Analysis and numerical simulation of a real cell merger using a three-dimensional cloud resolving model T. Karacostas a, ⁎, V. Spiridonov b , D. Bampzelis a , I. Pytharoulis a , I. Tegoulias a , K. Tymbanidis a a Department of Meteorology and Climatology, School of Geology, A.U.Th., Thessaloniki 541 24, Greece b Faculty of Natural Sciences and Mathematics, Institute of Physics, Gazi Baba bb, 1000 Skopje, The Former Yugoslav Republic of Macedonia abstract article info Article history: Received 31 December 2014 Received in revised form 9 September 2015 Accepted 10 September 2015 Available online 30 September 2015 Keywords: Cloud model Simulated cloud seeding Storm characteristics A three-dimensional cloud resolving model is used to study a real cell merger case that occurred on 10 August, 2008 over north-central Greece, causing heavy rainfall, hailfall and high-frequency lightning. Firstly, the storm is observed, analyzed and recorded using a C-band weather radar. Secondly, three distinct simulations are performed using a cloud resolving model. An unseeded simulation, in order to test the ability of the model to reproduce the structural and evolutionary properties of the storm and two seeded simulations in which seeding occurred before and after cell merging. Reflectivity fields are analyzed, horizontally and vertically, at different simulation times. The 3-D numerical simulations suggest that the merger process occurred by two or three iso- lated single-cells and formed during their SW–NE motion. The merging process apparently alters dynamical and microphysical properties through low and middle level forcing; increases cloud diameters and cloud depths, pro- ducing more graupel and ice particles and increases radar reflectivity values. Processed radar images depict a similar view of the storm structure, evolution and interactions of such merging processes. The model calculated maximum radar reflectivity values coincide with the recorded ones. Results indicate that seeding the cloud be- fore its merging produces more positive effects on hail suppression than seeding after merging. These findings are quite important, in order to document the value of the cloud resolving model and its capability to simulate and reproduce the realistic storm processes and to provide a better understanding of the cloud dynamical and microphysical features related to different seeding approaches. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Airborne cloud seeding of convective clouds aiming for hail suppres- sion is a big challenge and common practice in many areas over the world for more than 60 years. There is a strong scientific evidence and assessment on the status of weather modification from literature (Rosenfeld and Woodley, 1989, 1993; Silverman, 2001; Pocakal and Stalec, 2003; Cotton and Pielke, 2007; Levin and Cotton, 2008). A num- ber of projects applied, related to hail suppression and rain augmenta- tion, have showed optimistic results over the years, applying static glaciogenic seeding of cumulus convective clouds (Dennis, 1980; Mason, 1980; Isaac et al., 1982; Silverman, 1986; Makitov, 2007; Krauss and Santos, 2004). Nevertheless, further research and improve- ments are required, since weather modification projects still have asso- ciated risks and the results may remain uncertain. Numerical modeling capabilities offer new opportunities and permit a more detailed examination and practice on weather modification activities. Modeling of the seeding procedure is a valuable tool at investigating responses on cloud dynamical and microphysical features. Moreover, hail reduc- tion and precipitation enhancement can be measured and tested against different seeding methods and strategies, seeding rates and seeding locations. Northern and central Greece is frequently affected by severe storms, accompanied by hail, during the warm period of the year (April to September). For this reason, the Greek National Hail Suppression Pro- gram (NHSP) was designed (Karacostas, 1984) and applied in these areas since 1984, with the objective to reduce hail damages on agricul- tural products. Several studies of convective storm characteristics and hailstorms over northern Greece have been based on the NHSP program (Karacostas, 1989; Karacostas, 1991; Foris et al., 2006; Bampzelis and Karacostas, 2012). A set of sensitivity experiments have been conducted to examine the cloud seeding effects of different convective clouds, under different atmospheric environments (mid-latitude, tropical) using both, single and double-moment microphysics schemes. An im- portant aspect in the study of convective activity is the identification of situations, such as cloud splitting or merging, which lead to intense Atmospheric Research 169 (2016) 547–555 ⁎ Corresponding author. E-mail address: karac@geo.auth.gr (T. Karacostas). http://dx.doi.org/10.1016/j.atmosres.2015.09.011 0169-8095/© 2015 Elsevier B.V. All rights reserved. Contents lists available at ScienceDirect Atmospheric Research journal homepage: www.elsevier.com/locate/atmos