Abstract
Operational cloud seeding programs have been increasingly deployed in several countries to augment natural rainfall amounts, particularly over water-scarce and arid regions. However, evaluating operational programs by quantifying seeding impacts remains a challenging task subject to complex uncertainties. In this study, we investigate seeding impacts using both long-term rain gauge records and event-based weather radar retrievals within the framework of the United Arab Emirates (UAE) National Center of Meteorology’s operational cloud seeding program. First, seasonal rain gauge records are inter-compared between unseeded (1981–2002) and seeded (2003–2019) periods, after which a posteriori target/control regression is developed to decouple natural and seeded rainfall time series. Next, trend analyses and change point detection are carried out over the July-October seeding periods using the modified Mann-Kendall (mMK) test and the Cumulative Sum (CUSUM) method, respectively. Results indicate an average increase of 23% in annual surface rainfall over the seeded target area, along with statistically significant change points detected during 2011 with decreasing/increasing rainfall trends for pre-/post-change point periods, respectively. Alternatively, rain gauge records over the control (non-seeded) area show non-significant change points. In line with the gauge-based statistical findings, a physical analysis using an archive of seeded (65) and unseeded (87) storms shows enhancements in radar-based storm properties within 15–25 min of seeding. The largest increases are recorded in storm volume (159%), area cover (72%), and lifetime (65%). The work provides new insights for assessing long-term seeding impacts and has significant implications for policy- and decision-making related to cloud seeding research and operational programs in arid regions.
Highlights
In response to shortages in water resources, exacerbated by growing populations and a changing climate, an increasing number of countries have invested in weather modification research and applications [1]
Precipitation enhancement is a subset of weather modification that aims to augment natural rainfall amounts through airborne or groundbased interventions in the microphysical processes of specific cloud types [2,3]
According to the most recent review on global precipitation enhancement activities conducted by the World Meteorological Organization (WMO) Expert Team on Weather Modification, cloud seeding from aircraft platforms is generally considered more effective compared to other techniques such as ground-based generators, customized rockets, and artillery shells [1]
Summary
In response to shortages in water resources, exacerbated by growing populations and a changing climate, an increasing number of countries have invested in weather modification research and applications [1]. Hygroscopic cloud seeding entails introducing large artificial (hygroscopic) aerosol particles into clouds to increase the uptake of available cloud liquid water beyond that expected from the natural background aerosol population with relatively smaller diameter sizes [4,5,6]. The larger seeding particles are expected to trigger a “competition effect” which favors the production of large drops that can activate the collision-coalescence process and enhance rainfall generation [7,8,9]. Evaluating the effectiveness of operational cloud seeding programs is critical to advance weather modification research as well as to provide policymakers with realistic cost-benefit metrics. Results from operational cloud seeding programs spanning several countries, including Australia [10], China [11], India [12], Israel [13], South
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