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Cloud microphysical processes, such as droplet activation, condensational growth, and collisional growth, play a central role in the evolution of clouds and precipitation. Accurate representations of these processes in numerical models are challenging partially due to incomplete understanding of them at the process level arising from limited systematic observations. Most surface-based active remote sensors, including today's operational cloud radars and lidars, have a resolution on the order of tens of meters. This resolution is insufficient to resolve cloud microphysical processes that manifest at finer (meter and submeter) scales. A new set of ultra-high-resolution ground-based radar and lidar systems have been developed to address this observational gap. The newly developed 94-GHz cloud radar has a range resolution down to 2.8 m, or a factor of 10 finer than typical radars, using a large bandwidth and quadratic phase coding techniques. The lidar has a range resolution down to 10 cm, or a factor of 100 finer than typical lidars, using a time-gated time-correlated single-photon-counting technique. Such high-resolution observations were previously only achievable through in situ aircraft measurements. Even then, aircraft measurements do not permit continuous long-term cloud observation as is possible with ground- based remote sensing instruments. In this study, the first-light cloud observations from the new radar and lidar systems are shown to reveal detailed cloud structures that conventional sensors could only perceive in a bulk sense, thus providing new avenues to investigate cloud microphysical processes and their impact on weather and climate. SIGNIFICANCE STATEMENT: In this study, we introduced two novel remote sensing instruments capable of high-resolution cloud observations: a cloud radar system with a range resolution down to 2.8 m and a lidar system with a range resolution down to 10 cm. Observations show that the radar and lidar systems resolve detailed cloud structures and microphysical processes which would not be obtained from traditional remote sensing systems. The high-resolution measurements provide insights toward the understanding of cloud microphysics at a fundamental level. KEYWORDS: Cloud microphysics; Clouds; Lidars/Lidar observations; Radars/Radar observations; Remote sensing, 1. Introduction Clouds cover approximately two-thirds of Earth's surface at scales ranging from tens of meters to thousands of kilometers. Contrastingly, the fundamental physical mechanisms that control cloud formation and [...]

Marine cloud brightening (MCB) is the deliberate injection of aerosol particles into shallow marine clouds to increase their reflection of solar radiation and reduce the amount of energy absorbed by the climate system. From the physical science perspective, the consensus of a broad international group of scientists is that the viability of MCB will ultimately depend on whether observations and models can robustly assess the scale-up of local-to-global brightening in todays climate and identify strategies that will ensure an equitable geographical distribution of the benefits and risks associated with projected regional changes in temperature and precipitation. To address the physical science knowledge gaps required to assess the societal implications of MCB, we propose a substantial and targeted program of research-field and laboratory experiments, monitoring, and numerical modeling across a range of scales.

Abstract: We report on a multiyear set of airborne field campaigns (2005–16) off the California coast to examine aerosols, clouds, and meteorology, and how lessons learned tie into the upcoming NASA Earth Venture Suborbital (EVS-3) campaign: Aerosol Cloud meTeorology Interactions oVer the western ATlantic Experiment (ACTIVATE; 2019–23). The largest uncertainty in estimating global anthropogenic radiative forcing is associated with the interactions of aerosol particles with clouds, which stems from the variability of cloud systems and the multiple feedbacks that affect and hamper efforts to ascribe changes in cloud properties to aerosol perturbations. While past campaigns have been limited in flight hours and the ability to fly in and around clouds, efforts sponsored by the Office of Naval Research have resulted in 113 single aircraft flights (>500 flight hours) in a fixed region with warm marine boundary layer clouds. All flights used nearly the same payload of instruments on a Twin Otter to fly below, in, and above clouds, producing an unprecedented dataset. We provide here i) an overview of statistics of aerosol, cloud, and meteorological conditions encountered in those campaigns and ii) quantification of model-relevant metrics associated with aerosol–cloud interactions leveraging the high data volume and statistics. Based on lessons learned from those flights, we describe the pragmatic innovation in sampling strategy (dual-aircraft approach with combined in situ and remote sensing) that will be used in ACTIVATE to generate a dataset that can advance scientific understanding and improve physical parameterizations for Earth system and weather forecasting models, and for assessing next-generation remote sensing retrieval algorithms.

In the decade before the U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) facility was established in 1992, early modern climate models exhibited large differences in the climate response [...]

