Thursday, 31 August 2023
Boundary Waters (Hyatt Regency Minneapolis)
Natural cloud composition and variability can have a significant impact on the effectiveness of cloud seeding at enhancing snowfall over complex terrain. In theory, cloud seeding with silver iodide can increase the effectiveness of snow growth and fallout in clouds containing supercooled liquid water due to low natural abundances of ice nuclei. The mesoscale and microscale dynamics within orographic wintertime clouds influence natural ice initiation and abundance, particle and snow growth, and thus the resulting snowfall. Examples of some of these dynamic phenomena include terrain-induced circulations (i.e., gravity waves), cloud top generating cells, and turbulence over complex terrain. These phenomena as well as variables relating to ice initiation and ice growth (liquid and ice water content, cloud droplet concentration, ice particle habits), and variables relating to snow growth/fallout were captured extensively throughout the Seeded Natural and Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE) field campaign in the Payette mountains of Idaho between January and March 2017.
SNOWIE utilized ground-based dual-polarization radars, airborne in-situ cloud droplet probes, and an airborne vertically-pointing Doppler radar to capture natural and seeded orographic clouds. In addition to a research aircraft, which flew parallel to the prevailing wind to capture downstream vertical velocity, reflectivity cross-sections, and the overall evolution of natural and seeded clouds and precipitation, there was also a seeding aircraft, which flew in a perpendicular flight path further upstream of the study region and released ejectable and burn-in-place silver iodide flares to disperse seeding material.
In this study, we focus on a subset of the intensive observations periods (IOPs) from SNOWIE and conduct radar-based microphysical case studies, to qualitatively differentiate the evolving snow growth and fallout processes within natural and seeded clouds. To incorporate the impact of seeding, the amount of seeding material released, and the seeding time/duration is included in these analyses. Preliminary results show enhancements of reflectivity in aircraft cross-sections and ground-based range height indicator (RHI) radar scans associated with fine-scale kinematic structures such as cloud top generating cells, turbulence, embedded convection, and isolated convective cells. During some events, ice particle production from seeding enhanced the natural ice particle production in these updraft structures forming distinct seeding signatures. During other events, natural ice production was sufficient to generate precipitation and seeding signatures were weak or not apparent.
SNOWIE utilized ground-based dual-polarization radars, airborne in-situ cloud droplet probes, and an airborne vertically-pointing Doppler radar to capture natural and seeded orographic clouds. In addition to a research aircraft, which flew parallel to the prevailing wind to capture downstream vertical velocity, reflectivity cross-sections, and the overall evolution of natural and seeded clouds and precipitation, there was also a seeding aircraft, which flew in a perpendicular flight path further upstream of the study region and released ejectable and burn-in-place silver iodide flares to disperse seeding material.
In this study, we focus on a subset of the intensive observations periods (IOPs) from SNOWIE and conduct radar-based microphysical case studies, to qualitatively differentiate the evolving snow growth and fallout processes within natural and seeded clouds. To incorporate the impact of seeding, the amount of seeding material released, and the seeding time/duration is included in these analyses. Preliminary results show enhancements of reflectivity in aircraft cross-sections and ground-based range height indicator (RHI) radar scans associated with fine-scale kinematic structures such as cloud top generating cells, turbulence, embedded convection, and isolated convective cells. During some events, ice particle production from seeding enhanced the natural ice particle production in these updraft structures forming distinct seeding signatures. During other events, natural ice production was sufficient to generate precipitation and seeding signatures were weak or not apparent.
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