Lynn McMurdie

Radar scans of winter storms frequently show enhancements in equivalent radar reflectivity factor Z _e within the −18° to −12°C cloud layer, often referred to as the “dendritic growth layer” (DGL). However, the microphysical structures responsible for these radar signatures remain poorly understood due to limited in-cloud in situ validation. This study leverages coordinated airborne radar and in situ observations collected during the NASA Investigation of Microphysics and Precipitation in Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. We analyzed 581 vertical profiles of Ku-band Z _e gradient ( dZ _Ku / dz ), each averaged over 10 km (∼1 min) segments and grouped into five clusters using a k -means clustering algorithm. Two clusters exhibited local maxima in the magnitude of dZ _Ku / dz ≥ 10 dB Z _e km ⁻¹ and corresponding increases in Ku–Ka dual-frequency ratio (DFR) ≥ 1.5 dB within the DGL. Coincident in situ observations from one of these clusters revealed larger particle sizes and the presence of aggregates across the DGL. However, habit analyses from three independent imaging probes showed that pristine dendrites were rare across all clusters, comprising only a small fraction of observed particles. Instead, complex polycrystals, particularly side planes and polycrystalline plates, dominated the habit population in the DGL, with large aggregates prevalent only in the cluster exhibiting enhanced radar signatures. Thermodynamic observations showed that ice supersaturation largely remained below the criterion for dendritic growth. We hypothesize that the aggregation of predominately nondendritic polycrystals produced the observed radar enhancements in the DGL, with only a minor contribution from dendrites. Significance Statement: In winter storms, radars often detect enhanced signals in a narrow temperature range within a cloud layer where side-branched, dendritic particles are assumed to grow. These radar signatures have often been attributed to dendritic growth despite limited in-cloud observations to confirm their presence. In this study, we used coordinated radar and in situ aircraft measurements to investigate this assumption. We find that, even in the radar-indicated zone of increased radar reflectivity, dendrites were largely absent. Instead, the aggregation of plate-like polycrystals was more common and likely responsible for the radar enhancements previously linked to dendrites. These findings offer new insight into interpreting radar observations in winter storms.

Ice crystal chain aggregates—linear structures of conjoined monomer crystals—have previously been observed in strongly electrified deep convection, likely forming via electric‐field‐enhanced aggregation. The NASA Investigation of Microphysics and Precipitation for Atlantic Coast‐Threatening Snowstorms field campaign conducted aircraft‐based sampling of cold‐season storms using cloud probes. These measurements reveal chain aggregates in weakly electrified winter storms. Chain aggregates were identified in 28 of 34 research flights from 2020 to 2023 (∼10% of total P‐3 flight time), spanning −38°C to +2.5°C and 1.5–9.7 km. High‐resolution Cloud Particle Imager and Particle Habit Imaging and Polar Scattering imagery revealed sublimation signatures on chain aggregates, consistent across instruments. The widespread occurrence of chain aggregates in weakly electrified clouds suggests alternative formation pathways than electric‐field‐enhanced aggregation.

Ice crystal chain aggregates—linear structures of conjoined monomer crystals—have previously been observed in strongly electrified deep convection, likely forming via electric-field-enhanced aggregation. The NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign conducted aircraft-based sampling of cold-season storms using cloud probes. These measurements reveal chain aggregates in weakly electrified winter storms. Chain aggregates were identified in 28 of 34 research flights from 2020 to 2023 (~10 % of total P-3 flight time), spanning –38 to +2.5 °C and 1.5 to 9.7 km. High-resolution CPI and PHIPS imagery revealed sublimation signatures on chain aggregates, consistent across instruments. The widespread occurrence of chain aggregates in weakly electrified clouds suggests alternative formation pathways than electric-field-enhanced aggregation.

