Radar Retrieval Evaluation and Investigation of Dendritic Growth Layer Polarimetric Signatures in a Winter Storm

Abstract

This study evaluates ice particle size distribution and aspect ratio (φ) Multi-Radar/Multi-Sensor (MRMS) dual-polarization radar retrievals through a direct comparison with two legs of observational aircraft data obtained during a winter storm case from the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign. In situ cloud probes, satellite, and MRMS observations illustrate that the often-observed Kdp and ZDR enhancement regions in the dendritic growth layer can either indicate a local number concentration increase of dry ice particles or the presence of ice particles mixed with a significant number of supercooled liquid droplets. Compared to in situ measurements, MRMS retrievals on average underestimated mean volume diameters by 50% and overestimated number concentrations by over 100%. IWC retrievals using ZDR and Kdp within the dendritic growth layer were minimally biased compared to in situ calculations where retrievals yielded −2% median relative error for the entire aircraft leg. Incorporating φ retrievals decreased both the magnitude and spread of polarimetric retrievals below the dendritic growth layer. While φ radar retrievals suggest that observed dendritic growth layer particles were rather non-spherical (0.1 ≤ φ ≤ 0.2), in situ projected aspect ratios, idealized numerical simulations, and habit classifications from cloud probe images suggest that the population mean φ was generally much higher. Coordinated aircraft radar reflectivity with in situ observations suggests that the MRMS systematically underestimated reflectivity and could not resolve local peaks in mean volume diameter sizes. These results highlight the need to consider particle assumptions and radar limitations when performing retrievals.

Evaluation of polarimetric ice microphysical retrievals with OLYMPEX campaign data

“…Instead of D m , the aircraft microphysical probes usually measure either median mass diameter D mm or median volume diameter D mv of the distribution of maximal particle dimension (Hu and Ryzhkov, 2022), which is given by…”

“…(4a) assuming particle density being inversely proportional to its size. Assuming φ = 0.6 and an exponential distribution (µ = 0), the following two D m relationships (Hu and Ryzhkov, 2022) are obtained: In addition to the quantitative information provided by the 2D-S and HVPS, high-resolution in situ particle imagery data from the Cloud Particle Imager (CPI; SPEC Inc, USA) can be used to accurately identify particle types and characteristics. Only with the CPI it is possible to directly monitor supercooled liquid water (SLW) droplets of micron-size attached to ice particles and thus diagnose riming unambiguously and estimate the degree of riming.…”

“…One reason for the bad performance of non-polarimetric retrievals is that the horizontal reflectivity Z H in snow is approximately proportional to the fourth moment of the particle size distribution (PSD) (Hu and Ryzhkov, 2022), hence Z H is insensitive to small particles, whereas other moments, such as the IWC, are sensitive to the small particle contributions. In contrast, specific differential phase K DP which is proportional to the first moment of the PSD and thus the whole spectrum is considered.…”

Development of Simulation Models Supporting Next-Generation Airborne Weather Radar for High Ice Water Content Monitoring

“…4 is a coefficient related to wavelength. Based on the assumption that in Equation (1), for any i ̸ = j, ψ i and ψ j are independent and uniformly distributed random variables, the Appendix shows that Equation (7) leads to…”

Evaluation of polarimetric ice microphysical retrievals with OLYMPEX campaign data

“…One reason for the bad performance of non-polarimetric retrievals is that the horizontal reflectivity Z H in snow is approximately proportional to the fourth moment of the particle size distribution (PSD) (Hu and Ryzhkov, 2022); hence Z H is insensitive to small particles, whereas other moments, such as the IWC, are sensitive to the small particle contributions. In contrast, specific differential phase K DP is proportional to the first moment of the PSD and thus the whole spectrum is considered.…”