Multi-sensor analysis of convective activity in central Italy during the HyMeX SOP 1.1

Atmos. Meas. Tech., 9, 535–552, 2016 www.atmos-meas-tech.net/9/535/2016/ doi:10.5194/amt-9-535-2016 © Author(s) 2016. CC Attribution 3.0 License. Multi-sensor analysis of convective activity in central Italy during the HyMeX SOP 1.1 N. Roberto1 , E. Adirosi1 , L. Baldini1 , D. Casella1 , S. Dietrich1 , P. Gatlin2 , G. Panegrossi1 , M. Petracca1,3 , P. Sanò1 , and A. Tokay4,5 1 CNR – Istituto di Scienze dell’Atmosfera e del Clima, Rome, Italy 2 NASA Marshall Space Flight Center, Huntsville, AL, USA 3 Department of Physics, University of Ferrara, Ferrara, Italy 4 Joint Center for Earth Systems Technology, University of Maryland Baltimore County, Baltimore, MD, USA 5 NASA Goddard Space Flight Center, Greenbelt, MD, USA Correspondence to: N. Roberto () Received: 30 July 2015 – Published in Atmos. Meas. Tech. Discuss.: 7 September 2015 Revised: 12 January 2016 – Accepted: 14 January 2016 – Published: 17 February 2016 Abstract. A multi-sensor analysis of convective precipitation events that occurred in central Italy in autumn 2012 during the HyMeX (Hydrological cycle in the Mediterranean experiment) Special Observation Period (SOP) 1.1 is presented. Various microphysical properties of liquid and solid hydrometeors are examined to assess their relationship with lightning activity. The instrumentation used consisted of a C-band dual-polarization weather radar, a 2-D video disdrometer, and the LINET lightning network. Results of Tmatrix simulation for graupel were used to (i) tune a fuzzy logic hydrometeor classification algorithm based on Liu and Chandrasekar (2000) for the detection of graupel from Cband dual-polarization radar measurements and (ii) to retrieve graupel ice water content. Graupel mass from radar measurements was related to lightning activity. Three significant case studies were analyzed and linear relations between the total mass of graupel and number of LINET strokes were found with different slopes depending on the nature of the convective event (such as updraft strength and freezing level height) and the radar observational geometry. A high coefficient of determination (R 2 = 0.856) and a slope in agreement with satellite measurements and model results for one of the case studies (15 October 2012) were found. Results confirm that one of the key features in the electrical charging of convective clouds is the ice content, although it is not the only one. Parameters of the gamma raindrop size distribution measured by a 2-D video disdrometer revealed the transition from a convective to a stratiform regime. The raindrop size spectra measured by a 2-D video disdrometer were used to partition rain into stratiform and convective classes. These results are further analyzed in relation to radar measurements and to the number of strokes. Lightning activity was not always recorded when the precipitation regime was classified as convective rain. High statistical scores were found for relationships relating lightning activity to graupel aloft. 1 Introduction Cloud microphysical processes and their relation to the electrical activity during intense convective precipitation events is an issue of strong interest, especially for its impact on numerical weather prediction (NWP) models. The contribution of very fine-scale kinematic and microphysical processes and their nonlinear interactions with larger scale processes limit the ability of current NWP models to predict these phenomena (e.g., Weisman et al., 2008; Miglietta and Rotunno, 2012). Many projects and fields campaigns aimed at investigating convection by deploying ground and airborne instruments have been conducted especially in the USA and in tropical regions, the most recent one being the multi-year and multi-site Chuva in Brazil (Machado et al., 2013), or the Midlatitude Continental Convective Clouds Experiment (MC3E) in Oklahoma (Petersen and Jensen, 2012). Although similar experiments are not so frequent in the Mediterranean, the ongoing Hydrological cycle in the Mediterranean ex- Published by Copernicus Publications on behalf of the European Geosciences Union. 536 N. Roberto et al.: Multi-sensor analysis of convective activity in central Italy periment (HyMeX) includes some experimental activities to investigate convection. In particular, the Special Observation Period (SOP) 1.1 that took place between 5 September and 6 November 2012 in target regions of the Mediterranean region was dedicated to observing heavy precipitation and flash floods (Ducrocq et al., 2014). Several instrumented hydrometeorological sites, three of them in Italy – Liguria–Tuscany (LT), northeastern Italy (NEI), and central Italy (CI) – were set up to investigate the mechanisms responsible for the heavy precipitation events in these areas (see Ferretti et al., 2014; for an overview of the SOP activities in Italy). The CI site is of particular interest both for its central position between the Adriatic Sea and the Tyrrhenian Sea, and because it includes the densely populated urban area of Rome consisting of approximately 4 million inhabitants. In the late summer–early autumn, severe weather conditions are quite frequent in CI and are mostly related to the development of intense convective systems (Melani et al., 2013). The correct forecast of precipitation associated with these events is linked to the ability of numerical models to initiate convection, which is especially challenging over the Tyrrhenian Sea, and with the correct representation of cloud microphysical processes (Ferretti et al., 2014). The microphysical schemes used by NWP models often misrepresent the auto-conversion processes as well as the characteristics of cloud particles (e.g., size distribution, densities). The microphysical parameterization should be tuned to the different types of precipitation regimes (i.e., convective vs. stratiform) throughout the life cycle of the simulated event (Lang et al., 2003; Wu et al., 2013). Ground-based instruments such as weather radars and disdrometers can be used to gain insights about the microphysical structure of a precipitating cloud and to derive the parameters required by microphysics schemes. Lightning information is useful to monitor the evolution of convective events (e.g., Gatlin and Goodman, 2010) as well as to assess the NWP model ability to reproduce their intensity and predict their evolution in time (e.g., Federico et al., 2014). Robust relationships of lightning activity to cloud microphysics and dynamical properties (such as updraft strength) could be exploited by assimilating lightning data into NWP models to improve the initiation and forecast in both time and space of convective activity (Lynn et al., 2012; Lagouvardos et al., 2013). Numerous studies have investigated the relationship between lightning and heavy precipitation, and, more specifically, between the volume of precipitable water and lightning flashes (or lightning strokes), and it was found to depend on climatology as well as on geographica (...truncated)