Ilan Koren

• PhD
• Professor at Weizmann Institute of Science

Cloud droplet mobility is referred to here as a measure of the droplets’ ability to move with ambient air. We claim that an important part of the aerosol effect on convective clouds is driven by changes in droplet mobility. We show that the mass-weighted average droplet terminal velocity, defined here as the ‘effective terminal velocity’(η) and its...

Among all cloud-aerosol interactions, the invigoration effect is the most elusive. Most of the studies that do suggest this effect link it to deep convective clouds with a warm base and cold top. Here, we provide evidence from observations and numerical modeling of a dramatic aerosol effect on warm clouds. We propose that convective-cloud invigorat...

Warm rain production as the end result of all clouds' processes is highly affected by aerosol loading and properties. Here an axisymmetric bin microphysics cloud model is used to study the aerosol's effects on the competition and synergy between processes in a single convective cloud, to provide a baseline for studies of aerosol effects on cloud fi...

The cloud invigoration effect refers here to the link between an increase in aerosol loading and deepening of convective clouds. The effect can be reflected also in a larger cloud fraction and an increase in the condensate mass that is distributed higher in the atmospheric column. Identifying the invigoration effect by aerosols requires attributing...

Phytoplankton blooms are ephemeral events of exceptionally high primary productivity that regulate the flux of carbon across marine food webs [1, 2 and 3]. Quantification of bloom turnover [4] is limited by a fundamental difficulty to decouple between physical and biological processes as observed by ocean color satellite data. This limitation hinde...

The prediction of climate has been a long-standing problem in contemporary science. One of the reasons stems from a gap in the ability to obtain 3D mapping of clouds, especially shallow scattered clouds. These clouds are strongly affected by mixing processes with their surroundings, rendering their internal volumetric structure highly heterogeneous...

Shallow, sparse, non-precipitating convective clouds forming over the ocean are considered among the least organized cloud fields. The formation mechanism of these clouds is associated with random, local perturbations that create buoyant parcels. Their sparseness suggests no or very weak interactions between clouds. Here, we show that such clouds f...

Plain Language Summary Glories appear as colored rings around the perfect backscatter angle. The pattern can often be seen in mountains where the sun is behind the observer, projecting the observer's shadow onto a cloud. Faint glories can be found in non‐polarized satellite images, forming over marine Stratocumulus clouds. We can translate the opti...

This study employs a high-resolution (10m) System for Atmospheric Modeling (SAM) coupled with the Spectral Bin Microphysical (SBM) scheme to thoroughly investigate the processes governing the evolution of aerosol properties within and outside a shallow cumulus cloud. The model encompasses the complete life cycle of cloud droplets, starting from the...

Cloud microphysics studies include how tiny cloud droplets grow and become rain. This is crucial for understanding cloud properties like size, life span, and impact on climate through radiative effects. Small weak-updraft clouds near the haze-to-cloud transition are especially difficult to measure and understand. They are abundant but hard to captu...

Inverse problems in scientific imaging often seek physical characterization of heterogeneous scene materials. The scene is thus represented by physical quantities, such as the density and sizes of particles (microphysics) across a domain. Moreover, the forward image formation model is physical. An important case is that of clouds, where microphysic...

Aerosols and clouds are key components of the marine atmosphere, impacting the Earth’s radiative budget with a net cooling effect over the industrial era that counterbalances greenhouse gas warming, yet with an uncertain amplitude. Here we report recent advances in our understanding of how open ocean aerosol sources are modulated by ocean biogeoche...

Shallow convective clouds play a crucial role in Earth's energy budget, as they modulate the radiative transfer in the atmosphere and participate in the vertical transport of aerosols, energy, and humidity. The parameterizations representing these complex, vital players in weather and climate models are mostly based on a description of steady‐state...

The cloud responses to global warming are captured in various global climate models with distinct inferences on changes in cloud vertical structure as function of surface warming. However, long term observational evidences are scarce to validate the model outputs. Here, we have studied the changes in radiosonde derived cloud macro-physical properti...

Plain Language Summary The spatial arrangement of clouds within a cloud field plays a significant role in influencing the field's radiative effects, cloud properties, and precipitation patterns. Thus, cloud organization, determined by both cloud spacing and sizes, is a critical aspect to consider. Here, we introduce a novel 2D method, termed the Lv...

