The winter 2001 IMPROVE II site at McKenzie Bridge in the foothills of the Oregon Cascades experienced several periods when the melting layer between ice and rain intersected the surface. The site was equipped with NOAA ETL's vertically-pointing S-band Doppler radar, a wind profiler, and a suite of surface meteorology instrumentation including two types of disdrometers. The PARSIVEL M300 disdrometer simultaneously measures particle size and fall velocity. The PARSIVEL can measure particles with diameters from 0.31 to 24.5 mm and fall velocities from 0.25 to 20.8 m/s. Snow particles up to 13 mm diameter were observed at McKenzie Bridge. Particles larger than 2.5 mm diameter are difficult to observe with aircraft probes because of sample volume and turbulence constraints. The vertically-pointing S-band radar and wind profiler are used to characterize the kinematic and microphysical structures associated with the analyzed in situ samples.

The observed joint frequency distributions of particle size and fall speed are compared to empirically determined size-fall speed relations of Gunn and Kinzer (1949) for rain, and Locatelli and Hobbs (1974) for several types of ice particles. Most particles observed at temperatures at or near 0 deg C have characteristics corresponding to the empirical curve for rain. However, subsets of particles have characteristics between the empirical curves for rain and for graupel and between the curves for graupel and for dendrites.

Quantitative descriptions of the joint particle size and fall speed distribution in the region of the melting layer will aid in the refinement of microphysics parameterizations within numerical models and radiative transfer calculations.