Potential impacts of lightning-induced plasma on cloud ice formation and precipitation have been a subject of debate for decades. Here, we report on the interaction of laser-generated plasma channels with water and ice clouds observed in a large cloud simulation chamber. Under the conditions of a typical storm cloud, in which ice and supercooled water coexist, no direct influence of the plasma channels on ice formation or precipitation processes could be detected. Under conditions typical for thin cirrus ice clouds, however, the plasma channels induced a surprisingly strong effect of ice multiplication. Within a few minutes, the laser action led to a strong enhancement of the total ice particle number density in the chamber by up to a factor of 100, even though only a 10 −9 fraction of the chamber volume was exposed to the plasma channels. The newly formed ice particles quickly reduced the water vapor pressure to ice saturation, thereby increasing the cloud optical thickness by up to three orders of magnitude. A model relying on the complete vaporization of ice particles in the laser filament and the condensation of the resulting water vapor on plasma ions reproduces our experimental findings. This surprising effect might open new perspectives for remote sensing of water vapor and ice in the upper troposphere.nonlinear optics | secondary ice | lightning C louds and their feedbacks in the climate system are the largest source of uncertainty in our ability to predict future climate (1). At the same time, they play an important role in the atmospheric part of the fresh water cycle. In both cases, cloud ice formation is of central importance.Cirrus clouds are formed over large areas of the upper troposphere at altitudes between 6 and 12 km at temperatures below −37°C (2) where only ice can exist as all water freezes by homogeneous nucleation (3). They cool the earth surface by reflecting incoming solar radiation and at the same time warm it by absorbing outgoing thermal radiation. Although, on average, the warming effect seems to prevail, the magnitude and sign of the net climatic effect of cirrus clouds depend on the height and temperature of the cirrus cloud as well as on the size distribution and shape of the ice crystals (4). In contrast to liquid phase clouds, the nucleation of ice clouds is often kinetically hindered, so that large areas of supersaturation with respect to ice seem common especially in the upper tropical troposphere (5). Relative humidity and saturation is difficult to measure at these low temperatures, however, both by in situ measurements and by remote sensing.Precipitation in midlatitude clouds is predominantly initiated via the Wegener-Bergeron-Findeisen process (6), which relies on the heterogeneous freezing of supercooled cloud droplets promoted by ice-active aerosol particles and the subsequent growth of the ice particles by water vapor deposition and riming. As good ice nuclei are very rare in the atmosphere (7), ice and liquid cloud particles can coexist at temperatures above −37°C and...