This study employs a high-resolution (10m) System for Atmospheric Modeling (SAM) coupled with the Spectral Bin Microphysical (SBM) scheme to thoroughly investigate the processes governing the evolution of aerosol properties within and outside a shallow cumulus cloud. The model encompasses the complete life cycle of cloud droplets, starting from the...

Clouds microphysics describes the formation and evolution of cloud droplets, rain, and ice particles. It is among the most critical factors in determining the cloud's size, lifetime, precipitation, and radiative effect. Among all cloud types, the small clouds, characterized by weak updrafts, that are close to the haze-to-cloud transition pose chall...

Ubiquitous, yet elusive to a complete understanding: Tiny, warm clouds with faint visual signatures play a critical role in Earth's energy balance. These "twilight clouds", as they are sometimes called, form under weak updraft conditions. Their constituent particles exist in a precarious state, teetering between hazy wisps and activated droplets. T...

Ocean microbes are involved in global processes such as nutrient and carbon cycling. Recent studies indicated diverse modes of algal–bacterial interactions, including mutualism and pathogenicity, which have a substantial impact on ecology and oceanic carbon sequestration, and hence, on climate. However, the airborne dispersal and pathogenicity of b...

Understanding the nature of mixing between cloudy air and its surroundings is an important and yet, open question. In this research, we use high‐resolution (10 m) bin‐microphysics Large Eddy Simulation of a cumulus cloud, together with a Lagrangian passive tracer tracking method, to study mixing. We analyze the passive tracers as a function of thei...

Novel tomographic principles yield 3D atmospheric fields using multi-view imagery. Turbulence strength is mapped by observing scintillation of bulbs. Extinction in clouds is mapped volumetrically from polarimetric cameras onboard a satellite formation.

Detection of trends in cloud properties and radiative effects is crucial for understanding the climate and predicting its future state. The El Niño-Southern Oscillation (ENSO), a climate phenomenon affecting temperature and clouds globally, can obscure the detection of cloud feedback to global warming. Therefore, accurately decoupling ENSO's signat...

Shallow cloud fields exhibit different patterns, such as closed or open hexagonal cells and cloud streets. These patterns play a key role in determining the cloud fields’ radiative effects, thereby affecting the climate. Here, we show that a large subset of shallow cloud fields forms organized, mesoscale-sized, regular patterns that persist for ext...

Obtaining the response of cloud top temperature (CTT) to global warming correctly is crucial for understanding the current and future energy budget of the climate system. For a given cloud fraction, colder CTT implies more longwave radiation being capped within the Earth-atmosphere system, consequently heating it. Current climate models predict an...

Understanding the nature of mixing between cloudy air and its surroundings is an important and yet, open question. In this research, we use high-resolution (10m) bin-microphysics LES of a cumulus cloud, together with a Lagrangian passive tracer tracking method, to study mixing. We analyze the passive tracers as a function of their trajectories and...

Clouds play a key role in Earth's energy budget and water cycle. Their response to global warming contributes the largest uncertainty to climate prediction. Here, by performing an empirical orthogonal function analysis on 42 years of reanalysis data of global cloud coverage, we extract an unambiguous trend and El-Niño–Southern-Oscillation-associate...

The Tara Pacific expedition (2016–2018) sampled coral ecosystems around 32 islands in the Pacific Ocean and the ocean surface waters at 249 locations, resulting in the collection of nearly 58 000 samples. The expedition was designed to systematically study warm-water coral reefs and included the collection of corals, fish, plankton, and seawater sa...

The emergence of organized multiscale patterns resulting from convection is ubiquitous, observed throughout different cloud types. The reproduction of such patterns by general circulation models remains a challenge due to the complex nature of clouds, characterized by processes interacting over a wide range of spatio-temporal scales. The new advanc...

Record-breaking statistics are combined here with a geographic mode of exploration to introduce a record-breaking map. We examine time series of sea surface temperature (SST) values and show that high SST records have been broken far more frequently than the expected rate for a trend-free random variable (TFRV) over the vast majority of oceans (83...

Clouds play a key role in Earth's energy budget and water cycle. Their response to global warming contributes the largest uncertainty to climate prediction. Here, by performing an empirical orthogonal function analysis on 42 years of reanalysis data of global cloud coverage, we extract clear trend and ENSO-associated modes. The trend mode translate...

