A passive scalar-like model for rain applicable up to storm scale

Analysis of the data collected during the AMMA (African Monsoon Multidisciplinary Analyses) campaign shows that rain storms typical of the African monsoon have multifractal properties, and can be modelled by fractionally integrated multiplicative cascades. The originality of the present study lies in the application of a constraint, which results in the interior only of storms being investigated, such that the multifractal analysis is not affected by the presence of numerous zero values. The model is validated in the time domain by means of disdrometer measurements, and in the spatial domain with co-localized meteorological radar rain maps. The non-conservation parameter obtained in the spatial domain is found to be consistent with the assumption that the rain rate follows a passive scalar-like scaling law up to the scale of storms, including corrections due to fluxes intermittencies. Comparison of the value of this parameter with that obtained in the temporal domain indicates the presence of a space-time anisotropy, which could be explained by turbulent advection.