Winter Storms over the San Juan Mountains. Part II: Microphysical Processes

Abstract

Airborne observations of wintertime storms in southeastern Colorado have shown that the microphysical structure of those storms evolved in close relationship to the dynamical structure. The storms evolved from deep synoptic-scale systems to shallower orographic systems, and in the course of this evolution became unstable. During early storm stages the precipitation developed primarily by diffusional growth of ice crystals, but during later storm stages accretional growth was at least as important as diffusional growth. Early storm stages were almost completely glaciated, but extensive regions of supercooled water appeared in later storm stages. Weak embedded convection also produced important regions of supercooled water in the later storm stages. Ice crystal concentrations were high, often in excess of 50 L?1. The observed ice crystal concentrations were far above the corresponding ice nucleus measurements, and the discrepancy could not be attributed to known ice multiplication processes.