Abstract The potential for cloud seeding to induce dynamic changes that alter cloud structure beyond basic ice formation processes has remained theoretical. While previous research hypothesized the presence of dynamic responses in seeded clouds, this study presents the first direct observational evidence that seeding can generate buoyant forces strong enough to deepen and deform clouds. In situ aircraft measurements and W‐band radar analysis shows how the buoyant force increased cloud tops by hundreds of meters and induced secondary circulations that altered the cloud and precipitation structure. These dynamic changes triggered additional ice formation and precipitation not captured in current conceptual models. The results demonstrate that dynamic responses can be induced through glaciogenic seeding, representing foundational research that will significantly improve understanding of seeding mechanisms and precipitation formation in commonly seeded clouds.

Cloud droplet temperature plays a key role in fundamental cloud microphysical and radiative processes. The supercooled droplet temperature and lifetime can impact cloud ice and precipitation formation via homogeneous freezing and activation of ice-nucleating particles through contact and immersion freezing. While most observational and modeling studies often assume droplet temperature to be spatially uniform and equal to the ambient temperature (Ta), this assumption may not always be valid, particularly when droplets experience strong relative humidity (RH) gradients at cloud boundaries.For a wide range of ambient conditions, we model the coupled heat and mass transfer between the droplet and its environment and quantify the decrease in droplet temperature (ΔT) from that of the far-away ambient temperature (Ta), and the increase in droplet lifetime due to reduced droplet surface temperatures, compared to Maxwellian diffusion-limited evaporation estimates. ΔT is found to increase with Ta, and decrease with increase in ambient relative humidity (RH), and pressure (P). For a prescribed environment and assuming the droplet has infinite thermal heat conductivity, ΔT was typically 1-5°C lower than Ta, with highest values (~10.3°C) for very low RH, low P, and Ta closer to 0ºC. For higher RH and larger droplets, droplet lifetimes can increase by more than 100s compared to the diffusion-limited evaporation approach, which ignores droplet cooling. The steady state temperature of evaporating droplets can be approximated by environmental thermodynamic wet-bulb temperature. Radiation was found to play a minor role in influencing droplet temperatures, except for larger droplets in environments close to saturation. If we resolve the spatiotemporally varying thermal and vapor density gradients near the evaporating droplet, results demonstrate a higher subsaturation-dependent decrease in the droplet temperature as well as the envelope of air in the vicinity of the droplet surface. For an ambient environment specified far away, with Ta = -5°C, RH = 10%, 40%, and 70%, the decrease in droplet temperatures due to evaporative cooling is ~ 24, 11, and 5°C, respectively and the evaporatively cooled droplets survive longer compared to previous estimates. The implications of evaporative cooling and increased lifetimes of supercooled cloud droplets on potential enhancement of ice nucleation near evaporating cloud edges, such as cloud-top generating cells, and especially for moderately supercooled ambient temperatures, are discussed. The importance of using accurate droplet temperatures to improve activated ice nuclei number concentrations from existing primary ice nucleation parameterization schemes, especially in sub-saturated environments, is highlighted. Finally, using high-resolution direct numerical simulations of moderately supercooled cloud boundaries, we discuss the impacts of droplet evaporative cooling on the evolution of supercooled droplet size distributions, which critically impacts ice nucleation.

Abstract Radar scans of winter storms frequently show enhancements in equivalent radar reflectivity factor (Z e ) with in the -18°C 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 minute) 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 dBZ e km −1 and corresponding increases in Ku-Ka dual-frequency ratio (DFR) ≥ 1.5 dB with in 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 non-dendritic polycrystals produced the observed radar enhancements in the DGL, with only a minor contribution from dendrites.

