Potential of natural seeding by ice clouds over Switzerland

<p><span><span>The ice phase in clouds d</span></span><span><span>etermines</span></span><span><span> many of their key properties </span></span><span><span>and </span></span><span><span>influences </span></span><span><span>the water cycle, since</span></span> <span><span>most precipitation globally originates from the ice phase</span></span><span><span>. I</span></span><span><span>ce crystals falling as seeds from an ice cloud into a lower lying mixed-phase or liquid cloud can </span></span><span><span>influence i</span></span><span><span>ce and precipitation formation</span></span><span><span>. In the lower lying cloud, the </span></span><span><span>ice crystals</span></span> <span><span>feed on the liquid, grow and enhance precipitation </span></span><span><span>(seeder-feeder mechanism)</span></span><span><span> or trigger glaciation </span></span><span><span>(natural cloud seeding)</span></span><span><span>. Th</span></span><span><span>e</span></span><span><span> seeder-feeder mechanism has been associated with the </span></span><span><span>intensification</span></span><span><span> of extreme precipitation and flooding.</span></span></p><p><span><span>Even though t</span><span>here have been </span><span>multiple</span><span> case studies of the seeder-feeder mechanism </span><span>and a few on natural cloud seeding,</span><span> estimate</span><span>s</span><span> of </span><span>the </span><span>occurrence frequency </span><span>of these processes are</span><span> lacking.</span></span></p><p><span><span>We derive</span></span><span><span>d</span></span><span><span> t</span></span><span><span>he frequency of </span></span><span><span>possible seeding situations (</span></span><span><span>ice-layer above liquid or mixed-phase cloud layer</span></span><span><span>)</span></span><span><span> from </span></span><span><span>radar/lidar </span></span><span><span>satellite observations over Switzerland. </span></span><span><span>We found an ice layer to be present above a</span></span><span><span>nother</span></span><span><span> cloud layer about 20% of </span></span><span><span>the time</span></span><span><span>. The distance between the cloud layers was </span></span><span><span>uniform</span></span><span><span>ly distributed between 100m and 10km. In </span></span><span><span>sublimation</span></span><span><span> calculations </span></span><span><span>we used the mean effective ice crystal radius from the satellite observations and calculated the crystals&#8217; sublimation height, assuming a spherical shape. I</span></span><span><span>n a significant number of cases ice crystals </span></span><span><span>would</span></span><span><span> survive the fall between the two cloud layers. </span></span><span><span>We investigated the effect of the falling ice crystals on the lower lying cloud layer and on precipitation formation i</span></span><span><span>n sensitivity studies of s</span></span><span><span>elected</span></span><span><span> situations </span></span><span><span>with the regional climate model</span></span><span><span> COSMO.</span></span></p><p><span><span>T</span></span><span><span>he high occurrence frequency of seeding situations and the survival of the ice crystals </span></span><span><span>i</span></span><span><span>ndicate</span></span><span><span> the</span></span><span><span> seeder-feeder process </span></span><span><span>and natural cloud seeding</span></span> <span><span>as widespread</span></span><span><span> phenomen</span></span><span><span>a</span></span><span><span>. To infer </span></span><span><span>their</span></span><span><span> importance, the magnitude of the seeding ice crystals&#8217; effect </span></span><span><span>on lower lying clouds and precipitation </span></span><span><span>needs to be established.</span></span></p>