Graduate Seminar

The physics of clouds involves an enormous range of spatial scales from global scale to droplet scale (10⁷ m to 10^-6 m).  Even if we restrict ourselves to the scales of motion in a single small cumulus cloud, the range is still very large (10³ m to 10^-3 m).  Current methods for simulating the physics of clouds typically focus on high-fidelity representations of either the large turbulent eddies or the droplet microphysics, through 3D large-eddy simulation (LES), 3D direct numerical simulation (DNS), or explicit droplet microphysics models.  The EMPM (Explicit Mixing Parcel Model) predicts the evolving in-cloud variability due to entrainment and finite-rate turbulent mixing using a 1D representation of a rising cloudy parcel.  The 1D formulation allows the model to resolve fine-scale variability down to the smallest turbulent scales (about 1 mm).  The EMPM calculates the condensational growth of thousands of individual cloud droplets based on each droplets local environment.  EMPM results can be used to address two fundamental difficulties that the large-eddy simulation LES) approach faces when attempting to represent the effects of entrainment and mixing on droplet microphysics.  One is representing the subgrid-scale (SGS) variability of subsaturation and its impact on droplet size distribution (DSD) evolution.  Another is accounting for the finite rate SGS mixing and therefore of droplet evaporation.  The response of small droplets to turbulence has important features at scales as small as the droplet radius.  Namely, droplet motion relative to the fluid at scales less than the Kolmogorov microscale induces droplet clustering and increases droplet relative velocities that increase droplet collision rates significantly.  We have developed, implemented, and tested an economical method for simulating these effects of turbulence on droplets.  We have also implemented a collision detection algorithm into the model so that we can simulate collisions and coalescence between droplets. we can simulate collisions and coalescence between droplets.