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Colloquium

Department of Atmospheric Science

Tue., Apr. 10, 3:10 pm, EN6085

A tale about supercooled liquid water in wintertime clouds

Dr. Sarah Tessendorf

Researchg Applications Laboratory, National Center for Atmospheric Research

Abstract

Conventional wisdom suggests that water freezes at 0°C (32°F). Yet, it can only do so with an appropriate ice nucleus. Therefore, most water in the atmosphere can supercool, existing at temperatures well below the freezing point. The presence of supercooled liquid water (SLW) in clouds can be a sign that ice-phase precipitation formation processes are not efficient, and therefore such clouds might be suitable for potential precipitation enhancement via cloud seeding techniques. In fact, for over 60 years, water managers around the world have attempted to use cloud seeding to augment water resources. Yet, despite decades of research, clear evidence of the effectiveness of cloud seeding to enhance precipitation still does not exist. Another outcome of SLW in clouds is that it poses a serious threat to aviation due to the potential for aircraft icing. In fact, some wintertime clouds produce supercooled drizzle-size drops, also known as supercooled large drops (SLD), which pose a unique icing hazard for aviation.

The Seeded and Natural Orographic Wintertime clouds: the Idaho Experiment (SNOWIE) field program took place January–March 2017 to investigate natural orographic precipitation processes, with a focus on ice initiation, snow growth, and the impacts of orography on the development of SLW, as well as to evaluate the impact of cloud seeding on orographic clouds. Measurements from SNOWIE provide the first direct, incontrovertible evidence of the effects of cloud seeding from the initiation of first ice through growth of crystals by deposition and riming to the eventual fallout of precipitation.

Additionally, clouds with SLD were frequently observed during the SNOWIE field campaign and these data are also being used to support aircraft icing research at NCAR. In particular, the goals of the icing research using SNOWIE data are to better understand conditions that lead to SLD icing and to evaluate and improve tools for predicting and diagnosing aircraft icing conditions.

This seminar will present an overview of the SNOWIE field campaign, including a summary of the data collected during the campaign and a few highlights from the preliminary analysis completed to date, as well as how the data is being used for aircraft icing research.


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