Evaporation of Rain Falling from Convective Clouds as Derived from Radar Measurements

The rainshafts of about 3000 summer afternoon convective rain cells in the semiarid region of central south Africa were tracked and measured with a volume scanning radar. The area and time integrated rain volume of each individual rain cell was obtained at the cloud base level and at a lower level, assuming a fixed radar reflectivity-rain intensity (Z-R) relationship. The ratio of the rain volumes at the two levels arid, thereby, the cumulative fractional evaporation of the failing rain was found to depend on the rain intensity at the cloud base level and on the fall distance from the cloud base. With a small lifetime peak rain intensity at the cloud base (1.0 mm h−1), about 50% of the rain evaporated by 1 km below the cloud base and all of the rain evaporated by 1.6 km. With a medium rain intensity (10 mm h−1), about 25% evaporated by 1 km and 50% by 1.6 km. With very heavy rain intensity (80 mm h−1), about 15% evaporated by 1 km and 30% by 1.6 km below the cloud base level. These must be regarded as lower limits of the evaporation, because the more rapid evaporational depletion of the smaller drops with increasing fall distance causes a relative radar overestimate of the rain intensity at the lower level and, therefore, an underestimate of the evaporation, when using the vertically fixed Z-R relationship.