P14.22 Rain heterogeneity studies and specific Z-R relationships determination with x-band and k-band radars to improve rain rate retrieval. Frédéric TRIDON, Joël VAN BAELEN*, and Yves POINTIN Laboratoire de Météorologie Physique (LaMP), CNRS / Université Blaise Pascal Clermont-Ferrand II 24, avenue des Landais, 63177 Aubière cedex, France Email: F.Tridon@opgc.univ-bpclermont.fr ABSTRACT In this work, we use the combination of an X- band high time and spatial resolution local area weather radar (typically 30 seconds in time and 60 meters in range and 2° in azimuth up to 20 kilometers) and a K-band vertically looking Micro Rain Radar to study the temporal and spatial heterogeneity of rain and its corresponding drop size distribution within precipitating systems. Then, defining simple classification criteria (based on rain intensity, trend, time variability, etc...), we determine separate rain regimes for which we can derive specific Z-R relationships. Applying these relationships to the X band reflectivity measurements allows us to improve rain rate restitutions over the size of a small urban basin after adequate correction for the attenuation effects. INTRODUCTION During more than 50 years of development, ground-based weather radars have become a research and operational tool well suited for precipitation surveillance and, eventually, quantitative rainfall measurements. The major source of errors lies in the conversion of the radar reflectivity factor Z (mm 6 m -3 ) to rain rate R (mm h -1 ). These two parameters are related to each other via the raindrop size distribution (DSD) which cannot be inferred by conventional weather radar measurements. Hence, it has been common practice to take a simple power law relationship between Z and R, like the well known Marshall-Palmer relationship. However, DSD is extremely variable in time and space even within a single precipitating event. Thus, many of these relationships have been proposed, nevertheless, in most of the cases, a unique relationship is used for one precipitating event. The aim of this work is to categorize the different rain regimes that might occur even within individual precipitation cells to derive the corresponding specific relationships. Then we will confront the rain estimates with these specific relationships to the classical approach using one single relationship in order to investigate their potential for improve rain estimation. To do so, we analyse the simultaneous measurements of a scanning X- band radar and a vertically pointing K-band radar in their common volume. In section 2, we will describe the radars and data. In section 3, we will present the methods and their performances. Finally, we will outline some conclusions and perspective for future developments. EXPERIMENTAL SET-UP During the summer of 2007, for the international COPS campaign (Convective and Orographically induced Precipitations Study, Wulfmeyer et al., 2008), the X-band radar was installed on a small hill at the foot of the Vosges mountains in order to overlook a largely instrumented site where we had installed the K-band radar, but also to monitor