FORMATION OF INHOMOGENEITY IN DROP CONCENTRATION INDUCED BY THE INERTIA OF DROPS FALLING IN A TURBULENT-FLOW, AND THE INFLUENCE OF THE INHOMOGENEITY ON THE DROP-SPECTRUM BROADENING

The mechanisms of drop-concentration inhomogeneity formation are studi ed using both a numerical simulation with a model of isotropic and hom ogeneous turbulence, and analytical methods. It is shown that atmosphe ric turbulence can create a significant drop-concentration inhomogenei ty due to the effects of drop inertia. Two types of area in the turbul ent flow are revealed. Drops tend to leave the areas of 'drop vortices ' and collect within the zones out of the vortices. As a result, the z ones of drop-track collection turn out to be the zones of enhanced dro p concentration. The rare of concentration enhancement is studied for drops of different sizes using the Monte Carlo method. It is shown tha t the drop flux velocity divergence and droplet-concentration variatio ns reach their maximum at 100 mu m drop radius. Zones of enhanced drop concentrations are stretched along the drop tracks. Characteristic sc ales of drop-concentration fluctuations along the drop tracks are of t he order of several metres or even a few tens of metres. Across the dr op tracks, the characteristic scale of concentration pulsations is of the order of a few centimetres. The existence of the areas of drop-con centration enhancement and the areas of decreased concentration means that drop collisions are not distributed uniformly, but are concentrat ed within the areas with greater drop concentration. The possible effe cts of droplet inhomogeneity is simulated by solving a stochastic coal escence equation for the water drop-size-distribution function in area s of enhanced and decreased droplet concentration with the subsequent mixing of the corresponding spectra at each 10 s. It is shown that due to the nonlinear nature of collision processes the effects of the inc rease of collision frequency in the areas of enhanced concentration do minate over those of its decrease. The effects of the drop-concentrati on inhomogeneity are accumulated with time and lead to substantial acc eleration of droplet spectrum broadening. It is supposed that the inho mogeneity of rain rates with time and space can be attributed to these turbulence effects determining drop-track collection and drop-concent ration inhomogeneity. These turbulence effects can contribute to the f ormation of inhomogeneity of ice and aerosol particles as well.