Precipitation enhancement over complex terrain is predominantly driven by quasi-stationary, terrain-tied vertical motions, making their variability a critical factor in shaping precipitation distributions and accumulation. This study quantifies the dominant modes of terrain-tied vertical motion variability over the Payette River basin of Idaho. Principal component analysis is applied to a seasonal simulation spanning November 2016-April 2017, which encompassed the Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE) field campaign (January-March 2017). The first mode, accounting for more than 20% of the variance in vertical motion, captures ridge-tied updrafts and represents the primary pattern of terrain-induced ascent. The second mode (8%) reflects how synoptic-scale variations modulate updraft orientation, distinguishing between north-south and east-west ridgelines. The third mode (6%) isolates variability in updraft width and magnitude. These three dominant modes of variability, which explain over one-third of the vertical velocity variance in the seasonal simulation, strongly influence the distribution of supercooled liquid water (SLW) and precipitation over the terrain. Results show that the dominant modes of vertical motion variability were consistent with patterns commonly observed during SNOWIE research flights. Additionally, we quantified vertical motion, SLW, and precipitation means as a function of phase space between the modes, demonstrating that enhanced SLW and precipitation occurred when quasi-stationary waves were present over the terrain.

Part I of this study demonstrated how terrain-induced gravity waves triggered elevated convection, with tops up to 6-7 km above sea level, in a potentially unstable layer during a winter storm event over the Idaho Central Mountains on 7 February 2017. Herein, this case is explored further with a large-eddy simulation (LES) at 100-m grid spacing to examine the detailed structure and evolution of convective cells emergent from shallow stratiform clouds, their interaction with complex terrain, and the resulting precipitation processes. The 100-m LES produced fine-scale precipitation structures similar in depth and width to radar observations, with vertical velocity distributions and cloud microphysical properties matching airborne observations. The 100-m LES confirmed the role of vertically propagating gravity waves over the highest terrain ridges in providing the initial lift necessary to release potential instability. Unlike coarser-resolution simulations, the 100-m LES produced clusters of convective towers,-2 km wide, roughly matching observations, although they were more regularly spaced than observed. Cospectral analysis of these towers confirms their convective nature. The small-scale convective updrafts, locally exceeding 2 ms-1 and mostly within the-10 degrees to-20 degrees C temperature zone, enabled snow particles to grow rapidly through depositional growth and riming, and a significant fraction of the simulated precipitation fell as graupel, according to the LES model. Precipitation from this emergent convection occurred primarily in the lee of the main terrain ridge on account of the strong flow above mountain top level. Cumulatively, the LES produced 18% more precipitation than non-LES models in this case. SIGNIFICANCE STATEMENT: This study advances our understanding of cold-season precipitation processes over complex terrain. The key physical mechanisms identified herein are gravity wave-driven potential instability release, multiscale interactions between terrain and convection, and enhanced mixed-phase precipitation growth. This study demonstrates that large-eddy simulations with a grid spacing of-100 m are needed to properly capture the observed small convective towers and the snowfall they produce over and downwind of mountain ridges. In this case study, the large-eddy simulations (<= 300-m grid spacing) produced slightly more precipitation than less-resolved convection-permitting simulations with parameterized eddy exchanges. A more systematic analysis of precipitation from small-scale convection in winter storms is warranted since orographic precipitation has important implications for water resource management in the western United States.

The Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms field campaign comprised three deployments in January and February of 2020, 2022, and 2023. Throughout these deployments, the NASA Earth Resources-2 (ER-2) aircraft conducted 26 research flights, equipped with three vertically pointing radars. These radars sampled the vertical structure of extratropical cyclone clouds at four distinct radar wavelengths, enabling a finer scale analysis of reflectivity and radial velocity structures within extratropical cyclones with a vertical sampling resolution of 26.5 m. In this analysis, we introduce a novel technique utilizing vertical gradients in radial velocity and reflectivity, which proved effective in identifying turbulence, waves, and layers of ascent over 132.5 m layers for all flight legs conducted during the campaign. The spatial scale of 132.5 m was chosen to capture fine-scale variations associated with small-scale turbulent eddies and shear zones in frontal regions. The gradient analysis aided in detecting small scale changes in reflectivity and radial velocity that might have gone unnoticed otherwise. Moreover, the corresponding gradients in reflectivity suggest potential interactions of falling ice crystals with turbulence, waves, and shear layers, possibly influencing the microphysical characteristics and the vertical spatial distribution of falling snow. The observed vertical gradients in radial velocity often exhibited linear, layered patterns, but in some instances displayed wave-like appearances, sloped patterns along frontal boundaries, or magnitude fluctuations. This paper focuses on presenting and detailing the vertical gradient technique to examine winter storms. Future work will involve a comprehensive analysis of these gradients in relation to dual-frequency radar measurements and in situ microphysical characteristics.

