An Improved Methodology for classifying convective and stratiform rain

One of the main errors affecting the rainfall estimation by weather radar is due to the fact that the radar measures the value of reflectivity at a certain height instead of doing it at the ground level. Usually the correction of this error is realized extrapolating the observed values to the ground by supposing that the Vertical Profile of Reflectivity (VPR) agrees a determined shape. From this point of view is very convenient to distinguish between convective and stratifom precipitation since each kind of rain shows different characteristic VPR. A first Methodology is currently operational at the Catalan Weather Service (METEOCAT) in the framework of the EHIMI system (Integrated Tool for HydroMeteorological Forecasting). This methodology consists of applying the algorithm for identifying stratiform rain developed by Sánchez-Diezma et al. (2000), and the algorithm for identifying convective rain developed by Steiner et al. (1995) and implemented by Sánchez-Diezma (2001) for being used at the weather radars of the Catalan Network. The results obtained over several real time situations show some limitations, specially related to the stratiform rain detection far from the radar and to the erroneous identification of stratiform pixels where the first PPI intercepts the bright band as convective ones. Here taking into account the strong vertical development which characterizes the convective rain is proposed as a new complementary criterion for distinguishing between convective and stratiform precipitation. In this sense, a variable that is showed to be useful is the average reflectivity (measured in mm6/m3) over the height where is placed the bright band of the stratiform rain. To obtain this variable the VPR is integrated from a certain height above the bright band to the top of the precipitation and the result is divided by the integration height interval. According to this, the pixels where the average elevated reflectivity is greater than a chosen threshold are classified as convective and the pixels where this variable takes values bellow a second lower threshold are classified as stratiform. In case that they belong to the interval between these two thresholds the precipitation could be low convection or stratiform rain with a high bright band. An improved Methodology combining the previous identification algorithms and this new criterion is presented. The analysis of its performance over a number of selected radar images shows promising results.