The European RainGain project and Paris Region RadX@IDF project will enable us to asses in four European metropolis (Brussels, London, Paris and Rotterdam) the interest for urban hydrology of rainfall data at a much finer resolution (about a factor ten) than usually provided by national weather services. However, there are a few scientific challenges to overcome to take advantage of the full extent of this recent X-band radar technology. We will emphasize the importance for the quality of retrieval algorithms of not only the dual polarimetry, due to the strong absorption in the X-band, but also to the strong heterogeneity of the raindrop distribution contrary to the usual assumptions. Indeed, the latter may introduce important statistical biases.