Exploring a Stratocumulus-to-Cumulus Transition: A Perturbed Parameter Ensemble of Large-Eddy Simulations

Abstract

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