Laboratoire de Mathématiques Raphaël Salem

UMR 6085 CNRS - University of Rouen Normandy

Clouds are one of the large uncertainties of weather forecasting and climate prediction. Because of difficulty of observation and experiment our knowledge of the cloud micro physical processes, such as cloud condensation nucleation, condensational growth, collision-coalescence, turbulent mixing of moisture and temperature and so on, is very limited.  Recently we have developed a simulation code, Cloud Microphysics Simulator (CMS), in the spirit of the first principle in that the code computes evolution of cloud droplets in the Lagrangian frame and turbulence in the Eulerian frame by using the direct numerical simulation. The CMS has successfully simulated their evolution and found that the distribution function of the droplet size having single peak at initial time moves toward larger size and then has the secondary peak at latter stage, the formation of the rain drops. An interesting feature found in the CMS computation is the interaction between the cloud droplets and turbulence. Unlike the traditional phenomenology of Kolmogorov for turbulence that the excitation is applied at large scales and transferred toward small scales, the modification (excitation) of the spectra of the water vapor and temperature fluctuations begins in the diffusive range, typically at scales smaller than 1 mm, and reaches the inertial-convective range as time goes on. This modification is explained in terms for spectral theory of turbulence.