Airborne multifrequency radars provide valuable information regarding the size, density, and shape of ice hydrometeors. Higher values of the dual-frequency reflectivity ratio (DFR), for instance, are often associated with the presence of larger ice crystals with potential implications for snowfall at the surface. The 3-yr Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign aimed to address the linkage between airborne remote sensing and in situ microphysics by collecting data obtained through the close coordination of a “satellite-simulating” ER-2 and “storm-penetrating” P-3 aircraft. In regions of prominently higher Ku-band and Ka-band DFR along the P-3 flight track, the solid-phase mass-weighted mean diameter ( D _m ) was 66% larger, the liquid-equivalent normalized intercept parameter ( N _w ) was 123% lower, the effective density ( ρ _e ) was 73% smaller, and the ice water content (IWC) was 46% higher for the 21 events studied. Use of a preexisting neural network radar retrieval allowed for the vertical structure of microphysical properties to be compared to the larger DFR signatures, with similar conclusions valid for D _m and N _w at all altitudes studied. The analysis of the in situ microphysics data and the radar retrieval complement the relationship between bulk microphysical properties and multifrequency radar signatures that allow for microphysical processes such as aggregation and deposition to be inferred for different environments (e.g., at particular temperature ranges or depths below cloud top). Significance Statement: Radars operating at multiple frequencies can provide useful information on the sizes, shapes, and concentrations of ice crystals in winter snowstorms and can be related to banded precipitation structures with implications for snowfall at the ground. Results from a recent 3-yr field project show that ice crystals were typically larger and less dense when the reflectivity difference between two frequencies was larger and supports the process by which crystals aggregate to form larger ice particles.

This study uses a convection‐permitting model simulation to describe the environmental conditions under which convective upscale growth occurs in central Argentina, particularly examining environmental parameters when deep convection initially forms that could differentiate the rate of initial upscale growth. Simulated mesoscale convective systems (MCSs) are separated into slow and rapid growth by the rate of spatial growth from convection initiation until reaching the MCS scale. A low‐level jet (LLJ) is found more frequently near the deep convection that experiences rapid growth to an MCS, but its presence alone is not predictive of rapid growth. Using spatially‐averaged parameters, we find that rapid growth to MCSs also occurs in environments that are significantly more thermodynamically favorable with greater low‐level moisture and instability. Fewer significant differences are found in the kinematic environment with only the 0–2 km vertical wind shear magnitude being significantly larger for rapid growth MCSs compared to slow growth MCSs, potentially related to LLJs often peaking near this height. When focusing only on MCSs with the slowest and fastest growth rates, elevated‐layer shear is significantly smaller for very rapid growth MCSs, suggesting elevated‐layer shear may help discriminate between the upper and lower bounds of growth rate. Finally, when upscale growth occurs near the Sierras de Córdoba (SDC) with a LLJ present, rapid growth is also supported by favorable wind shear orientation. However, this does not hold for upscale growth occurring away from the SDC, highlighting the importance of interpreting shear direction relative to the orientation of features initiating deep convection.

Ice‐ and mixed‐phase clouds play an important role in the global radiative budget and hydrological cycle, yet the complexity of ice crystal shapes and the presence of supercooled liquid water (SCLW) present challenges for retrieving cloud properties from airborne and spaceborne remote sensing instruments. Airborne lidar measurements of the backscatter coefficient ( β ), color ratio ( χ ), and volume depolarization ratio ( δ ) provide additional spatial context for the cloud phase and to some extent the particle shapes both vertically and horizontally through clouds. Coordination between a NASA P‐3 and ER‐2 aircraft during the Investigation of Microphysics and Precipitation for Atlantic Coast‐Threatening Snowstorms field campaign provided 3.3 hr of Cloud Physics Lidar observations from 16 events to be spatially related to the particle size and morphological properties from the Cloud Particle Imager, in addition to temperature and SCLW measurements obtained by the P‐3 aircraft. After the lidar data were matched to the P‐3 location, in situ microphysics data were mapped in β‐δ and triple‐wavelength frameworks involving χ . Compared to SCLW regions, regions dominated by ice typically exhibited lower β (−2 km ⁻¹ sr ⁻¹ ), lower 532/355‐nm χ (0.2) and were associated with particle sizes that were on average 105% larger and area ratios that were 40% lower. The relationships between cloud properties and lidar measurements established in this study have implications for future cloud phase and particle habit algorithms using airborne and spaceborne lidar data.

