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Abstract The discovery of a polarimetric radar signature indicative of hydrometeor refreezing has shown promise in its utility to identify periods of ice pellet production. Uniquely characterized well below the melting layer by locally enhanced values of differential reflectivity ( Z DR ) within a layer of decreasing radar reflectivity factor at horizontal polarization ( Z H ), the signature has been documented in cases where hydrometeors were completely melted prior to refreezing. However, polarimetric radar features associated with the refreezing of partially melted hydrometeors have not been examined as rigorously in either an observational or microphysical modeling framework. Here, polarimetric radar data—including vertically pointing Doppler spectral data from the Ka-band Scanning Polarimetric Radar (KASPR)—are analyzed for an ice pellets and rain mixture event where the ice pellets formed via the refreezing of partially melted hydrometeors. Observations show that no such distinct localized Z DR enhancement is present, and that values instead decrease directly beneath enhanced values associated with melting. A simplified, explicit bin microphysical model is then developed to simulate the refreezing of partially melted hydrometeors, and coupled to a polarimetric radar forward operator to examine the impacts of such refreezing on simulated radar variables. Simulated vertical profiles of polarimetric radar variables and Doppler spectra have similar features to observations, and confirm that a Z DR enhancement is not produced. This suggests the possibility of two distinct polarimetric features of hydrometeor refreezing: ones associated with refreezing of completely melted hydrometeors, and those associated with refreezing of partially melted hydrometeors. Significance Statement There exist two pathways for the formation of ice pellets: refreezing of fully melted hydrometeors, and refreezing of partially melted hydrometeors. A polarimetric radar signature indicative of fully melted hydrometeor refreezing has been extensively documented in the past, yet no study has documented the refreezing of partially melted hydrometeors. Here, observations and idealized modeling simulations are presented to show different polarimetric radar features associated with partially melted hydrometeor refreezing. The distinction in polarimetric features may be beneficial to identifying layers of supercooled liquid drops within transitional winter storms.
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This content will become publicly available on May 1, 2026
Reflectivity-Inferred Microphysical Response to Turbulent Layers in Winter Mountain Storms
Abstract Shear and buoyancy gradients, often observed in midlatitude baroclinic and orographic winter storms, produce discrete layers of turbulence. These turbulent layers modify the distribution of supercooled liquid water (SLW), whose presence enables hydrometeors to grow to precipitation sizes faster than through vapor-ice deposition alone. Both the Wegener–Bergeron–Findeisen process and riming require SLW—heterogeneously distributed in response to the dynamic forcings at all superposed scales. The University of Wyoming W-band cloud radar Doppler spectrum width characterizes the air motion turbulence intensity in mixed-phase layer clouds after avoiding fall speed dominated regions through comparison to coarser radar turbulence metrics. Embedded layers of turbulent air motion are compared to quiescent cloud regions in either/both the upwind and downwind directions. Median radar reflectivity profiles characterize the vertical growth of hydrometeors in the vicinity of identified layers, and differences in these vertical reflectivity gradients comparing turbulent to nonturbulent regions quantify enhanced hydrometeor growth over the layer. Over the entirety of the Seeded and Natural Orographic Wintertime Clouds—the Idaho Experiment, this parameter demonstrates a statistically significant increase, −13.6 dBZekm−1(from −1.7 to −24.5, 95% computed confidence), in radar reflectivity echo power with distance downward for embedded turbulent layers compared to quiescent cloud nearby. The increased vertical particle growth rate for turbulent layers appears to result from spatially heterogeneous phase partitioning, increased SLW mass and extent, and enhanced collision/collection rates in these layers. These first two conditions are examined individually where turbulent layers or fall streaks are sampled in situ, while the latter agrees with modeling results but can only be inferred herein. Significance StatementTurbulent mixing in mixed-phase clouds is understood to enhance cloud hydrometeor growth. This study quantifies these effects on observed airborne W-band radar reflectivity over an entire field campaign targeting midlatitude winter storms and calls into question whether this linkage is diagnosed properly if at all in forecast and bulk microphysical models with coarse (greater than 500 m) grid spacing or vertical resolution.
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- Award ID(s):
- 2016077
- PAR ID:
- 10656714
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of the Atmospheric Sciences
- Volume:
- 82
- Issue:
- 5
- ISSN:
- 0022-4928
- Page Range / eLocation ID:
- 1015 to 1032
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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