocean Expedition 1991 which must have been a large factor in the frequent occurrence of fogs. Fogs were divided into groups of summer, transition and winter types depending on whether the advecting air, the ice surface or sea surface respectively was warmest and the source of heat. The probability of summer and transition fogs increased at air temperatures near 0°C while winter fogs had a maximum probability of occurrence at air temperatures between - 5 and - 10 °C. Advection from the open sea was the primary cause of the summer group, the probability of occurrence being high during the 1st day’s travel and appreciable until the end of 3 days. Transition fogs reached its maximum probability of formation on the 4th day of advection. Radiation heating and cooling of the ice both appeared to have influenced summer and transition fogs, while winter fogs were strongly favoured by the long wave radiation loss at clear sky conditions. Another cause of winter fogs was the heat and moisture source of open leads. Wind speed was also a factor in the probability of fog formation, summer and transition fogs being favoured by winds between 2 and 6 ms^-1, while winter fogs were favoured by wind speeds of only 1 ms^-1. Concentrations of fog drops were generally lower than those of the cloud condensation nuclei active at 0.1%, having a median of 3 cm^-3. While a well-defined modal diameter of 20–25 μm was found in all fogs, a second transient mode at about 100 μm was also frequently observed. The observation of fog bows with supernumerary arcs pointed to the existence of fog droplets as large as 200–300 μm in diameter at fog top. It is suggested that the large drops originated from droplets grown near the fog top and were brought to near the surface by an overturning of the fog layer. Shear induced wave motions and roll vortices were found to cause perturbations in the near-surface layer and appeared to influence fog formation and dissipation. The low observed droplet concentration in fogs limits their ability to modify aerosol number concentrations and size distributions, the persistent overlying stratus being a more likely site for effective interactions. It is suggested that variations in the fog formation described in this paper may be a useful indicator of circulation changes in the arctic consequent upon a global warming.