Invited Speaker: Evaluation of the Wyoming winter orographic cloud seeding program: Design of the randomized seeding experiment

The Wyoming Weather Modification Pilot Program (WWMPP), funded by the State of Wyoming through the Wyoming Water Development Commission, is unique among state-sponsored programs in that it includes a substantial evaluation component. The main purposes of the WWMPP are to establish an orographic cloud seeding program in three target areas (the Medicine Bow Range, Sierra Madre Range and Wind River Range) and evaluate the feasibility and effectiveness of the cloud seeding. The logistics, infrastructure, and operations of the program are covered under a contract with Weather Modification Inc., while the evaluation activities fall under a separate contract with the Research Applications Laboratory of the National Center for Atmospheric Research.

The WWMPP builds off of past research into winter orographic cloud seeding that established and refined a generally-accepted cloud seeding conceptual model. Two general approaches are guiding the evaluation of the WWMPP: a) a randomized experiment that builds distributions of seeded and control (unseeded) cases, and b) exploratory studies to investigate a wide variety of ideas on detecting seeding effects, including physical studies to document the precipitation formation events hypothesized to be important to snowfall production in orographic storms. In this paper, we focus on the design aspects of the randomized experiment.

The first two winter seasons of the program (2005-06, 2006-07) were needed for performing exploratory studies to develop the evaluation plan, permitting activities, installing equipment, and peer review of the scientific experimental design. The initial seeding season (2007-2008) revealed some design issues that were subsequently addressed. Two seeding seasons under the experimental design have been completed. Elements of the final experimental design, both logistical aspects and the statistically design, are detailed in the paper. Some physical measurements are presented that describe the Wyoming program in the context of the conceptual model. Preliminary results will be presented to highlight strengths and weaknesses in the design and to report on progress to date.