This is a field campaign, referred to as ASCII (AgI Seeding of Clouds Impact Investigation), to investigate how glaciogenic seeding affects the cloud and precipitation physics of winter orographic clouds. ASCII takes advantage of an operational cloud seeding project in Wyoming and will deploy an airborne Doppler profiling cloud radar and lidar. This project combines the analysis of data from new yet proven instruments with high-resolution cloud- and aerosol-resolving simulations, in order to evaluate the model, to understand how competing microphysical processes are affected by the injection of ice nuclei, and to assess how upstream conditions (moisture, temperature, stability, ice nuclei and cloud condensation nuclei concentrations) influence the impact of ground-based glaciogenic seeding on cloud properties and surface precipitation. It also examines two processes that may be important in precipitation formation in mixed-phase orographic clouds, i.e. boundary-layer turbulence and surface-induced ice initiation.
Intellectual merit: Verification of precipitation enhancement has proven to be extremely difficult, in part because of a high level of "noise" in naturally precipitating cloud systems, compared to the magnitude of the signal. Yet in the past two decades new tools have been developed to more effectively study the impact of cloud seeding, in particular ground-based glaciogenic seeding of orographic clouds. On the one hand, there are new observational tools, in particular airborne cloud radars and lidars, allowing a detailed view of processes just above the complex terrain, where most of the natural hydrometeor growth occurs. On the other hand, dramatic advances in the numerical modeling of aerosol and cloud processes now enable the simulation of the ice nuclei seeding process. Model output can be processed such that simulations can be compared directly with composite radar and lidar data. This study is relevant not only to the practical question of efficacy of precipitation enhancement by means of glaciogenic cloud seeding, but also to the much broader question about how the concentration of cloud-active aerosol affects the precipitation efficiency of clouds, thereby affecting the latent and the radiative heat forcing of the climate system. The latter question is very complex, and thus can be productively addressed by means of relatively controlled experiments such as glaciogenic cloud seeding.
Broader Impacts: Cloud seeding has been the most widely practiced method of advertent weather modification, mainly with the purpose of enhancing precipitation. It is remarkable that notwithstanding a series of targeted field campaigns and the stronger experimental control than in field work dealing with natural cloud and precipitation processes, the effectiveness of cloud seeding in enhancing precipitation remains uncertain. Nonetheless, seeding clouds to enhance precipitation remains a thriving commercial activity, which simply points to the high potential benefit, given the cost of water in water-limited regions. We are confident that the project will shed new light into how the injection of ice nuclei in orographic clouds affects cloud properties and surface precipitation. The field work will train several graduate students in atmospheric measurement techniques.
