Effects of Cloud Seeding, Latent Heat of Fusion, and Condensate Loading on Cloud Dynamics and Precipitation Evolution: A Numerical Study

Abstract

This study attempts to isolate the dynamic and microphysical effects of seeding. A two-dimensional, time-dependent cloud model has been used to simulate silver iodide (AgI) seeding of convective clouds. Two major dynamic effects (latent heat of fusion and condensate loading) are separated through a sequence of differential processes to examine their individual effect. A High Plains sounding is used for the tests. The effects of condensate loading and latent heat of fusion are due to natural processes as well as to cloud seeding. Separate discussions and comparisons are made of both processes. Condensate loading has the greater influence on cloud development. A method of differencing the results from different cases is used to illustrate the overall seeding effects and to isolate those portions of the latent heat of fusion and loading effects which are due solely to ice-phase cloud seeding. The results indicate significant fusion and loading effects due to seeding, but at 10 min or so after the seeding. Glaciation via accretional freezing of the cloud water is accomplished at this time. Direct seeding glaciation of this vigorous cloud within a minute or so of seeding time is not accomplished. The model results show a natural cloud system that consists of three cloud cycles during the period of integration (covering a real-time period of about one hour). The first cloud cycle is produced by a model thermal and humidity perturbation, the two second-cycle clouds are set off by the acceleration stage of the first cloud, and the third-cycle clouds are initiated by downdraft outflow induced by falling precipitation in the boundary layer. Major cloud growth and precipitation formation are caused by interactions of the second-cycle clouds with the first cloud. The seeded cloud system, although identical to the unseeded system until seeding is simulated (at 19 min of simulated real-time), forms precipitation earlier than the unseeded system and produces four cloud cycles during the 60 min of simulated real-time. The first-cycle cloud is more vigorous than its unseeded counterpart, but the earlier formed precipitation interacts destructively with the two second-cycle clouds, denying their liquid water contents to the later growth stages of the first-cycle cloud. The third- and fourth-cycle clouds are produced by precipitation-induced downdraft outflow, but are weak and produce little precipitation. Those cases with the loading effect turned off develop much more rapidly than those with the latent heat of fusion effect turned off. Compared with a ?normal? run (i.e., all effects turned on), peak values of domain-averaged kinetic energy increase by nearly 100% with the loading effect off and decrease by 40% with the latent heat of fusion effect turned off. Of course, loading is always present in a cloud?latent heat of fusion may or may not depend on the development of the cells. The premature sweepout of the second-cycle cloud liquid by first-cycle precipitation from above, before the cloud liquid has a chance to freeze, thus decreases the intensity of storm development.