Physical science research needed to evaluate the viability and risks of marine cloud brightening.

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 today's 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.

Cloud microphysical response to entrainment and mixing is locally inhomogeneous and globally homogeneous: Evidence from the lab.

Entrainment of dry air into clouds strongly influences cloud optical and precipitation properties and the response of clouds to aerosol perturbations. The response of cloud droplet size distributions to entrainment-mixing is examined in the Pi convection-cloud chamber that creates a turbulent, steady-state cloud. The experiments are conducted by injecting dry air with temperature (Te) and flow rate (Qe) through a flange in the top boundary, into the otherwise well-mixed cloud, to mimic the entrainment-mixing process. Due to the large-scale circulation, the downwind region is directly affected by entrained dry air, whereas the upwind region is representative of the background conditions. Droplet concentration (Cn) and liquid water content (L) decrease in the downwind region, but the difference in the mean diameter of droplets (Dm) is small. The shape of cloud droplet size distributions relative to the injection point is unchanged, to within statistical uncertainty, resulting in a signature of inhomogeneous mixing, as expected for droplet evaporation times small compared to mixing time scales. As Te and Qe of entrained air increase, however, Cn, L, and Dm of the whole cloud system decrease, resulting in a signature of homogeneous mixing. The apparent contradiction is understood as the cloud microphysical responses to entrainment and mixing differing on local and global scales: locally inhomogeneous and globally homogeneous. This implies that global versus local sampling of clouds can lead to seemingly contradictory results for mixing, which informs the long-standing debate about the microphysical response to entrainment and the parameterization of this process for coarse-resolution models.

The role of turbulent fluctuations in aerosol activation and cloud formation.

Aerosol indirect effects are one of the leading contributors to cloud radiative properties relevant to climate. Aerosol particles become cloud droplets when the ambient relative humidity (saturation ratio) exceeds a critical value, which depends on the particle size and chemical composition. In the traditional formulation of this problem, only average, uniform saturation ratios are considered. Using experiments and theory, we examine the effects of fluctuations, produced by turbulence. Our measurements, from a multiphase, turbulent cloud chamber, show a clear transition from a regime in which the mean saturation ratio dominates to one in which the fluctuations determine cloud properties. The laboratory measurements demonstrate cloud formation in mean-subsaturated conditions (i.e., relative humidity <100%) in the fluctuation-dominant activation regime. The theoretical framework developed to interpret these measurements predicts a transition from a mean- to a fluctuation-dominated regime, based on the relative values of the mean and standard deviation of the environmental saturation ratio and the critical saturation ratio at which aerosol particles activate or become droplets. The theory is similar to the concept of stochastic condensation and can be used in the context of the atmosphere to explore the conditions under which droplet activation is driven by fluctuations as opposed to mean supersaturation. It provides a basis for future development of cloud droplet activation parameterizations that go beyond the internally homogeneous parcel calculations that have been used in the past.

Hierarchical Metal-Organic Frameworks with Macroporosity: Synthesis, Achievements, and Challenges.

Introduction of multiple pore size regimes into metal-organic frameworks (MOFs) to form hierarchical porous structures can lead to improved performance of the material in various applications. In many cases, where interactions with bulky molecules are involved, enlarging the pore size of typically microporous MOF adsorbents or MOF catalysts is crucial for enhancing both mass transfer and molecular accessibility. In this review, we examine the range of synthetic strategies which have been reported thus far to prepare hierarchical MOFs or MOF composites with added macroporosity. These fabrication techniques can be either pre- or post-synthetic and include using hard or soft structural template agents, defect formation, routes involving supercritical CO2, and 3D printing. We also discuss potential applications and some of the challenges involved with current techniques, which must be addressed if any of these approaches are to be taken forward for industrial applications.

Can Hail and Rain Nucleate Cloud Droplets?

We present results from moist convection in a mixture of pressurized sulfur hexafluoride (liquid and vapor), and helium (gas) to model the wet and dry components of the Earth's atmosphere. To allow for homogeneous nucleation, we operate the experiment close to critical conditions. We report on the nucleation of microdroplets in the wake of large cold liquid drops falling through the supersaturated atmosphere and show that the homogeneous nucleation is caused by isobaric cooling of the saturated sulfur hexafluoride vapor. Our results carry over to atmospheric clouds: falling hail and cold rain drops may enhance the heterogeneous nucleation of microdroplets in their wake under supersaturated atmospheric conditions. We also observed that under appropriate circumstances settling microdroplets form a rather stable horizontal cloud layer, which separates regions of super- and subcritical saturation.

Autonomic modulations during 5 hours at 4574 m (15,000 ft) breathing 40% oxygen.

INTRODUCTION: This study was done to evaluate the effects of exposure to hyperoxic hypobaria on the autonomic reactivity and hence G tolerance of high-performance aircraft aircrews. METHODS: Heart rate variability (HRV) indices and traditional measures of autonomic functions were measured during normobaric baseline, hourly at hyperoxic hypobaria for 5 h, and during normobaric recovery in 15 subjects at an equivalent altitude of 4574 m breathing 40% oxygen. RESULTS: Supine and resting-sitting R-R interval, E:I ratio, and time domain indices of HRV increased significantly, with no change in Valsalva ratio. Power in both low- (LF) and high-frequency (HF) bands of HRV spectra increased significantly, which persisted even after normalization with mean R-R2 (LF 1177 +/- 801 at baseline to 1837 +/- 1032 at 5 h and HF 944 +/- 683 at baseline to 1545 +/- 1095 at 5 h). Pressor response to handgrip was attenuated, as evident from a significant interaction between the effects of handgrip and hypobaria. DISCUSSION: Increases in R-R interval, E:I ratio, and all the time domain indices of HRV and HF power suggest an increase in parasympathetic influences. Increase in LF power is explained by the ability of the parasympathetic system to modulate the HRV spectrum in regions lower than respiratory frequency. Attenuation of pressor response to isometric handgrip contraction could have resulted from an increase in the transmural pressure gradient across the carotid sinuses due to hypobaria, which can adversely affect the anti-G straining maneuver.