On the sensitivity of hail accretion rates in numerical modeling

Numerical models of convective clouds with a bulk-water parameterization scheme extend the hail spectrum unrealistically to zero size. Since some of the microphysical characteristics of clouds depend critically on the hail accretional growth, it is desirable to see how the accretion rates may be improved by considering only hail sized particles, equal or greater than 0.5 cm in diameter. An equation for hail accretion rates which includes only hail particles larger than 0.5 cm is derived. Calculations show that the new hail accretion rates may differ from their earlier values by a factor of more than 3. The new equation for hail accretion rates is tested in a 1-D time-dependent model. The numerical results show that the modeled cloud with the new formula is more effective to produce the amount of rainfall and hail in shorter time than in previous case. The result is in fair agreement with observations.