Characterization of Ice and Snow in-Situ Properties from GPM Field Campaigns

Microphysical models and satellite retrievals require a priori assumptions of particle size distribution (PSD) of hydrometeors. In ice clouds, these distributions are particularly poorly constrained by observations. Most microphysical parameterizations present in current atmospheric models and satellite retrievals assume a particular form of the Gamma PSD (i.e. exponential or other prescribed values of the shape parameter μ, or assumed relationship of slope parameter lambda with mixing ratio, etc.). In this presentation, the dependence of the gamma fit parameters is investigated, as a function of environmental parameters, synoptic situation, orographic forcing, and other factors using data acquired during the Olympic Mountain Experiment (OLYMPEX).

OLYMPEX took place in the Pacific Northwest in 2015 and sampled a unique variety of mid-latitude precipitation types as frontal systems moved from the ocean to the coast to the mountains. During OLYMPEX, the University of North Dakota (UND) Citation Research Aircraft made in-situ cloud microphysics and thermodynamic measurements and the National Aeronautics and Space Administration (NASA) DC-8 with the Airborne Precipitation Radar Third Generation (APR-3) onboard remotely-sensed profiles of reflectivity (Z_e) and dual-frequency ratios (DFRs). Identification of in-situ data coincident with the DC-8 retrievals allows for the relationship(s) between environmental indicators and PSDs to be determined. In this work the microphysical characteristics and associated environments of ice and snow in relation to the region of the extratropical cyclone (i.e.: warm front, warm sector, and postfrontal), and geographic locations (e.g.: over the ocean or mountain regions) are examined.

Results show that the warm sector is characterized by large IWC (mean of 0.055 g m^-3), large D_m, (mean of 1.57mm), and μ close to zero, consistent with aggregation being the dominant process in the region. In contrast, observations from the postfrontal sector show smaller IWCs (0.005 g m^-3), smaller D_m (0.125mm), and negative μ (-1.2), consistent with rimed particles dominating the region. Evidence for this was also seen with particle images from the in-situ probes showing large aggregates present in the warm sector and rimed particles in the postfrontal sector. Observations over the mountains showed significantly higher values of μ in comparison to oceanic regions. Furthermore, these results are compared to observations from previous field campaigns to investigate whether similar PSD parameterizations should be used in mid-latitude snow and high-latitude precipitating ice clouds.