The Initiation of Precipitation in Updrafts

Abstract

The initial stages of the formation of precipitation in uniform updrafts under natural and seeded conditions are examined using a detailed microphysical model of parcel ascent. The effects of seeding are related to the initiation of precipitation which is considered to be the rate-limiting step in the precipitation process. The times required to reach a 20 dBZ radar reflectivity factor are compared between natural and seeded cases. Three regions of ice-nucleant seedability are defined on the bases of cloud-base temperature and updraft velocity. These are 1) no precipitation under natural or seeded conditions, 2) easy to overseed, and 3) difficult to overseed. It is suggested that these regions define a physically valid stratification for the analysis of cloud seeding projects. The optimal ice crystal concentration which yields the minimum time to form precipitation is estimated to be between 100 and 1000 crystals per liter. Seeding with large drops is shown to produce precipitation more efficiently than AgI in all cases with cloud bases warmer than 0°C. Increasing the seeding rate always reduces the time required to produce precipitation (at the 20 dBZ level), i.e., there is no optimal seeding rate as with ice nucleant seeding. The time required to produce precipitation also decreases with increasing seed drop size (for a constant seeding rate); however, the effects of sedimentation may be expected to reverse this trend and to produce an optimal seed drop size for a given updraft. An examination of the dilution or entrainment into the parcel shows a marked reduction in the production of large drops by the coalescence process as the dilution is increased. It is concluded that the coalescence process in the cloud is dominated by the rare regions of nearly adiabatic liquid water content.