Abstract
The uncertainty in polar cloud feedbacks calls for process understanding
of the cloud response to climate warming. As an initial step toward
improved process understanding, we investigate the seasonal cycle of
polar clouds in the current climate by adopting a novel modeling
framework using large eddy simulations (LES), which explicitly resolve
cloud dynamics. Resolved horizontal and vertical advection of heat and
moisture from an idealized general circulation model (GCM) are
prescribed as forcing in the LES. The LES are also forced with
prescribed sea ice thickness, but surface temperature, atmospheric
temperature, and moisture evolve freely without nudging. A semigray
radiative transfer scheme without water vapor and cloud feedbacks allows
the GCM and LES to achieve closed energy budgets more easily than would
be possible with more complex schemes. This enables the mean states in
the two models to be consistently compared, without the added
complications from interaction with more comprehensive radiation. We
show that the LES closely follow the GCM seasonal cycle, and the
seasonal cycle of low-level clouds in the LES resembles observations:
maximum cloud liquid occurs in late summer and early autumn, and winter
clouds are dominated by ice in the upper troposphere. Large-scale
advection of moisture provides the main source of water vapor for the
liquid-containing clouds in summer, while a temperature advection peak
in winter makes the atmosphere relatively dry and reduces cloud
condensate. The framework we develop and employ can be used broadly for
studying cloud processes and the response of polar clouds to climate
warming.