Particles in the Lower Troposphere over the High Plains of the United States. Part III: Ice Nuclei

Abstract

One hundred and fifty-five ice nucleus (IN) spectra were measured from an aircraft at various locations over the High Plains of the United States during the springs and summers of 1975 and 1976. Frequency distributions of the IN concentrations at each test temperature reveal two modes, each of which is apparently lognormally distributed. The dominant mode occurs at high IN concentrations, and the other mode at low concentrations. The characteristic IN spectrum was represented by N(T) = 2 ? 10?4 exp(?0.3?T), where N(T) is the concentration (per liter) of IN at temperature T and supercooling ?T. The IN generally appear to derive from a widespread and fairly uniform ground source, which accounts for the dominant mode. However, concentrations can be increased by duststorms and industrial pollution and decreased in the vicinity of localized IN sinks (e.g., clouds and precipitation). As a consequence, IN concentrations can vary markedly during the course of a day, from day-to-day, and cyclically over periods of days to weeks.