Multimodal Raindrop Size Distributions

Abstract

The raindrop size distributions (DSDs) observed over a short span usually have an erratic shape, with several relative maxima. This multimodal structure is studied from disdrometer data acquired in tropical and midlatitude areas. It is shown that some modes of DSDs have a persistence larger than several minutes and can be spotted from one DSD to the next one as they migrate through the size classes. It is demonstrated that Nm, the number of modes of DSDs, for diameter larger than 2 mm, is not related to the mean rain rate but depends on the rain-rate fluctuations. Statistical evidence of such a relation is given. The spread of DSDs is found to be dependent on its multimodal structure, that is, on Nm. The large values of Nm are associated with low values of slope ? and intercept N0 of the fitted exponential distribution. In order to explain the dependence of the DSD shape on Nm, a conceptual model is proposed in which the modes are interpreted as resulting from an overlapping of rain shafts. The associated DSD is termed a synthetic drop size distribution (SDSD). It is shown that the overlapping of rain shafts generated from a sequence of rain cells of increasing intensity, such as observed at the leading edge of a convective system, results in undersloping SDSDs. In the reverse configuration, that is, with a sequence of rain cells with decreasing intensity, such as observed at the ending edge of a convective system, it results in oversloping SDSDs. Observations in agreement with these conclusions are presented. The readability of the modal structure of the DSDs depends on several factors in such a way that an apparent multimodal structure is not necessary for a DSD to be an SDSD. It is suggested that most of the DSDs observed at the ground are synthetic DSDs.