With dry deposition and below-cloud scavenging, in-cloud scavenging is one of the three components of aerosol transfer from the atmosphere to the ground. There is no experimental validation of in-cloud particle scavenging models for all cloud types that is not impacted by uncertainties concerning below-cloud scavenging. In this article, the choice was made to start with a recognised and validated microphysical cloud formation model (the DEtailed SCAvenging Model, DESCAM) to extract a scheme of aerosol scavenging by clouds, valid for different cloud types. The resulting model works for the two most extreme precipitation clouds: from cumulonimbus to stratus. It is based on data accessible a priori from numerical weather prediction (NWP) outputs, i.e. the intensity of the rain and the relative humidity in the cloud. The diagnostic of the altitude of the cloud base proves to be a key parameter, and accuracy in this regard is vital. This new in-cloud scavenging scheme is intended for use in long-distance (> 100 km) atmospheric transport models (ATMs) or global climate models (GCMs).

An increasing number of countries are planning to carry out rain-enhancement activities in response to water shortages and other societal needs. Rain enhancement can work with reasonable cost–benefit ratios under the right conditions. However, many components of the technology need improvement and testing, and many physical processes are not yet fully understood due to their complexity. Global research on cloud-seeding technology indicates that precipitation can be increased up to 15% of the annual norm, depending on the available cloud resources and technical systems used. However, there is still ambiguity in the results of the studies conducted and the effects and scale of the rain enhancement. When evaluating the results of rain enhancement projects, it is necessary to adhere to rigorous scientific approaches and proven methods.

A new In-Cloud Aerosol Scavenging Experiment (In-CASE) has been conceived to measure the collection efficiency (CE) of submicron aerosol particles by cloud droplets. In this setup, droplets fall at their terminal velocity through a 1 m high chamber in a laminar flow containing aerosol particles. At the bottom of the In-CASE chamber, the droplet train is separated from the aerosol particle flow – droplets are collected in an impaction cup, whereas aerosol particles are deposited on a high-efficiency particulate air (HEPA) filter. The collected droplets and the filter are then analysed by fluorescence spectrometry since the aerosol particles are atomised from a sodium fluorescein salt solution (C20H10Na2O5). In-CASE fully controls all the parameters which affect the CE – the droplets and aerosol particles size distributions are monodispersed, the electric charges of droplets and aerosol particles are controlled, and the relative humidity is indirectly set via the chamber's temperature. This novel In-CASE setup is presented here as well as the first measurements obtained to study the impact of relative humidity on CE. For this purpose, droplets and particles are electrically neutralised. A droplet radius of 49.6±1.3 µm has been considered for six particle dry radii between 50 and 250 nm and three relative humidity levels of 71.1±1.3 %, 82.4±1.4 % and 93.5±0.9 %. These new CE measurements have been compared to theoretical models from literature which adequately describe the relative humidity influence on the measured CE.

A new In-Cloud Aerosol Scavenging Experiment (In-CASE) has been developed to measure the collection efficiency (CE) of submicron aerosol particles by cloud droplets. Droplets fall at their terminal velocity through a 1 m high chamber in a laminar flow containing aerosol particles. At the bottom of the In-CASE chamber, the droplet train is separated from the aerosol particles flow, and the droplets are collected in an impaction cup, whereas aerosol particles are deposited on a high-efficiency particulate air (HEPA) filter. The collected droplets and the filter are then analysed by fluorescence spectrometry since the aerosol particles are atomised from a sodium fluorescein salt solution (C20H10Na2O5). In-CASE fully controls all the parameters which affect the CE – the droplets and aerosol particles size distributions are monodispersed, the electric charges of droplets and aerosol particles are known and set, and the relative humidity is indirectly controlled via the chamber's temperature. This paper details the In-CASE setup and the dataset of 70 measurements obtained to study the impact of the electric charges on CE. For this purpose, droplets and particles charges are controlled through two charging systems developed in this work – both chargers are detailed below. The droplet charge varies from -3.0×104±1.4×103 to +9.6×104±4.3×103 elementary charges, while the particle charge ranges from zero to -90±9 elementary charges depending on the particle radius. A droplet radius of 48.5±1.1 µm has been considered for four particle dry radii between 100 and 250 nm while the relative humidity level during experiments is 95.1±0.2 %. The measurements are then compared to theoretical models from literature – showing good agreement.

