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.

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The coalescence efficiency for accretion was assumed to be governed by a collisional Weber number. This concept was used with the aid of theoretical collision efficiences to organize collection efficiency data from several sources into a unified set. The resulting collection efficiencies show a maximum of 71% for a collector drop of 115 μm and a cloud droplet of 12‐μm radius. Accretion calculations were made for a monodisperse cloud to illustrate the effects of cloud droplet sizes and collection efficiencies on drop growth. The time average collection efficiency for growth from 50 to 500 μm by accretion of 30‐μm droplets is reduced by 50% when compared to the time average collision efficiency. A maximum of 65% in the time average collection efficiency occurs for accretion of 10‐μm droplets. The contribution to accretion by cloud droplets smaller than 5 μm appears to be negligible.

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The microphysical processes that lead to the development of precipitation in small, warm cumulus are examined using data from the Small Cumulus Microphysics Study near Cape Canaveral, Florida. Aircraft measurements are used to determine the concentration and size distribution of giant and ultragiant nuclei in clear air as a function of relative humidity, altitude, wind speed, and wind direction. The clear-air particle distributions show that ultragiant particles (radii extending from 10 to 150 m) exist from the surface to cloud base in concentrations that correspond to the concentrations of raindrops observed during drizzle to moderate rainfall events. A shift of the spectra toward larger size with increasing relative humidity was observed, suggesting that the spectra are composed of deliquesced particles growing by condensation. The small cumulus clouds are shown to contain cores where the observed liquid water content was nearly adiabatic. The observed evolution of the cloud droplet distribution within the near-adiabatic cores as a function of height showed an increase in the small droplet mode associated with condensation and an increase in the concentration of larger droplets associated with growth by accretion. Droplets with radii extending to nearly 100 m were present just above cloud base. These measurements were consistent with the clear-air measurements and provided evidence that the ultragiant nuclei can immediately act as embryos for raindrop growth by accretion upon entering cloud base. Comparisons of reflectivity computed from the cumulus core composite droplet distributions with the radar-observed reflectivity data provided independent evidence that the composite spectra reasonably represented the evolving microstructure of the cores of small cumulus clouds as they grew vertically.The analyses provide strong evidence of an efficient process for the initial development of precipitation in small Florida cumuli. This process consists of raindrop embryo formation on ultragiant nuclei followed by growth by accretion as the newly formed drops proceed upward through the adiabatic cores of the cumulus clouds. These data support the conceptual model of raindrop formation in marine clouds first proposed by Woodcock a half century ago.

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The first reliable aircraft measurements of characteristic cloud drop charges were obtained by utilizing a counterflow virtual impactor to substantially increase charge sensitivity and eliminate spurious contact charging that contaminated previous aircraft measurements. We find average drop charges more than an order of magnitude larger than expected from mountain surface measurements in similar clouds. Our evaluation of the data indicates that the high average charges on cloud drops originate in charge layers at the cloud boundaries and are carried into the cloud layer by vertical motions. These initial aircraft results demonstrate that cloud drop charges in layer clouds may be high enough to influence microphysical processes that promote precipitation.