Combining cirrus cloud CALIPSO (IIR-CALIOP) and aircraft measurements with climate modeling to evaluate the spatial and seasonal radiative impact of homogeneous ice nucleation
Authors: Mitchell David L. , Mejia John F. , Garnier Anne , Tomii Yuta , Krämer Martina , Hosseinpour Farnaz 
Published: March 2021

&lt;p&gt;Many global climate modeling studies over the last decade have attempted to evaluate the relative contributions of homo- and heterogeneous ice nucleation (henceforth hom and het) in cirrus clouds, and the radiative contribution of hom relative to het.&amp;#160; There is likely a spatial and seasonal dependence here.&amp;#160; Since the microphysical and radiative properties of hom- and het-dominated cirrus clouds are likely very different, the outcome of such studies may be important to climate science.&amp;#160; But since the physics determining the competition between hom and het is very complex, involving poorly constrained variables, results from such modeling studies have often contradicted each other.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;This study takes a different approach by using CALIPSO satellite effective diameter (D&lt;sub&gt;e&lt;/sub&gt;) retrievals from cirrus clouds, validated by recent in situ measurements (obtained from 24 field campaigns consisting of 150 flights), to constrain the cloud microphysics module (i.e., version 2 of the Morrison-Gettelman scheme or MG2) in the Whole Atmosphere Community Climate Model version 6 (WACCM6). [As a side-note, the ice particle number concentration N was calculated from the retrieved D&lt;sub&gt;e&lt;/sub&gt; and the in situ climatological ice water content and shown to be consistent with N retrievals based on a CloudSat-CALIPSO lidar-radar method]. The MG2 cirrus cloud ice particle size distribution was constrained to conform with these D&lt;sub&gt;e&lt;/sub&gt; retrievals that depend on temperature (T), latitude, season and land fraction (land vs. ocean).&amp;#160; The treatment of ice particle fall speeds was also revised.&amp;#160; Two 40-year WACCM6 simulations were differenced to obtain the radiative contribution of hom; one based on the retrieved D&lt;sub&gt;e&lt;/sub&gt; and one based on retrieved D&lt;sub&gt;e&lt;/sub&gt; corresponding to het conditions (where retrieved N was minimal).&amp;#160; The experimental design assumes hom-affected cirrus occur only outside the &amp;#177; 30 &amp;#176;latitude zone since cirrus within this zone exhibited the lowest N and were thus used to produce the D&lt;sub&gt;e&lt;/sub&gt; &amp;#8211; T look-up tables corresponding to het conditions.&amp;#160; These D&lt;sub&gt;e&lt;/sub&gt; &amp;#8211; T relationships for het conditions were applied to the entire planet in one simulation (labeled HET) while the other simulation (labeled CALCAL for CALIPSO-calibrated) is based on the actual D&lt;sub&gt;e&lt;/sub&gt; retrievals.&amp;#160; CALCAL &amp;#8211; HET differences in the cloud radiative effect (CRE) reveal the estimated CRE effect due to hom.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The results show CALCAL &amp;#8211; HET CRE differences of 2.4 and 2.5 W m&lt;sup&gt;-2&lt;/sup&gt; in the northern and southern hemispheres, respectively.&amp;#160; These CRE differences are largely due to cirrus-induced changes in mixed phase clouds.&amp;#160; However, top-of-model (TOM) CALCAL &amp;#8211; HET differences in total net forcing did not match these CRE differences due to mid-level increases in relative humidity in HET relative to CALCAL, so that these TOM differences were 1.8 and 2.0 W m&lt;sup&gt;-2&lt;/sup&gt; in the northern and southern hemispheres, respectively.&amp;#160; Radiative contributions from hom were minimal during the summer months (JJA) since shortwave and longwave cloud forcing tends to cancel then.&amp;#160; Other studies show this is true for the tropics (reinforcing the realism of our experimental design from a radiation purview).&amp;#160; During non-summer months, the TOM CALCAL &amp;#8211; HET difference in total net forcing was 2.4 W m&lt;sup&gt;-2&lt;/sup&gt; in both hemispheres.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;