Satellite-Retrieved Microstructure of AgI Seeding Tracks in Supercooled Layer Clouds

Abstract

NOAA Advanced Very High Resolution Radiometer (AVHRR) images revealed conspicuous tracks of glaciated cloud in thick supercooled layer clouds over central China. These tracks were identified as being artificially produced by cloud-seeding operations at the ?10°C isotherm, less than 1 km below cloud tops, aimed at precipitation enhancement, by means of AgI acetone generators. The cloud composition was deduced by retrieving the cloud-top effective radius (re) and analyzing its spatial relations with cloud-top temperatures and with the visible reflectance. Cloud-top temperature varied between ?13° and ?17°C. The glaciation became apparent at cloud tops about 22 min after seeding. The glaciated tops sank and formed a channel in the supercooled layer cloud. The rate of sinking of about 40 cm?s?1 is compatible with the fall velocity of ice crystals that are likely to form at these conditions. A thin line of new water clouds formed in the middle of the channel of the seeded track between 38 and 63 min after seeding, probably as a result of rising motions induced by the released latent heat of freezing. These clouds disappeared in the more mature segments of the seeded track, which continued to expand throughout the observation period of more than 80 min. Eventually the seeding tracks started to dissipate by expansion of the ambient cloud tops inward from the sides. Using the brightness temperature difference between 10.8 and 12.0 ?m allowed for observation of the seeding signature deep in the clouds, even when it was obscured under thin supercooled layer clouds. This is the third and most detailed report of effects of advertent cloud seeding for precipitation enhancement being detected and analyzed based on satellite observations. It opens new possibilities of using satellites for directing and monitoring weather modification experiments and operations.