Potential of natural seeding by ice clouds over Switzerland
Authors: Proske Ulrike , Bessenbacher Verena , Dedekind Zane , Lohmann Ulrike , Neubauer David 
Published: March 2020

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