Observations and Model Simulations of Transport and Precipitation Development in a Seeded Cumulus Congestus Cloud

Abstract

Observations made by three instrumented aircraft, a Doppler radar, and other data sources were used to follow the initiation and development of precipitation in a small cumulus congestus cloud. The cloud was seeded at its base using an airborne silver iodide solution burner. Sulfur hexafluoride tracer gas was released along with the seeding material. Analyzers on two instrumented aircraft detected the tracer gas during subsequent cloud penetrations as it was carried up into the cloud along with the seeding agent. Ice developed initially in the upper regions of the cloud near the ?10°C level ?15 min after the commencement of seeding. This is consistent with primary nucleation by the seeding agent. The cloud developed millimeter-size graupel within the following few minutes. A radar echo approaching 40 dBZ subsequently developed. The echo was observed to descend through the cloud as the cloud dissipated. One-dimensional, steady-state and two-dimensional, time-dependent bulk water models were used to simulate this cloud. The one-dimensional model produced realistic values for updraft speeds allowing credible estimates of time required for transport from cloud base to upper regions of the cloud. The development of precipitation in the two-dimensional simulation resembled that in the observed cloud. Precipitation developed through riming of snow to graupel. In both the observed and simulated clouds, precipitation development was limited by cloud lifetime. Both clouds collapsed at a time when they were still generating ample supercooled water in their updrafts. Total precipitation on the ground from the seeded cloud simulations was ?5 times the radar estimated rainfall total of 0.5 mm from the observed seeded cloud. This occurred despite the fact that the simulated cloud went through an accelerated life cycle compared to the observed cloud. A comparison between simulations with a natural ice process and with cloud base release of silver iodide shows that seeding accelerated precipitation formation in the model cloud leading to a fourfold increase in total precipitation for the seeded cases compared to the natural one.