Shallow, maritime cumuli are ubiquitous over much of the tropical oceans, and characterizing their properties is important to understanding weather and climate. The Rain in Cumulus over the Ocean (RICO) field campaign, which took place during November 2004–January 2005 in the trades over the western Atlantic, emphasized measurements of processes related to the formation of rain in shallow cumuli, and how rain subsequently modifies the structure and ensemble statistics of trade wind clouds. Eight weeks of nearly continuous S-band polarimetric radar sampling, 57 flights from three heavily instrumented research aircraft, and a suite of ground- and ship-based instrumentation provided data on trade wind clouds with unprecedented resolution. Observational strategies employed during RICO capitalized on the advances in remote sensing and other instrumentation to provide insight into processes that span a range of scales and that lie at the heart of questions relating to the cause and effects of rain from shallow maritime cumuli.

SUMMARYQuantitative predictions of the relationship between the droplet size-distribution width and entrainment in warm cumulus have been elusive, largely because of the difficulty in representing the extent of the scales involved. A new modelling framework is presented as a first step toward quantitative predictions of droplet size distributions resulting from entrainment, consisting of a three-dimensional cloud model coupled with a Lagrangian microphysical parcel model. The cloud model represents turbulent cloud dynamics but parametrizes microphysical processes such as condensation, and the parcel model complements this approach by performing explicit microphysical calculations within the kinematic and thermodynamic constraints established by the cloud model. The parcel model is run along trajectories all ending at the same point in the cloud, and the individual droplet size distributions are averaged together at this point to represent the turbulent mixing together of the droplets produced by these different parcel trajectories.The results replicate some important features of observed cloud droplet size distributions, including large widths, the continued presence of small droplets high in the clouds, and the bimodal structure. The origin of these features in these calculations is the variability introduced by entrainment, which leads to possibilities for droplets to encounter varying supersaturation histories during their transit through the cloud to the point of observation. Droplet sizes larger than those calculated for adiabatic ascent are also produced, with possible implications for coalescence initiation.

Measured ice crystal concentrations in natural clouds at modest supercooling (temperature ;.2108C) are often orders of magnitude 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.

Weather and climate models are challenged by uncertainties and biases in simulating Southern Ocean (SO) radiative fluxes that trace to a poor understanding of cloud, aerosol, precipitation and radiative processes, and their interactions. Projects between 2016 and 2018 used in-situ probes, radar, lidar and other instruments to make comprehensive measurements of thermodynamics, surface radiation, cloud, precipitation, aerosol, cloud condensation nuclei (CCN) and ice nucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase cloudsnucleating particles over the SO cold waters, and in ubiquitous liquid and mixed-phase clouds common to this pristine environment. Data including soundings were collected from the NSF/NCAR G-V aircraft flying north-south gradients south of Tasmania, at Macquarie Island, and on the RV Investigator and RSV Aurora Australis. Synergistically these data characterize boundary layer and free troposphere environmental properties, and represent the most comprehensive data of this type available south of the oceanic polar front, in the cold sector of SO cyclones, and across seasons.Results show a largely pristine environments with numerous small and few large aerosols above cloud, suggesting new particle formation and limited long-range transport from continents, high variability in CCN and cloud droplet concentrations, and ubiquitous supercooled water in thin, multi-layered clouds, often with small-scale generating cells near cloud top. These observations demonstrate how cloud properties depend on aerosols while highlighting the importance of confirmed low clouds were responsible for radiation biases. The combination of models and observations is examining how aerosols and meteorology couple to control SO water and energy budgets.

The objective of this study is to address the problem of the production of rain in warm cumulus clouds that has been observed to occur within about 20 min. A hybrid model approach is used where a microphysical parcel model is run along trajectories produced by a 3D cloud model, with sufficiently high resolution to allow explicit representation of the effects of entrainment and mixing. The model calculations take the next step from the previous study, which showed that entrainment and mixing can accelerate the diffusional growth of cloud droplets to the production of raindrops by collision and coalescence. The mechanism depends on the variability in droplet trajectories arriving at a given location and time in a cumulus cloud. The resulting broadening favors collisions among droplets in the main peak of the droplet size distribution, which leads to the production of raindrop embryos. However, this production and the subsequent growth of the embryos to become raindrops only occur in regions of relatively high cloud water content. The modeling framework allows an objective test of this sequence of events that explain the seemingly contradictory notions of the enhancement of cloud droplet growth as a result of entrainment and mixing and the need for substantial cloud water content for collision and coalescence growth. The results show that raindrops can be produced within 20 min in warm cumulus clouds. The rain produced is sensitive to giant aerosols, but modification of the modeling framework is required to conduct a more robust test of their relative importance.Corresponding author address: Alan M. Blyth, NCAS, University of Leeds,