On the use of GPM constellation for monitoring heavy precipitation events over the Mediterranean region

Precipitation retrievals exploiting the available passive microwave (PMW) observations by cross-track and conically scanning satellite-borne radiometers in the Global Precipitation Measurement (GPM) mission era are used to monitor and characterize heavy precipitation events that occurred during the Fall 2014 in Italy. The analysis focuses on specific features of three selected case studies over the Italian area, in order to evidence suitability and limitations of PMW precipitation retrievals from the different radiometers to estimate quantitatively the precipitation rate and to characterize extreme events. Different physically based PMW precipitation retrieval algorithms are used: the Cloud Dynamics and Radiation Database (CDRD) and Passive microwave Neural network Precipitation Retrieval (PNPR), used operationally in the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Satellite Application Facility on support to Operational Hydrology and Water Management (H-SAF), and the National Aeronautics and Space Administration (NASA) Goddard PROFiling algorithm (GPROF). The assessment of the accuracy of the satellite precipitation estimates has been carried out using the available polarimetric ground-based radar and/or raingauge measurements. The different characteristics of the radiometers (i.e., viewing geometry, spatial resolution, channel assortment) and of retrieval techniques, as well as the limitations of the ground-based reference datasets, are taken into consideration in the evaluation of the accuracy and consistency of the retrievals. Results show that PMW precipitation retrievals from the GPM constellation of radiometers provide a reliable and quantitative description of the precipitation (instantaneous and on the daily scale) throughout the evolution of the precipitation systems in the Mediterranean region. The comparable relative errors among gauges, radar, and combination of radiometer overpasses legitimize the use of PMW estimates as a valuable and independent tool for monitoring precipitation. The pixel-based comparison with radars and raingauges indicates the ability of the different sensors to identify different precipitation areas and regimes (0.60 < POD < 0.76; 0.28 < FAR < 0.45; 0.42 < ETS < 0.59; -1.6 mm/h < ME < 1.1 mm/h, with values depending on the radiometer and on the precipitation product). This is particularly relevant in the presence of complex orography in proximity of coastal areas, as for the analyzed cases. For both CDRD and PNPR, some weaknesses in the PMW detection of light precipitation are found, while the areas affected by the most intense precipitation are generally well identified (the opposite is found for the GPROF products). Good consistency in CDRD and PNPR precipitation retrievals is found for (near) coincident overpasses of the different radiometers, with discrepancies that are comparable to the differences between ground-based instruments (i.e., raingauges and radar). In spite of their low spatial resolution and relatively poor temporal sampling, PMW precipitation retrievals from CDRD and PNPR, specifically designed for the Mediterranean area, provide good estimates of total precipitation amounts for the areas that are affected by the most intense precipitation, especially when the events are localized and persistent.