A Multiscale Modelign Framework Study of Changes in Clouds and Atmospheric Circulation Associated with Rapid Adjustment Induced by Increasing Atmospheric CO2

The radiative heating increase due to increased CO₂ is the primary source for the rapid adjustment of atmospheric circulation and clouds. In this study, we investigate the rapid adjustment resulting from doubling of CO₂ and its physical mechanism using a multiscale modeling framework (MMF). The MMF includes an advanced higher-order turbulence closure in its cloud-resolving model component and simulates realistic shallow and deep cloud climatology and boundary layer turbulence. The rapid adjustment is characterized by 1) reduced ascent and descent strengths over the ocean, 2) increased lower tropospheric stability (LTS) over the subsidence region, 3) shoaling of planetary boundary layers over the ocean, 4) increased deep convection over lands and shift of cloud coverage from the ocean to lands, and 5) reduced sensible (SH) and latent heat (LH) fluxes over the ocean. Unlike conventional general circulation models and another MMF, reduction of the global-mean shortwave cloud radiative cooling is not simulated, due to the increase in low clouds at lower altitudes over the ocean resulting from increases in LTS of the subsidence regions. Changes in regional atmospheric circulations play a key role in cloud changes. Weaker energy transport resulting from CO₂ cloud and water vapor masking effect in the oceanic regions with strong large-scale ascent reduces the upward motion and convective clouds. The ocean-land transports are linked to the partitioning of surface SH and LH fluxes that increases humidity over lands and enhances deep convection over the tropical lands. Low-level clouds over non-desert lands increase rather than decrease over desert lands.