Some Surprising Results from Simulated Seeding of Stratiform-Type Clouds

Abstract

Studies have been conducted to determine the cloud seeding potential of stratiform type clouds using a two-dimensional, time-dependent cloud model. An atmospheric sounding from Villanubla, Spain, in February 1980, was used to initialize the model. The model is designed to allow mesoscale convergence in the lower levels and divergence in the upper levels, which results in a stratiform-type cloud in this Spanish situation. The seeding of clouds using either dry ice or silver iodide has been tested and rather surprising results are indicated. The silver iodide seeding simulations produce strong dynamic responses in the model clouds, even with small amounts of supercooled liquid available and a few natural ice crystals per liter in the cloud. These effects occur in a nearly moist adiabatic layer as well as in a convectively unstable layer. The effects appear to be due to the heat released as the liquid freezes and the cloudy environment switches from liquid saturation to ice saturation. Cloud vertical motions of a few to several m s?1 are produced in the seeded cloud region. Vertical motions of 10 to 20 cm s?1 exist in comparable regions of the unseeded cloud. Precipitation is strongly affected. Consequently, this heat release is much more significant in terms of the overall energetics of the cloud than has been evident in our seeding simulation conducted in pure convective situations with much stronger updrafts. The tests of the dry ice seeding indicate small effects, but this is largely due to the rapid fall of the dry ice pellets through the cloud and to the short time period available for the seeding to take effect.