A Continental Storm with a Steady, Adiabatic Updraft and High Concentrations of Small Ice Particles: 6 July 1976 Case Study

Abstract

A storm in southeastern Wyoming was investigated by the NCAR/NOAA sailplane which spiraled up to 5.6 km above cloud base in and above a weak echo region. The updraft associated with the weak echo region was remarkably steady (about 8 m s?1) and the temperature lapse rate was nearly adiabatic up to at least 4.4 km above cloud base (?30°C), indicating that the updraft air was unmixed. If the nearest sounding (45 km away in the direction of the storm's inflow) is representative of the storm's low-level environment, then the updraft air should have originated from a subcloud layer about a kilometer above ground where the low-level winds with respect to the storm were the strongest, and not from surface levels where the water vapor mixing ratios were the highest. The updraft did not accelerate even though the temperature measurements indicate that the updraft air had significant amounts of buoyancy (with corrections for adiabatic liquid or frozen water loading). Presumably nonhydrostatic pressure forces had balanced the buoyancy force because the cloud top was rising through a stably stratified environment. Outside. the main updraft region in the lower cloud levels, them were smaller updrafts surrounded by downdrafts, and the cloud air was diluted by mixing. Analysis of mixed air properties suggests downward mixing from the middle cloud levels where mixing of cloud air with the environment can produce strong negative buoyancy. In the main updraft region at temperatures lower than ?18°C, the sailplane encountered small panicles (200?400 ?m mean dimension) in highly inhomogeneous concentrations which often exceeded 100 L?1. It is argued that these ice panicles could not have formed through the Hallett-Mossop ice multiplication mechanism or through collisional breakup, nor could they have been the result of downward mixing of frozen cloud droplets from above the level of homogeneous freezing. Small ice particles in similarly high concentrations have been observed in several other clouds during the 1976 field season. In all cases the high concentrations were observed during the later stages of cloud development, after precipitation had fallen below cloud base. The possibility that some precipitation had descended into the inflow air below cloud base and increased the ice nucleus concentration is discussed.