Nonlinear time delay systems produce inherently delay-induced periodic oscillations, which are, however, too idealistic compared to observations. We exhibit a unified stochastic framework to systematically rectify such oscillations into oscillatory patterns with enriched temporal variabilities through generic, nonlinear responses to stochastic pert...

Glory is a beautiful optical phenomenon observed in an atmosphere as concentric colored rings reflected by clouds or fog around an antisolar point. Here we report that true color glories, although faint, are discernible in raw unpolarized satellite images by a naked eye on a daily basis, thus constituting a large and untapped reservoir of cloud dat...

Record-breaking statistics are combined here with geographic mode of exploration to construct a new object: a record-breaking map. Such maps are shown here to reveal surprisingly robust statistical results and spatial content. Specifically, we examine a time series of sea surface temperature (SST) values and show that high SST records have been bro...

The Tara Pacific expedition (2016-2018) sampled coral ecosystems around 32 islands in the Pacific Ocean and the ocean surface waters at 249 locations, resulting in the collection of nearly 58,000 samples. The expedition was designed to systematically study warm coral reefs and included the collection of corals, fish, plankton, and seawater samples...

The diversity of microbes and their transmission between ocean and atmosphere are poorly understood despite the implications for microbial global dispersion and biogeochemical processes. Here, we survey the genetic diversity of airborne and surface ocean bacterial communities sampled during springtime transects across the northwest Pacific and subt...

One of the major sources of uncertainty in climate prediction results from the limitations in representing shallow cumulus (Cu) in models. Recently, a class of continental shallow convective Cu was shown to share distinct morphological properties and to emerge globally mostly over forests and vegetated areas, thus named greenCu. Using machine‐learn...

In this work, we analyze time series of lightning strokes as detected by an LF/VLF network, over the Eastern Mediterranean during winter storms. The strokes' raw data is examined without pre-grouping it into flashes. A distance-versus-time differential space (termed dRvsdT) is introduced, examining the intervals between successive strokes. While it...

The dynamic structure of a small trade wind cumulus (Cu) is analyzed using a novel approach. Cu developing in a shear-free environment is simulated by 10-m-resolution LES model with spectral bin microphysics. The aim is to clarify the dynamical nature of cloud updraft zone (CUZ) including entrainment and mixing in growing Cu. The validity of concep...

We introduce a comprehensive method for space-borne 3D volumetric scattering-tomography of cloud micro-physics, developed for the CloudCT mission. The retrieved micro-physical properties are the liquid-water-content and effective droplet radius within a cloud. We include a model for a perspective polarization imager, and an assumption of 3D variati...

Open cloud cells can be described in ideal form as connected clouds that surround spots of isolated clear skies in their centers. This cloud pattern is typically associated with marine stratocumulus (MSc) that form in the oceanic boundary layer. However, it can form in deeper convective clouds as well. Here, we focus on deep-open-cells (with tops r...

Aerosols play a significant role in regional scale pollution that alters the cloud formation process, radiation budget, and climate. Here, using long-term (2003-2019) observations from multi-satellite and ground-based remote sensors, we show robust aerosol-induced instantaneous daytime lower tropospheric cooling during the pre-monsoon season over t...

SubLCL clouds are defined here as clouds that form below the estimated lifting condensation level (LCL), on days that are predicted to be cloud‐free. On more than 50% of those days we observed clouds. Measurements of thermodynamic and sky conditions are used here together with numerical simulations to study subLCL clouds’ formation. It was previous...

Shallow convective clouds are important players in Earth’s energy budget and hydrological cycle, and are abundant in the tropical and subtropical belts. They greatly contribute to the uncertainty in climate predictions due to their unresolved, complex processes that include coupling between the dynamics and microphysics. Analysis of cloud structure...

The process of mixing in warm convective clouds and its effects on microphysics are crucial for an accurate description of cloud fields, weather, and climate. Still, they remain open questions in the field of cloud physics. Adiabatic regions in the cloud could be considered non-mixed areas and therefore serve as an important reference to mixing. Fo...