Mesoscale bands develop within winter cyclones as concentrated regions of locally enhanced radar reflectivity, often corresponding to intensified precipitation rates lasting several hours. Surface precipitation characteristics are governed by the microphysical properties of the ice-phase particles aloft, yet their unique microphysical evolutionary pathways and ambient environmental dependencies in banded regions remain poorly understood, in part due to a paucity of observations within clouds. Addressing this need, the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms measured properties of winter cyclones from airborne in situ and remote sensing platforms. Observations collected within a banded region of a decaying-stage northeast United States cyclone revealed a microphysical pathway characterized by precipitation fallout from a weak generating cell layer through an ∼ 2 km deep subsaturated downdraft region. Sublimation was a dominant evolutionary process, resulting in a > 70 % reduction in the initial characteristic ice water content (IWC). This vertical evolution was reproduced by a one-dimensional (1D) particle-based model simulation constrained by observations, conveying accuracy in the process representation. Four sensitivity simulations assessed evolutionary dependencies based on observationally informed perturbations of the ambient relative humidity, RH, and vertical air motion, w. Perturbations of ∼ 2 % RH significantly varied the resultant characteristic IWC loss, by as much as 29 %, whereas comparable perturbations of w had negligible effects. Intrinsic particle evolution during sublimation demonstrated a notable imprint on vertical profiles of radar reflectivity, but the Doppler velocity was more strongly governed by the ambient w profile. These findings contextualize radar-based discrimination of sublimation from other ice-phase processes, including riming and aggregation.

This study uses a convection-permitting model simulation to describe the environmental conditions under which convective upscale growth occurs in central Argentina, particularly examining environmental parameters when deep convection initially forms that could differentiate the rate of initial upscale growth. Simulated MCSs are separated into slow and rapid growth by the rate of spatial growth from convective initiation until reaching the mesoscale convective system (MCS) scale. A low-level jet (LLJ) is found more frequently near the deep convection that experiences rapid growth to an MCS, but its presence alone is not predictive of rapid growth. Using spatially-averaged parameters, we find that rapid growth to MCS also occurs in environments that are significantly more thermodynamically favorable with greater low-level moisture and instability. Fewer significant differences are found in the kinematic environment with only the 0-2 km vertical wind shear magnitude being significantly larger for rapid growth MCSs compared to slow growth MCSs, potentially related to LLJs often peaking near this height. When focusing only on MCSs with the slowest and fastest growth rates, elevated-layer shear is significantly smaller for very rapid growth MCSs, suggesting elevated-layer shear may help discriminate between the upper and lower bounds of growth rate. Finally, when upscale growth occurs near the Sierras de Córdoba (SDC) with a LLJ present, rapid growth is also supported by favorable wind shear orientation. However, this does not hold for upscale growth occurring away from the SDC, highlighting the importance of interpreting shear direction relative to the orientation of features initiating deep convection.

Mesoscale bands develop within winter cyclones as concentrated regions of locally enhanced radar reflectivity, often producing intensified precipitation rates lasting several hours. Surface precipitation characteristics are governed by the microphysical properties of the ice-phase particles aloft, yet their unique microphysical evolutionary pathways and ambient environmental dependencies in banded regions remain poorly understood, in part due to a paucity of observations within natural clouds. Addressing this need, the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms recently measured properties of winter cyclones from airborne in situ and remote sensing platforms. Observations collected within a banded region of a decaying-stage northeast United States cyclone revealed a microphysical pathway characterized by precipitation fallout from a weak generating cell layer through an ~2 km deep subsaturated downdraft region. Sublimation was a dominant evolutionary process, resulting in > 70 % reduction of the initial ice water content (IWC). This vertical evolution was reproduced by a 1D particle-based model simulation constrained by observations, conveying accuracy in the process representation. Four sensitivity simulations assessed evolutionary dependencies based on observationally-informed perturbations of the ambient relative humidity, RH, and vertical air motion, w. Perturbations of ~2 % RH significantly varied the resultant IWC loss, as much as 29 %, whereas comparable perturbations of w had negligible effects. Intrinsic particle evolution during sublimation demonstrated a notable imprint on vertical profiles of radar reflectivity, but Doppler velocity was more strongly governed by the ambient w profile. These findings contextualize radar-based discrimination of sublimation from other ice-phase processes, including riming and aggregation.