On 1-2 February 2001 a strong cyclonic storm system developed over the northeastern Pacific Ocean, which included a vigorous frontal rainband that moved onto the Washington coast. This storm was one of several studied during the first field phase of the Improvement of Microphysical Parameterizations through Observational Verification Experiments (IMPROVE). A wide variety of in situ and remotely sensed measurements were collected during the IMPROVE field studies to create a comprehensive data bank of both meteorological state parameters (temperature, pressure, humidity, winds, and vertical velocity) and microphysical parameters (particle concentrations, size spectra, particle imagery, and liquid water content), for the purpose of testing and improving bulk microphysical schemes used in mesoscale models.

In the 1-2 February case, soundings and wind profiler observations showed that the frontal rainband was located at the leading edge of an upper cold front in a classical warm-occluded structure. Ground-based radar observations revealed two separate layers of precipitation generating cells within this band, one at 5-7 km and one at 9-10 km. The lower layer, referred to as the “altocumulus generating cell layer”, produced fall streaks that could be traced from the generating cells down to the radar brightband at 2 km. The microphysical structure of the rainband was elucidated by imagery from a Cloud Particle Imaging (CPI) probe, mounted aboard the University of Washington’s Convair-580 research aircraft. These images provide detailed information on crystal habits and degrees of riming throughout the depth of the precipitating cloud. The crystal habits reveal temperature and saturation conditions where particle growth occurred, and they help in estimating particle fall speeds, which are important for deriving precipitation rates from radar-measured quantities. These measurements will be used to quantitatively describe particle growth processes observed in this storm.