Formation flying enables innovative Earth observation missions. Synchronised images from several viewpoints can be captured with a fleet of satellites in close formation. CloudCT is implementing such a mission to observe fields of warm, small clouds. These images are post-processed to retrieve their 3D micro-physical properties using computer tomog...

Sea spray aerosol (SSA) formation have a major role in the climate system, but measurements at a global-scale of this micro-scale process are highly challenging. We measured high-resolution temporal patterns of SSA number concentration over the Atlantic Ocean, Caribbean Sea, and the Pacific Ocean covering over 42,000 km. We discovered a ubiquitous...

Glaciation in clouds is a fundamental phenomenon in determining Earth’s radiation fluxes, sensible and latent heat budgets in the atmosphere, the water cycle, cloud development and lifetime. Nevertheless, the main mechanisms that govern the temperature of glaciation in clouds have not been fully identified. Here we present an analysis of 15 years (...

A subset of continental shallow convective cumulus (Cu) cloud fields has been shown to have distinct spatial properties and to form mostly over forests and vegetated areas, thus referred to as “green Cu” . Green Cu fields are known to form organized mesoscale patterns, yet the underlying mechanisms, as well as the time variability of these patterns...

Microbes play essential roles in biogeochemical processes in the oceans and atmosphere. Studying the interplay between these two ecosystems can provide important insights into microbial biogeography and diversity. We simultaneously mapped the microbial diversity of airborne and marine bacterial communities across 15,000 kilometers in the Atlantic a...

The process of mixing in warm convective clouds and its effects on microphysics, is crucial for an accurate description of cloud fields, weather, and climate. Still, it remains an open question in the field of cloud physics. Adiabatic regions in the cloud could be considered as non-mixed areas and therefore serve as an important reference to mixing...

Smoke alarm Intense, widespread bushfires in Australia injected huge amounts of smoke into the stratosphere in 2020. Hirsch and Koren found that this smoke caused record-breaking levels of aerosols over the Southern Hemisphere, as much as that from a moderate volcanic eruption. The severity was caused by a combination of the vigor of the fires and...

A Correction to this paper has been published: https://doi.org/10.1038/s43247-021-00119-5

Atmospheric motions in clouds and cloud surrounding have a wide range of scales, from several kilometers to centimeters. These motions have different impacts on cloud dynamics and microphysics. Larger-scale motions (hereafter referred to as convective motions) are responsible for mass transport over distances comparable with cloud scale, while moti...

A subset of continental shallow convective Cumulus (Cu) cloud fields were shown to have unique spatial properties and to form mostly over forests and vegetated areas, thus referred to as green Cu. Green Cu fields are known to form organized mesoscale patterns, yet the underlying mechanisms as well as the time variability of these patterns are still...

Anthropogenic pollution from marine microplastic particles is a growing concern, both as a source of toxic compounds, and because they can transport pathogens and other pollutants. Airborne microplastic particles were previously observed over terrestrial and coastal locations, but not in the remote ocean. Here, we collected ambient aerosol samples...

Aerosol size distribution has major effects on warm cloud processes. Here, we use newly acquired marine aerosol size distributions (MSDs), measured in situ over the open ocean during the Tara Pacific expedition (2016–2018), to examine how the total aerosol concentration (Ntot) and the shape of the MSDs change warm clouds' properties. For this, we u...

Clouds play a key role in Earth’s radiation budget, covering more than 50% of the planet. However, the binary delineation of cloudy and clear sky is not clearly defined due to the presence of a transitionary zone, known as the cloud twilight zone, consisting of liquid droplets and humidified to dry aerosols. The twilight zone is an inherent compone...

Warm convective clouds play a significant role in the earth's energy and water budgets. However, they still pose a challenge in climate research as their feedback to predicted thermodynamic changes is highly uncertain and considered critical to the overall climate system's response. The focus of this study is continental, organized shallow convecti...

Ocean-atmosphere interactions such as sea spray aerosol (SSA) formation have a major role in the climate system, but a global-scale assessment of this micro-scale process is highly challenging. We measured high-resolution temporal patterns of SSA number concentration over the Atlantic Ocean, Caribbean Sea, and the Pacific Ocean covering 42,000 km o...

