Elise Rosky

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled a cold and polluted closed-to-open cell transition with cloud-top temperatures below −20 °C and upstream cloud condensation nuclei concentrations surpassing 600 cm ⁻³ . The aerosol likely originates from Siberian industrial emissions. As the boundary layer deepens prior to and across the transition, driven by strong turbulent surface fluxes and aided by a weakening cloud-top inversion, aerosol number concentrations decrease and supercooled droplet sizes increase, causing ice particles to shift from primarily vapor depositional growth towards riming. Precipitation shafts within open-cells contain abundant large graupel (>5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm h ⁻¹ . Ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Updrafts support liquid water production along convergence boundaries at the upwind cold pool edge. Nonetheless, strong buoyancy fluxes exceeding 200 W m ⁻² prevent sub-cloud decoupling. Ice particle number concentrations increase tenfold across the transition, surpassing 10 L ⁻¹ within open-cells, more than 1000× ice nucleating particle number concentrations. Given that cloud temperatures are too cold for Hallett-Mossop rime splintering, this indicates the importance of other secondary ice production mechanisms. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm^-3, decreasing to 90 cm^-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m^-2 to 270 g m^-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (N_i) are generally 2 L^-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m^-2 collocated with strong updrafts, adjacent to glaciated cloud. N_i surpasses 10 L^-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr^-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m^-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm^-3, decreasing to 90 cm^-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m^-2 to 270 g m^-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (N_i) are generally 2 L^-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m^-2 collocated with strong updrafts, adjacent to glaciated cloud. N_i surpasses 10 L^-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr^-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m^-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm^-3, decreasing to 90 cm^-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m^-2 to 270 g m^-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (N_i) are generally 2 L^-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m^-2 collocated with strong updrafts, adjacent to glaciated cloud. N_i surpasses 10 L^-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr^-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m^-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

Closed-to-open cell mixed-phase cloud transitions within marine cold air outbreaks subjected to strong turbulent surface fluxes remain poorly understood despite their importance to high-latitude climate. The Cold-Air outbreak Experiment in the Sub-Arctic Region (CAESAR) research aircraft sampled closed-cells with cloud condensation nuclei concentrations surpassing 680 cm-3, decreasing to 90 cm-3 across a transition to open-cells. The aerosol likely originated from Siberian industrial emissions. With fetch, liquid water paths (LWPs) increase from 120 g m-2 to 270 g m-2 and cloud-top effective diameters increase from 10 μm to 16 μm, coincident with more riming. Ice particle number concentrations (Ni) are generally 2 L-1 or less, but exceed ice nucleating particle number concentrations by 100x. As the cloud-top inversion weakens and the boundary layer deepens further, ice precipitation co-exists with lidar-observed surface cold pools, modulated by entrainment events, juxtaposed with surface-based plumes of warm moist air. Open-cells contain isolated LWP peaks surpassing 500 g m-2 collocated with strong updrafts, adjacent to glaciated cloud. Ni surpasses 10 L-1 at cloud temperatures < -15 °C. Precipitation shafts contain abundant large graupel (> 5 mm diameter) with liquid-equivalent precipitation intensities reaching 3 mm hr-1 developing cold pools with virtual potential temperature depressions reaching 1.3 K. Nonetheless, buoyancy fluxes of 200-250 W m-2 prevent sub-cloud decoupling. The updrafts supporting liquid water production occur at the upwind edge of the cold pools. This case expands the observations needed to better understand mixed-phase Arctic cloud processes.

How cloud droplets evaporate when mixed with the dry surrounding air is fundamental to cloud optical properties and lifetime. We find from observations in cumulus clouds made during the ESCAPE field campaign that this mixing process appears strongly inhomogeneous-like, where a subset of droplets evaporate completely as mixing proceeds, rather than all droplets partially evaporating. We visualize the microphysical properties in a two-dimensional evaporation-phase-relaxation space and find that a diffusive turbulent-evaporation model is able to capture the dynamic evolution of the entrainment process. The results indicate that the first evaporating droplets humidify the region around the cloud so that the unmixed dry air rarely reaches the core, explaining why most mixing events appear inhomogeneous. A mixing slope parameter also confirms the nature of the mixing process. On the basis of the inhomogeneous mixing model, we propose a simple parameterization of cloud optical properties suitable for coarse-resolution models.

