Text S1.
MISR Simulator Base State Climatology
This section of the supplement is dedicated to understanding how the
mean cloud climatology or base state of the models differs from
observations. Here and in later analysis, the MISR output is averaged
over 5 latitude bands: Tropics -20° to 20°, Subtropics ±20° to ±40°, and
Midlatitudes ±40° to ±60°. The mean CTH-OD histogram observed by MISR,
along with simulated histograms for CESM2 and IPSL SSP 585 simulations
are shown in Figures S1 and S2 . All three histograms are
averages taken over the period 2001 to 2015, which is the portion of the
SSP 585 simulation that is based on historical emissions (rather than
projected emissions) that overlaps with the MISR record. The MISR
observations are restricted to ocean, and the model averages are
restricted to ocean to match.
Figure S1 shows that distribution of clouds in CESM2 and IPSL
differ noticeably between each other and with the observations. Of
particular note is that IPSL does not produce much mid-level cloud
(between 5 and 7 km), which have a cooling effect on the tropical
climate (Bourgeois et al., 2016), and produces more optically thin high
clouds (OD less than 3.6) than is observed by MISR. In contrast, CESM2
has more mid-level clouds and has more high cloud with optical depths
between 1.3 and 23 than observations. In both models high clouds are
predominantly located between 11 and 13 kilometers (in agreement with
observations), but the observed distribution is wider, with more cloud
occurring outside of the 11-13 km bin, than the models (especially
IPSL). The thin distribution of IPSL has implication for our WCTH that
are discussed below.
The histograms averaged over the subtropics and midlatitudes are shown
in Figure S2 . Total cloud occurrence in the subtropics
(20o to 40o) is dominated by low
clouds, with much less high and mid-level clouds as compared to the
tropics. High clouds are lower in altitude and optically thinner on
average than in the tropics. The most occupied vertical bin is between 9
and 11 km in the Norther Hemisphere (NH) and 7 to 9 km in the Southern
Hemisphere (SH). In both models and observations there are fewer
optically thin clouds above 5 km in the SH than in the NH.
In the midlatitudes the cloud patterns are dominated by midlatitude
cyclones. Optically thick high clouds occur lower in the atmosphere than
in the subtropics or tropics in both observations and models. The
presence of a mid-level cloud peak remains visible in the MISR
observations and CESM2 simulations, located in the 3 to 4 km height bin.
In IPSL, high clouds are optically thicker than observations, with a
significant fraction of high clouds having optical depth greater than
60.
Bourgeois, Q., Ekman, A. M. L., Igel, M. R., & Krejci, R. (2016).
Ubiquity and impact of thin mid-level clouds in the tropics.Nature Communications , 7 (1), 1–6.
https://doi.org/10.1038/ncomms12432