Numerical Simulation of Airborne Cloud Seeding for the DAPHNE Precipitation Enhancement Project in Central Greece

The continued increase in water needs, in relation to the semiarid conditions observed over central Greece and the imminent climate change have alerted scientists in finding new ways to augment water resources and to mitigate drought conditions. Rain enhancement can act as a supplementary tool to increase water potential, by means of weather modification, incorporating new sophisticated atmospheric models and observational systems. The project DAPHNE aims at mitigating the problem of drought in the most vital agricultural area of Greece, Thessaly, by means of rain enhancement, utilizing state-of-the-science modeling tools, performing measurement campaigns and cloud seeding experiments, and investigating the impact of its implementation on the environment. This study, as part of the project, focuses in comparing actual and simulated storm characteristics using the cell tracker TITAN (Thunderstorm Identification, Tracking, Analysis, and Nowcasting) and a three-dimensional cloud model over two subjectively selected case studies over central Greece. Moreover the cloud model is used to simulate AgI seeding by aircraft, according to the seeding methodology, seeding hypothesis and seeding rates, adopted for the project. The analyses are performed in order to verify and document the ability of the cloud model to: a) simulate the initiation, growth and behavior of such storms and b) to evaluate the effects of seeding with respect to rainfall increase.

The WRF model (WRF-ARW ver3.5.1) is applied to simulate the airflow characteristics and the convective activity in the area of interest. It is integrated in three domains covering Europe, Eastern Mediterranean and Central-Northern Greece, using telescoping nesting with grid spacing of 15km, 5km and 1km. Simulated soundings, derived from the WRF outputs, are used to initialize the convective cloud model at time and space, where the actual storms developed within the area of interest for a more realistic storm depiction. The convective cloud model, that is used to simulate the actual cases and assess cloud seeding, is a three-dimensional, non-hydrostatic, time-dependant, compressible, homogeneous system. Storm characteristics are observed using a C-band weather radar located within the third domain (39.674N, 21.837E). The selected cases correspond to southwest (SW) and northwest (NW) synoptic conditions; which seems to have the highest frequency of appearance as storm producing synoptic patterns over the area during the warm season.

The seeding optimization for each selected case is conducted by analyzing thermodynamic characteristics of the atmosphere as well as observed radar reflectivity fields. Preliminary results indicate the ability of the cloud model to simulate such convective storms, since model calculated maximum radar reflectivity values, as well as storm horizontal and vertical extend, coincide with the recorded ones. Regarding cloud seeding, results have shown positive effects, with respect to rainfall increase, after successful seeding procedures. Seeded clouds exhibit earlier development of precipitation along with an updraft enhancement. Rainfall increase is estimated at about 10%. A hailfall decrease of about 15% is also depicted. Results emphasize a strong interaction between cloud dynamics and microphysics, especially in small scale processes, which influence seeding agent transport and diffusion within the storm's complex environment. Seeding effects are strongly depended upon the seeding strategy, relying on the exact seeding time, the right placement of the seeding agent and the appropriate seeding rate.

Acknowledgments: This research work is part of DAPHNE project (11SYN_8_1088_TPE) which is co-funded by the European Union (European Regional Development Fund) and Greek National Funds, through the action "COOPERATION 2011: Partnerships of Production and Research Institutions in Focused Research and Technology Sectors" in the framework of the operational programme "Competitiveness and Enterpreneurship" and Regions in Transition (OPC II, NSRF 2007-2013).