Andrew J. Heymsfield

Ice replicas of natural hailstones together with earlier measurements for natural hailstones are used to develop a relationship that characterizes the drag coefficient and terminal velocity of hailstones through their stages of development and melting. The ice replicas were produced based on 3D scans of natural hailstones. These particles were then levitated in the wind tunnel, and their terminal velocity, mass, and dimensions were measured as the particles were melting. Constant temperatures are used to characterize the properties of the hailstones as they melt. A primary goal of this study was to find a relationship that includes the apparent increase in drag coefficient found at relatively high Reynolds numbers (Re) and associated terminal velocities found in the wind tunnel for nonmelting hailstones, which has not been considered in earlier representations of hail terminal velocities. For this reason, several drag coefficient models for regularly and irregularly shaped particles using particle sphericity and shape are tested against the observations. Although several of the sphericity/shape models fit the data reasonably well, the increase in the drag coefficient at the higher Re is overpredicted in most of the models. Models and drag coefficients dependent only on the Re are also tested against the observations. A consistency is found between the observations and a relationship between the Reynolds number and maximum or equivalent diameters fit to the data. Significance Statement: Earlier studies have largely assumed that hail is spherical with solid ice density and that unmelted and melted hail fall equally fast. This study develops consistent relationships across the range of conditions encountered by unmelted and melting hailstones that can be readily incorporated into models.

Two supercell thunderstorms that occurred in Del Rio and Burkburnett, Texas in the spring of 2020 produced greater than 5 cm diameter hailstones. Fourteen hailstones, eight from Del Rio and six from Burkburnett, were cut in half along the maximum principal axis. Half the hailstone was dissected at 0.5 cm increments, providing 198 subsamples of meltwater (0.25 ml) for stable isotope analysis (ẟ ² H and ẟ ¹⁸ O) using a Picarro L2130‐i water analyzer and half used to create a thin section for identification and analysis of ice crystal morphology. Deuterium serves as a proxy for relative altitude, providing a record of hailstone formation and growth trajectory, while crystallographic analysis differentiates wet and dry growth. Results indicate significant variations in isotopic composition and ice crystal morphology, reflecting distinct growth trajectories in the supercell environment, providing evidence that hailstone formation and growth occurred at varying altitudes and reflect recycling in the updraft. The formation of a hailstone embryo at higher altitudes challenges the paradigm of embryo formation only at the cloud base or lower altitudes compared to the outer growth layers. Dendritic crystal morphologies observed in Del Rio hailstones are consistent with rapid freezing in the presence of elevated supercooled liquid water content. Mass ratio analysis provided that most of the hailstones approached the density of bubble free ice while the sphericity index reflected that most hailstones approached 0.80 sphericity but can more accurately be described as triaxial ellipsoids. The goal of this research was to provide better information on hailstone growth and trajectory through supercell convective thunderstorms.

Marine fog plays an important role in ship and aircraft operations as well as in marine ecosystems and climate change. Fog is affected by boundary layer processes occurring at varying time and space scales. Its monitoring and forecasting can be challenging in marine environments because of the limited number of observation sites, such as buoys and ships. Therefore, the Fog and Turbulence in Marine Atmosphere (FATIMA) field campaign was designed to study the marine fog life cycle with high fog occurrence. FATIMA‐YS was established over the Yellow Sea region of the Republic of Korea from 20 June to 9 July 2023. Observations were collected using the South Korean research vessel (R/V) Onnuri , the Atmospheric Research Aircraft (NARA), three Korean Ocean Research Stations supported by R/V Gisang‐1 , meteorological stations including buoys, and the Korean Meteorological Administration weather stations. In‐situ instruments on the R/V and aircraft provided extensive observations of various microphysical and dynamical parameters, aerosols and gases, and radiation parameters. Satellite platforms (Himawari and Geostationary Ocean Color Image) together with ship‐based millimeter cloud radar, lidar, and microwave radiometer have also been used for fog monitoring and simulation validation. During the campaign, eight Intensive Observational Periods (IOPs) were performed, and visibility changed from tens of meters up to tens of kilometers, and liquid water content reached up to 0.5 g·m ⁻³ during heavy fog conditions. Marine fog usually occurs due to advection processes, but fog formation was found to be highly related to the mixing of air masses, turbulence, and ocean cold‐water upwelling, whereas its maturity and dissipation were strongly correlated with mixing processes. Preliminary results, extensive new technologies, and the project campaign description with IOP summaries are provided. Finally, the future work and challenges are discussed.

