THE SHAPE OF AVERAGED DROP SIZE DISTRIBUTIONS

The shape of averaged drop size distributions (DSD) is studied from a large sample of data (892 h) collected at several sites of various lat itudes. The results show that neither the hypothesis of an exponential distribution to represent rainfall with a high rain rate (R) nor the concept of equilibrium distribution arising from the various models us ing the parameterization of Low and List is compatible with the observ ations. To describe the DSD two regions have to be distinguished: a sm all-drop region, for diameter D smaller than a threshold D-c, and a la rge-drop region, for diameter D larger than D-c. For D < D-c the distr ibutions are strongly dependent on R and on z(0), the height of fall o f the rain from the base of the melting level. The decrease in tile re lative number of small drops with increasing R suggests that in this r egion the depletion of the small drops by the big ones is not totally compensated for by the input of small drops due to the collisional bre akup process; there is no stationary state for D < D-c. In the big-dro p region, for D > D-c, the slope of the distributions is almost indepe ndent of D. At low values of R (<20 mm h(-1)) it decreases when R incr eases, and not much depends on z(0). For increasing values of R beyond about 20 mm h(-1), the slope of the distributions tends toward a cons tant value of about 2.2-2.3 mm(-1). This suggests a certain stationari ty between the coalescence and collisional breakup processes in intens e rainfalls. It also appears that the two regions are not much influen ced by each Ether. The value of D-c increases with R and with z(0). Th e relation between the radar reflectivity factor (Z) and R obtained fr om the averaged DSDs are close to those calculated from nonaveraged da ta and compatible with those proposed in the literature. The differenc es observed between the coefficients of Z-R relations for various type s of rain are essentially due to the differences in small-drop concent ration.