This article presents an experimental study, performed in the BERGAME setup, dedicated to studying the collection of submicron aerosol particles by raindrops. The initial aim was to focus on the influence of the electrical charges of raindrops on the efficiency with which they collect aerosol particles. However, in the relative humidity range considered in this article (26–36%), measurements highlight a first-order role of phoretic effect for submicron aerosol particles. Indeed, measurements highlight a 100% increase in the collection efficiency for each percentage decrease in the atmospheric relative humidity. Phoretic effects are known to play a role in collection by drops; however, none of the models found in the literature predicts the same magnitude as the one presently measured. Characterization of the aerosol trajectories around the drop, accelerated to terminal velocity, seems to show a coupling between phoretic effects and rear capture. This interaction, already suggested by Grover et al., is a line of explanation for such a sharp unpredicted increase of the collection efficiency with moisture decrease.

For the last 25 years, CO-PDD (Cézeaux-Aulnat-Opme-puy de Dôme) has evolved to become a full instrumented platform for atmospheric research. It has received credentials as a national observing platform in France and is internationally recognized as a global station in the GAW (Global Atmosphere Watch) network. It is a reference site of European and national research infrastructures ACTRIS (Aerosol Cloud and Trace gases Research Infrastructure) and ICOS (Integrated Carbon Observing System). The site located on top of the puy de Dôme mountain (1465 m a.s.l.) is completed by additional sites located at lower altitudes and adding the vertical dimension to the atmospheric observations: Opme (660 m a.s.l.), Cézeaux (410 m), and Aulnat (330 m). The integration of different sites offers a unique combination of in situ and remote sensing measurements capturing and documenting the variability of particulate and gaseous atmospheric composition, but also the optical, biochemical, and physical properties of aerosol particles, clouds, and precipitations. Given its location far away from any major emission sources, its altitude, and the mountain orography, the puy de Dôme station is ideally located to sample different air masses in the boundary layer or in the free troposphere depending on time of day and seasons. It is also an ideal place to study cloud properties with frequent presence of clouds at the top in fall and winter. As a result of the natural conditions prevailing at the site and of the very exhaustive instrumental deployment, scientific studies at the puy de Dôme strongly contribute to improving knowledge in atmospheric sciences, including the characterization of trends and variability, the understanding of complex and interconnected processes (microphysical, chemical, biological, chemical and dynamical), and the provision of reference information for climate/chemistry models. In this context, CO-PDD is a pilot site to conduct instrumental development inside its wind tunnel for testing liquid and ice cloud probes in natural conditions, or in situ systems to collect aerosol and cloud. This paper reviews 25 years (1995–2020) of atmospheric observation at the station and related scientific research contributing to atmospheric and climate science.

Over the Cévennes–Vivarais region in southern France 5 h intensive rainfall covering an area of 1000 km2 with more than 50 mm of rain accumulation was observed during IOP7a of HyMeX. This study evaluates the performance of a bin-resolved cloud model for simulating this heavy-precipitation event. The simulation results were compared with observations of rain accumulation, radar reflectivity, temporal and spatial evolution of precipitation, 5 min rain rates, and raindrop size distributions (RSDs). The different scenarios for aerosol number concentrations range from 1000 to 2900 cm−3 and represent realistic conditions for this region. Model results reproduce the heavy-precipitation event with respect to maximum rain intensity, surface area covered by intense rain and the duration, as well as the RSD. Differences occur in the short-term rainfall rates, as well as in the drop number concentration. The cloud condensation number concentration has a notable influence on the simulated rainfall, on both the surface amount and intensity but also on the RSD properties, and should be taken into account in microphysics parameterizations.

