Ice Microphysical Processes in Winter Storms Encountering Complex Terrain

Winter storms in complex terrain are often associated with hazardous weather conditions and remain a challenging forecast scenario. Some uncertainty in precipitation forecasts is due to poor parameterization of ice-phase microphysics in numerical weather prediction models. Additionally, the intrinsic variability of ice particles limits the accuracy of satellite-based precipitation measurements in complex terrain that are derived from remote sensing algorithms based on fixed assumptions of ice particles. In this study, we focus on observations of ice-phase microphysical processes occurring above the melting level in precipitating clouds as they encounter complex terrain to improve the representation of ice microphysical properties and our understanding of the processes that modify them. In particular, ground-based dual-polarization radar and in-situ aircraft microphysical measurements from the Olympic Mountains Experiment (OLYMPEX) during winter 2015-16 are used to analyze ice-phase precipitation processes in frontal systems as flow is modified by the Olympic Mountains.

To objectively distinguish stratiform radar echo and approximate the melting level height, we have designed a bright band detection technique specific to the NASA S-band dual-polarization radar (NPOL) that scanned over the ocean and over the terrain of the Olympic Mountains. We find that an elevated, local maximum in radar reflectivity regularly occurs at approximately 2 to 2.5 km above the bright band and is often enhanced in magnitude over the terrain when compared to over the ocean. By objectively identifying local reflectivity maxima in the ice region above the bright band, we find that it regularly occurs within the -10°C to -15°C range. This layer is often, but not always, just below a local maximum in differential reflectivity which is suggestive of dendritic ice growth. However, coincident in-situ aircraft observations from OLYMPEX flights indicate that these regions often also contain appreciable amounts of supercooled liquid water and rimed ice particles. In this study we explore the elevated reflectivity maximum and the frequency of its association with supercooled liquid water or rimed ice particles. We also study how that frequency varies in the context of environmental flow patterns. This will develop a better understanding of the orographic processes that lead to precipitation enhancement over the Olympic Mountains. Ground-based precipitation measurements from OLYMPEX at varying elevations will also be studied to describe the role of ice-phase processes in modulating the intensity and distribution of precipitation reaching the surface across windward and lee slopes of the terrain and the variability of those processes with synoptic environment.