INFLUENCES ON FORMATION AND DISSIPATION OF HIGH ARCTIC FOGS DURING SUMMER AND AUTUMN AND THEIR INTERACTION WITH AEROSOL

Citation
Ed. Nilsson et Ek. Bigg, INFLUENCES ON FORMATION AND DISSIPATION OF HIGH ARCTIC FOGS DURING SUMMER AND AUTUMN AND THEIR INTERACTION WITH AEROSOL, Tellus. Series B, Chemical and physical meteorology, 48(2), 1996, pp. 234-253
Citations number
33
Categorie Soggetti
Metereology & Atmospheric Sciences
ISSN journal
02806509
Volume
48
Issue
2
Year of publication
1996
Pages
234 - 253
Database
ISI
SICI code
0280-6509(1996)48:2<234:IOFADO>2.0.ZU;2-3
Abstract
Radiosondes established that the air in the near surface mixed layer w as very frequently near saturation during the International Arctic Oce an Expedition 1991 which must have been a large factor in the frequent occurrence of fogs. Fogs were divided into groups of summer, transiti on and winter types depending on whether the advecting air, the ice su rface or sea surface respectively was warmest and the source of heat. The probability of summer and transition fogs increased at air tempera tures near 0 degrees C while winter fogs had a maximum probability of occurrence at air temperatures between -5 and -10 degrees C. Advection from the open sea was the primary cause of the summer group, the prob ability of occurrence being high during the Ist day's travel and appre ciable until the end of 3 days. Transition fogs reached its maximum pr obability of formation on the 4th day of advection. Radiation heating and cooling of the ice both appeared to have influenced summer and tra nsition fogs, while winter fogs were strongly Favoured by the long wav e 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 fog s being favoured by winds between 2 and 6 ms(-1), while winter fogs we re favoured by wind speeds of only 1 ms(-1). Concentrations of fog dro ps were generally lower than those of the cloud condensation nuclei ac tive at 0.1%, having a median of 3 cm(-3). While a well-defined modal diameter of 20-25 mu m was found in all fogs, a second transient mode at about 100 mu m was also frequently observed. The observation of fog bows with supernumerary arcs pointed to the existence of fog droplets as large as 200-300 mu m in diameter at fog top. It is suggested that the large drops originated from droplets grown near the fog top and w ere brought to near the surface by an overturning of the fog layer. Sh ear induced wave motions and roll vortices were found to cause perturb ations in the near-surface layer and appeared to influence fog formati on and dissipation. The low observed droplet concentration in fogs lim its their ability to modify aerosol number concentrations and size dis tributions, 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 circul ation changes in the arctic consequent upon a global warming.