Process behind ice formation in clouds decoded

In a first, researchers have discovered how ice formation in clouds is initiated by the turbulence from vertical air motions, an advance that may lead to better understanding of how rain and snowfall are initiated in clouds.

The researchers, including those from Leibniz Institute for Tropospheric Research (TROPOS) in Germany, said it was important to understand this process in clouds since without it no precipitation would occur in the middle latitudes of the Earth - which includes parts of southern India.

The study, published in the journal npj Climate and Atmospheric Science, said as hot air is less dense than colder air, it rises and forms vertical air motions, and these induce ice formation in mixed-phase clouds that contain water vapour, ice particles, and supercooled liquid droplets.

The researchers used laser and radar equipment to measure the vertical air velocity, and ice formation in thin mixed-phase clouds. The researchers said it is difficult to separately study the ice formation process in clouds due to their interactions with aerosol particles, air motion, and the complexity of other smaller physical processes in clouds.

Using radar and laser technology, they assessed large cloud fields at about 2 to 8 kilometres height, with a width of about 100 to 200 metres. The study noted that the clouds contained extremely little ice in the range of micrograms per cubic metre.

The researchers said the clouds were thin enough to be penetrated by laser beams and radar. They found that increased turbulence does not directly increase ice formation, but enhanced the efficiency of water droplets coalescing to larger drops with a higher probability for ice formation.

Higher turbulence in the clouds, "enhances the entrainment of cloud condensation nuclei (CCN) and ice-nucleating particles into the cloud layer," the study noted. "The effect only became visible when we observed the ice directly below the clouds' top layer," said study co-author Johannes Buhl of TROPOS.

"Our findings enable for the first time quantitative and well constraint insights into the relationship between turbulence and ice formation in the atmosphere. The stronger a cloud is 'shaken' by vertical air motions, the more ice falls out of it," Buhl said. In the following studies, the researchers plan to explore the influence of aerosols in ice formation by taking a closer look at the beginning and end of the process.