Polarimetric and Vertically Pointing Radar Observations of Snow Growth in Colorado Winter Storms

Dendritic growth and aggregation are important processes within stratiform precipitation systems that can influence precipitation rates at the surface. Polarimetric and vertically pointing X-band radar observations taken during the Front Range Orographic Storms (FROST) project in Colorado are used in this study to explore these processes. The polarimetric data support the presence of dendrites in a number of cases, with maxima in Z_DR and K_DP found at temperatures near -15 °C. The vertically pointing observations of radial velocity suggest a decrease in the Z_H-weighted mean particle fall speed occurs at temperatures between -20 °C and -15 °C. However, Z_H at these temperatures increases towards the ground, suggesting that the mean particle size is increasing.

To reconcile the growth of particles with the slowing observed in radial velocity, previous studies have postulated that a persistent updraft exists at -15 °C. We present an alternate explanation to account for this signature in which two classes of ice particles are present: small crystals experiencing dendritic growth, and larger, faster-falling particles. In favorable dendritic growth conditions, the faster-falling particles grow much slower than the small crystals, causing these growing dendrites to provide the majority of the increase in Z_H. Thus, the differential growth between the two particle classes causes the mean radial velocity to shift towards the slower-falling dendrites. Simplified calculations based on the observed vertical Z_H and Doppler velocity gradients support this explanation. The use of vertically pointing Z_H and radial velocity observations to identify areas of dendritic growth and further constrain microphysical models will be discussed.