Microphysical Mechanisms in Stratiform Precipitation as Observed in OLYMPEX

Abstract

This thesis presents results from a strong, rain-producing storm that passed over the Olympic Mountains during the 2015–16 Olympic Mountains Experiment (OLYMPEX). The microphysical characteristics of the stratiform component occurring during this 12–13 November event were investigated using simultaneous in-situ aircraft microphysical sampling and ground-based dual-polarization radar measurements. On 12 November, the University of North Dakota Citation research aircraft flew through a widespread region of stratiform precipitation in the pre-warm frontal sector of the storm. In general, the precipitating clouds of the 12 November case adhered to the basic layered stratiform precipitation model in terms of particle type, ice particle concentrations, and liquid water content (LWC). However, the dual-polarization radar and in-situ aircraft data indicated certain differences from the generally accepted model. Aircraft particle sampling showed large- and medium-sized ice particles within a secondary maximum in reflectivity aloft. A maximum in differential reflectivity was collocated with the maximum in reflectivity, suggesting the presence of horizontally oriented particles. The particle images showed high concentrations of plate-like and irregularly shaped particles corresponding to a relative decrease in the correlation coefficient at this level. On 13 November, the Citation sampled the warm sector of the system after the warm front passed and before the cold front moved through the DOW sector. The Citation instrumentation measured cloud LWC as high as 0.45 g m-3 within generating cells embedded in the stratiform echo on this day. Within the generating cells, a negative correlation of reflectivity and differential reflectivity was observed, similar to findings of Kumjian et al. (2014), who hypothesized that the anti-correlation could be explained by riming, aggregation, or both. Particle probe imagery showed many small liquid water particles, plate-like particles, and rimed particles in the central turret of an observed cell. The existence of rimed particles, led to the increase in reflectivity and decrease in differential reflectivity in this case.