Size Distributions of Precipitation Particles in Frontal Clouds

Measurements of the size spectra of precipitation particles have been made with Particle Measuring Systems probes aboard an aircraft flying through frontal clouds as part of the CYCLES (Cyclonic Extra-tropical Storms) PROJECT. These measurements were obtained while the aircraft flew through the clouds associated with mesoscale rainbands at temperatures ranging from −42 to +6°C. Particles ≳1.5 mm in diameter closely follow an exponential size distribution. Above the melting level precipitation occurs mainly in the form of ice particles. In this region the mean particle size of the exponential distribution increases with increasing temperature, indicating that the ice particles grow as they drift downward. The variance of the exponential distribution also increases with increasing temperature above the melting level, indicating that the particles grow particularly well by collection as they fall at various speeds. Passage of the failing particles through the melting level is accompanied by a sudden decrease in the mean and variance of the exponential size distribution.