Retrievals of Riming and Snow Density From Vertically Pointing Doppler Radars

Abstract

Retrievals of ice and snow are made from Ka‐ and W‐band zenith‐pointing Doppler radars at Hyytiälä, Finland, during the snow experiment component of the Biogenic Aerosols: Effects on Clouds and Climate (2014) field campaign. In a novel optimal estimation retrieval, mean Doppler velocity is exploited to retrieve a density factor parameter, which modulates the mass, shape, terminal velocity, and backscatter cross sections of ice particles. In a case study including aggregate snow and graupel we find that snow rate and ensemble mean ice density can be retrieved to within 50% of in situ measurements at the surface using dual‐frequency Doppler radar retrievals. While Doppler measurements are essential to the retrieval of particle density, the dual‐frequency ratio provides a strong constraint on particle size. The retrieved density factor is strongly correlated with liquid water path, indicating that riming is the primary process by which the density factor is modulated. Using liquid water path as a proxy for riming, profiles classified as unrimed snow, rimed snow, and graupel exhibit distinct features characteristic of aggregation and riming processes, suggesting the potential to make estimates of process rates from these retrievals. We discuss the potential application of the technique to future satellite missions.

Integrative and comprehensive Understanding on Polar Environments (iCUPE): the concept and initial results

“…These relations were applied in the validation of ice cloud and precipitation retrieval algorithms. For example, the performance of the "unified" algorithm that combines the radar and lidar observations of EarthCARE, in a variety of snowfall conditions (Moisseev et al, 2017;Li et al, 2018), was tested using the snowfall data as 720 shown in Mason et al (2018;. Using these studies showed that the measurements collected at Hyytiälä, namely combined observations of the surface snowfall observations and multi-frequency radar observations, are ideally suited for verification of satellite cloud and precipitation retrieval algorithms.…”

The importance of particle size distribution shape for triple-frequency radar retrievals of the morphology of snow

“…Szyrmer and Zawadzki, 2014a;Petäjä et al, 2016). Doppler velocity measurements of the terminal fallspeed of snow have been used to constrain variations in particle density by which riming of snow, and the mixed-phase cloud in which it occurs, can be diagnosed (Mosimann, 1995; 20 Szyrmer and Zawadzki, 2014a, b;Mason et al, 2018). Variations in the density of ice particles are also related to changes in their shape and structure-whether due to aggregation, riming or a combination of processes-which are reflected in their radar backscatter cross-sections (Leinonen and Szyrmer, 2015).…”

“…Variations in the density of ice particles are also related to changes in their shape and structure-whether due to aggregation, riming or a combination of processes-which are reflected in their radar backscatter cross-sections (Leinonen and Szyrmer, 2015). Mason et al (2018) formulated a single parameter modulating the density, shape and structure of ice particles which was retrieved in single-and dual-frequency Doppler radar retrievals, and was chiefly constrained by the mean Doppler velocity. This representation was based on a large database of ice particle mass-25 and area-size relations and insights from remote-sensed and in-situ measurements of snow events (Kneifel et al, 2015), but it remains to explore in more detail how the parameters controlling the size distribution and morphology of snow particles relate to one another and their radar scattering characteristics.…”

“…Combining triple-frequency and Doppler velocity information, Kneifel et al (2016) used triple-frequency Doppler spectra to identify the spectral signatures 5 of rimed and unrimed snow. The insights provided by triple-frequency radar techniques have contributed to the development of expanded scattering databases for representing and evaluating a wide range of ice particles (Kneifel et al, 2018).…”

“…Triple-frequency radar retrievals have been demonstrated in which the structure and density of ice particles 10 are allowed to vary (Leinonen et al, 2018;Tridon et al, 2019); however, it is not yet clear which ice particle properties are best constrained by the triple-frequency radar measurements; Leinonen et al (2018) found that a triple-frequency radar retrieval did not differ significantly from that of a dual-frequency retrieval, suggesting that the problem is over-constrained. To our knowledge, a triple-frequency Doppler retrieval of ice has not yet been described-this may have the advantages of estimating particle density constrained by Doppler velocity (as in Mason et al, 2018) and using triple-frequency radar signatures to 15 constrain some parameters of particle morphology.…”