In the first part of this study, a Weather Research and Forecasting (WRF) Model simulation of the 20 June 2015 Plains Elevated Convection at Night (PECAN) mesoscale convective system (MCS) was analyzed using a novel strategy for analyzing air parcel trajectories to show that the storm transitioned from surface-based to elevated as a response to the evolving nighttime environment, despite the surface cold pool remaining strong even after the system became elevated. To better understand the role of the cold pool in the propagation of an elevated MCS, three additional WRF simulations were carried out in which the magnitude of latent cooling due to evaporation was either doubled, halved, or removed entirely, effectively controlling the strength of the cold pool. The novel trajectory analysis developed in the first part of this study was then repeated for the additional simulations. It was found that in an environment where both surface-based and elevated instability were present, a stronger cold pool led to less surface-based convection, while a weaker cold pool led to more surface-based convection. During the second half of the simulation, when only elevated instability existed, all simulated storms remained elevated, but the mechanism by which the elevated convection propagated differed. A strong cold pool led to a bore-like feature developing in response to the forcing of the cold pool, which displaced the stable boundary layer and forced elevated, unstable air upward to its level of free convection. A weaker cold pool led to waves developing atop the stable boundary layer and propagating ahead of the surface outflow and initiating new convective updrafts. SIGNIFICANCE STATEMENT: The mechanisms by which summertime nocturnal thunderstorms can remain longlived are not as well understood compared to daytime thunderstorms, making them difficult to forecast. While signifi-cant progress has been made toward understanding these nocturnal storms, there are still questions about how they develop and are maintained. This paper investigates how a nocturnal storm's ability to ingest air from different sources changes as a response to the strength of its cold pool. During periods that there were energy sources available near the surface and aloft, a stronger cold pool made the storm less likely to be fueled by surface air, while a weaker cold pool led to the storm being more likely to be fueled by surface air. These results counter conventional understanding of how these storms remain long-lived, and explanations for such counterintuitive behavior are given.

Cloud seeding of wintertime orographic clouds in the western United States has been attempted to enhance snow production and snowpack. Due to the scarcity of long-term, high-resolution cloud and precipitation observations over complex terrain, few studies have explored variations in orographic snowfall amounts by comparing environmental conditions and cloud characteristics with surface snowfall distribution and quantity. This study analyzes the environmental conditions and cloud characteristics in relation to surface snowfall patterns for the 24 snowfall events observed during the 2017 Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE). The investigation aims to understand: 1) What is the influence, if any, of wind, turbulence, and updraft strength on snowfall amounts, rates, and distribution? 2) What is the relationship, if any, of cloud properties and precipitation-forming effectiveness? and 3) Can cloud seeding modify controlling cloud characteristics sufficiently to increase precipitation in otherwise inefficient orographic clouds? The analysis over a 7200-km(2) observational domain revealed that the accumulated liquid-equivalent snowfall was G0.9 x 10(7) m(3) and snowfall rates were G0.45 mm h(-1) for about half of the events. Low snowfall events were characterized by cloud-top temperatures >-20 degrees C, fewer larger droplets, higher liquid water content, and lower ice water content compared to the other events. Cases with minimal background natural snowfall also permitted radar observation of seeding lines. In these cases, cloud seeding was mainly responsible for snowfall. The amount of silver iodide (AgI) released during cloud seeding did not correlate well with snowfall amount and rate.

