Orographic modification of ice-phase precipitation processes during the Olympic Mountains Experiment (OLYMPEX)

Abstract:

Estimating precipitation from wintertime storms in mountainous terrain is often a very challenging forecast scenario. Understanding both the liquid- and ice-phase responses to the modification of flow encountering complex terrain is needed to improve precipitation forecasts in mountainous regions. Previous studies using OLYMPEX data identified enhancements of radar reflectivity above the melting layer over the windward slopes of the Olympic Mountains when compared to over the upstream ocean. Our study takes advantage of research quality ground-based radar and coincident in situ aircraft measurements collected during OLYMPEX to relate the reflectivity structure to microphysical properties in precipitating clouds in midlatitude wintertime storms encountering the Olympic Mountains.

In stratiform radar echo, we regularly observed an upper-level reflectivity maximum within a layer around 2 to 2.5 km above the bright band. We found that this secondary reflectivity maximum aloft was often enhanced over the Olympic Mountains when compared to over the upstream ocean. This layer is typically at temperatures just warmer than expected for dendritic growth and is often, but not always, observed below a local differential reflectivity maximum, further suggesting dendritic growth. Previous attempts to model comparable radar structures in other wintertime storms using electromagnetic scattering calculations often concluded that diffusional growth of dendrites increases aggregation efficiency to form larger particles, explaining the observed reflectivity enhancement. During OLYMPEX, in situ measurements within this layer not only reveal a relative increase in ice mass dominated by dendrite-like particles but also the presence of supercooled liquid water and rimed ice particles, apparently associated with a low-level jet that becomes orographically forced aloft. Supercooled liquid water appears to accelerate particle growth and aggregation efficiency, likely modulating the precipitation rate and distribution on the windward and lee slopes of the mountains. Ground-based precipitation measurements from OLYMPEX will be studied to explore the role of ice-phase processes within the enhanced reflectivity layer in modulating and redistributing precipitation at the surface.