Abstract
We conduct a global assessment of the spatial heterogeneity of cloud
phase within the temperature range where liquid and ice can coexist.
Single-shot CALIOP lidar retrievals are used to examine cloud phase at
scales as fine as 333 m, and horizontal heterogeneity is quantified
according to the frequency of switches between liquid and ice along the
satellite’s path. In the global mean, heterogeneity is greatest between
$-$15 and $-$4 \textdegree C with a peak at $-$5
\textdegree C, when small patches of ice are prevalent
within liquid-dominated clouds. Heterogeneity “hot spots” are typically
found over the extratropical continents, whereas phase is relatively
homogeneous over the Southern Ocean and the eastern subtropical ocean
basins, where supercooled liquid clouds dominate. Even at a fixed
temperature, heterogeneity undergoes a pronounced annual cycle that, in
most places, consists of a minimum during autumn or winter and a maximum
during spring or summer. Based on this spatial and temporal variability,
it is hypothesized that heterogeneity is affected by the availability of
ice nucleating particles. These results can be used to improve the
representation of subgrid-scale heterogeneity in general circulation
models, which has the potential to reduce longstanding model biases in
cloud phase partitioning and radiative fluxes.