Thanks to the authors for the detailed answers. With the correction on aerosol populations, the paper now effectively demonstrates both that the bin scheme DESCAM is able to produce reasonable amounts of precipitation in a 3D, real-case simulation of a convective system, and that there is a sensitivity to the aerosol population, therefore pushing for the use of aerosol-aware cloud physics schemes. My remaining comments are:

Measurements of the ratio of oxygen‐18 to oxygen‐16 are often used to reconstruct past climate or to quantify phase changes. However, in most models, the evolution of the isotopic ratio is calculated using the Rayleigh parameterization. The Descam model has recently been extended to simulate isotopic fractionation from occurring condensation/evaporation processes and the mass of oxygen‐18 in aerosol particles, droplets, and ice crystals explicitly as a function of their size. In a first step, isotopic calculations are implemented into the 1.5 d dynamical framework where several sensitivity studies are conducted to test the evolution of the isotopic ratio as a function of the different phase changes in a convective cloud. Though an isotopic signature can be identified for different microphysical pathways, the predominant factor for the isotopic ratio in the precipitation seems to be the dynamical evolution of the considered cloud.

The Mediterranean region is frequently affected in autumn by heavy precipitation that causes flash-floods or landslides leading to important material damage and casualties. Within the framework of the international HyMeX program (HYdrological cycle in Mediterranean EXperiment), this study aims to evaluate the capabilities of two models, WRF (Weather Research and Forecasting) and DESCAM (DEtailed SCAvenging Model), which use two different representations of the microphysics to reproduce the observed atmospheric properties (thermodynamics, wind fields, radar reflectivities and precipitation features) of the HyMeX-IOP7a intense precipitating event (26 September 2012). The DESCAM model, which uses a bin resolved representation of the microphysics, shows results comparable to the observations for the precipitation field at the surface. On the contrary, the simulations made with the WRF model using a bulk representation of the microphysics (either the Thompson scheme or the Morrison scheme), commonly employed in NWP models, reproduce neither the intensity nor the distribution of the observed precipitation—the rain amount is overestimated and the most intense cell is shifted to the East. The different simulation results show that the divergence in the surface precipitation features seems to be due to different mechanisms involved in the onset of the precipitating system: the convective system is triggered by the topography of the Cévennes mountains (i.e., south-eastern part of the Massif Central) in DESCAM and by a low-level flux convergence in WRF. A sensitivity study indicates that the microphysics properties have impacted the thermodynamics and dynamics fields inducing the low-level wind convergence simulated with WRF for this HyMeX event.

Cloud processing of aerosol particles is an important process and is, for example, thought to be responsible for the so-called “Hoppel-minimum” in the marine aerosol particle distribution or contribute to the cell organization of marine boundary layer clouds. A numerical study of the temporal and spatial scales of the processing of aerosol particles by typical marine stratocumulus clouds is presented. The dynamical framework is inspired by observations during the VOCALS (Variability of the American Monsoon System Ocean-Cloud-Atmosphere-Land Study) Regional Experiment in the Southeast Pacific. The 3-D mesoscale model version of DESCAM (Detailed Scavenging Model) follows cloud microphysics of the stratocumulus deck in a bin-resolved manner and has been extended to keep track of cloud-processed particles in addition to non-processed aerosol particles in the air and inside the cloud drops. The simulation follows the evolution of the processing of aerosol particles by the cloud. It is found that within one hour almost all boundary layer aerosol particles have passed through at least one cloud cycle. However, as the in-cloud residence times of the particles in the considered case are only on the order of minutes, the aerosol particles remain essentially unchanged. Our findings suggest that in order to produce noticeable microphysical and dynamical effects in the marine boundary layer clouds, cloud processing needs to continue for extended periods of time, exceeding largely the time period considered in the present study. A second model study is dedicated to the interaction of ship track particles with marine boundary layer clouds. The model simulates quite satisfactorily the incorporation of the ship plume particles into the cloud. The observed time and spatial scales and a possible Twomey effect were reproduced.

This paper describes a theoretical model which computes the collection efficiency of aerosol particles by droplets, due to the combined action of dynamic (inertia, weight and drag) and electrophoresis forces acting on an aerosol particle of radius 0.004 <= a <= 1.3 mu m around a droplet of radius 15 <= A <= 100 mu m. The electrostatic forces are defined following the concept of image charges. In the given particle range, the Brownian motion must be considered and was consequently added to the model. A novel approach is developed, based on the Langevin's theory and solved using an Ito process. Results of electroscavenging related to natural atmospheric ionisation, as well as for strongly charged aerosol particles released after a nuclear accident, are presented with pressure and temperature representative of the mid-troposphere (-17 degrees C, 540 hPa). The droplet charges considered in the paper are representative of weakly and strongly electrified clouds. The values of collection efficiency computed are convenient for incorporation into cloud models and to study scavenging of aerosol particles by clouds whether for climate, pollution or nuclear safety issues.