The method of ice crystal replication in Formvar (polyvinyl formal resin) was introduced by Vincent Schaefer in 1941 [1]. At that time no aircraft based optical instrumentation was available to study the morphology of ice crystals. In spite of the rapid advance of the sophisticated optical particle probes, the formvar replication technique, applied in its very original form, turns out to be a valuable complimentary method for the ice crystal habit characterization [4].In addition to habit and surface morphology, some formvar replicas preserve residual particles that may have acted as ice nucleating particles (INPs). Early attempts to identify these nuclei (e.g. Kumai, 1951 [2] and Koenig, 1960 [3]) were limited by poor instrumental resolution and lack of accurate elemental analysis. Modern scanning electron microscopy and X-Ray spectroscopic techniques allows us to revisit this approach. The ice nucleating particles preserved within the replicas can be characterized and attributed to the ice crystal habits and sampling environmental conditions. Based on several case studies, including ice nucleation experiments conducted in AIDA chamber and analysis of formvar replicas of ice crystals collected from free atmosphere and by airborne probes, we evaluate the potential of the ice replication method combined with SEM analysis for INP identification.References:[1] Schaefer, V. J.: A method for making snowflake replicas. Science, 93 (1941) pp. 239-240.[2] Kumai, M.: Electron-microscope study of snow-crystal nuclei. J. Atmos. Sci., 8 (1951) pp. 151-156.[3] Koenig, L. R.: The chemical identification of silver-iodide ice nuclei: a laboratory and preliminary field study. J. Atmos. Sci., 17 (1960) pp. 426-434.[4] Miloshevich, L. M. and Heymsfield, A. J.: A Balloon-Borne Continuous Cloud Particle Replicator for Measuring Vertical Profiles of Cloud Microphysical Properties: Instrument Design, Performance, and Collection Efficiency Analysis, J. Atmos. and Oceanic Tech., 14 (1997), pp. 753-768.

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.

Hail is the leading driver of severe convective storm losses in the United States and globally, with early estimates though June in 2025 approaching 20 billion dollars insured loss in the U.S. alone. Despite this impact, many questions remain unanswered within hail science, ranging from hailstone properties to swath properties, to environmental drivers and storm kinematic and microphysical structures and how these can be remotely sensed. To address this gap the In Situ Collaborative Experiment for the Collection of Hail in the Plains (ICECHIP) campaign was conducted between May 15 and June 28th of 2025. Active periods of convection persisted throughout the campaign yielding over 20 intensive observation periods.ICECHIP field assets included over 100 ground deployable instruments, multiple atmospheric profiling systems, mobile doppler radars with a team of 15 institutions and 6 countries. Numerous cases ranging from accumulating small to storms with hail in excess of 75mm were sampled, with the largest hailstone observed >140mm and weighing 462 grams. ICECHIP addressed observations necessary for 5 major research themes: 1) embryo development and hailstone growth and fall behavior; 2) in-storm hail trajectory and convective updraft relationships; 3) environmental impacts on hail processes and predictability; 4) surface properties of hailstones and associated impacts; and 5) relationship of hailstone physical properties and growth processes to radar observations.This presentation will focus on an overview of major project cases and preliminary significant findings. Examples of the data collected include ground observed arrays at horizontal resolutions of a few hundred meters across hailswaths, updraft measurements of both supercell and non-supercellular hail producing storms, dual-doppler and high-resolution radar coverage overlapping detailed ground mapping, and numerous examples of near storm environmental profiles.

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 analyzes in situ aircraft microphysical measurements in deep snow-producing clouds during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field programs in winter 2020, 2022, and 2023 to characterize and compare the microphysics in the updraft and downdraft regions. Measurements were acquired from the NASA P-3 aircraft equipped with a full complement of particle probes and instruments for direct measurements of condensed water content and thermodynamic and 3D wind measurements. We identified the P-3 aircraft flight days collected within snowbands, generating cells, and peripheral regions from 14 flights. These are composited and used for the analysis. Temperatures sampled ranged between −27° and +1°C. The data are partitioned by air vertical velocity, with strong updrafts defined as >0.5 m s⁻¹, very strong updrafts as >1 m s⁻¹, and strong downdrafts as <−0.5 m s⁻¹. This partitioning revealed precipitation mass concentrations that were 2x higher in the strong updrafts and 3x higher in the very strong updrafts than in the downdrafts, a result of particle growth and relative fallout within the updrafts. Total particle concentrations at the concentrations > 1 mm were about the same in each region. However, fallout of the larger aggregates through the updrafts at temperatures > −5°C and into the melting layer results in previously unreported shedding and lofting of the shed particles in the updrafts to subfreezing temperatures. This observation is supported by the overflying NASA ER-2 Doppler radar measurements. Significance Statement: This study uses data from 3 years of wintertime field programs during the Investigation of Microphysics and Precipitation for Atlantic Coast-Threatening Snowstorms (IMPACTS) field program. New insights are gained into differences in the properties of the updraft and downdraft regions and the lofting of drops created from melting snow aggregates that are lofted back by updrafts to subfreezing temperatures.

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.