The Plains Elevated Convection at Night (PECAN) project was designed to investigate the environmental factors responsible for nocturnal mesoscale convective systems (MCSs) exhibiting either surface-based or elevated convection and to identify the mechanisms by which these MCSs can propagate when conditions conducive to the propagation of similar daytime storms are absent. In this paper, we analyze the 20 June 2015 nocturnal MCS and its response to the evolving nighttime environment. Because the system matured before the nocturnal transition, this case provides a unique opportunity to explore the direct impact of the nocturnal environment on the MCS's tendency to ingest either elevated or near-surface air. A high-resolution Weather Research and Forecasting (WRF) Model simulation of the event, together with a novel strategy for analyzing air parcel trajectories, is employed to determine the origin levels of air entering updrafts within the storm. The trajectory analysis demonstrated that, during the first half of the simulation, between 0300 and 0530 UTC, the MCS was ingesting over 80% of parcels originating from anywhere near the surface up to 2.5 km MSL. In the second half of the simulation after the nocturnal transition occurred, which happened simultaneously with the stabilization of the boundary layer and intensification of the low-level jet, parcels originating from within the boundary layer were no longer being ingested by the strongest updrafts, while parcels originating from above the boundary layer continued to be efficiently ingested by the MCS's strongest updrafts, signifying a clear transition to elevated convection following the nocturnal transition. Evidence of bore-driven propagation was found during this elevated period.

Microphysical measurements within winter storms are commonly analyzed using two-dimensional radar cross sections from airborne vertically pointing radars or ground-based scanning radars. While these radars offer valuable insights, they provide limited insights into the storm's microphysical characteristics within the context of the storm's threedimensional structure. To address this limitation, this analysis uses conically scanning X-band radar data to investigate the three-dimensional structure of a shallow generating cell (GC) driven snowstorm (,6 km deep) sampled over central Illinois and Indiana on 25 February 2020 during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign. The observed microphysical properties and reflectivity structures along the nadir-pointing radar cross section represent the superposition of 3D trajectories of fall streaks originating in GCs upwind of the aircraft's flight track. GCs formed in a potentially unstable layer near cloud top based on HRRR analysis, where supercooled water formed and created a droplet-rich environment for ice crystal nucleation, growth, and fallout. In situ micro-physics measurements beneath cloud top allowed for the assessment of particle aspect ratios within and outside of GC fall streaks. When sampled 2-3 km below cloud top, fall streaks typically contained larger ice crystals and aggregates with higher aspect ratios compared to the surrounding cloud, and increased reflectivity in nadir and plan-view scans. The southern end of the storm lacked GCs, was supercooled, and contained smaller, low-aspect-ratio ice crystals in high concentrations.

The Dual-Frequency Precipitation Radar (DPR) onboard the Global Precipitation Measurement (GPM) core mission provides the opportunity to estimate rain microphysics properties globally, including precipitation amount, intensity, and type. These properties can be derived from particle size distributions (PSDs), traditionally modeled using a three-parameter gamma-shaped size distribution. However, DPR measurements provide only two observables: reflectivity at Ku and Ka bands. Although the dual-frequency ratio (DFR) is commonly used in retrieval algorithms, it is a derived quantity and does not constitute a third independent measurement. This limitation necessitates the use of simplified or underconstrained retrieval methods. Here, we assessed whether these limitations and assumptions in baseline methods (GPM-analytical and GPM-operational) contribute significantly to uncertainties in estimating rain microphysical properties. Using PSD data from NASA's Clouds, Aerosol, and Monsoon Processes Philippines Experiment (CAMP2Ex) field campaign, we computed bulk properties and normalized gamma size distribution (NGSD) parameters and compared them against synthetic retrievals. Two deep neural network (DNN) models were tested: one using only ZKu and ZKa (matching DPR capabilities) and a second using ZKu , DFR, and Ka-band specific attenuation (kKa ). The three-input DNN signifi-cantly improved rainfall retrieval accuracy, yielding MAE and RMSE values of 1.7 and 4.2mm h-1, respectively, compared to 3.6 and 12.1 for GPM-analytical and 6.2 and 15.8 for GPM-operational. The two-input model performed worse (MAE = 7.8 mm h-1, RMSE = 14.5 mm h-1) but remained comparable to GPM-operational, demonstrating that even with limited inputs, a machine learning framework can provide skillful retrievals relative to legacy methods.