A closed to open-celled transition of mixed-phase clouds within a marine cold air outbreak (MCAO), driven by an occluded cyclone over the Nordic Seas, is interrogated with data acquired during the Cold Air Outbreak Experiment in the Sub-Arctic Region (CAESAR) field campaign on 29 February 2024. The understanding of these transitions is a challenge for numerical prediction models due to their small scale but important for weather and climate prediction, as open celled conditions have stronger updrafts supporting locally high precipitation rates along with a lower cloud fraction (lower albedo) than closed celled conditions. Measurements indicate that a stratiform (closed cell convective) cloud deck with cloud top heights of ~1230 m and liquid water paths (LWP) of ~130 gm-2, within a boundary layer with aerosol number/CCN surpassing 600 cm-3, deepen to heights of ~1520 m with LWPs of ~270 gm-2 over 250 km of fetch, before transitioning into an open-celled convective structure with cloud tops reaching up to 2220 m. Cooling free tropospheric temperatures with fetch, which reduce the inversion strength thereby enhancing growth by entrainment may encourage boundary layer growth. Open-cells are more glaciated than closed-cells with mean LWPs falling from 270 gm-2 to 80 gm-2 across the transition, however isolated peaks of LWP within updrafts of open cells occasionally surpass 500 gm-2. Minimal secondary ice production (SIP) is observed in closed cells with ice nucleating particle and ice number concentrations ~2 L-1 with cloud temperatures between -20oC and -15oC. In open cellular convection (cloud temperatures between -22oC and -15oC), ice number concentrations reach ~10 L-1 indicating SIP. High aerosol concentrations are hypothesized to support the maintenance of closed-celled convection, with 80% of drops smaller than 10 μm reducing the riming efficiency. Small droplets also limit the production of freezing drizzle, which is hypothesized to limit the potential of SIP due to freezing/fragmentation within closed cell convection. Only after aerosol concentrations are depleted through scavenging and/or entrainment are SIP processes able to become more effective and precipitation particles able to grow large and dense enough to reach the surface and form cold pools breaking up the cloud deck. Plumes of warm moist air lifting off the ocean surface, juxtaposed with cold pools and entrainment events penetrating to the surface are documented using the Multi-function Airborne Raman Lidar (MARLi) in the first observations of its kind.

In-situ cloud measurement techniques, particularly those collected by airborne platforms, capture microphysical characteristics of mixed-phase clouds but are unable to directly measure ice formation mechanisms and particle growth histories. Addressing this observational gap, we demonstrate that in-situ measurement of stable water isotopes can be used to quantify ice growth processes more directly. By analyzing the isotopic composition of ice hydrometeors, we can identify their dominant growth pathway: direct vapor deposition, riming, or through Wegener-Bergeron (WBF) conditions.Stable water isotopologues are water molecules which contain deuterium (D) or oxygen-18 (O18). They are present within water everywhere, and are termed “heavy” due to their larger molecular mass. The concentration of heavy water isotopes found in atmospheric ice particles is dependent on the thermodynamic conditions experienced during growth. Specifically, the in-situ temperature, relative humidity, and thermodynamic phase (liquid or ice) dictate the amount of heavy isotopes that enter the cloud condensate.Water isotopes within mixed-phase clouds were measured in-situ during the CAESAR 2024 (Cold Air Outbreak Experiment in the Sub-Arctic Region) airborne field campaign. We first provide an overview of stable water isotopes and their dependence on environmental conditions. Then, we present the use of isotopic measurements to identify vapor deposition, riming, and WBF growth conditions inside mixed-phase clouds from CAESAR. A suite of in-situ cloud probes (PHIPS, HOLODEC, and Optical Array Probes) is used to validate the results of the isotopic analysis. This observational technique can be leveraged to study each ice growth mechanism’s influence on cloud properties.

A THz radar, with its wide bandwidth, is capable of high‐resolution imaging down to the centimeter scale. In this study, a THz radar is applied to detect hydrometeors generated in a spray chamber. The observed backscattering signals show fluctuations at centimeter scales, indicating various hydrometeor distribution patterns along the radar beam. A co‐located High‐Speed Imaging (HSI) sensor is used to measure the Drop Size Distributions (DSD) in the spray chamber. The radar sampling beam is well aligned with the HSI probes, allowing an objective comparison between the remote sensing and in situ observations. In this study, the observed radar power is compared with the power estimated from the HSI measurements. Results show great consistency, with power difference smaller than 0.5 dB. This study demonstrates the feasibility and great potential of using a THz radar for ultra‐high‐resolution observations of clouds in a laboratory facility, and in the real atmosphere.

From the molecular dynamics of water molecules to the global processes that control atmospheric circulation: describing the evolution of atmospheric clouds necessitates physics that spans vast spatial scales. In this work, steps are taken towards connecting the molecular physics of ice-nucleation to the growth and subsequent freezing of cloud droplets in convective cumulus clouds. Motivated by experimental evidence of ice nucleation triggered by agitation and distortion of a water droplet surface, the first study in this dissertation uses molecular dynamics simulations to demonstrate that negative Laplace pressure from a curved water surface leads to heterogeneous ice nucleation at higher temperatures. A linear approximation, dependent on latent heat and the molar volume difference between liquid and ice, provides a reasonable estimate of the increase in freezing temperature with decreasing pressure. In the atmosphere, agitation and curvature of water surfaces is likely to occur when cloud droplets reach large enough sizes to experience droplet collisions or breakup. The second study in this dissertation uses in-situ measurements of developing cumulus to address the prominent theory that mixing of cloudy air with dry air at cloud edges produces larger droplets than those found in the undiluted cloud core. Droplet size distributions measured by the Holographic Detector for Clouds (HOLODEC) during the Secondary Production of Ice in Cumulus Experiment (SPICULE) show that diluted cloud edges consistently contain droplets biased towards larger diameters compared to droplets in the adjacent undiluted updraft. Further investigation is required to identify the physical mechanisms responsible for these observed trends.