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In situ data collected by three research aircraft in four geographical locations are analyzed to determine the relationship between cloud-base temperature, drop size distribution, and the development of supercooled water drops and ice in strong updraft cores of convective clouds. Data were collected in towering cumulus and feeder cells in the Caribbean, over the Gulf of Mexico, over land near the Gulf Coast, over land in the southeastern United States, and the high plains in Colorado and Wyoming. Convective clouds in the Caribbean, over the Gulf of Mexico and its coast, and over the southeastern United States all develop millimeter-diameter supercooled drops in updraft cores. Clouds over the high plains do not generate supercooled large drops, and rarely are drops >70 μ m observed in updraft cores. Commensurate with the production of supercooled large drops, ice is generated and rapidly glaciates updraft cores through a hypothesized secondary ice process that is based on laboratory observations of large drops freezing and emitting tiny ice particles. Clouds over the high plains do not experience the secondary ice process and significant concentrations of supercooled liquid in the form of small drops are carried much higher (up to −35.5°C) in the updraft cores. An empirical relationship that estimates the maximum level to which supercooled liquid water will be transported, based on cloud-base drop size distribution and temperature, is developed. Implications have applications for modeling the transport of water vapor and particles into the upper troposphere and hygroscopic seeding of cumulus clouds.

In situ carbonyl sulfide (OCS) measurements from the Stratospheric Aerosol processes, Budget and Radiative Effects (SABRE) 2023 airborne campaign are used to evaluate the sulfate budget in the Arctic stratosphere during boreal winter. The strong correspondence between these measurements and remote retrievals from the Atmospheric Chemistry Experiment–Fourier Transform Spectrometer provide robust validation of the satellite's capability to monitor stratospheric OCS globally. We demonstrate how trends in the tropical tropopause layer and National Oceanic and Atmospheric Administration OCS surface data reveal a post‐2016 ∼8% global decline in OCS abundance, which is absent from many global climate models. New simulations with a revised planetary boundary layer OCS abundance show improved agreement with remote retrievals and in situ data across multiple stratospheric layers, but remaining model biases highlight the need for additional in situ OCS observations. The revised representation reduces the stratospheric sulfate burden, resulting in an increased shortwave solar flux at the tropical tropopause by as much as 0.3 Wm−2 locally, with implications for stratospheric circulation, radiative forcing, and climate feedbacks.

Two-dimensional (2D) imaging probes, such as the 2D stereo (2D-S) probe and the cloud imaging probe (CIP), are routinely used to provide in situ measurements of cloud particle properties. The basic measurement is shadowgraphs of water drops and ice particles from which particle size distributions, projected particle area, and mass concentrations are determined. These data permeate data archives of domestic and foreign government agencies, universities, and the private sector. This paper provides results from laboratory tests and flight tests on a Learjet research aircraft that give new insights into the performance of the 2D imaging probes, and how their performance may have impacted measurements collected in data archives. The laboratory tests are conducted with the aid of two devices: 1) a droplet generator that provides known concentrations of water drops from 15 to 65 µ m ± 1 µ m that can be positioned in the probe’s sample volume with 10- µ m precision; and 2) a motorized spinning platform that supports transparent disks with small opaque features (i.e., a “spinning disk”), which replicates the effect of particles transecting the probe’s sample volume at translational speeds up to 190 m s −1 . The flight tests were conducted with a Learjet research aircraft that collected cloud particle data at true airspeeds from 99 to 170 m s −1 . The results provide new insights into how probe optics, time response, and data throughput of the 2D-S and CIP electro-optics impact the measurements of cloud particles. The results, summarized in the conclusions, suggest how archived data are impacted.

Understanding ice development in cumulus congestus (CuCg) clouds, which are ubiquitous globally, is critical for improving our knowledge of cloud physics, precipitation and climate prediction models. Results presented here are representative of data collected in 1008 penetrations of moderate to strong updrafts in CuCg clouds by five research aircraft in six geographic locations. The results show that CuCg with warm (∼23°C) cloud-base temperatures, such as in tropical marine environments, experience a strong collision–coalescence process. Development of coalescence is also correlated with drop effective radius >∼12 to 14 μ m in diameter. Increasing the cloud-base drop concentration with diameters from 15 to 35 μ m and decreasing the drop concentration < 15 μ m appears to enhance coalescence. While the boundary layer aerosol population is not a determinate factor in development of coalescence in most tropical marine environments, its impact on coalescence is not yet fully determined. Some supercooled large drops generated via coalescence fracture when freezing, producing a secondary ice process (SIP) with production of copious small ice particles that naturally seed the cloud. The SIP produces an avalanche effect, freezing the majority of supercooled liquid water before fresh updrafts reach the −16°C level. Conversely, CuCg with cloud-base temperatures ≤ ∼8°C develop significant concentrations of ice particles at colder temperatures, so that small supercooled water drops are lofted to higher elevations before freezing. Recirculation of ice in downdrafts at the edges of updrafts appears to be the primary mechanism for development of precipitation in CuCg with colder cloud-base temperatures.

Anthropogenic emissions over China have recently declined due to environmental actions. This work estimates the sensitivity of sulfate aerosol (SO4 ${\\text{SO}}_{4}$) concentration to the amount of SO2 ${\\text{SO}}_{2}$ emissions reduction over China from 2010 to 2020 using an Earth system model with two different aerosol representations. We find that a larger rate of SO2 ${\\text{SO}}_{2}$ emissions decline over 2010–2020 from an updated Chinese emission inventory leads to improvement in modeled SO2 ${\\text{SO}}_{2}$ and SO4 ${\\text{SO}}_{4}$ concentrations when evaluated with targeted airborne observations from the Asian summer monsoon region from the 2022 Asian summer monsoon Chemical and Climate Impact Project. Updated Chinese SO2 ${\\text{SO}}_{2}$ emissions reduce SO4 ${\\text{SO}}_{4}$ concentration by >20% at 200 hPa over the North Pacific, and by >7% at 100 hPa throughout the tropics. These SO4 ${\\text{SO}}_{4}$ reductions result in an increase to global net instantaneous radiative forcing of ∼ ${\\sim} $0.10–0.15 W m−2 ${\\mathrm{m}}^{-2}$ by 2020, with regional effects up to ∼ ${\\sim} $6 times greater.

While nucleation of solids in supercooled liquids is ubiquitous, surface crystallization, the tendency for freezing to begin preferentially at the liquid-gas interface, has remained puzzling. Here we employ high-speed imaging of supercooled water drops to study the phenomenon of heterogeneous surface crystallization. Our geometry avoids the \"point-like contact\" of prior experiments by providing a simple, symmetric contact line (triple line defined by the substrate-liquid-air interface) for a drop resting on a homogeneous silicon substrate. We examine three possible mechanisms that might explain these laboratory observations: (i) Line Tension at the triple line, (ii) Thermal Gradients within the droplets and (iii) Surface Texture. In our first study we record nearly perfect spatial uniformity in the immersed (liquid-substrate) region and, thereby, no preference for nucleation at the triple line. In our second study, no influence of thermal gradients on the preference for freezing at the triple line was observed. Motivated by the conjectured importance of line tension (τ) for heterogeneous nucleation, we also searched for evidence of a transition to surface crystallization at length scales on the order of δ ~ τ /σ, where σ is the surface tension; poorly constrained τ leads to δ ranging from microns to nanometers.