Springtime Visibility in the Arctic

Abstract

Since the Ptarmigan flights in the 1950s, the springtime visibility reduction in the Arctic has been identified with pollution aerosol. However, observed values of the dry aerosol extinction coefficient are too small to explain the observed visibility reductions. Water uptake by the aerosol appears to be an important component of the Arctic turbidity. Furthermore, the presence of lower-tropospheric ice crystals may have a dominant effect on the visibility in the Arctic. Data obtained from a series of meteorological, chemical, and cloud microphysical measurements made by the University of Washington research aircraft during April 1983 and 1986 are used as input for a visibility model. The model calculates the water uptake by the dry aerosols as a function of relative humidity and determines the single-scattering properties of the aerosols and the ice crystals. Path radiances are calculated using an extension to the delta-Eddington approximation introduced by Hering. Modeled visual ranges indicate that aerosols alone cannot account for the low visibilities. In certain cases, the inclusion of ice crystals along with the aerosols can produce visual ranges which are as low as those observed. A comparison between visual ranges obtained using the model and estimated using Koschmieder's equation showed that Koschmieder's equation will generally underestimate the visual range.