Ice/Snow Microphysics
Snow clouds, which are commonly produce a variety of ice particle types such as pristine ice crystals, aggregates, graupel, and ice pellets in the same cloudy volume, is challenging research. It is impossible to separate and quantify their relative amounts if a single radar variable such as radar reflectivity is utilized. Doppler spectra measured by profiling millimeter-wavelength radars offer the ability to identify and separate the contributions of different hydrometeor species to the radar returns and perform microphysical and dynamical retrievals. Polarimetric observables from cloud and precipitation radars offer the capability to identify ice hydrometeor species as well as their spatial distributions. These observations at different radar wavelengths utilize the frequency-dependent attenuation and backscattering properties of hydrometeors. We illustrated a new approach to qualitative and quantitative analysis of the complexity of ice and mixed-phase microphysical processes in Arctic deep precipitating systems using the combination of Ka-band zenith-pointing radar Doppler spectra and polarimetric radar variables measured by a Ka/W-band scanning radar.
Frequent occurrences of the multimodal Doppler spectra were observed in dendritic/planar ice growth layers and mixed-phase layers, indicating that slower- and faster-falling different types of ice particles coexisted in each layer. They had different contributions to the radar polarimetry: differential reflectivity (ZDR, representing aspect ratio of ice particles) and specific differential phase (KDP, proportional to aspect ratio of ice particles and water content). In the dendritic/planar growth layers, the slower-falling particle populations contributed to an increase of ZDR, while an enhanced KDP was caused by both the slower and faster-falling particle populations (A–A’ and region A in Fig. 1). In the mixed-phase layers, particle fall speeds were higher than those in the dendritic growth layer, and the enhancements of KDP and ZDR were weak, suggesting that compact, high density, spherical particles produced by riming dominated and those with different sizes were mixed together (B–B’ and region B in Fig. 1). Oue, M., P. Kollias, A. Ryzhkov, and E. P. Luke, 2018: Towards exploring the synergy between cloud radar polarimetry and Doppler spectral analysis in deep cold precipitating systems in the Arctic. Journal of Geophysical Research, 123, DOI: 10.1002/2017JD027717. |
Figure 2: Time versus height cross sections of (a) KAZR reflectivity, (b) KAZR mean Doppler velocity, and (c) mean Doppler velocity of slow-falling subpeaks, (d) microwave radiometer (MWR)-retrieved liquid water path (LWP), and (e) Images of ice particles recorded by the MASC at the ground for April 29, 2016 (left column) and May 24, 2016 (right column). Boxes in (c) represent the dendritic/planar growth layers (DGLs, blue) and the mixed-phase layer (MPL, red) analyzed in this study. Black dots in (a), (b), and zoomed plots in (c) represent ceilometer-observed cloud base heights indicating the presence of liquid droplet layers. Gray dashed lines in (a-c) represent temperature with a 5 K increment from soundings at Oliktok.
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Figure 1: (Left) Example of KAZR Doppler velocity spectra showing a profile and single spectrum along A-A’ and B-B’. Color shade in the profile plot represents reflectivity. Horizontal gray dashed lines represent heights of temperature of -10°C, -15°C, and -20°C. (Right) Quasi Vertical Profiles (QVPs) of KDP and ZDR estimated from a Ka (black lines) and W (gray lines) SACR2 plan position indicator (PPI) scan at an elevation angle of 20°. Adapted from Oue et al. (2018)
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The triple frequency space can be used to compare radar observations and scattering models. Three frequencies can be compared in 2 dimensional space using a combination of two Dual Wavelength Ratio (DWR) i.e. the difference of the logarithmic effective reflectivity factor from two frequencies. Triple frequency space contains rich information since the position of snowflakes in this framework depends on snow particle structure, bulk snowfall density, and characteristic size of the PSD.
Until now, studies were unable to relate the observed scattering properties from radars to particle shape. This limitation is overcome in this studies by combining radar observations with in-situ measurement of snowflakes at the ground. Three cases collected during the recent Finland reveal previously published triple frequency signatures such as a bending-away in the triple frequency from the curve of classical spheroid scattering models in the presence of large (>5 mm) snow aggregates. This study also reveals new signatures for rimed particles which appear along an almost horizontal line in the triple frequency space.
Kneifel S., A. V. Lerber, J. Tiira, D. Moisseev, P. Kollias, J. Leinonen, (2015) Observed Relations between Snowfall Microphysics and Triple-frequency Radar Measurements. Accepted to the Journal of Geophysical Research