This paper provides a summary of the assessment report of the World Meteorological Organization (WMO) Expert Team on Weather Modification that discusses recent progress on precipitation enhancement research. The progress has been underpinned by advances in our understanding of cloud processes and interactions between clouds and their environment, which, in turn, have been enabled by substantial developments in technical capabilities to both observe and simulate clouds from the microphysical to the mesoscale. We focus on the two cloud types most commonly seeded in the past: winter orographic cloud systems and convective cloud systems. A key issue for cloud seeding is the extension from cloud-scale research to water catchment–scale impacts on precipitation on the ground. Consequently, the requirements for the design, implementation, and evaluation of a catchment-scale precipitation enhancement campaign are discussed. The paper concludes by indicating the most important gaps in our knowledge. Some recommendations regarding the most urgent research topics are given to stimulate further research.

This article presents new measurements of the efficiency with which aerosol particles of accumulation mode size are collected by a 1.25 mm sized raindrop. These laboratory measurements provide the link to reconcile the scavenging coefficients obtained from theoretical approaches with those from experimental studies. We provide here experimental proof of the rear capture mechanism in the flow around drops, which has a fundamental effect on sub-microscopic particles. These experiments thus confirm the efficiencies theoretically simulated by Beard (1974). Finally, we propose a semi-analytical expression to take into account this essential mechanism to calculate the collection efficiency for drops within the rain size range.

Are clouds able to modify the ambient aerosol particle spectrum so that the number concentration is reduced and the size and chemical composition of the particles is changed? Can they do it in such a way that the cloud forming on these particles will finally be able to precipitate and remove the water and pollution from the atmosphere?

Measured ice crystal concentrations in natural clouds at modest supercooling (temperature similar to>-10 degrees C) are often orders ofmagnitude greater than the number concentration of primary ice nucleating particles. Therefore, it has long been proposed that a secondary ice production process must exist that is able to rapidly enhance the number concentration of the ice population following initial primary ice nucleation events. Secondary ice production is important for the prediction of ice crystal concentration and the subsequent evolution of some types of clouds, but the physical basis of the process is not understood and the production rates are not well constrained. In November 2015 an international workshop was held to discuss the current state of the science and future work to constrain and improve our understanding of secondary ice production processes. Examples and recommendations for in situ observations, remote sensing, laboratory investigations, and modeling approaches are presented.

Even though ice formation mechanisms in clouds probably obey all the same thermodynamic principles, the associated mechanical and thermal energy transfers differ with respect to the exact pathway and the associated phases. Consequently, heterogeneous ice nucleation parameterizations play an important role in cloud modeling.The 1.5D bin-resolved microphysics Detailed Scavenging Model (DESCAM) was used to assess the role of the parameterizations for different ice initiation processes. Homogeneous nucleation, deposition freezing, contact freezing, immersion freezing, and condensation freezing were treated explicitly, and their impacts alone and in competition with each other on cloud microphysics and precipitation were studied. The role of efficiently ice-nucleating bacteria on cloud evolution was addressed, as well as means to consider different chemical natures of ice nucleation particles.For the conditions studied, it was found that deposition and contact freezing only played a negligible role with respect to the other ice-nucleating mechanisms. Homogeneous freezing and classical immersion freezing showed a similar behavior. Both freezing rates increase with increasing drop age (i.e., size). This suggests a possibility for regrouping processes in future parameterized cloud models. Condensation freezing parameterization, however, acts at much warmer temperatures in clouds and for much smaller drops. The associated release of latent heat at lower altitudes caused significantly different cloud dynamics with respect to homogeneous/immersion freezing. This suggests that, in future parameterized models, the condensation freezing process needs particular attention, as well as the fact that ice-forming nuclei (IN) are a subset of aerosol particles that are depleted and replenished like the rest of the population.