Features of radar echoes formed by aircraft cloud-seeding with liquid carbon dioxide

The management of water resources is inevitable for agricultural activities including irrigation in semi-arid region, industrial use, hydroelectric power generation, and human life. Anticipated climate change caused by future global warming has large possibility to lead to the shortage of water in many areas in the world. Therefore, further development of technologies for effectively obtaining water resources are required. Many challenges of cloud-seeding projects have been conducted until the present, because of many uncertainties in the temporally and spatially-changing complicated micro-physical and dynamical processes occurring in clouds. However, cloud-seeding technologies have not reached the stage of practical use for obtaining a large amount of water resources. From the background, towards the future, the accumulation of significant evidences induced by cloud-seeding is important for the development of cloud-seeding technology. Therefore, this study investigated significant features of radar-based signals obtained by the seeding of liquid carbon dioxide (LCD). A cloud seeding experiment using LCD was carried out near Fukuoka City in Northern Kyushu on December 26, 2013. The small airplane took off the Saga Airport in Saga City toward Karatsu City near Fukuoka City at 1300JST. The weather condition of the experimented day was after the passage of low pressure depression. The air temperature of the ground in Fukuoka City was 7.3C. And the wind-speed and wind-direction (850hPa to 700hPa) above Fukuoka City were from 15ms-1 to 20ms-1 and west-southwest (WSW). Prior to LCD-seeding, stratocumulus or layer clouds consisting of two layers could be confirmed by aircraft observation. The thickness of the first cloud layer was about 2000m (cloud top: 3460m, cloud base: 1500m). The cloud top temperature of the first layer was -9.5C. Since the second cloud layer was located at high altitude, this experiment selected the first cloud layer as a target of the LCD- seeding. The seeding operations were carried out three times (total seeding duration: 9 minutes between 1335 and 1344JST) at an altitude of 2600m (-4 C) with a seeding rate of 11.1g/s. (1)The first LCD-seeding experiment was carried out from south-southeast (SSE) to north-northwest (NNW) over Karatsu City. The seeding altitude was 2480m (-2.1C) and the seeding time was 120 seconds (1335 to 1337JST). (2)The second LCD-seeding experiment was carried out from SSE to NNE in almost same course as the first experiment. The seedind altitude was 2650m (-4.1C) and the seeding time was 120 seconds (1338 to 1340JST). (3)The last LCD-seeding experiment was carried out from WSW to ENE over Karatsu City. This flight pass was almost right-angled of the first and second flight pathes. The seeding altitude was 2596m (-3.3C) and the seeding time 120 seconds (1342 to 1344JST). Seeded clouds moved towards the northeast, and further development of the cloud top height could be visually recognized from the equipped aircraft. At 1440JST (55 min. after seeding), suddenly appearance of artificially induced radar echoes could be recognized by the radar observation of the Japan Meteorological Agency (JMA). The radar echoes continued until 1550JST. And the maximum area of artificial radar echoes was abou 400km2 on 1500JST (75min. after seeding). It is considered that the sudden appearance of the radar echoes implies similar tracking and similar growth history of artificially-induced ice particles because all the ice particles were formed at a constant altitude by the LCD-seeding. It is considered that this is one of signals of artificially-induced radar echoes by using the LCD-seeding.