Using a Bulk Model to Study the Influence of Local Topography on the Rainfall Structure

During, the first Special Observation Period (SOP-1) of the HyMeX campaign, in fall 2012, a specific observational network has been set-up in southern France with the aim to better understand the role and the impact of a complex terrain on the horizontal and vertical structure of rainfall and the associated microphysical processes. From a case study (IOP7a), the analyses of drop size distribution (DSD) parameters as well as the observed spectra show that the topography of the region has an impact on the rainfall structure at fine scale and that rainfall regimes also have an influence. Those results were then generalized to the fall 2012 SOP period. A detailed study based on the evolution of the shape of the DSD reveals that the microphysical processes are affected differently considering the complexity of the terrain. Thus, the local orography and the rainfall regimes impact both the rainfall structure and the associated microphysical processes. In this presentation, we will aim to evaluate the representativity of the rainfall structure and the associated microphysical mechanisms within a bulk model such as WRF. Using the WRF model and a two-moment microphysics scheme, we can calculate DSD spectra from the diagnostic variables of the model. A detailed comparison between these calculated DSD and the observed ones show that the impact of the underlying relief on the horizontal rainfall structure is well represented by the model. However, the vertical structure of precipitations from the simulation lacks the variability observed during an event. Nevertheless, a study of three variables from the model allows us to associate the microphysical processes with the rainfall structure and discuss of the utility of a bulk model in order to investigate the influence of the orography on the microphysical processes.