The detection and properties of drizzle onset in marine stratus clouds using Doppler cloud radar observations in the Azores

Understanding the onset of coalescence in warm clouds is key in our effort toward improving cloud representation in numerical models. Coalescence acts at small scales, and its study requires detailed high-resolution dynamical and microphysical measurements from a comprehensive suite of instruments over a wide range of environmental conditions (e.g., aerosol loading, liquid water path and vertical air motion). The first ARM Mobile Facility (AMF) operated by the US Department of Energy's ARM Climate Research Facility is currently in its second year of a two-year deployment at Graciosa Island in the Azores, offering the opportunity to collect a long data set from a stable land-based platform in a marine stratocumulus regime.

The presented analysis is based on two novel observational techniques. The first uses moments from the profiling cloud radar for the objective determination of the radar reflectivity threshold that distinguishes drizzle-free from drizzling cloud volumes, on an hourly basis. This enables the determination of such areas in shallow stratiform clouds. The second technique uses radar Doppler spectra to separate the cloud and drizzle radar moments (e.g., reflectivity, mean Doppler velocity). This creates a unique opportunity to study the microphysical processes that are responsible for the change of radar reflectivity profile in drizzle-free versus drizzling clouds. Preliminary results indicate that in pristine (low aerosol loading) conditions, drizzle has radar signatures vastly different from those reported in the past. Furthermore, the observations show the drizzle growth rates for different environmental conditions.