Dr. Fred Burnet, Meteo-France
The onset of precipitation in convective clouds has not yet been adequatly explained. The current theory of droplet condensationnal growth predicts narrow spectra, while production of precipitation embryos needs broad spectra with large droplets.
In situ measurements of droplet distributions are critical for understanding cloud microphysics. The SCMS experiment was then especially dedicated to the study of precipitation formation in non precipitating warm cumulus cloud (without ice phase). We present a validation of microphysical data collected during SCMS, where new airborne devices were tested. The primary problem is the lack of absolute standard for calibration of these instruments. Our approach is to compare probe responses, in various conditions, to evaluate the performances of several instruments based on different measuring techniques (single particle counters, integrators and hot wire probe).
Entrainment and mixing of environmental dry air in the cloud can be responsible for the broadening of the spectra. Droplet distribution measurements are analysed to study the formation of large droplets from these processes. We develop a new analysis method for characterizing the statistical properties of the mixing process, homogeneous or heterogeneous. It is shown that heterogeneous concept provides a satisfactory description of the statistical properties. Based on this result, we build a simple model for reproducing the evolution of the droplet size distribution in the mixing cells with constant total concentration. Conditions which lead to the formation of superadiabatic droplets growth are identified. Finally, results from the model are compared with observations.