Comments about the drag laws used in hail growth simulations

The values of the drag coefficient (CD), experimentally obtained by different authors for graupels and hailstones, are carefully analyzed, in order to investigate how the numerical simulations of hail trajectories, which markedly depend on the CD values assigned to the growing particle, can be improved. It is found that new measurements of CD should be performed in the whole range of the Reynolds number (Re) of the cloud ice particles. These measurements should be carried out applying the same criterion to define the characteristic variables (particle cross section and dimensions) used to calculate CD and Re. Furthermore, it is found that the CD fitting curves should be expressed as a function not only of Re but also of other parameters which represent the particle surface roughness and the departure from the spherical form. Based on this finding, a plausible definition of these variables and the way to introduce them in a hail trajectory calculation are proposed.