Combining cirrus cloud CALIPSO (IIR-CALIOP) and aircraft measurements with climate modeling to evaluate the spatial and seasonal radiative impact of homogeneous ice nucleation

Abstract

<p>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.  There is likely a spatial and seasonal dependence here.  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.  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.</p><p> </p><p>This study takes a different approach by using CALIPSO satellite effective diameter (D<sub>e</sub>) 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<sub>e</sub> 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<sub>e</sub> retrievals that depend on temperature (T), latitude, season and land fraction (land vs. ocean).  The treatment of ice particle fall speeds was also revised.  Two 40-year WACCM6 simulations were differenced to obtain the radiative contribution of hom; one based on the retrieved D<sub>e</sub> and one based on retrieved D<sub>e</sub> corresponding to het conditions (where retrieved N was minimal).  The experimental design assumes hom-affected cirrus occur only outside the ± 30 °latitude zone since cirrus within this zone exhibited the lowest N and were thus used to produce the D<sub>e</sub> – T look-up tables corresponding to het conditions.  These D<sub>e</sub> – 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<sub>e</sub> retrievals.  CALCAL – HET differences in the cloud radiative effect (CRE) reveal the estimated CRE effect due to hom.</p><p> </p><p>The results show CALCAL – HET CRE differences of 2.4 and 2.5 W m<sup>-2</sup> in the northern and southern hemispheres, respectively.  These CRE differences are largely due to cirrus-induced changes in mixed phase clouds.  However, top-of-model (TOM) CALCAL – 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<sup>-2</sup> in the northern and southern hemispheres, respectively.  Radiative contributions from hom were minimal during the summer months (JJA) since shortwave and longwave cloud forcing tends to cancel then.  Other studies show this is true for the tropics (reinforcing the realism of our experimental design from a radiation purview).  During non-summer months, the TOM CALCAL – HET difference in total net forcing was 2.4 W m<sup>-2</sup> in both hemispheres.</p><p> </p>