Exploring a Stratocumulus-to-Cumulus Transition: A Perturbed Parameter Ensemble of Large-Eddy Simulations
Authors: Sansom Rachel , Lee Lindsay , Johnson Jill , Regayre Leighton , Carslaw Ken 
Published: March 2022

&lt;p&gt;&lt;span xml:lang=&quot;EN-US&quot; data-contrast=&quot;auto&quot;&gt;&lt;span data-ccp-parastyle-defn='{&quot;ObjectId&quot;:&quot;8c112871-d111-4339-b713-ce16d1de72eb124&quot;,&quot;ClassId&quot;:1073872969,&quot;Properties&quot;:[469775450,&quot;Text body&quot;,201340122,&quot;2&quot;,134233614,&quot;true&quot;,469778129,&quot;Textbody&quot;,335559740,&quot;276&quot;,201341983,&quot;0&quot;,335559739,&quot;140&quot;,469778324,&quot;Normal&quot;]}'&gt;The transition from stratocumulus&lt;/span&gt; &lt;span&gt;to&lt;/span&gt; &lt;span&gt;cumulus clouds that takes place &lt;/span&gt;&lt;span&gt;as air is advected from the subtropics towards the equator causes a decrease in cloud radiative effect&lt;/span&gt;&lt;span&gt;, with cloud fraction halving from start to finish. &lt;/span&gt;&lt;span&gt;The transition is initiated by &lt;/span&gt;&lt;span&gt;increas&lt;/span&gt;&lt;span&gt;ing sea surface temperatures&lt;/span&gt;&lt;span&gt;, and &lt;/span&gt;&lt;span&gt;it is widely agreed that &lt;/span&gt;&lt;span&gt;the lower tropospheric stability plays a key role in the tim&lt;/span&gt;&lt;span&gt;ing of the transition.&lt;/span&gt;&lt;span&gt; In this work, we study the relative importance of five atmospheric initial cond&lt;/span&gt;&lt;span&gt;itions&lt;/span&gt;&lt;span&gt;:&lt;/span&gt;&lt;span&gt; specific humidity in the boundary layer and free troposphere, free tropospheric potential temperature, inversion height and initial aerosol distribution. &lt;/span&gt;&lt;span&gt;We&lt;/span&gt; &lt;span&gt;simulate&lt;/span&gt; &lt;span&gt;a &lt;/span&gt;&lt;span&gt;Lagrangian&lt;/span&gt;&lt;span&gt; trajectory of a stratocumulus-to-cumulus transition, &lt;/span&gt;&lt;/span&gt;&lt;span xml:lang=&quot;EN-GB&quot; data-contrast=&quot;none&quot;&gt;&lt;span&gt;using the Met Office/NERC cloud model coupled with a &lt;/span&gt;&lt;span&gt;bulk microphysics scheme and a radiation scheme. &lt;/span&gt;&lt;span&gt;From this base simulation we &lt;/span&gt;&lt;span&gt;make 60 perturbations to simulate the transition under different combinations of &lt;/span&gt;&lt;span&gt;the &lt;/span&gt;&lt;span&gt;atmospheric &lt;/span&gt;&lt;span&gt;initial &lt;/span&gt;&lt;span&gt;conditions&lt;/span&gt;&lt;span&gt; mentioned&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;/span&gt;&lt;span xml:lang=&quot;EN-US&quot; data-contrast=&quot;auto&quot;&gt;&lt;span&gt;Additionally, we include a model parameter from the &lt;/span&gt;&lt;span&gt;Khairoutdinov&lt;/span&gt;&lt;span&gt; and Kogan &lt;/span&gt;&lt;span&gt;autoconversion&lt;/span&gt; &lt;span&gt;parameterisation&lt;/span&gt;&lt;span&gt; from 2000.&lt;/span&gt; &lt;/span&gt;&lt;span xml:lang=&quot;EN-GB&quot; data-contrast=&quot;none&quot;&gt;&lt;span&gt;We discuss here the relative importance of these so-called parameters, in particular the role of aerosol, and we explore whether a much faster transition by drizzle takes place in simulations with lower aerosol concentrations.&lt;/span&gt;&lt;/span&gt;&lt;span&gt;&amp;#160;&lt;/span&gt;&lt;/p&gt;