This study uses airborne, vertical W-band radial velocity (Vr) radar data from seven ER-2 flights during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign together with High-Resolution Rapid Refresh (HRRR) model initialization data to investigate hydrometeor vertical motions within elevated potentially unstable and stable layers in winter extratropical cyclones. Cohen's D test (Cd) is used to evaluate how distributions of Vr vary with cyclone type and intensity, within stable and unstable layers, and with characteristics of elevated potential instability (EPI) described in Part I. In general, hydrometeor vertical motions rarely exceeded 2 ms-1 within stable and EPI layers. The Vr distributions, including stable and EPI layers, varied more by cyclone intensity than cyclone type. The Vr distributions shifted toward positive values and broadened in stronger cyclones. Surprisingly, Vr distributions were similar in stable and EPI layers (Cd = 0.15). The hydrometeor vertical motions in stable layers were associated with orographically induced gravity waves, shear-induced turbulence, and cloud-top generating cells. In general, the distance from the low pressure center, region within the comma head, depth of EPI layers, and number of EPI layers had little influence on the Vr distributions. The Vr distributions varied most by base height of the EPI layer (Cd = 0.28-0.68) followed by EPI magnitude (Cd = 0.37-0.66) where the higher the layer base, the more positive the Vr mode. The stronger the instability, the more negative the Vr mode, likely due to riming within elevated convection.

The combination of simultaneous, collocated aircraft in situ measurements and remote sensing data at multiple wavelengths is of tremendous value in physical process studies but is hard to obtain in practice. Appropriate multiaircraft and multisensor resources for a given project must be coupled with agile mission support (people and tools) and close coordination with the Federal Aviation Administration to implement successfully. Obtaining closely coordinated in situ and remote sensing measurements was key to meeting the science objectives for the NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS), and it required a team effort. IMPACTS flew a complementary suite of remote sensing and in situ instruments in three 6-week deployments on the NASA ER-2 and P-3 aircraft to provide observations critical to understanding the mechanisms of snowband formation, organization, and evolution. The collocated IMPACTS data subset encompassed 106 flight legs during 22 storms, and it included over 21 h where the ER-2 and P-3 were only up to 5 min and 4 km apart. This unique dataset on winter storm conditions in the Northeast and Midwest United States provides a wealth of information, which will have lasting value for the community. This paper explains how the science team, engineers, aircrews, and NASA mission support accomplished the measurement goals and key aspects of the IMPACTS coordinated dataset. Future field campaigns with similar science applications can maximize their flight hours by leveraging the lessons learned from IMPACTS coordination.

It is not uncommon for layers within the warm conveyor belt in a frontal system to become potentially unstable, releasing elevated convection. The present study examines this destabilization process over complex terrain, and resulting precipitation, with a focus on the surface coupling, orographic ascent, and the initiation and evolution of convective cells. This study uses detailed observations combined with numerical modeling of a baroclinic system passing over the Central Idaho Mountains in the United States on 7 February 2017. The data were collected as part of the Seeded and Natural Orographic Wintertime Clouds: the Idaho Experiment (SNOWIE). Specifically, observations from a ground-based scanning X-band radar and an airborne profiling Doppler W-band radar along ;100-km-long flight tracks aligned with the wind describe the development and evolution of convective cells above shallow stratiform orographic clouds. Convection-permitting numerical simulations of this event, with an inner domain grid resolution of 0.9 km, capture the emergence and vertical structure of the convective cells. Therefore, they are used to describe the advection of warm, moist air over a retreating warm front, cold-air pooling within the Snake River basin and adjacent valleys, destabilization in a moist layer above this shallow stable layer, and instability release in orographic gravity wave updrafts. In this case, the convective cells topped out near 6 km MSL, and the resulting precipitation fell mostly leeward of the ridge where convection was triggered, on account of strong cross-barrier flow. Sequential convection initiation over terrain ridges and rapid downwind transport led to banded precipitation structures.

