Microphysical Processes and Synergistic Interaction between Frontal and Orographic Forcing of Precipitation during the 13 December 2001 IMPROVE-2 Event over the Oregon Cascades

Abstract On 13–14 December 2001 a vigorous cyclonic storm passed over the Pacific Northwest, producing heavy orographic precipitation over the Cascade Mountains. This storm was one of several studied during the second field phase of the Improvement of Microphysical Parameterization through Observational Verification Experiment (IMPROVE). A wide variety of in situ and remotely sensed measurements were obtained as this storm passed over the Oregon Cascades. These measurements provided a comprehensive dataset of meteorological state parameters (temperature, pressure, humidity, winds, and vertical air velocity), polarization Doppler radar measurements, and cloud microphysical parameters (cloud liquid water, particle concentrations, size spectra, and imagery). The 13–14 December case was characterized by the passage of a tipped-forward lower-tropospheric front that extended upward to a preceding vigorous upper cold-frontal rainband, which produced clouds up to ∼8–9 km. An important difference between this stor...