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.

Desert dust is one of the most important atmospheric ice‐nucleating aerosol species around the globe. However, there have been very few measurements of ice‐nucleating particle (INP) concentrations in dusty air close to desert sources. In this study we report the concentration of INPs in dust laden air over the tropical Atlantic within a few days' transport of one of the world's most important atmospheric sources of desert dust, the Sahara. These measurements were performed as part of the Ice in Clouds Experiment‐Dust campaign based in Cape Verde, during August 2015. INP concentrations active in the immersion mode, determined using a droplet‐on‐filter technique, ranged from around 102 m−3 at −12°C to around 105 m−3 at −23°C. There is about 2 orders of magnitude variability in INP concentration for a particular temperature, which is determined largely by the variability in atmospheric dust loading. These measurements were made at altitudes from 30 to 3,500 m in air containing a range of dust loadings. The ice active site density (ns) for desert dust dominated aerosol derived from our measurements agrees with several laboratory‐based parameterizations for ice nucleation by desert dust within 1 to 2 orders of magnitude. The small variability in ns values determined from our measurements (within about 1 order of magnitude) is striking given that the back trajectory analysis suggests that the sources of dust were geographically diverse. This is consistent with previous work, which indicates that desert dust's ice‐nucleating activity is only weakly dependent on source.