Introducing cloud horizontal overlap at NWP scales (1-10 km) in a fast 3D radiative transfer model
Authors: Manev Mihail , Mayer Bernhard , Emde Claudia , Voigt Aiko 
Published: March 2021

&lt;p&gt;&lt;span&gt;Interactions between radiation and clouds are a source of significant uncertainty in both numerical weather prediction &lt;/span&gt;&lt;span&gt;(NWP) &lt;/span&gt;&lt;span&gt;and climate models. &lt;/span&gt;&lt;span&gt;Future models need to both incorporate more realistic description of physical processes and be computationally efficient. &lt;/span&gt;&lt;span&gt;With the &lt;/span&gt;&lt;span&gt;steadi&lt;/span&gt;&lt;span&gt;ly&lt;/span&gt; &lt;span&gt;increasing&lt;/span&gt;&lt;span&gt; resolution of NWP models, &lt;/span&gt;&lt;span&gt;previously neglected effects like the horizontal propagation of radiation become more important. &lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Here we present a &lt;/span&gt;&lt;span&gt;hybrid&lt;/span&gt;&lt;span&gt; radiative transfer model that combines &lt;/span&gt;&lt;span&gt;a traditional twostream maximum random overlap &lt;/span&gt;&lt;span&gt;(twomaxrnd)&lt;/span&gt;&lt;span&gt; radiative solver &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;&amp;#268;&lt;/span&gt;&lt;span&gt;rnivec and Mayer, 20&lt;/span&gt;&lt;span&gt;19&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; with a Neighbouring Column Approximation &lt;/span&gt;&lt;span&gt;(NCA)&lt;/span&gt; &lt;span&gt;model &lt;/span&gt;&lt;span&gt;(Klinger and Mayer, 20&lt;/span&gt;&lt;span&gt;19&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt;, which parametrizes horizontal photon transport between adjacent grid-cells. &lt;/span&gt;&lt;span&gt;Thereby &lt;/span&gt;&lt;span&gt;the hybrid&lt;/span&gt; &lt;span&gt;includes&lt;/span&gt;&lt;span&gt; both subgrid-scale effects and grid-scale horizontal transport. &lt;/span&gt;&lt;span&gt;In addition&lt;/span&gt;&lt;span&gt; we introduced &lt;/span&gt;&lt;span&gt;a&lt;/span&gt;&lt;span&gt; horizontal &lt;/span&gt;&lt;span&gt;cloud &lt;/span&gt;&lt;span&gt;overlap scheme &lt;/span&gt;&lt;span&gt;to the hybrid model&lt;/span&gt;&lt;span&gt;. In order to &lt;/span&gt;&lt;span&gt;differentiate between different overlap concepts and deduce optimal overlap coefficients we used&lt;/span&gt;&lt;span&gt; high resolution radiative &lt;/span&gt;&lt;span&gt;transfer &lt;/span&gt;&lt;span&gt;simulations &lt;/span&gt;&lt;span&gt;of LES cloud fields &lt;/span&gt;&lt;span&gt;(horizontal resolution of 100-300 m)&lt;/span&gt; &lt;span&gt;deploying &lt;/span&gt;&lt;span&gt;a very accurate Monte Carlo &lt;/span&gt;&lt;span&gt;(MYSTIC)&lt;/span&gt;&lt;span&gt; model &lt;/span&gt;&lt;span&gt;(&lt;/span&gt;&lt;span&gt;Mayer, 2009&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt;.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&lt;span&gt;Further we assess the performance of the &lt;/span&gt;&lt;span&gt;hybrid&lt;/span&gt;&lt;span&gt; model &lt;/span&gt;&lt;span&gt;at the NWP scale (1-10 km)&lt;/span&gt;&lt;span&gt; for various &lt;/span&gt;&lt;span&gt;realistic &lt;/span&gt;&lt;span&gt;cloud configurations &lt;/span&gt;&lt;span&gt;using&lt;/span&gt;&lt;span&gt; results from the benchmark MYSTIC model&lt;/span&gt; &lt;span&gt;and determine the differences compared to other solvers that only consider either grid-scale or subgrid-scale effects, twomaxrnd, Tripleclouds and NCA.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;