These facts have motivated the World Meteorological Organization (WMO) Expert Team on Weather Modification to review the progress made on precipitation enhancement since the last published assessments in 2000 (WMO) and 2003 (NRC). The presentation will summarize the findings of the report, as well as the subsequently published publication in BAMS.
The presentation focuses on the two cloud types most seeded in the past: winter orographic cloud systems and convective cloud systems; and also discusses different seeding techniques. A key issue for cloud seeding is the extension from cloud-scale research to water catchment-scale impacts on precipitation on the ground. Consequently, the requirements for the design, implementation and evaluation of a catchment-scale precipitation enhancement campaign are discussed.
It was noted that the most promising results pertain to wintertime orographic clouds. Their distribution of natural precipitation suitable for seeding is largely determined by the orography interacting with synoptic-scale systems. Thus, the spatial and temporal distribution of precipitation on the ground can often be estimated with sufficient accuracy to allow the impact of seeding to be quantified. Seeding with glaciogenic particles near mountainous water catchments aims to convert orographically-induced supercooled liquid water to ice, which in turn leads to snowfall and increased precipitation on the ground. Careful analysis of randomized campaigns identifies a possible increase of precipitation ranging from essentially zero to more than 20%. Higher values tend to be associated with aircraft-based seeding. The most promising results are obtained for clouds that have already a natural tendency for precipitation formation.
Mixed phase convective clouds have also been seeded with hygroscopic or glaciogenic particles with the aim of triggering liquid or mixed phase precipitation. As these clouds are generally driven by surface heating, the variability of their natural precipitation means that it is difficult to quantify any increase in local precipitation due to seeding.
Only once the range of exploratory studies has been completed and deemed successful, the seeding can be extended to larger areas and time periods to obtain an economic benefit. This upscaling of an exploratory seeding campaign to a catchment basin-sized region requires again a strict protocol. Before a catchment-scale experiment is undertaken, historical data should be analysed to estimate the probability of detection of enhanced precipitation, that is, to determine the minimum duration of the experiment. Randomization of seeding and a consistent methodology are essential to support a rigorous statistical analysis of the data collected during an experiment. High-resolution modelling can be used to support all phases of an experiment. Furthermore, possible toxicological, ecological, sociological and legal issues, as well as extra-area effects need to be considered.
The presentation concludes by indicating some of the most important gaps in our knowledge. Some recommendations regarding the most urgent research topics are given to stimulate further research.
The WMO Expert Team on Weather Modification acknowledges the support provided by the National Center of Meteorology, Abu Dhabi, UAE under the UAE Research Program for Rain Enhancement Science.
References:
Flossmann, A.I., M. J. Manton, A. Abshaev, R. Bruintjes, M. Murakami, Th. Prabhakaran, Z. Yao, 2019: Review of Advances in Precipitation Enhancement Research; Bull. Atm. Met. Soc., DOI: 10.1175/BAMS-D-18-0160.1
Flossmann, A.I., Michael Manton, Ali Abshaev, Roelof Bruintjes, Masataka Murakami, Thara Prabhakaran and Zhanyu Yao, 2018: Peer Review Report on Global Precipitation Enhancement Activities; 129pp, WMO, WWRP; http://www.wmo.int/pages/prog/arep/wwrp/new/documents/FINAL_WWRP_2018_1.pdf
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