Elevated potential instability (EPI) often occurs in the comma head of wintertime extratropical cyclones as air within the storm's dry slot moves above a warm or occluded frontal zone. Lifting of EPI layers may result in elevated convection, enhanced snowfall, and thundersnow. High-Resolution Rapid Refresh (HRRR) initialization values of equivalent potential temperature theta(e) are used to analyze EPI characteristics along tracks of the NASA Earth Resources-2 (ER-2) aircraft within the comma head of 14 cyclones sampled during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign. EPI was found in 53% of HRRR 1-km-wide vertical columns along ER-2 flight legs, typically 210-410 km from the low pressure center within the northern comma head region, but 0-400 km from the low in the western region of strong cyclones [sea level pressure gradients > 4 hPa (100 km)(-1)]. The highest and lowest frequencies of EPI were in Miller type B (MB) and Great Plains cyclones, respectively. In 67% of 1-km-long columns exhibiting EPI, EPI occurred within a single layer. The median base of the potentially unstable layers (Delta theta(e) /Delta z < 0) was 4.2 km, the median depth was 0.59 km, and the median equilibrium level was 0.94 km above the base. MBs had more, shallower EPI layers compared to fewer, deeper layers in Miller type A and Gulf Coast cyclones. The median value of Delta theta(e) /Delta z within potentially unstable layers was -0.6 K km(-1), the 95th percentile was -2.8 K km(-1), and the greatest values of EPI occurred in MBs and cyclones with sea level pressure gradients < 2 hPa (100 km)(-1). SIGNIFICANCE STATEMENT: Snowfall associated with wintertime extratropical cyclones can cause large disruptions to everyday life, impacting transportation, schools, businesses, and power. Snowfall arises from different circulations in the atmosphere ranging from broad-scale ascent of air to small-scale buoyant circulations associated with elevated potential instability (EPI) above frontal zones. This paper examines the distribution, frequency, and intensity of EPI within storms sampled during the 3-yr NASA Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign and the relationship of the instability to cyclone type and intensity. We address two questions: 1) How common is EPI and where does it develop within the comma head region of cyclones? Does the occurrence of EPI vary based on cyclone type and strength? And 2) what are the characteristics of EPI (number of layers, depth of layers, and strength of EPI)? How do these characteristics vary based on cyclone type and strength? Detailed, quantitative answers to these questions are provided.

The mesoscale and microphysical structure of a cloud system associated with an Arctic front is analyzed using data from two research aircraft, two WSR-88D radars, the HYSPLIT model, and initialization fields from the RAP model. The flights, conducted during the NASA Investigation of Microphysics and Precipitation in Atlantic Coast-Threatening Snowstorms (IMPACTS) campaign, collected in situ and remote sensing data as the cloud system moved across Illinois. The system developed within an air mass that, based on back trajectory analysis, originated over the subtropical eastern Pacific before being lifted over the Arctic front. This led to a region of potential instability extending upward over the frontal zone. The ascending flow triggered the release of the instability that manifested as elevated convection in the storm's southern sector. In the convective region, supercooled water was found in cloud towers, leading to saturated conditions that supported growth of a range of particle habits and growth by riming. Within this region, and in shallower clouds between convective towers, needle particle habits, supercooled water, and high ice particle concentrations implied active secondary ice processes. Two snowbands formed north of the convective region, with radar evidence suggesting that precipitation within these bands originated in cloud towers at altitudes of 4-6 km in a near-neutral to weakly unstable region. Water saturated conditions, evidenced by supercooled water at the sampling level, permitted the growth of a range of particle habits. Despite ice particle concentrations , 15 L21 within the bands, some aggregated particles exceeding a centimeter in maximum dimension were observed at 258C, likely contributing to the 21-27 dBZe reflectivity characteristic of the bands.