Aerosol size distribution has major effects on warm cloud processes. Here, we use newly acquired marine aerosol size distributions (MSD), measured in-situ over the open ocean during the Tara Pacific expedition (2016–2018), to examine how the total aerosol concentration (Ntot) and the shape of the MSD change warm cloud's properties. For this, we use...

By means of Galerkin–Koornwinder (GK) approximations, an efficient reduction approach to the Stuart–Landau (SL) normal form and center manifold is presented for a broad class of nonlinear systems of delay differential equations that covers the cases of discrete as well as distributed delays. The focus is on the Hopf bifurcation as a consequence of...

Marine aerosols play a significant role in the global radiative budget, in clouds’ processes, and in the chemistry of the marine atmosphere. There is a critical need to better understand their production mechanisms, composition, chemical properties, and the contribution of ocean-derived biogenic matter to their mass and number concentration. Here w...

Interactions between the ocean and the atmosphere occur at the air-sea interface through the transfer of momentum, heat, gases and particulate matter, and through the impact of the upper-ocean biology on the composition and radiative properties of this boundary layer. The Tara Pacific expedition, launched in May 2016 aboard the schooner Tara, was a...

The effects of aerosol on warm convective cloud cores are evaluated using single cloud and cloud field simulations. Three core definitions are examined: positive vertical velocity (Wcore), supersaturation (RHcore), and positive buoyancy (Bcore). As presented in Part 1 (Heiblum et al., 2019), the property Bcore⊆RHcore⊆Wcore is seen during growth of...

The properties of a warm convective cloud are determined by the competition between the growth and dissipation processes occurring within it. One way to observe and follow this competition is by partitioning the cloud to core and margin regions. Here we look at three core definitions, namely positive vertical velocity (Wcore), supersaturation (RHco...

Aerosol effects on convective clouds and associated precipitation constitute an important open-ended question in climate research. Previous studies have linked an increase in aerosol concentration to a delay in the onset of rain, invigorated clouds and stronger rain rates. Here, using observational data, we show that the aerosol effect on convectiv...

Shallow convection is a subgrid process in cloud-resolving models for which their grid box is larger than the size of small cumulus clouds (Cu). At the same time such Cu substantially affect radiation properties and thermodynamic parameters of the low atmosphere. The main microphysical parameters used for calculation of radiative properties of Cu i...

Earth observing systems have proven to be a unique source of long-term synoptic information on numerous physical, chemical and biological parameters on a global scale. Merging this information for integrated studies that peruse key questions about the ocean-atmosphere interface is, however, very challenging. Such studies require interdisciplinary f...

Clouds control much of the Earth's energy and water budgets. Aerosols, suspended in the atmosphere, interact with clouds and affect their properties. Recent studies have suggested that the aerosol effect on warm convective cloud systems evolve in time and eventually approach a steady state for which the overall effects of aerosols can be considered...

Physical insights into processes governing temporal organization and evolution of cloud-fields are of great importance for climate research. Here, using large eddy simulations with a bin- microphysics scheme, we show that warm convective cloud fields exhibit oscillations with two distinct periods (~10 and ~90 min, for the case studied here). The sh...

The properties of a warm convective cloud are determined by the competition between the growth and dissipation processes occurring within it. One way to observe and follow this competition is by partitioning the cloud to core and margin regions. Here we look at three core definitions: positive vertical velocity (Wcore), supersaturation (RHcore), an...

The effects of aerosol on warm convective cloud cores are evaluated using single cloud and cloud field simulations. As presented in Part I, the Bcore⊆RHcore⊆Wcore property is seen during growth of warm convective clouds. We show that this property is kept irrespective of aerosol concentration. During dissipation core fractions gen...

An understanding of sea spray aerosol (SSA) production is needed to better assess its influence on climate. Using satellite data, we investigated the production of the coarse mode of aerosol optical depth (AODc), a proxy for SSA, over the pristine South Pacific Gyre. The analysis was done on three time scales: daily, seasonal, and interannual. Scal...

Data S1. Coccolith Mass and Volume Concentration Calculations, Related to Table 1

Sea spray aerosols (SSA), have a profound effect on the climate; however, the contribution of oceanic microbial activity to SSA is not fully established. We assessed aerosolization of the calcite units (coccoliths) that compose the exoskeleton of the cosmopolitan bloom-forming coccolithophore, Emiliania huxleyi. Airborne coccolith emission occurs i...