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
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.