Characteristics of Cloud Ice and Precipitation during Wintertime Storms over the Mountains of Northern Colorado

Abstract

This article describe cloud ice and precipitation process in 17 wintertime storm systems that occurred over the mountains of northwestern Colorado. Surface, remote sensing and aircraft measurements are used to study cloud and precipitation characteristics. Ice particle concentrations; secondary ice particle production; and ice panicle growth, habits and aggregation are studied in the context of cloud system structure. The region of the cloud near cloud top was found to be a primary source region for ice particles in these clouds. Ice particle concentrations were found to increase over elevated topography and to increase at lower altitudes in deep cloud systems. Approximately 80% of the measured concentrations were within the range of 1-100 L?1, and 50% were less than 20 L?1. The activity of three secondary ice particle production mechanisms was evaluated. Fragmentation was found to be important in cloud systems that contained dendritic crystals. Evidence suggested that the Hallett-Mossop rime splintering mechanism occurred only occasionally in these clouds and contributed little to the total precipitation. Droplet fragmentation during freezing did not appear to be an important mechanism. Surface precipitation from these cloud systems consisted of irregular crystal habits and dendritic snowfall. Irregular particles accounted for approximately 70% of the snowfall. The large number of irregular particles was attributed to (a) continuous changes in vapor density excess, temperature and concentration of liquid water droplets encountered by particles during their fall; (b) fragmentation; and (c) riming. Dendrites accounted for approximately 20% of the snowfall. A common feature of all but one major dendritic snowfall event was that cloud top was in or near the dendritic growth temperature regime (?13° to ?17°C). Aircraft penetrations near the tops of these clouds found the cloud top region to contain liquid water. In these cases, the region near cloud top was found to have ideal conditions for dendritic growth. Warm temperature sheaths and needles contributed little to the total precipitation in these cloud systems. The primary components of aggregates in Park Range cloud systems were planar dendritic crystals and radiating assemblages of dendrites. When these particles were present in the snowfall, aggregation occurred independent of surface temperature and precipitation rate. Significant aggregation was not observed when dendritic crystals were not present.