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Study Investigates Evolution of Drop Size Distributions in High-based Cumulus Congestus Clouds over the UAE

‘Warm rain’ generation, a process that does not involve an ice phase, depends on the characteristics of the cloud Droplet Size Distribution (DSD). Observations show that the formation of warm rain requires large droplets with a radius exceeding 12 - 14 micrometers prior to the development of warm rain. Another key factor in warm rain formation is the width of DSD, as wide DSDs promote the growth of drizzle-sized drops through collision-coalescence.

To better understand DSD evolution in cumulus clouds given their widespread occurrence around the globe, Dr. Paul Lawson, Senior Research Scientist at SPEC Incorporated, USA, and one of the second cycle awardees of the UAE Research Program for Rain Enhancement Science, investigated a high-based cumulus congestus case observed over the UAE. A notable feature of this case was the lack of warm rain generation despite the deep liquid layer (> 3.5 km).

The study analyzed aircraft observations as well as Large Eddy Simulations (LES) coupled with a bin microphysics model. It addressed several outstanding questions concerning key processes driving DSD evolution for this case and focused especially on the factors that may have limited the production of mm-sized drops despite the cloud depth.

The UAE aircraft observations in actively growing cloud turrets comprised of liquid droplets obtained at four different levels ranging from about 9.5 degrees Celsius near cloud base and up to the -12 degrees Celsius level.

The LES-modeled cloud produced very few mm-sized drops similar to the observations, but copious large drops with reduced aerosol loading. Most of the cloud volume was strongly diluted from entrainment and mixing which drives droplet activation above cloud base but limits liquid water content. Neglecting droplet activation above cloud base decreased modeled droplet concentrations aloft, but did little to increase rain production.

A key result from the simulations conducted by the study is that secondary activation had only a limited impact on the tail of DSDs and thus on drop growth via collision-coalescence. Thus, one of its main conclusions from the simulations is that warm rain generation (or lack thereof in this case) was determined more by the sub-cloud aerosol and DSDs near cloud base than DSD evolution aloft from secondary droplet activation.

In future work, the team aims to continue exploring warm rain processes in cumulus congestus using Lagrangian microphysics to address some of the shortcomings noted with bin microphysics.

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