In-situ and Radar Investigation of Snow Aggregation in Pacific Northwest Midlatitude Cyclones

The Olympic Mountains Experiment (OLYMPEX) was conducted in late 2015 over the west coast of Washington State. In the experiment, first-of-their-kind airborne observations of triple-frequency radar reflectivities coincident with airborne in-situ observations of the particle size distribution were collected. This provides an opportunity to evaluate whether aggregates and dendrites occupy distinct regions of the Ku-Ka-W band Dual-Frequency Ratio (DFR) plane as previously suggested by scattering results from the Discrete Dipole Approximation (DDA).

In this study, triple-frequency radar observations (Ku-, Ka- and W-band) from OLYMPEX are used to discriminate particle types to create a multi-frequency Hydrometeor Characterization Algorithm (MFHCA). First, an evaluation of this algorithm relative to ground-based dual-polarization Hydrometeor Classification Algorithms (DPHCA) is performed using coincident in-situ microphysical data, aircraft multi-frequency radar observations and ground-based dual-polarization radar measurements. Case studies of collocated in-situ particle images evaluate the retrieval of aggregate locations derived from the MFHCA and DPHCA. Profiles of the relative fraction of aggregates derived from MFHCA as a function of temperature quantify regions of ice crystal growth and aggregation. As a complement to the particle shape retrieval, dual-frequency and triple-frequency empirical retrievals of bulk effective density (ρ) and mean mass dimension (Dmm) are constructed from the OLYMPEX in-situ dataset and applied to the entire radar dataset. The empirical retrievals of ρ and Dmm are used to evaluate and constrain the assumptions of the retrieval of ice mass and ice mass flux, with the goal to improve spaceborne retrievals of ice-phase precipitation. Retrieved profiles of aggregates, ρ, and Dmm characterize snow and ice over both the ocean and complex topography and are compared against in-situ profiles of ρ and Dmm to better characterize the hydrometeor properties occurring in landfalling Pacific Northwest midlatitude cyclones