Recent studies from the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) demonstrated definitive radar evidence of seeding signatures in winter orographic clouds during three inten-sive operation periods (IOPs) where the background signal from natural precipitation was weak anda radar signal attribut-able to seeding could be identified as traceable seeding lines. Except for the three IOPs where seeding was detected, background natural snowfall was present during seeding operations and no clear seeding signatures were detected. This paper provides a quantitative analysis to assess if orographic cloud seeding effects are detectable using radar when back-ground precipitation is present. We show that a 5-dB change in equivalent reflectivity factor Ze is required to stand out against background natural Ze variability. This analysis considers four radar wavelengths, a range of background ice water contents (IWC) from 0.012 to 1.214 g m-3, and additional IWC introduced by seeding ranging from 0.012 to 0.486 g m-3. The upper-limit values of seeded IWC are based on measurements of IWC from the Nevzorov probe employed on the University of Wyoming King Air aircraft during SNOWIE. This analysis implies that seeding effects will be undetectable using radar within background snowfall unless the background IWC is small, and the seeding effects are large. It therefore remains uncertain whether seeding had no effect on cloud microstructure, and therefore produced no signature on radar, or whether seeding did have an effect, but that effect was undetectable against the background reflectivity associated with naturally produced precipitation. SIGNIFICANCE STATEMENT: Operational glaciogenic seeding programs targeting wintertime orographic clouds are funded by a range of stakeholders to increase snowpack. Glaciogenic seeding signatures have been observed by ra-dar when natural background snowfall is weak but never when heavy background precipitation was present. This analysis quantitatively shows that seeding effects will be undetectable using radar reflectivity under conditions of background snowfall unless the background snowfall is weak, and the seeding effects are large. It therefore remains uncertain whether seeding had no effect on cloud microstructure, and therefore produced no signature on radar, or whether seeding did have an effect, but that effect was undetectable against the background reflectivity associated with naturally produced pre-cipitation. Alternative assessment methods such as trace element analysis in snow, aircraft measurements, precipitation measurements, and modeling should be used to determine the efficacy of orographic cloud seeding when heavy back-ground precipitation is present.

Cloud-top phase (CTP) impacts cloud albedo and pathways for ice particle nucleation, growth, and fallout within extratropical cyclones. This study uses airborne lidar, radar, and Rapid Refresh analysis data to characterize CTP within extratropical cyclones as a function of cloud-top temperature (CTT). During the 2020, 2022, and 2023 Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field campaign deployments, the Earth Resources 2 (ER-2) aircraft flew 26 research flights over the northeast and midwest United States to sample the cloud tops of a variety of extratropical cyclones. A training dataset was developed to create probabilistic phase classifications based on Cloud Physics Lidar measurements of known ice and liquid clouds. These classifications were then used to quantify dominant CTP in the top 150 m of clouds sampled by the Cloud Physics Lidar in storms during IMPACTS. Case studies are presented illustrating examples of supercooled liquid water at cloud top at different CTT ranges (-3 degrees < CTTs < -35 degrees C) within extratropical cyclones. During IMPACTS, 19.2% of clouds had supercooled liquid water present at cloud top. Supercooled liquid was the dominant phase in extratropical cyclone cloud tops when CTTs were >-20 degrees C. Liquid-bearing cloud tops were found at CTTs as cold as -37 degrees C.

Arctic cyclones (ACs) are an important component of the Arctic climate system. While previous studies focused on case studies or samples of intense ACs, an AC tracking algorithm is applied here to ERA5 to provide more than 9300 tracks. This large sample enables evaluations of seasonality, latitudinal dependence, and the structural evolution of ACs using storm-centered composite analysis and phase space analysis. The structures of ACs of different genesis regions, polar versus midlatitude, are also examined and compared. The results show that ACs typically have an asymmetric horizontal structure with cold air to the west and warm air to the east of the cyclone center. Cyclone asymmetry decreases, and the circulation becomes more barotropic in higher latitudes. ACs of polar origin are more symmetric than ACs of midlatitude origin and dominate the cyclone occurrences over the Arctic Ocean. Regarding seasonality, winter ACs are more intense and have a stronger horizontal asymmetry, and the cyclonic circulation extends higher into the stratosphere than summer ACs. In contrast, summer ACs have stronger warm anomalies in the lower stratosphere associated with subsidence above the cyclone center, and the cyclonic circulation typically does not extend beyond 50 hPa. The latitudinal and seasonal variations of AC structure are consistent with the latitudinal and seasonal differences in environmental baroclinicity. Additionally, our analyses show that the structural evolution of ACs is characterized by reduced vertical tilt and asymmetry, weakened temperature contrast between west and east sectors in troposphere, and reduced updraft strength in the later stage of the AC life cycle.