SEEDING CONVECTIVE CLOUDS WITH ICE NUCLEI OR HYGROSCOPIC PARTICLES - A NUMERICAL STUDY USING A MODEL WITH DETAILED MICROPHYSICS

Simulations of seeding clouds for rain enhancement with ice nuclei (IN ) or hygroscopic particles were conducted using a numerical model of a n axisymmetric convective cloud with detailed treatment of both warm a nd cold microphysical processes. The simulations were performed for th ree clouds that differed in their cloud condensation nuclei (CCN) conc entrations and spectra. Tests were carried out on clouds characterized as maritime (100 CCN cm(-3)), moderate continental (600 CCN cm(-3)), and extreme continental (1100 CCN cm(-3)) using two different initial conditions in which cloud tops reached -20 degrees and -12 degrees C. The seeding time was found to be a critical parameter for obtaining po sitive results. The optimal ''time window'' for IN seeding was found t o be very short and to correspond to the time at which the natural ice began to form. Seeding after this time reduced the rain. The optimal concentration of seeding material was about 75-125 L(-1). In the marit ime clouds rain formation processes were very efficient, and seeding d id not produce any significant increase in rain amounts. In the modera te and extreme continental clouds, with tops at -20 degrees C, seeding with IN at the optimal time and location increased the precipitation by 9% and 35%, respectively. Ice nuclei seeding of a warmer cloud with a top temperature of -12 degrees C did not change the rainfall when s eeding rook place in the optimal time window. Seeding with hygroscopic particles had a dramatic effect on the rainfall. In the moderate and extreme continental clouds increases of 65% and 109% in rain amounts w ere obtained. In these cases, the optimal time window was longer, and even clouds with tops at -12 degrees C doubled their rain amounts.