A continuous decrease in aerosols over the Western North Atlantic Ocean (WNAO) on a decadal timescale provides a long‐term benchmark to evaluate how various natural and anthropogenic processes affect the manifestation of aerosol‐cloud interactions in this region. Furthermore, the WNAO serves as a natural laboratory with diverse aerosol sources, marine boundary layer clouds more variable than those in marine stratocumulus deck regions, and unique flow regimes established by the Gulf Stream and the semi‐permanent Bermuda High. We investigate how satellite‐retrieved macrophysical and microphysical properties of low clouds and the surface shortwave irradiance changed from 2003 to 2020, in tandem with this aerosol decrease. The decadal changes in large‐scale meteorology related to the North Atlantic Oscillation (NAO) are also examined. We find a reduction in low‐cloud optical thickness, accompanied by fewer and larger cloud droplets, yet observe no significant changes in low‐cloud fraction and liquid water path. Despite the reduction in low‐cloud optical thickness together with aerosol decrease, a corresponding increase in the trends of surface shortwave irradiance, also known as surface brightening, is lacking. This absence of brightening is potentially related to concomitant changes found in large‐scale meteorology associated with NAO—Bermuda High strengthening, sea surface warming, and atmospheric moistening— as well as an increase in high‐level cloud fraction that can counteract the surface brightening. Ultimately, our findings suggest that spatial patterns of decadal meteorological variability introduce complexities in the surface cloud radiative effect over the WNAO, thereby complicating the isolation and examination of aerosol‐cloud interactions. Plain Language Summary: Over recent decades, the Western North Atlantic Ocean has experienced a steady decrease in aerosols—tiny particles in the air. This situation offers a chance to study how natural phenomena and human activities influence cloud‐aerosol interactions. The area is known for its diverse aerosol sources, complex cloud types, and distinct oceanic and atmospheric flow patterns influenced by the Gulf Stream and Bermuda High. By analyzing satellite data from 2003 to 2020, we investigated how low‐lying clouds and sunlight penetration have changed in response to fewer aerosols, and how the North Atlantic Oscillation (NAO) affects weather patterns. Our findings show that while low‐lying clouds have become more transparent with fewer, larger droplets, their coverage and juiciness have not changed significantly. Contrary to expectations, these changes have not lead to an increase in sunlight reaching the Earth's surface, a phenomenon known as "surface brightening." This could be due to NAO‐related warmer sea surfaces, increased moisture, and more high‐level clouds, which might obscure any brightening. Our study shows that weather shifts in the Western North Atlantic Ocean add complexity to understanding how clouds influence sunlight, challenging the study of aerosol‐cloud interactions. Key Points: In tandem with the continuous decrease in aerosol, low‐cloud droplets became fewer and larger over the western North AtlanticDespite concurrent changes in aerosol and low‐cloud optical properties, surface shortwave irradiance lacked reciprocal significant trendsMeteorological fields—temperature and humidity—correlated with the North Atlantic Oscillation had changed, exhibiting tripolar patterns [ABSTRACT FROM AUTHOR]

Surface-based measurements of broadband shortwave (solar) and longwave (infrared) radiative fluxes using thermopile radiometers are made regularly around the globe for scientific and operational environmental monitoring. The occurrence of ice on sensor windows in cold environments - whether snow, rime, or frost - is a common problem that is difficult to prevent as well as difficult to correct in post-processing. The Baseline Surface Radiation Network (BSRN) community recognizes radiometer icing as a major outstanding measurement uncertainty. Towards constraining this uncertainty, the De-Icing Comparison Experiment (D-ICE) was carried out at the NOAA Atmospheric Baseline Observatory in Utqiaġvik (formerly Barrow), Alaska, from August 2017 to July 2018. The purpose of D-ICE was to evaluate existing ventilation and heating technologies developed to mitigate radiometer icing. D-ICE consisted of 20 pyranometers and 5 pyrgeometers operating in various ventilator housings alongside operational systems that are part of NOAA's Barrow BSRN station and the U.S. Dept. of Energy Atmospheric Radiation Measurement (ARM) Program North Slope of Alaska and Oliktok Point observatories. To detect icing, radiometers were monitored continuously using cameras, with a total of more than 1 million images of radiometer domes archived. Ventilator and ventilator/heater performance overall was skilful with the average of the systems mitigating 77 % of icing and many being 90+ % effective. Ventilators without heating elements were also effective and capable of providing heat through roughly equal contributions of waste energy from the ventilator fan and adiabatic heating downstream of the fan. This provided ~ 0.6 C of warming, enough to subsaturate the air up to a relative humidity (w.r.t. ice) of ~ 105 %. Because the mitigation technologies performed well, a near complete record of verified ice-free radiometric fluxes were assembled for the duration of the campaign. This well-characterized data set is suitable for model evaluation, in particular for the Year of Polar Prediction (YOPP) first Special Observing Period (SOP1). We used the data set to calculate short- and long-term biases in iced sensors, finding that biases can be up to +60 W m-2 (longwave) and -211 to +188 W m-2 (shortwave). However, because of the frequency of icing, mitigation of ice by ventilators, cloud conditions, and the timing of icing relative to available sunlight, the biases in the monthly means were generally less than the aggregate uncertainty attributed to other conventional sources. [ABSTRACT FROM AUTHOR]

The rapidly-warming Arctic is sensitive to perturbations in the surface energy budget, which can be caused by clouds and aerosols. However, the interactions between clouds and aerosols are poorly quantified in the Arctic, in part due to: (1) limited observations of vertical structure of aerosols relative to clouds and (2) ground-based observations often being inadequate for assessing aerosol impacts on cloud formation in the characteristically stratified Arctic atmosphere. Here, we present a novel evaluation of Arctic aerosol vertical distributions using almost 3 years' worth of tethered balloon system (TBS) measurements spanning multiple seasons. The TBS was deployed at the U.S. Department of Energy Atmospheric Radiation Measurement Program's facility at Oliktok Point, Alaska. Aerosols were examined in tandem with atmospheric stability and ground-based remote sensing of cloud macrophysical properties to specifically address the representativeness of near-surface aerosols to those at cloud base. Based on a statistical analysis of the TBS profiles, ground-based aerosol number concentrations were unequal to those at cloud base 86 % of the time. Intermittent aerosol layers were observed 63 % of the time due to poorly mixed below-cloud environments, mostly in the spring, causing a decoupling of the surface from the cloud layer. A uniform distribution of aerosol below cloud was observed only 14 % of the time due to a well-mixed below-cloud environment, mostly during the fall. The equivalent potential temperature profiles of the below-cloud environment reflected the aerosol profile 89 % of the time whereby a mixed or stratified below-cloud environment was observed during a uniform or layered aerosol profile, respectively. In general, a combination of aerosol sources, thermodynamic structure, and wet removal processes from clouds and precipitation likely played a key role in establishing observed aerosol vertical structure. Results such as these could be used to improve future parameterizations of aerosols and their impacts on Arctic cloud formation and radiative properties. [ABSTRACT FROM AUTHOR]